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Tuesday, 12 May 2020

Spinosaurus 2020: thoughts for artists

A 2020 take on some dinosaur or another. I forget its name. This individual has recently gorged itself, resulting in a distended belly and sleepy demeanour.

Unless you've been living under a rock for the last fortnight you cannot have escaped news on one of the most famous and controversial of all dinosaurs: Spinosaurus aegyptiacus. The appearance of Spinosaurus has once again transformed via the discovery of new fossils unearthed from the Late Cretaceous Kem Kem beds of Morocco: chiefly, a long paddle-like tail of superficially newt or crocodylian-like flavour. Keen interest in Spinosaurus, as well as a large National Geographic-led PR campaign for the new study, has seen social media awash with discussion about the new discovery, and illustrations of the latest in spinosaurine fashion have swamped online galleries since. As Chris Dipiazza eloquently explained on Twitter, it hasn't been the best two weeks if you aren't a Spinosaurus fan.
At the risk of numbing everyone further to Spinosaurus, I want to share some thoughts and reactions to this new research here. We've covered a few (but not all) of the twists and turns of Spinosaurus research in recent years (posts 1, 2, 3, 4) but, rather than simply writing another blog summary or popular rehashing of the new findings, I thought I'd write this from an artistic perspective, based on reading I conducted to produce my own take on "Spinosaurus 2020", shown above. As keen-eyed readers will note, I've not slavishly stuck to the same interpretations currently circulating the press circuit because - as we'll see - our takes on Spinosaurus are more complex than ever. Even with the tremendous amount of new data published on Spinosaurus in the last two decades, it remains the ultimate moving target for dinosaur palaeaoartists.

Spinosaurus 2020: where are we now?

Before we dive into this post, it makes sense to go beyond the recent Spinosaurus press coverage to look at what's in the new papers, as there's a lot more to them than what is being reported in the popular press. Needless to say, Spinosaurus has been an especially hot topic in dinosaur palaeontology since 2014 when Nizar Ibrahim and colleagues placed a newly discovered partial skeleton from the Moroccan Kem Kem beds at the core of a radical reinterpretation of a genuinely enigmatic animal. This was our introduction to Spinosaurus as a potentially short-legged semi-aquatic species, as well as proposals that spinosaurine material from across Northern Africa should be collected into one species, S. aegyptiacus, sinking several named taxa in the process (Ibrahim et al. 2014). It also proposed that the new partial skeleton should be the replacement exemplar specimen - the neotype - for S. aegyptiacus, after the original was destroyed in the Second World War to leave us with only Ernst Stromer's original descriptive work and photographs as records of its existence. Though widely publicised and catalysing a wave of public interest in Spinosaurus, the proposals of Ibrahim et al. (2014) proved controversial among academics. Numerous responses - some supportive, some critical - have been published by theropod researchers since.

Semiaquatic adaptations in a giant predatory dinosaur | Science
Spinosaurus as illustrated by Ibrahim et al. 2014. Many aspects of this reconstruction have been questioned and scrutinised in the last six years, but new data suggests that this may be closer to the appearance and proportions of certain spinosaurines than many of us initially believed.

Fast forward to today, and we've got not one, but two new papers by Nizar and colleagues that push discussions of all things Spinosaurus along significantly. The first is not Spinosaurus specific, but is an extensive monograph on the geology and palaeontology of the Kem Kem beds, now formally termed the Kem Kem Group (Ibrahim et al. 2020a). This is an important paper that brings some long-needed clarity and formality to details of Kem Kem stratigraphy and geology, including a new nomenclatural scheme to divide the Kem Kem into the Gara Sbaa and Douira formations. This is an important step for understanding the provenance of Kem Kem fossils which clarifies details of palaeoenvironments, relative ages of fossils, and comparisons with other fossil faunas (Ibrahim et al. 2020a). The entire fossil record of the Kem Kem Group is also reviewed, including a large discussion about Spinosaurus and its status as a Kem Kem theropod. Anyone interested in Spinosaurus and its world will need to check this paper out.

The second paper concerns additional material of the same 'neotype'* individual published in 2014, recovered from the same locality in more recent fieldwork (Ibrahim et al. 2020b). Among other finds, this includes a nearly complete tail that reinvents the appearance of Spinosaurus for the second time in six years. In addition to being short-limbed, it now seems that Spinosaurus had a deep, fin-like tail comprising narrow vertebrae with long and reclined neural spines and chevrons. This is interpreted as evidence of a swimming predatory ecology by the authors, it being argued that Spinosaurus could have swum like a crocodylian or newt to chase prey. The associated PR provides large amounts of media - videos, artwork etc. - showing Spinosaurus as a deep-diving species adapted to chasing large fish. This is not the first time Spinosaurus has been interpreted as a strong swimmer in recent years (e.g. Gimsa et al. 2016; Arden et al. 2019) but the recovery of a fin-like caudal skeleton adds a lot more weight to this argument.

*I'm going to refer to this specimen as the 'neotype' throughout this article for readability, as it's a catchier name than FSAC-KK 11888. That the proposal that FSAC-KK 11888 should be the Spinosaurus neotype remains controversial however, and will likely remain so until it's fully described and we can properly evaluate its similarity to Spinosaurus. I don't have a horse in this race but, for what it's worth, FSAC-KK 11888 looks like a member of Spinosaurus to me, although it has several differences from S. aegyptiacus that require investigation.

Artistic speculations that Spinosaurus may have borne some sort of tail fin have been common since the 2014 reinterpretation of the genus. Here's my finned version from 2016.

Both papers also provide comments in defence of Spinosaurus palaeobiology as proposed by Ibrahim et al. (2014). Criticism of their work included doubts about the authenticity and scaling of the neotype skeleton (Evers et al. 2015; Henderson 2018); its biomechanical feasibility as a swimming animal (Henderson 2018); the appropriateness of collating widely-dispersed and anatomically-distinguished North African spinosaurine material into one species (a question with particular reference to the overall number of spinosaurines in the Kem Kem) (Evers et al. 2015; Hone and Holtz 2015; Hendrickx et al. 2016; Maganuco and Dal Sasso 2018; Lakin and Longrich 2019); the suitability of the proposed Spinosaurus neotype (Evers et al. 2015; Maganuco and Dal Sasso 2018); and the general ecology of spinosaurines (Hone and Holtz 2015, 2019; Henderson 2018). The new data presented by Ibrahim et al. (2020a, b) addresses some of these concerns to an extent that some criticism - as we'll explore below - can probably be laid to rest. However, the enhanced debate around all things Spinosaurus means that these new papers have arrived in a much busier and more heated academic realm than their 2014 counterpart, and initial impressions from key players in spinosaurine research imply conversations will remain ongoing about aspects of lifestyle and taxonomy. For artists, this complicates our view of what Spinosaurus and other spinosaurines may have looked like, as well as what we can show it doing. While contributing important primary data on Spinosaurus, we have to remember that these new papers represent one interpretation of the appearance and lifestyle of a most unusual dinosaur in an increasingly busy academic debate, and that the ball is still in play.

Body plan and proportions

With that set up, it's time to dig into some art-relevant details. Firstly, I think Ibrahim et al. (2020b) adds a lot more confidence to the proposed strange proportions of Spinosaurus. The authenticity and scaling of the neotype have been questioned on grounds that it was collected, purchased and excavated by different people at different times (Evers et al. 2015; Henderson 2018), but the recovery the new tail and other elements in the same site as the pelvic, hindlimb and torso material, as well as their concordant proportions, suggests that all these remains were genuinely associated and likely belong to one individual (Ibrahim et al. 2020b). There is no evidence of other species in the bonebed and many broken bones of the neotype have now been reunited with once-missing pieces. Their histology and inferred growth stage are also matching. Courtesy of a quarry map illustration, we have a good idea of how these elements were associated in the field and how they relate to the material published in 2014.

Quarry map of the neotype locality and skeletal reconstruction of Spinosaurus, from Ibrahim et al. (2020b). Note the large area in which bones were found, the absence of non-spinosaurine bones, and the absence of bone duplicates: this is good evidence of the neotype representing a single individual, no matter how peculiar its proportions are. Known elements of the neotype are shaded in the skeletal, with different colours reflecting different field seasons and quarry locations. Scale bar represents 1 m.

