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.

Okay fine I did the #Spinosaurus with it’s new paddle tail. Everyone’s debating on what the purpose of this wonderous tail was. Well I think it’s quite obvious, don’t you? We just observed it in real time. #scicomm #paleontology #theropods pic.twitter.com/wi8GjxKJnf

— Christopher DiPiazza (@ChrisDPiazza) April 30, 2020

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.

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.

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 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?

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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• Renesto, S., Spielmann, J. A., Lucas, S. G., & Spagnoli, G. T. (2010). The taxonomy and paleobiology of the Late Triassic (Carnian-Norian: Adamanian-Apachean) drepanosaurs (Diapsida: Archosauromorpha: Drepanosauromorpha): Bulletin 46 (Vol. 46). New Mexico Museum of Natural History and Science.
• Smith, J. B., Lamanna, M. C., Mayr, H., & Lacovara, K. J. (2006). New information regarding the holotype of Spinosaurus aegyptiacus Stromer, 1915. Journal of Paleontology, 80(2), 400-406.
• Taylor, M. P., & Wedel, M. J. (2013). Why sauropods had long necks; and why giraffes have short necks. PeerJ, 1, e36.

Jaw gaping in Spinosaurus hinges on modern birds

A north African spinosaurine, with obvious nods to recent work suggesting some of these animals might've had short legs and a semi-aquatic lifestyle. The pterosaurs are azhdarchids, which are known to coexist with some African spinosaurines.

Honestly, I don't intend to cover every new paper which comes out on Spinosaurus. This fourth blog post, covering the third successive paper on this animal since 2014, is not the latest instalment of a stealthily-implemented new blog feature (check out this, this and this for previous non-instalments). Rather, north African spinosaurs are simply 'in' right now, hard to miss on palaeontological social media and the topic of widespread conversation - the dinosaur equivalent of skinny jeans, adult colouring books or whatever ITV runs on a Saturday night nowadays.

The latest paper on these animals is that of Christophe Hendrickx et al. (2016), a piece which provides another interpretation on Moroccan spinosaurine diversity based on isolated quadrate bones. These are elements from the back of the skull which, among other things, articulate with the lower jaw. I don't really want to go into the ins and outs of their primarily descriptive and systematic assessment as the paper is a) a bit of a beast and b) we've spoken a lot about spinosaurine taxonomy of late and I'm desiring fresh topics. It will suffice to summarise that Hendrickx et al. (2016) provide compelling evidence for at least two spinosaurines being present in the Moroccan Kem Kem Beds, one of which is Sigilmassasaurus brevicollis and the other is - in their interpretation - Spinosaurus aegyptiacus. These results are not exactly the same as those presented in the recent Evers et al. (2015) paper, as Hendrickx et al. shuffle and deal north African spinsosaurid fossils among named taxa in another unique way. However, it certainly adds further evidence against the concept of a single spinosaurine species ruling Late Cretaceous north Africa proposed by Ibrahim et al. (2014). And yes, for those interested in scientific responses to the famous quadrupedal Spinosaurus reconstruction publicised in 2014, Hendrickx et al. (2016) specifically comment on the likelihood of it being a chimera of animals from across time and space. There's lots more in the paper - spinosaur skull morphology, body size, ontogeny, loads of illustrations and it's 100% open access - interested parties should definitely check it out.

The pelican-like foraging anatomy of Spinosaurus, as illustrated in Hendricks et al. (2016).

Moving on to fresh, but still spinosaurine-filled waters, let's talk about something more fun - functional morphology that is*! One of the neater parts of the Hendrickx et al. analysis is that they pay a lot of attention to the jaw articulation of spinosaurines, providing detailed descriptions of how it may have influenced jaw operation and prey capture (above). Some readers may be aware that spinosaurids have 'helical' or 'asymmetric' jaw articulations, in which the quadrate condyles force the jaw somewhat outwards as it opens. Hendrickx et al. quantify this, noting that the lateral displacement is somewhere in the region of 20% of the quadrate width when the jaw is opened 45°. This equates to approximately 20 mm of motion on each side of the jaw in a c. 1 m long skull. As a rule, non-avian theropods do not have these helical joints, although they do occur in some pterodactyloid pterosaurs and, perhaps more famously, modern pelicans. Hendrickx et al. (2016) also note that the anterior connection between spinosaurid mandibular rami - the mandibular symphysis - has a fibrous texture indicative of being somewhat loose and flexible, enhancing the pelecanid comparisons further.