With these data, and the fact that another spinosaurine specimen (Stromer's "Spinosaurus B") shows the same short-limbed morphology (Ibrahim et al. 2014), I think we can be fairly confident that at least some spinosaurines really were long-bodied, short-legged creatures with a body plan basically akin to that outlined by Ibrahim et al. (2014, 2020b). I know some folks are still holding out for data proving that the pelvis and hindlimbs belong with the vertebral column, but I think the burden of proof has shifted in light of these new data. Why aren't these legs associated with the body, given what we now know about the taphonomy of the site? A common question online is how much bearing the new tail has on other spinosaurids. We have sufficient skeletal remains of baryonychine spinosaurids (e.g. Baryonyx, Suchomimus) to suggest that they weren't fin-tailed, but the tails of spinosaurine spinosaurids aren't well known. The dorsal and caudal vertebrae of Ichthyovenator compare well with Spinosaurus, however (Allain et al. 2012), and it may have sported similar tail anatomy.

Posture and balance

Within the supplementary data of Ibrahim et al. (2020b) is a discussion of Spinosaurus mass and centre of gravity based on the (estimated) 11 m long neotype individual. Using a digital model and varying takes on tissue density, a mass of 3,219-4,173 kg was predicted and the centre of gravity was found to be just over one femur-length from the pelvic limb joint. This is fractionally more posterior than modelled in the 2014 model, if not quite as close to the pelvis as predicted by Henderson (2018). The cause of this shift is the larger tail and, although subtle, this difference has forced a reassessment of one of the most controversial aspects of the 2014 study: the presentation of Spinosaurus as a quadruped. Ibrahim et al. (2020b) now favour a facultative, rather than obligate, quadrupedal gait for terrestrial locomotion.

For artists, this means we can be a little more comfortable posing Spinosaurus as a biped, and I wonder if further work will substantiate bipedal poses further. Elsewhere in the supplementary data, Ibrahim et al. (2020b) suggest that the volume of restored tail musculature is conservative, and it stands to reason that models with more substantive tail volumes will pull the centre of gravity rearwards. Moreover, I wonder if the restored neck bulk is a little on the thick side, making the model more front-heavy. Among the neotype elements are long cervical ribs which, assuming typical tetrapod neck anatomy, could indicate displacement of some ventral neck muscles towards the torso (Taylor and Wedel 2013). Given that Spinosaurus already seems to have had a longish, low neck skeleton, displacing some of the neck muscle fraction posteriorly could have made for a relatively slender neck that would lighten the front end. If Spinosaurus also walked a little more upright than a typical theropod - using poses proposed by Andrea Cau, say - it might have avoided quadrupedality altogether.

Extended Data Fig. 8
Centre of mass estimates from Ibrahim et al. (2020b), compared to that of Henderson 2018 (C) and Ibrahim et al. 2014 (D).

I'm aware that some people feel that the legs of the neotype specimen are too slender to support the weight of Spinosaurus on land. The predicted 3 - 4-tonne masses of the neotype individual are relatively lightweight compared to theropods of similar length (>10 m theropods in the dataset of Benson et al. 2014 mass at 6-7 tonnes, for instance) and the hindlimbs would have to be held pretty straight to clear the animal from the ground (see illustrations, above). If so, the hindlimbs might have been loaded more like columns and imparted greater support than a traditionally bent theropod limb. Using hindlimb measurements from Ibrahim et al. (2014), I ran some very basic calculations on the strength of the neotype femur and found it critically weak against bending: it would fail when loaded with less than one 4-tonne body weight. When loaded as a column, however, it could take multiple 4-tonne masses. These calculations were very basic and ignore a lot of the nuance associated with theropod femoral posture but, if basically accurate, they suggest that the hindlimbs were strong enough to support Spinosaurus on land without help from weight-bearing forelimbs. I won't share the full details of these sums here as this post is already very long, but I can produce a follow-up article if it's of interest. Furthermore, while the hindlimbs themselves are small, there is evidence that aspects of their musculature - such as the caudofemoralis (a powerful hindlimb retractor) - were not reduced. In occupying much of the top half of the femur, the fourth trochanter of the neotype Spinosaurus femur is proportionate to the rest of the body (see for yourself in the 2014 image above, panel I, label 'ft') and suggests that the legs were capable of propelling their owner forward with suitable force, perhaps without propulsive assistance from the forelimb.

A topic I'm going to avoid here is the swimming posture of Spinosaurus, as this is an area that warrants further investigation before anything concrete can be said. I feel that the digital floating experiments with the 2014 Spinosaurus reconstruction by Don Henderson (2018) presented several worthy criticisms of Spinosaurus as an underwater swimmer, including its inability to sink due to pneumatised skeletal components (though some bones of Spinosaurus were pachyostosic (Ibrahim et al. 2014), its skull, neck and dorsal vertebrae were not) and the elevated centre of mass created by the tall, dense sail. A caveat about this study is that Spinosaurus had a relatively wider torso than was factored into the floating model, which would likely impact placement of the centre of mass and thus stability. We shouldn't dismiss Don's work because we assume this will correct the tipping issue, however: we need to see this investigated. We also have to consider the impact a wider torso would have on the suggested 'unsinkable' nature of Spinosaurus, as a wider torso will increase the lung volume fraction and impact buoyancy. For the time being we perhaps need to recognise that the body plan of Spinosaurus, even with its new tail, is entirely unlike any swimming animals alive today and that it's challenging to know how it functioned in water. Our science on this unusual dinosaur is in its infancy, and forming robust ideas about its swimming pose and capability is going to take time.

Floating spinosaurids in lateral and dorsal views.
Floating spinosaurids from Henderson (2018). One of the take-homes from Don's work is that Spinosaurus did not have an unusual floating posture among theropods, and that theropods were, in general, capable of floating with their heads well clear of the water to breathe. This questions whether features of the Spinosaurus skull linked to aquatic lifestyles - like the position of the eyes and nose - were specific adaptations to aquatic lifestyles.

Sail shape

One area where I'm less certain about the proportions of our new Spinosaurus reconstruction is the shape of the torso sail. Reconstructing the sail shape of Spinosaurus has always been difficult because the original S. aegyptiacus vertebrae were already not in great shape before Allied bombs blew them to pieces. As shown in Stomer's 1915 plates, the Spinosaurus neural spines were mostly disassociated from their centra; some were broken or deformed at their tips; and their arrangement within the vertebral series was not clear, even to those who saw them in person (Smith et al. 2006). Accordingly, several ideas about Spinosaurus vertebral order and sail shape have been proposed in the last century. While we seem to have a reasonable handle on the arrangement of the anterior sail vertebrae (artists, note that the neural spines project somewhat forward as well as up here: this is a common mistake in spinosaurine art), the shape of the posterior sail slope is more open to interpretation. Originally mounted in the Paläontologische Staatssammlung as a short, tightly-arced sail, Stromer rearranged the vertebrae into a longer, more gently sloping sail in 1936. Later, noting the reclined nature of the posteriormost-known sail spine, others proposed that the sail extended onto the tail (proposed independently by Andrea Cau in 2008 and Jaime Headden in 2010; Paul (2016) shows a similar arrangement while also matching Stromer's 1936 interpretation). More recently, Ibrahim et al. (2014; 2020b) have revived aspects of the pre-1944 Munich arrangement which brings shorter, sometimes anteriorly-positioned spines into a more posterior position (below).

Various restored shape shapes from a century of Spinosaurus. Images from Smith et al. 2006 and Ibrahim et al. 2014. Be sure to check out other takes on this sail by Andrea Cau, Jaime Headen and Scott Hartman.

I don't want to pretend that I know which of these arrangements is correct. Arranging these vertebrae is complicated, and there are multiple, perhaps equally viable ways we can order them at present. Based on the new tail data, I suspect the interpretation of Stomer and Ibrahim et al. are correct in restoring the sail plunging sharply into the tail base, but I also see merit to Stomer's 1936 model where vertebra 'f' - the cause of the dip in the Ibrahim et al. model - is positioned more anteriorly.

Unfortunately, the neotype material seems to complicate the resolution of the sail shape further. The sail spines of the neotype are noticeably more slender than those of the holotype despite coming from animals of generally similar size (the neotype is an estimated 11 m long, vs 12 m for the holotype; Dal Sasso et al. 2005; Ibrahim et al. 2020b) and, as preserved, they are quite a bit shorter. Does this imply a lower, less robust sail in the neotype individual, or is this something to do with growth, sexual dimorphism, or another form of variation within Spinosaurus? It's here where our taxonomic assumptions start impacting our reconstructions. Ibrahim et al. (2020a, b) regard S. aegyptiacus as an anatomically variable species, suggesting that we might be OK to blend data from the holotype and neotype sails. Conversely, other schemes regard S. aegyptiacus as potentially confined to Egypt and cast the 'neotype' as a closely related animal (e.g. Evers et al. 2016; Maganuco and Dal Sasso 2018), in which case we might focus more on the sail shape specifically indicated by the Kem Kem specimen. I don't know that there's a clear answer to this conundrum, so artists probably have several options for Spinosaurus sail shapes at present. My own reconstruction follows a somewhat more Stromer 1936-compliant model, as well as a sail height conservatively modelled on the neotype specimen.