*It's OK, my shame about that pun is worse than any punishment you could deliver.

In both the paper and news outlets (example), these perceived similarities between spinosaurid and pelican jaws are being stated as evidence of spinosaurs feeding in a pelican-like fashion, splaying their jaws to enhance food capture and swallow larger prey. Regular readers may recall that Darren Naish and I published research on similar claims for pterosaurs in 2013 (although the paper was not 'officially' published until last year - Witton and Naish 2015), countering suggestions that some pterosaurs fed like pelicans because of their helical quadrate articulations (Averianov 2013). This background made me surprised that another group of fossil animals was being labelled as pelican-like, as much of our discussion on pterosaurs is applicable to non-avian theropods and we're even cited in the Hendrickx et al. paper! I thought it might be of interest to explain some of my reservations about this idea here**.

**I want to note that I feel a bit awkward writing this commentary in light of recent controversies on palaeontology blogs, so-called 'Post Publication Peer Review' and so on. I hope that it's clear that this article, as with all the writings here, are meant as constructive, well-meaning expressions of opinion from someone with an interest in these topics and experience in a similar research field. My disagreement with the functional analogy proposed by Hendrickx et al. and their PR work is a polite one, and doesn't mean I disrespect them as scientists or want to undermine the significance of their paper.

We should begin by familiarising ourselves with the jaws of some relevant modern birds. Quite a bit of research has been done into pelican jaw anatomy (Schreiber et al. 1975; Meyers and Myers 2005; Field et al. 2011), an unsurprising fact given how awesomely and specifically adapted their jaws are to their unusual foraging method. Researchers have identified a number of adaptations critical to pelicans being able to splay their jaws so widely. They include reduced mineralisation at the middle part of the lower jaw to make a long 'bending zone', and even further reduction of mineral content (down to 20%) adjacent to the mandibular symphysis. This makes a 'hinge' for the mandibular rami to swing outwards on despite the fact there is no articulation or joint in the jaw at this point. The mandibular symphyses are so short that virtually all the jaw length is permitted to splay. At the other end of the jaw, the tongue is similarly reduced so as not to get in the way when the mouth is opened. The connection between the dentary bone (forming the jaw anterior) and the complex of bones of the posterior jaw is long, loosely connected and obliquely oriented so as to aid motion when the mandible spreads outwards. Even the horny tissues of the beak are specialised, being very thin (described as 'skin-like' by some authors) so as not to impede jaw flexion.

Contrastingly, little mention is made of helical jaw joints when talking about pelican foraging strategies. Zusi (1993) mentions that jaw spreading at the joints may be important to this effect, but more recent literature states that the precise mechanic responsible for bowing pelican mandibles is not really understood (Meyers and Myers 2005). Three hypotheses are currently thought viable: forces and weight of water acting on the jaw (known to be only part of the solution, as pelicans can splay their jaws on land too), contraction of muscles in the gular pouch (pulling the chin backwards, pushing the jaw rami out) or twisting of the lower jaw by the pterygoideus jaw musculature (Meyers and Myers 2005 and references therein). I guess it's possible that lateral displacement of the jaws has some role too, but it's interesting that the pelican guys aren't championing it's role as essential to manibular bowing. It's always risky using negative evidence in this way, but it might be telling given how intensively studied pelican skulls are.

Mandibular bowing in modern birds, as illustrated by Zusi (1993). a, herring gull (Larus argentatus) with relaxed and bowed mandibles; b, common potoo (Nyctibius griseus) skull and mandible (arrows show zones of flexion; c, tawny frogmouth (Podargus strigoides), a potoo incapable of mandibular kinesis. Note contrasting morphology between the potoo mandibles - even as a fossil taxon, there would be no doubt that N. griseus was capable of mandibular flexion.