Tail flexion

Among the more interesting aspects of the new Spinosaurus tail is the reduction of zygapophyses in the distal region. This potentially allowed the tail to flex far more than was typical for a theropod and to be used for swimming (Ibrahim et al. 2020b). I was initially sceptical of this claim because the long neural spines of the tail extend not only upwards, but also backwards over several other vertebrae, meaning that any movement between vertebrae required the spines to bend in multiple places or else project at wide angles from the tail curve. This is not a novel observation on neural spine length in potentially aquatic animals: I'm basically rehashing arguments made by Silvio Renesto et al. (2010) about the unusual tail of the drepanosaur Hypuronector, and how its extremely long, backwards-projecting chevrons stiffened the tail against sculling-like swimming motions. What I forgot, however, was that Hypuronector also had very developed 'clamping' zygapophyses (Renesto et al. 2010), and what I didn't realise is that - according to folks who know a lot more about biomechanics than I do - the 15 mm wide neural spines of the Spinosaurus tail could probably bend quite far. The bones of healthy living animals are somewhat plastic and capable of flexion, but I was surprised to learn that muscles and ligaments binding the Spinosaurus tail together would let relatively thick bony rods bend considerably without failing. So perhaps there's less of a problem here than I anticipated, though I admit to wondering how this would work given that Ibrahim et al. (2020b) only reconstruct a very slight covering of soft-tissues on the distal neural spines (below). If muscles only extended up the basal portion of the spine, was this enough to hold the tail together as it sculled the animal through water?

Fig. 1
The new tail of Spinosaurus, as presented by Ibrahim et al. (2020b). Note the reduction of musculature in the distal tail ('e') in relation to the discussion of bone bending, above.

Based in part on these discussions, I've been wondering how much flexibility we can safely reconstruct in the tail. Some of the recent PR imagery has shown a degree of tail flexion that seems beyond that of crocodylians, which seems excessive even allowing for some plasticity in the tail bones. Crocodylian tails have relatively short neural spines and chevrons, as well as large transverse processes to anchor large, strong musculature along much of the tail length. This allows them to pull their tails into tight arcs but, as noted by Ibrahim et al. (2020b), the transverse processes in Spinosaurus are restricted to the anterior tail region in a pretty typical theropod fashion. This musculoskeletal arrangement is thus not very crocodylian-like, and I wonder if the tail was more flexible than usual for a theropod, but maybe not to the degree where it could form a tight, crocodylian-style arc. I also wonder if the energy stored in bending neural spines would spring the tail straight once muscular effort was relaxed, which might have been especially significant when the tail was unrestrained during walking or floating. Maybe, for all its potential flexibility, the tail was held largely straight unless it was actively being used in swimming, or braced against something in the environment.

Facial anatomy and lips

To close out this post, I want to briefly touch on a topic not directly covered in the recent Spinosaurus work, but that comes up whenever spinosaurid illustrations are discussed: did these animals have lipless, crocodylian-like faces? In my experience, lipless spinosaurids are justified by several lines of evidence: their superficially crocodylian-like jaws and teeth; the size and configuration of their anterior teeth (where large premaxillary teeth overbite the lower jaw and long dentary teeth - unusually for a theropod - protrude over the upper jaw during occlusion; Dal Sasso et al. 2005), and the development of liplessness in other semi-aquatic fishers, such as crocodylians and river dolphins.

But when looking at spinosaurid jaws with the same criteria generally used to predict extra-oral tissues in fossil animals (tooth size, tooth orientation, jaw bone foramina counts), spinosaurids do not seem unusual compared to other theropods. Their jaws appear peculiar in some ways - check out that foramina rich anterior rostrum, below - because of their atypical geometry, but beyond this, much of their jaw configuration is typically theropodan. Their jaw foramina counts, for example, are not significantly high. Foramina frequency in tetrapod jawbones (premaxilla, maxilla, dentary) have been provisionally hypothesised as indicating the presence of extra-oral soft-tissues in tetrapods (Morhardt 2009), so we can compare foramina counts of Spinosaurus to other tetrapods to infer their facial configuration. Ibrahim et al. (2014) give a Spinosaurus upper jaw foramina frequency of 125, which seems high, but this value represents four bones worth of foramina. Crocodylians have this many foramina, and perhaps many hundreds more, in a single jaw bone. Morhardt (2009) suggests that we need about 100 foramina per jaw bone to infer a lipless condition, which Spinosaurus is well short of. This point recalls comments that the foramina counts and inferred sensitivity of Spinosaurus jaws, which have been correlated to aquatic lifestyles by some authors (Ibrahim et al. 2014), may have been pretty standard for large theropods (Barker et al. 2017), and are possibly not related to aquatic lifestyles or unusual facial anatomy.

Spinosaurids are often suggested to be among the more likely dinosaurian candidates for liplessness and exposed teeth, but the key features we might look for regarding this condition - labial foramina counts and distribution, as well as jaw bone texture - are not atypical for theropods, nor are they especially crocodylian-like. Their large teeth, including those at the jaw anterior, are no larger (relatively speaking) than those of extant animals with immobile lips and sheathed dentitions (bottom row). Spinosaurus elements after Dal Sasso et al. (2005), Neovenator after Barker et al. (2017); American alligator cropped from original on Wikimedia by Didier Descouens, CC BY-SA 4.0.

We can also observe that the maxillary and dentary jaw foramina of Spinosaurus are arranged in a more lizard-like row along their oral margins, and not - as in crocodylians - distributed in a dense pattern across the entire jaw. In Spinosaurus at least, they seem to be placed some distance from the toothrow (Dal Sasso et al. 2005) in a lizard-like configuration. This would keep the nerves and blood vessels running into any lip tissues well clear of the overlapping dentary teeth when the mouth was closed, but - based on lizards with similarly displaced foramina - I don't think this means anything too radical for life appearance (lips, if present, would not look unusually big or weirdly anchored). The absence of unusual, epidermally-derived textures on Spinosaurus jaw bones is a further distinction from crocodylians. As we've discussed at length in other posts, the characteristic rugosity of crocodylian skulls is reflective of their facial skin and sensory tissues, so the absence of comparable characteristics in Spinosaurus is strong evidence of a different anatomical regime. I'm also not convinced that the teeth of Spinosaurus - so far as they are known (to my knowledge, Spinosaurus jaws with a complete set of teeth remain elusive) - are too large for sheathing behind lips. We have reptiles today with large teeth at their jaw tips and they do not protrude from their lips (above): to the contrary, you'd have no idea they were there from their external appearance.

Finally, what about the purported link between liplessness and fishing aquatic lifestyles? I feel that this reflects a focus on lipless semi-aquatic or aquatic tetrapods but ignorance of the great number of secondarily-aquatic fishers that have retained fully-sheathed dentitions. Yes, crocodylians and river dolphins have unsheathed teeth, but many other fishing swimmers - cetaceans, seals, otters, mink, water monitors, numerous snake species and so on - do not. In fact, many have facial tissues little different to their terrestrial relatives. This questions whether lifestyle is a useful predictor for facial anatomy in Spinosaurus. This is surely a problematic line of evidence anyway, given that it remains to be determined exactly what sort of habits were common to Spinosaurus. Was Spinosaurus an underwater pursuit predator (Ibrahim et al. 2014, 2020b; Gimsa et al. 2016), something more akin to a heron (Hone and Holtz 2015, 2019; Henderson 2016), or something in between? As with so much about Spinosaurus, we have a lot of primary questions to answer before we can start thinking about their implications for behaviour and life appearance.

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  • Taylor, M. P., & Wedel, M. J. (2013). Why sauropods had long necks; and why giraffes have short necks. PeerJ, 1, e36.

Wednesday, 22 April 2020

In pursuit of giant pliosaurids and whale-sized ichthyosaurs

Some marine reptiles - like Shonisaurus popularis - were big. So big, in fact, that you can't fit them all into a picture. But just how large did the largest marine reptiles get? Finding the answer can be more involved than you'd think. This image is a cropped version of a larger picture, which you can see at my Patreon.
The size of Mesozoic marine reptiles is undeniably a big part of their charm and popular appeal. And yet, if you want to know how big some of these animals really got - and by 'really', I mean 'size estimates preferred by contemporary scientists based on our latest data' - you might have a struggle on your hands. Marine reptile media is full of varying size estimates for different groups, some of which are accurate to modern science, some of which were accurate to old science, and many of which were never accurate to begin with. It's not always easy to tell which is which.