Perhaps a factor in ornithologists not paying much attention to pelican jaw joints is that helical quadrate articulations are not unique to them. In addition to pelicans, pterosaurs and spinosaurids, numerous bird groups are equipped with asymmetric quadrate condyles. These including herons, shoebills, certain hummingbirds and seed-eating songbirds, potoos and others. In all these species, the effect is the same - lateral displacement of the posterior jaws when the mouth is opened. Because these animals have very different diets, foraging strategies and lifestyles, their convergence on a similar jaw anatomy reflects a basic functional requirement: apprehending or swallowing large food (Zusi 1993). However, not all these species bow their anterior mandible regions in a significant way. To perform this trick, many non-pelecanid birds have mandibles with hinged regions (above). Some species, like certain potoos, are remarkable in this regard, perhaps surpassing pelicans in their ability to expand and even twist their throats into wide basins. It is not only specialised taxa which have remarkable lower jaws: the likes of seagulls can also bow their mandibles to an impressive extent. In these birds, a combination of the pull of the pterygoideus muscle and osteological specialisations allow the jaws to bulge sideways and large food items to enter the throat. The message here seems to be that helical jaw joints might have nothing to do with pelican-like jaw bowing, whereas other features - the development of pronounced mandibular hinges and specialised musculature - might be.

Modern birds give us a pretty good idea of what sort of features we're looking for in a pelican-like dinosaur. In doing so, as might already be obvious, they suggest major issues with the pelican-spinosaur analogy. Firstly, we need to acknowledge that helical jaw joints are not a special pelican feature. We could also call spinosaurid mandibular joints 'heron-like', 'potoo-like', or even 'hummingbird-like'. It seems more precise to suggest spinosaurid jaw joints are similar to those of several modern bird lineages and not over-emphasise anything to do with pelicans.

Our mandibular bowing experiments with pterosaurs and pelicans illustrated. Even when stretching the pterosaur jaws beyond the limits of their jaw joints, their area increase was negligible compared to that of a lazy pelican. From Witton and Naish 2015.

Secondly, given how ecologically variable modern birds with helical jaw joints are, and that there is no obvious correlation between asymmetric quadrate condyles and mandibular bowing, we need to treat them as part of a 'functional package' - a recipe of functional elements considered simultaneously to assess their overall effect on behaviour and lifestyle. The attempt here is to see the bigger picture of how such joints work with the rest of the jaw - do they function in a pelican-like fashion? It doesn't seem so. The amount of mandibular movement in spinosaurids is pretty negligible compared to what we see in modern birds. As noted above, quantification of of jaw motion by Hendrickx et al. suggests the lower jaws move only a tiny amount, each jaw splaying 20 mm from a skull approaching 1000 mm long. It's likely this motion would not even be noticeable in life. I'm reminded of the calculations Darren and I predicted for pterosaur jaw expansion (above) where we found pterosaur helical jaw joints boost jaw area, at most, by a few 10s of percent, but anterior jaw bowing in pelicans increases area by hundreds of percent (Witton and Naish 2015). These calculations seem to agree with current research that bowing of the anterior mandible is the most important agent in having widely distending jaws. No spinosaurid jaw yet known has adaptations for anterior mandible motion akin to those of pelicans, or even other birds capable of mandibular bowing. Many readers will know that we can, and have, assessed the kinetic potential of fossil animal skulls, including those of fossil pelicans (Louchart et al. 2011). we should be able to detect bowing mandibles in non-avian dinosaurs and other fossil reptiles, should they occur.