I've recently had cause to ascertain reliable size estimates for a number of marine reptile clades and found it far more involving than I anticipated, especially for pliosaurids and ichthyosaurs. This isn't necessarily because it's hard to find an article or paper giving figures of total length or mass, but because confused taxonomies, a deficit of complete skeletons and opaque comments in technical literature complicate efforts to recover and understand maximum size estimates from peer-reviewed, reliable sources. I thought it might be of interest to share some of my findings here, focusing specifically on those two aforementioned groups - pliosaurids and ichthyosaurs - as I feel their upper size range is perhaps most in need of clarification.

Before we get going, it's worth reminding ourselves of how estimating the size of very large extinct individuals is fraught with challenges, and how the numbers and stats we throw around are often based on relatively little data. Many well-known scaling problems apply to ichthyosaurs and pliosaurs. Our remains of especially large individuals are not only rare but are often relatively scrappy, and our capacity to scale reliably from other, smaller animals is frequently more limited than we might like. As is often necessary, there's a need to distinguish between size estimates that are generally reliable, being based on relatively complete large skeletons, and those which are only indicative, on account of being scaled from fragmentary remains. At the risk of spoiling details of the article below, there doesn't seem to be a clear answer to "what's the biggest pliosaurid" or "what's the biggest ichthyosaur" because the size estimate error bars for most very large specimens are too wide to give precise assessments.

Giant pliosaurids

The great size of certain Late Jurassic and Lower Cretaceous pliosaurids has made them famous, perhaps especially so in the UK and Australia where giant animals are particularly well represented. Since their discovery in the mid-19th century, there has been no doubt that certain pliosaurids attained enormous sizes and dwarfed virtually all other organisms in their respective palaeoenvironments (e.g. Owen 1841; Longman 1924; Tarlo 1959).

How the world met pliosaurids: plate 68 of Owen's Odontography (1841). That's a life-sized Pliosaurus tooth at the bottom of the page. Note the spelling - "Pleiosaurus". It's not clear why Owen changed the name later, but "Pleiosaurus" has not been widely used since the 1840s and Pliosaurus has, by rules of zoological nomenclature, won out.
But the question of how large pliosaurids were has, for a long time, not had a straightforward answer. An ongoing problem with estimating their size is that complete pliosaurids, in stark contrast to plesiosauroids, which are known from abundant complete remains, are actually pretty rare. Any attempt to estimate their size thus has to tackle uncertainty about their body proportions. It took over a century for palaeontologists to attempt the first calculations of pliosaurid body length, and these were still produced while aspects of vertebral number, torso length and so on remained unknown (Romer and Lewis 1959). This estimate was associated with the famous 12 m long Harvard Kronosaurus mounted skeleton, a reconstruction that has unfortunately covered the original fossil in so much plaster that it's now hard to examine in detail. It's now generally considered that the Harvard mount has too many vertebrae and is thus too long (McHenry 2009). The subsequent recovery of relatively complete pliosaurid specimens has supplied useful data for estimating their size (e.g. Noè et al. 2001; McHenry 2009) but we are still working with a very provisional dataset and assuming that the proportions of a handful of species apply to pliosaurids in general.

Newman and Tarlo's (1967) unnamed pliosaurid reconstruction, interpreted by McHenry (2009) as the 10 m long Pliosaurus macromerus (a species of uncertain validity today - see Knutsen 2012 vs. Benson et al. 2013) or Liopleurodon ferox, based on a near-complete, 4.88 m long specimen held in Tübingen, Germany (Noè 2001). I like the generous soft-tissue outline of this illustration - it looks appropriately bulky and well-muscled.
Famously, 1999 saw claims of whale-sized pliosaurs take root in public conscience courtesy of the BBC documentary Walking with Dinosaurs. The antagonist of the Jurassic marine-focused third episode was a 25 m, > 100-tonne Liopleurodon ferox which was, even given the general uncertainty about pliosaur size, a very controversial reconstruction for a species only known from much smaller remains. Our largest Liopleurodon skull (discovered in the early 20th century, so known to the producers of the programme) is 1.54 m long (Benson et al. 2013) so, assuming a skull:body length ratio of about 1:5 (Noè et al. 2001; McHenry 2009), we can predict that large Liopleurodon individuals were about 8 m long. How did this value become tripled for the documentary? Writing in 2000, two of my colleagues - David Martill (my PhD supervisor, also a consultant for WWD) and a pre-TetZoo Darren Naish explored the rationale of this decision:
This size created much debate in palaeontolgoical circles following the first airing of the programme, as no palaeontologist thinks Liopleurodon really got this big.
Although several complete [Liopleurodon] skeletons have been discovered, these are of individuals of between five and ten metres in length. It is less complete remains discovered in the Oxford Clay that indicate lengths greater than this, though here we move into an area of rough estimates and guesswork. A vertebra at the Peterborough Museum, brought to light in 1996, would seem to indicate a pliosaur of between seventeen and 20 metres in length, and various fragments of snout and lower jaw in other museum collections suggest specimens of similar size. Whether these fragments are actually from Liopleurodon is uncertain, and the animal to which they belonged has been nicknamed 'Megapleurodon'. Given that it is unlikely that these bones really represent the very biggest pliosaur specimens that ever lived, some experts cautiously suggest that Liopleurodon and related forms may have achieved total lengths of around 25 metres.
Martill and Naish 2000, p 80.

The WWD Liopleurodon was a showstopper back in 1999, and one of the stars of the series. Its arrival brings the end to this sequence showing Ophthalmosaurus giving birth. Clip from Walking with Dinosaurs, uploaded to Youtube by BBC Earth.

As is clear from this text and subsequent Tetrapod Zoology articles, both Dave and Darren knew that this size estimate was very speculative and an indulgent move by the programme-makers. But while a 25 m long pliosaur isn't defensible, supersized pliosaurs were not entirely out of the question 20 years ago. In addition to the specimens mentioned by Martill and Naish, a fragmentary Mexican pliosaurid (the so-called "Monster of Aramberri") was inferred as reaching 15 m just a few years after WWD aired (Buchy et al. 2003). While this is still some distance from 25 m, these data were pointing to pliosaurs of much larger sizes than generally anticipated during the late 1990s and early 2000s.

Unfortunately for those excited by the idea of a whale-sized pliosaurid, the specimens touted as rationalising the WWD monster have not delivered on their promise. In what is perhaps the most detailed assessment of maximum pliosaurid size conducted to date, Colin McHenry (2009) discussed all the fragmentary material from Britain and Mexico linked with supersized pliosaurids and found that they represented very large animals, but not whale-sized giants. Colin confirmed that the vertebra housed at Peterborough Museum was indeed very large - 252.5 mm wide by 219 mm tall - but its total body length estimate was just 11.6–14.2 m when scaled to well-known pliosaur remains [Note added 24/04/20: also see the comment from Colin below, for an update on this specimen). A large Jurassic mandibular symphysis archived in the Natural History Museum, London might represent an animal anywhere between 9.1 and 15.1 m long, while OUM J.10454, a near-complete, 2.8 m long lower jaw dubiously referred to Pliosaurus macromerus and known informally as the 'Cumnor mandible', scales to a surprisingly low 12.7 m. In subsequent years this estimate has also become questionable as we've realised how reconstructed the Cumnor mandible is: it remains to be determined what size range the original specimen represents (Benson et al. 2013).

The Tübingen University specimen of Liopleurodon ferox, a near-complete 4.8 m long juvenile. This is a key specimen for our understanding of Jurassic pliosaur proportions, and thus the size of the biggest individuals. From Wikimedia user Ghedoghedo, CC BY-SA 3.0.
The same story plays out across all other giant pliosaurid fragments, including the Mexican Monster of Aramberri. It seems that when scaled using our best data on pliosaurid proportions and growth allometry, most 'giant' pliosaurid fossils return total length estimates in the 10 m range, with only the upper bounds of our least reliable length estimates falling outside this (McHenry 2009). There's perhaps a moral here about reading too much into the size of giant but fragmentary bones. As stated by McHenry (2009):
Estimates made on incomplete series of vertebrae, or even a single vertebrae, are subject to the natural variation of vertebral dimensions and should be used with caution. The dimensions of individual vertebrae can be affected by taphonomic processes, in particular sedimentary compaction, and when size estimates are extrapolated from single elements small errors can be greatly magnified. The same applies to any allometric variation than is not accounted for in scaling models... any estimates based upon more complete pliosaurid material requires extrapolation over at least an order of magnitude of body mass, a leap that means even small errors in the estimate of allometric or intraspecific variation will produce a large range of results.
McHenry 2009, p. 422

My take on one of the largest Jurassic pliosaurids, Pliosaurus kevani. It was probably about 10 m long, assuming similar proportions to other pliosaurids. The smaller animal above is a calf.
So, putting the ideas of gigantic pliosaurs to the side, what are our most reliable estimates for the maximum sizes of these animals? Detailed assessments of the very large and relatively well-known pliosaur Kronosaurus queenslandicus suggest that a total length of 10.5 m and a mass of 11 tonnes is likely (McHenry 2009). Kronosaurus is one of the few large pliosaurids known from decent and articulated postcranial remains so this figure involves minimal extrapolation, and should be regarded as one of our more reliable insights into the maximum size of these animals. It seems that c. 10 m is about right for other large pliosaurids, too. The skulls of large Kronosaurus are 2.3 m long, a value comparable to measured and estimated skulls lengths of the largest Jurassic pliosaurid, Pliosaurus (2-2.5 m; Knutsen et al. 2011; Benson et al. 2013). If pliosaurid skulls account for roughly 20% of their body length, these are on target for a Kronosaurus-like size of 10 - 12.5 m. While that upper range is obviously higher than Kronosaurus, we shouldn't put much stock in it yet. Our 12.5 m Pliosaurus is a body length extrapolation based on a rough skull size calculation, and both these values hinge on the specimen in question having the same skull and body proportions as other, mostly non-Pliosaurus pliosaurids. Our 12.5 m Pliosaurus is thus perched on a stack of assumptions, each one compounding any errors of the one beneath it. Such methods are fine for getting ballpark size figures but they are far from precise and we should treat them with appropriate caution. Greater understanding of the size of the biggest pliosaurs will require discovery of more substantial skeletons of large pliosaur individuals.

Giant ichthyosaurs

It's remarkable to me that the giant ichthyosaurs, which are without doubt the largest marine reptiles of all time, appeared in the Late Triassic - just a few tens of millions of years after marine reptiles entered the seas. Ascertaining details of how large these animals got is complicated but not, as with pliosaurs, because of scant remains. To the contrary, we actually have several excellent fossils of very large ichthyosaurs (Camp 1980; Kosch 1990; McGowan and Motani 1999; Nicholls and Manabe 2004), and our challenge is instead related to working out what taxa they represent. The history of two ichthyosaur genera associated with giant size - Shastasaurus and Shonisaurus - is complex and intertwined. While there's no doubt that Shonisaurus contains at least one species of very large ichthyosaur (Sho. popularis), it's not always been clear whether Shastasaurus also contained giant animals, nor to which of these genera the largest ichthyosaur remains should be referred to. The confused treatment of these animals in technical literature has spread into the popular realm and it's very easy to find articles and art treating Shastasaurus and Shonisaurus interchangeably. This is unfortunate because, as we'll see, they're actually very different animals.

Quarry map of the holotype specimen of Shonisaurus sikanniensis, the biggest marine reptile yet known from a decent portion of skeleton, from Nicholls and Manabe (2004). The last decade has seen differing teams debating whether this animal is actually Shastasaurus or Shonisaurus. Note the 1 m scale bar at the bottom of the illustration - this animal was very big.
I won't recount the full taxonomic history of these genera here, it will suffice to say that long-running uncertainty about recognising and diagnosing Shastasaurus and Shonisaurus is the crux of this issue. Both genera have housed an ever-changing number of specimens and species in their long histories (Shastasaurus was named in 1895, Shonisaurus in 1976) and most of their new species have proved problematic one way or another: generally being undiagnostic, based on controversial specimen allocations, or being better referred to other named ichthyosaurs. The conflicted history of Shastasaurus and Shonisaurus continues today as authors continue to disagree over which specimens and species should be allocated to each genus (e.g. McGowan and Motani 1999, 2003; Nicholls and Manabe 2004; Sander et al. 2011; Ji et al. 2013).

This affects our assessment of ichthyosaur size because the taxonomic fate of several giant specimens are at stake in these disagreements, including the holotypes of "Shastasaurus careyi" and Sho. sikanniensis. This might not seem like a big deal - a giant ichthyosaur is a giant ichthyosaur, right? - but not all Triassic ichthyosaurs are alike, and the generic identification of these specimens influences our predictions of body dimensions. Although both Shonisaurus and Shastasaurus are generally considered to be shastasaurids, this group is not anatomically uniform enough that we can liberally borrow proportions from other species to calculate size. Some taxa, such as Shonisaurus, were long-snouted, deep-bodied forms, while other genera, such as Guanlingsaurus, were very long, slender animals with small skulls and short faces. For Shastasaurus in particular, trying to work out the anatomical characteristics of this genus is very challenging from all the material referred to it. Even the configuration of the skull seems open to question: was it a long-snouted, toothed form (e.g. Callaway and Massare 1989) or a short-snouted, edentulous form (Sander et al. 2011)?

Not all shastasaurids were deep-bodied, Shonisaurus-like animals. Several genera, like Guanlingsaurus and probably Shastasaurus, were slender, long-bodied animals with shortened faces, so it's important we know what sort of body plan our giants had to calculate their proportions. From Ji et al. 2013.
Happily, it seems that some clarity is emerging from this murk. A large number of ichthyosaur workers are now taking relatively conservative taxonomic approaches to Shastasaurus and Shonisaurus, restricting their specimen inventories to incontrovertibly assigned historic material and rationalising species taxonomy down to better-known, well-diagnosed fossils. This renders Shastasaurus a monospecific genus containing just Sha. pacificus, while Shonisaurus has two well-represented species, Sho. popularis and Sho. sikanniensis. This tidying up means we have a more concrete idea about their shape and form, and has had one surprising outcome: after decades of being touted as an ichthyosaur giant, it turns out that Shastasaurus was actually of unremarkable size. I couldn't find a published body length calculation for Shastasaurus, so I roughly predicted a total length of just 6-7 m using the partial skull illustrated by Sander et al. (2011) and the proportions of a large Guanlingsaurus. I admit to being a little disappointed, as I quite liked the idea of giant ichthyosaurs including both robust, deep-bodied forms like Shonisaurus as well as long, slender animals like Shastasaurus but... oh well. Unless something changes in future, we need to stop talking about Shastasaurus as a giant ichthyosaur.

The skull of Shastasaurus pacificus, illustrated by Sander et al. (2011). Stripped to the core hypodigm and type species, Shastasaurus is neither a well-known nor particularly big animal. I roughly estimate this skull to have been c. 60 cm long, which is big, but not remarkably so for a shastasaurid. Note the large eye socket and pinched snout: Shastasaurus had quite a different skull to the more traditionally ichthyosaurian Shonisaurus.

This leaves the Shonisaurus species as the named record holders of ichthyosaur size, and by some margin. Both species are known from substantial remains that allow us to be fairly confident in our body length estimates. We can actually get a lot of data from simply measuring their articulated skeletons. Our best size predictions for these animals shake out to 13-15 m for Sho. popularis (McGowan and Motani 1999) and a whopping 21 m for Sho. sikanniensis (Nicholls and Manabe 2004). Using data from Gutarra et al. (2019), these equate to approximate body masses of 20-30 and 80 tonnes, respectively. The Shonisaurus species were huge animals, among the largest to ever swim the seas.

As with our giant pliosaurs, several fragmentary Triassic ichthyosaur fossils are touted as indicating larger animals. Himalayasaurus tibetensis, if valid and not another representation of Shonisaurus, is not well known but seems to have rivalled or exceeded Sho. popularis in size, though by how much cannot be reliably ascertained (Motani et al. 1999). Giant remains dubbed the "Mount Potts ichthyosaur" from New Zealand, first reported in 1874, included vertebrae of some 450 mm diameter and ribs exceeding a metre long, making them comparably-sized or bigger than other known ichthyosaur fossils. Alas, they are now lost and these claims cannot be investigated further, making them fairly meaningless anecdotal evidence for extreme ichthyosaurian body size (Fleming et al. 1971). A portion of posterior lower jaw known informally as the "Lilstock ichthyosaur" from Somerset, UK, has been roughly estimated as being similar in size to Sho. sikanniensis at 20 - 25 m long, and another UK fragment (from Aust Cliff, Gloucestershire) possibly hints at an even larger or more robust individual (Lomax et al. 2018). These specimens are genuinely large chunks of bone and perhaps represent our most intriguing hint of even larger marine reptiles, but they're also such small pieces of evidently gigantic animals that we can only very roughly anticipate their size, especially since our scaling calculations for giant ichthyosaurs are still unable to factor proportional changes with growth (Lomax et al. 2018). It's thus hard to know exactly what to make of these specimens, except that they show additional evidence of roughly Sho. sikanniensis-sized creatures in the Late Triassic. For now, values around 21 m remain our most substantiated length estimates for giant ichthyosaurs, and the upper size threshold for marine reptiles as a whole.

My take on a 21 m long Shonisaurus sikanniensis, shown here with generic, 6 m long, Sha. pacificus-sized shastasaurids to stress the size difference between these two 'giant' genera. Shastasaurus is a little older than Shonisaurus and the two didn't live alongside one another but, if they had, Shastasaurus would have been dwarfed by its larger cousin.

What hope is there for blue whale-sized marine reptiles?

As an epilogue to the discussion above, I want to briefly share some thoughts on a marine reptile Holy Grail: a species comparable in size to the modern blue whale Balaenoptera musculus. Despite no marine reptile approaching the size of the largest blue whales (c. 30 m total length, over 100 tonnes) it's easy to find media comparing marine reptiles against our biggest modern cetaceans. Size estimates for the largest shastasaurids match the proportions of very large baleen whales, including certain Pacific blue whale populations - which generally reach 20-something metres in length - but obviously fall short of the largest, primarily North Atlantic and Antarctic B. musculus individuals. Should ever expect to find a 30 m long, 100+ tonne marine reptile?

It's easy to be blase about whales, but the fact we share the planet with the biggest animals to have ever lived on Earth is not to be taken for granted. It seems that the extreme sizes of large rorquals, like this blue whale, may be tied to certain unique biological and environmental properties, which means 30 m body lengths may not be attainable for just any marine tetrapod. (Image from Wikimedia, in public domain).
Studies on the factors influencing body size in marine animals indicate that food availability and foraging efficiency might be strong limiting factors on their upper size limits, and that increasing feeding efficiency via bulk filter-feeding gives rorquals a substantial edge over other species (Goldbogen et al. 2011; Gearty et al. 2018). Enhanced capacity to obtain energy from foraging means more resources to build body tissues, which feeds directly into physiological advantages of larger body size. Together, these strongly incentivise the development of gigantism where rorquals can find enough food, and it seems large blue whales have run with this about as far as they can, reaching predicted biomechanical limits for operating their jaws in water (Potvin et al. 2012). We can thus infer that the extreme size of living baleen whales is dependent on both their unique foraging mechanism and periods of high oceanic productivity (Gearty et al. 2018). This latter factor might explain why whales only developed extreme gigantism in the last few million years, despite their likely development of baleen 23 - 35 million years ago (Gearty et al. 2018; Peredo et al. 2018).

We should, not, therefore, look at the maximum size of rorquals and assume that they represent a universally obtainable figure for all marine vertebrates. As with all organisms, upper size limits are dependent on a complex interplay of anatomy, physiology and environment that are unique to every species, and it's not a given that marine reptiles had the same adaptive potential to reach the same size as the largest rorquals. While it's hard to know exactly how productive Mesozoic oceans were, we can certainly identify the lack of bulk filter-feeding mechanisms as a probable size-limiting factor for marine reptiles, and this may well explain why we've yet to find indications of ichthyosaurs much above 20 m long. Of course, we can't say that fossils of larger marine reptiles aren't out there, waiting to be found, but my guess is that, if giant blue whale-sized marine reptiles existed, they would be anatomically and ecologically very different to our currently known species.

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Friday, 27 March 2020

Realistic raptors: pop-culture dromaeosaurs vs current science, part 1

Recently, I shared this image of our greyhound, Beau, next to a Velociraptor* skeleton on social media. Twitter quickly lit up with likes and comments...
...many of which remarked on how, despite being dwarfed by a big, but not exceptionally large dog, Velociraptor was still a formidable animal that would turn Beau into mincemeat should the two ever meet in real life. Poor old Beau. She doesn't deserve that: she's barely a risk to a bag of kibble.

*Probably 'velociraptorine', to be honest: there's some uncertainty about the identification of the specimen this replica was cast from.

Reading through the comments on this tweet reinforced how Velociraptor-like dinosaurs have been mythologised in popular culture. Thanks largely to Jurassic Park, Raptor Red and other 1990s palaeo media, these dinosaurs are regarded as some of the fastest, most vicious and meanest creatures of all time. Commonly discussed aspects of their savagery include razor-sharp claws capable of ripping into and disembowelling prey; strong legs for delivering rapid slashing attacks; ferocious bites and flesh-rending teeth; cheetah-like speed and agility; and high intelligence. They sound, from this description, like some of the most terrifying predators to have ever existed.

As is often the case, our pop culture takes on Velociraptor and co. have not always aligned with scientific thinking. A case can be made for Velociraptor-like dinosaurs having attained a near-fantastical public reputation reminiscent of Western pop-culture takes on ninjas. Both are genuine historical entities capable of awesome and fascinating feats, but both are also so fundamentally and unrealistically overhyped by popular culture that they bear little resemblance to the real deal.

To attempt to shed some real light on what these famous but often misrepresented dinosaurs were like, I want to compare our pop-culture takes on Velociraptor and similar dinosaurs with some of our more robust recent science on these animals. My intention is not to provide a tried and tested "what Jurassic Park got wrong about Velociraptor-type post" or to discuss basic stuff you can read about elsewhere, like the undoubted presence of extensive feathering in these dinosaurs (Turner et al. 2007; DePalma et al. 2015), but to focus on aspects of lifestyle, biomechanics and ecology. There's a lot to talk about here, so I'm dividing this article into two posts.

The 1993 Jurassic Park Velociraptor, a Hollywood creation that introduced much of the world to dromaeosaurid dinosaurs, and which probably remains the chief point of public reference for their appearance and behaviour. There is, of course, a slew of well-known anatomical issues with the JP Velociraptor that conflict with science published in the last few decades, but that's not what I want to talk about here. Image © Universal, I'm not sure who originally put it online.
Before we begin we need to quickly discuss some aspects of terminology. Firstly, exactly what group of dinosaurs are we focusing on here? Velociraptor is a member of Dromaeosauridae, a group of bird-like feathered theropods generally characterised by large, sickle-shaped claws on their second toes; long, stiffened tails; and narrow, lightweight skulls lined with sharp, serrated teeth. Fossils show that their feathering was broadly comparable to living birds such that, in life, they probably looked like long-tailed, toothy avians. But dromaeosaurs were a diverse bunch which, in addition to Velociraptor-like forms, also included the long-skulled, long-legged unenlagiines, the small-bodied and sometimes 'four winged' microraptorines, and the newly identified Halszkaraptorinae, an enigmatic group containing the semi-aquatic Halszkaraptor. The "raptors" of popular convention - mid- to large-sized dromaeosaurs with large foot claws - are perhaps best matched among palaeontological classifications by Eudromosauria, a group generally considered to include several household names: Velociraptor, Deinonychus, Utahraptor, Dromaeosaurus and others. Eudromaeosauria is not a universally recognised group as the exact composition and arrangement of Dromaeosauridae remains the subject of ongoing study. However, it's a neat match for the public perception of dinosaurian "raptors" and will serve us well in our discussion, regardless of whether it's a true taxonomic group or merely a collection of anatomically similar, but unrelated species.

Secondly, a qualifier on the word "raptor". While "raptor" has been synonymous with "birds of prey" for about two centuries, it has increasingly been used to refer to dromaeosaurids and similar non-avian dinosaurs since the Jurassic Park franchise introduced it as shorthand for Velociraptor (see Farquhar 2017 for the complex history of this word). While some purists dislike the latter use (myself included, to be honest), technical palaeontological literature has started to adopt "raptor" as a colloquialism for Dromaeosauridae as well. Miserable old-fashioned folks like myself thus need to concede that "raptor" has grown to encompass members of Dromaeosauridae even though this complicates discussions where raptorial birds and dromaeosaurs are mentioned simultaneously. Thus, to avoid confusion, my use of "raptor" here is meant in the traditional avian sense of the word, unless otherwise specified.

These points in place, let's get moving through pop-culture takes on eudromaeosaurs to see how they stand up to scientific scrutiny.

Pop-culture concept. Eudromaeosaur species are basically all the same animal expressed at different body sizes.

It's difficult to mention dromaeosaurs online without discussion turning to the real taxonomic identity of the Velociraptor featured in the Jurassic Park series. Deinonychus and Utahraptor are often mentioned as the actual basis for the animals featured in those films*. These discussions imply that all eudromaeosaurs were generally similar in appearance to Deinonychus or Velociraptor: fairly gracile animals with long-ish limbs, large claws, long tails and low, slender skulls, and that size was their main distinguishing feature.

*It's worth taking a moment to give some additional information on this oft-discussed point. The preface of Robert Bakker's 1995 novel Raptor Red gives a behind-the-scenes insight on this matter, acknowledging that the size of the Jurassic Park Velociraptor was not based on anything other than the desires of the filmmakers, and that their scaling of these animals to sizes beyond those of Deinonychus - the biggest dromaeosaur known until 1993 - was cause for concern among some involved in the film. The Jurassic dromaeosaurs were made vastly bigger than both Velociraptor and Deinonychus because the real size of these animals wasn't considered intimidating enough. The 1993 discovery of Utahraptor, while giving the filmmakers a reprieve for making their dromaeosaurs so big, didn't fully justify their scaling either as this animal was initially thought to be around 7 m long: about twice the size of those in the film (Kirkland et al. 1993). The man-sized Jurassic 'raptors' thus lacked a good size match among Dromaeosauridae in the early 1990s and, in my view, are best viewed as 'generic' eudromaeosaurs shaped to the requirements of the film, rather than being based on any particular genus.

Not all eudromaeosaurs were variants on Velociraptor. Utahraptor ostrommaysorum sits at the other end of their anatomical range, being a large (5-6 m long), 300-500 kg predator with a proportionally large head, stout limbs and enormous claws. It almost resembles a dromaeosaur wanting to revert to a more traditional theropod body plan, without sacrificing some key dromaeosaur adaptations.
A fair degree of anatomical variation has been apparent in Eudromaeosauria since at least the early 1990s, however. Though sharing a similar body plan, eudromaeosaurs differ in attributes of limb length, limb bone proportions, head size, jaw depth, dental configuration, claw sizes, tail flexibility and many other smaller anatomical components (e.g. Turner et al. 2012; Paul 2016). They ranged from smallish animals less than 1.5 m in length and perhaps just 5 kg in weight (Bambiraptor) to grand species some 5-6 m long and exceeding 300 kg (Utahraptor). At least some of these large-bodied species, such as Utahraptor and Achillobator, were stocky, large-headed creatures with deep jaws, heavy hips, stout limbs, large sickle claws, and relatively powerful bites (above). Other giant eudromaeosaurs were not especially robust however, with Dakotaraptor being of similar build to Deinonychus-like morphs despite being one of the largest dromaeosaurs (DePalma et al. 2015). Smaller eudromaeosaurs were also anatomically varied, with genera such as Dromaeosaurus and Atrociraptor bearing short, deep snouts and robust teeth, instead of the long, slender jaws of Deinonychus-like species. Some real oddballs are also known, such as Adasaurus mongoliensis: a smallish eudromaeosaur with a somewhat reinforced posterior skull and a vastly reduced sickle claw (Turner et al. 2012).

The wimpy sickle claw of Adasaurus, as illustrated by Turner et al. 2012.
Although these differences are undeniably small compared to the disparity of theropods as a whole, they would surely be very obvious should we have seen these animals in life. Eudromaeosaurs were not merely differently-scaled variants on Velociraptor, but differently adapted species with a range of functional morphologies and behaviours. Eudromaeosauria was a widespread and long-lived group and these distinctions probably reflect adaptations to the range of prey species, as well as environmental and climatic regimes experienced by its members. We should probably view eudromaeosaurs as having a similar anatomical and ecological range to some living carnivore groups, such as felids or raptorial birds, which range from tiny hunters of small animals to heavyset predators of larger game. As with cats and raptors, those concerned with the accurate conveyance of eudromaeosaur biology have to be careful not to over-generalise details of their anatomy and appearance.

Pop culture concept. Eudromaeosaurs were lightning fast, streaking after their prey at speeds comparable to the fastest living land animals.

The silver-screen Velociraptor of Jurassic Park has, time and again, been shown as an incredibly swift animal. Described as having cheetah-like speed in the first film (which equates to a maximum speed of 109.4–120.7 kph, or 68.0–75.0 mph), we've since seen them running down hadrosaurs in Jurassic Park III and leading jeeps and motorcycles in Jurassic World. These pop-culture depictions align with energetic names and artwork associated with these animals for almost a century. Deinonychus was, of course, a poster child of the dinosaur renaissance and an animal which helped change thinking about dinosaur metabolism and activity rates. Robert Bakker's famous sprinting Deinonychus reconstruction (below, first published in Ostrom 1969) is a famous and influential palaeoartwork demonstrating eudromaeosaurs as fast, agile creatures. The names of classic eudromaeosaur taxa - Dromaeosaurus ("running lizard"), Velociraptor ("fast thief") - emphasise their swiftness and raptorial nature, implying speed and agility above the dinosaurian average.

Bakker's sprinting Deinonychus antirrhopus from Ostrom (1969). Now over 50 years old and dated in many respects, it remains an iconic image of the Dinosaur Renaissance and conveys the important message that dinosaurs - including dromaeosaurs - were fast, powerful creatures.
Perhaps surprisingly given their reputation, studies show that eudromaeosaurs weren't exactly speed-demons. A caveat here is that it's actually pretty difficult to know exactly how fast extinct animals could move because speed is influenced by a range of factors which are challenging to predict reliably from fossils. These include animal mass, muscle fractions, muscle speed, bone strength, stride length and others. Trackways can give an idea of velocity for a given individual - and they are known for dromaeosaurs - but they may not record animals moving at their maximum speeds. We can, however, make decent assessments of extinct animal speed from their limb proportions and by searching for anatomies that are common to fast runners today. From these, we've known for at least half a century that eudromaeosaurs were not among the quickest dinosaurs (Ostrom 1969; Paul 1988; Kirkland et al. 1993; Carrano 1999; Persons and Currie 2016), despite contrary claims in popular works and their treatment in some scientific literature.

What slows eudromaeosaurs down is that, in contrast to cursorial (= fast running) animals, they lack elongated distal limb segments, reduced and streamlined toe anatomy, and narrowed, fused metatarsals. Studies suggest that eudromaeosaur hindlimbs, although clearly well-muscled, sacrificed speed for strong, grasping foot anatomy (Ostrom 1969; Fowler et al. 2011). Biomechanical studies show that appendage strength and running speed are something of an adaptive fork in the road as they exert conflicting demands on muscle distribution, limb length and bone robustness. Close relatives of eudromaeosaurs, including the unenlagiines and troodontids, adapted towards greater cursorial abilities at the expense of foot power and were probably far nimbler, faster creatures than equivalently-sized eudromaeosaurs (Carrano 1999; Persons and Currie 2016).

Eudromaeosaurs were probably not the fastest dinosaurs, but they were lightly built, well-muscled animals that could surely move at a reasonable speed for at least a short amount of time to catch their prey. Here, Velociraptor chases down Zalambdalestes.
This all said, no-one thinks eudromaeosaurs were exactly slowpokes. As generally smallish, lightly built dinosaurs with somewhat elongated and well-muscled hindlimbs, eudromaeosaurs were probably capable of moving quickly at times, just not for sustained periods or at record-breaking speeds. Their stiffened tails are clear hallmarks of rapid locomotion, being ideally suited to facilitating quick changes in direction at speed (Ostrom 1969; Persons and Currie 2012). It seems reasonable to assume that eudromaeosaurs were adept at ambushing prey, relying on a short burst of speed and agility to catch fleeing animals from a covered position, but that target species might have had an advantage if a long pursuit was involved.

Certain eudromaeosaurs, such as the large-bodied Utahraptor, were probably not especially quick animals, however. Their hindlimb proportions are even less suited to running than other eudromaeosaurs and their tails were not significantly stiffened, suggesting lessened agility as well as speed (Kirkland et al. 1993). Judging from their proportions, it looks like these species compromised their running capabilities to facilitate greater body mass, hindlimb power and head size. Hopefully, as we learn more about these very large eudromaeosaur species we'll develop more insights into their locomotion.

Pop culture concept. Eudromaeosaurs had tremendously strong bites.

An idea popularized in at least the Jurassic Park novel is that eudromaeosaurs, in addition to being blade-wielding superninjas, were also armed with a bite that would make an alligator feel inadequate. In this book Velociraptor literally chews through steel bars in a noble but ultimately futile effort to kill one of fiction's most irritating characters, Ian Sodding Malcolm. While this doesn't seem to be a particularly widespread popular assumption about eudromaeosaurs, responses to the Beau vs. dromaeosaur tweet certainly included a few comments about powerful bites.

Terrifically preserved skull of Velociraptor mongoliensis showing the low, narrow, lightly built skull construction typical of most eudromaeosaurs. These are not the skulls of powerful biters, but of lightweight, fast-moving animals with teeth suited to rapid tearing of flesh. From Turner et al. (2012).
Dromaeosaur bite strength is something that we've addressed on this blog before so I won't dwell on it long here. Deep bite marks on a Tenontosaurus fossil have been attributed to Deinonychus and promoted as evidence for a powerful, alligator-grade bite in this genus by one set of authors (Gignac et al. 2010), but virtually all other studies conducted on the skull strength and bite forces of eudromaeosaurs have drawn conflicting conclusions (Therrien et al. 2005; Sakamoto 2010; Fowler et al. 2011). Eudromaeosaur skulls are generally lightweight structures composed of thin bars and sheets of bone, and were thus poorly suited to powerful biting. In all likelihood, those Tenontosaurus bones were bitten by another animal. Therrien et al. (2005) predicted the bite force of Deinonychus as being comparable to that of a 30 kg wolf, a value which seems impressive until we remember that Deinonychus was about twice that size (c. 80 kg). This difference likely reflects the fact that canids are adapted for chewing into bone, while eudromaeosaurs have the slender teeth and relatively delicate jaws of dedicated flesh-eaters. They surely ate around or swallowed skeletal elements whole so as not to damage their teeth chewing into bones. Thus, while it would not be wise to put your hand in a eudromaeosaur's mouth, there are plenty of other animals out there that could bite you harder. An unknown quantity here is how powerful the bite of something like Utahraptor was: there is good reason to think these animals had large, relatively strong skulls that may have allowed for greater bite forces, but we need more substantial fossils of these giant dromaeosaurs to understand their bite performance.

Pop culture concept. Eudromaeosaurs attacked their prey with razor-sharp claws, slicing deep into their flesh to leave long, bloody lacerations.

The eudromaeosaurs I knew from my childhood dinosaur books - both educational and fiction - were imagined as having ferociously sharp claws which could be deployed in an especially gory, grotesque fashion to dispatch prey. Palaeoart fans will not need to be reminded of the glut of 1990s dinosaur art showing this: swarms of dromaeosaurs using their claws to clamber over prey species, tearing into their hides to leave long, deep gashes. I have no doubt that these predatory scenarios were a major part of why dromaeosaurs became a firm favourite among dinosaur fans and the public alike. My own childhood sketchbooks were certainly full of bloody, gory dromaeosaur art inspired by these ideas.

Page from the 1993 comic serial Age of Reptiles showing Deinonynchus bringing down a sauropod with slashing, razor-sharp claws. Though portrayed in a comic book, this is pretty close to how dromaeosaur claw function was predicted by scientists in the early 1990s. Art by Ricardo Delgado, borrowed from Dark Horse Comics.
Flesh-ripping dromaeosaur claws have some actual basis in science, this being the accepted interpretation of sickle and hand claw function in the mid to late 20th century (e.g. Ostrom 1969; Bakker 1986, 1995; Paul 1988; Kirkland et al. 1993). Their deep, bladed nature and large flexor tubercles (the part of the claw anchoring flexing musculature) of dromaeosaur hand and sickle claws give this idea some credibility, and the size of most eudromaeosaurs claws is undeniably remarkable: there's no doubt that they were paramount to their predatory behaviour.

However, this concept has come under fire in recent years as we've started to assess the lifestyles of eudromaeosaurs in a more detailed and biomechanics-led fashion. It's quite well established, for instance, that while dromaeosaur claws are narrow, they aren't quite knife-like enough to facilitate easy cutting of skin and muscle tissue. Their cross-sections are somewhat like a stretched, inverted pear (below) with a narrow but distinctly rounded inner margin (Carpenter 2000; Farlow et al. 2011). It is also unlikely that their claws were shaped into razor-like cutting edges by keratinous sheaths, unless they had a sheath-claw bone relationship unlike anything seen in birds and reptiles today (Carpenter 2000; Manning et al. 2006). These are major problems for the slashing hypothesis because, as many of us know from personal dining experiences, it can be challenging to cut animal skin and flesh without a well-sharpened blade (Carpenter 2000). That dromaeosaurs could hone and maintain a razor-like claw edge against routine abrasion and wear is a naive assumption: claw tips can be sharpened by the removal of abraded and ragged sheath layers (as the trashed furniture of many cat owners will attest) but it's harder to hone the edge of an entire claw without dedicated technology (Carpenter 2000).

Eudromaeosaur claw shape as illustrated by Carpenter (2000). Note the width of the claws and lack of bladed cutting edges along their inner margins.
A further problem for the slashing hypothesis is the amount of force eudromaeosaurs could transmit to their sickle claws during kicks or other attacks with extended legs. Many of us are familiar with artwork of aggressive dromaeosaurs posed in this way, but it turns out that outstretched legs are actually the weakest configuration for application of claw force (Farlow et al. 2011; Bishop 2019). Eudromaeosaur hindlimbs actually delivered a lot more power through their sickle claws when the leg was crouched or otherwise flexed (Fowler et al. 2011; Bishop 2019) and, perhaps surprisingly, the overall force achieved at the claw tips was not great relative to the strength of prey animal tissues (Manning et al. 2006; Bishop 2019). Thus, even under optimal conditions, it's unlikely that eudromaeosaurs had sufficient strength to create long, deep wounds in animal flanks. Neither claw shape nor our understanding of hindlimb mechanics corroborates the use of eudromaeosaur claws as ripping and slashing structures.

A counterargument to this might be the commonality of foot slashing behaviour by sparring birds. Many avian species, including those with formidable claws for raptorial or perching behaviour, kick and slash at each other when settling disputes in a manner not entirely unlike that traditionally predicted for predatory dromaeosaurs. Might not similar movements, scaled up to the size of large eudromaeosaurs, have been effective means to bring prey down? In my mind, the behaviour of modern sparring birds might actually be further evidence against claws inflicting major injury through kicking actions. In all but the most serious bouts - where slashing and kicking turns to wrestling, pecking and eye-gouging, or where circumstances do not allow for escape for a weakened bird - avian sparring rarely leads to more than superficial injuries. In some species, including those with large talons and curved claws like eagles and seriemas, talon clashing is even employed in non-aggressive acts such as courtship and between parents and juveniles (e.g. Silva et al. 2016). The fact that birds can endure kicks from clawed feet without great concern is further evidenced by brutish humans equipping cockfighting roosters with artificial spurs - metal blades and so on - to allow them to inflict deeper, more critical wounds when sparring**. Though not a perfect analogue for eudromaeosaur slashing predation, these avian behaviours demonstrate that simply having large claws on powerful legs does not turn animals into deadly bladed assassins, and seemingly concurs with the predicted weak performance of claws in kicking or slashing. There's more to say on how dangerous bird claws can be when employed aggressively, and we'll return to this topic in the next article.

**In what might be seen as poetic justice, the addition of artificial spurs to fighting chickens turns them into animals that are also deadly to humans. At least three people have been recorded as dying after attacks or accidents involving sparring cockerels with razors added to their legs.

The flexed left foot of Deinonychus as illustrated by Fowler et al. (2011). I find this image absolutely compelling evidence of the powerful grip provided by these feet in life, and more than a little intimidating. Note the lateral flexion of the fourth toe afforded by the ball and socket-like joint at the end of the metatarsal. Scale bar is 100 mm.

So if they weren't for cutting and tearing, what were eudromaeosaur sickle claws for? A breakthrough interpretation of their function has stemmed from realising that we should focus on eudromaeosaur feet as a whole, and not just the impressive sickle claws on digit II (Fowler et al. 2011). Armed with this perspective, it becomes apparent that their whole foot structure is well adapted to piercing and powerful gripping. Their claws are mechanically strong against the forces associated with puncturing skin (Manning et al. 2009) and, although ill-suited to ripping flesh, physical modelling implies a great ability to dig into and hold bunched animal tissues (Manning et al. 2006, though see Fowler et al. 2011 for a critique of this research). Articulating well-preserved eudromaeosaur feet shows that they could form a formidable-looking 'fist' in which the middle toes (the sickle claw and digit III) clenched tightly in line with the long bones of the foot, and the lateral toes (the hallux, and a relatively mobile fourth digit) gripped from opposing sides (Fowler et al. 2011). This gripping function benefits enormously from the short, wide and unfused metatarsus of the eudromaeosaur foot as this provides room for multiple strong ligaments and gives a robust, strain-resistant base to the clenching digits. Presumably, this gripping adaptation is the payoff for reduced eudromaeosaur running speed (Fowler et al. 2011). An ability to form a tight fist with the foot is shared with many living raptors, and studies have found numerous hitherto unappreciated similarities between the feet of these species, with eagles among the best modern analogues (Fowler et al. 2009, 2011).

This revelation that dromaeosaur feet are more about gripping than slashing has important implications for how we imagine the ecology of these animals, and suggests many of our traditional concepts of eudromaeosaur prey apprehension are unlikely. It seems that the formidable claws of these animals were not quite the be-all and end-all of eudromaeosaur predation that we once thought, and that they were instead part of a prey immobilisation strategy that involved their entire bodies. Exactly what that predatory strategy might have been is something we'll get to in the second part of this series.

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