My points here might be countered by the observation that the mandibular symphyses of some spinosaurids are quite fibrous, perhaps indicating a loose and mobile connection between them (Hendrickx et al. 2016). This might be the case, but Hendrickx et al. also note that some spinosaurid symphyses are longer than those of other theropods, thus actually having a greater degree of anterior attachment between each lower jaw. Based on our understanding of modern bird jaws, surely this is the opposite of what we'd expect in an extinct pelican-analogue? Even if I'm wrong on that, a slightly spongy symphysis and helical jaw joints are several functional miles off the flexibility afforded by avian jaws - especially those with the most extreme adaptations for mandibular bowing, like pelicans.

In all, then, I'm not convinced on the idea that spinosaurids were pelican-like in their foraging habits. Rather, the adaptations outlined by Hendrickx et al. suggest a somewhat bird-like ability to increase gape for swallowing slightly larger food than usual. In actuality, spinosaurid jaw functionality contrasts so markedly with that of pelicans that it might be misleading to tout pelicans as spinosaur analogues. After all, the spinosaurid ability to bulge their jaws slightly is not especially pronounced or even rare, whereas what pelicans do is really both those things: an extreme and marked adaptation, and almost unique in nature. I come back to a point I've made about other instances of 'extreme' modern animals being used as analogues for extinct ones: we need to be as thorough as possible in our functional assessments before pointing to highly specialised and extremely adapted modern species as suitable analogues for long dead taxa.

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References

• Averianov, A. O. (2013). Reconstruction of the neck of Azhdarcho lancicollis and lifestyle of azhdarchids (Pterosauria, Azhdarchidae). Paleontological Journal, 47(2), 203-209.
• Field, D. J., Lin, S. C., Ben‐Zvi, M., Goldbogen, J. A., & Shadwick, R. E. (2011). Convergent evolution driven by similar feeding mechanics in balaenopterid whales and pelicans. The Anatomical Record, 294(8), 1273-1282.
• Hendrickx, C., Mateus, O., & Buffetaut, E. (2016). Morphofunctional Analysis of the Quadrate of Spinosauridae (Dinosauria: Theropoda) and the Presence of Spinosaurus and a Second Spinosaurine Taxon in the Cenomanian of North Africa. PloS one, 11(1), e0144695.
• Ibrahim, N., Sereno, P. C., Dal Sasso, C., Maganuco, S., Fabbri, M., Martill, D. M., Zouhri, S. Myhrvold, N. & Iurino, D. A. (2014). Semiaquatic adaptations in a giant predatory dinosaur. Science, 1258750.
• Louchart, A., Tourment, N., & Carrier, J. (2011). The earliest known pelican reveals 30 million years of evolutionary stasis in beak morphology. Journal of Ornithology, 152(1), 15-20.
• Meyers, R. A., & Myers, R. P. (2005). Mandibular bowing and mineralization in brown pelicans. The Condor, 107(2), 445-449.
• Schreiber, R. W., Woolfenden, G. E. & Curtsinger, W. E. (1975). Prey capture by the Brown Pelican. The Auk, 92(4), 649-654.
• Witton, M. P. and Naish, D. (2015) Azhdarchid pterosaurs: water-trawling pelican mimics or "terrestrial stalkers"? Acta Palaeontologica Polonica 60, 651-660.
• Zusi, R. L. (1993). Patterns of diversity in the avian skull. The skull, 2, 391-437.

The Spinosaurus saga continues

A year after the 'Spinosaurus reboot' as a small-legged, early whale-mimicking aquatic quadruped, experts remain divided over fundamental aspects of Spinosaurus palaeobiology. This depiction shows Spinosaurus aegyptiacus as generally imagined prior to 2014.

The long, tragic and occasionally controversial research history of the giant, enigmatic theropod Spinosaurus aegyptiacus will be familiar to many readers of this blog*. First named and described in the early 20th century by Ernst Stromer from remains found in Late Cretaceous strata of Egypt, our principle Spinosaurus material fell victim to Allied bombing raids in World War II and was completely destroyed. Stromer's detailed illustrations and descriptions are all that remains of this material, and these have formed a variably interpreted foundation of all subsequent Spinosaurus research. For much of the 20th century the life appearance of Spinosaurus remained mysterious. Depicted as a nondescript sailed giant theropod early on, discovery of well represented spinosaurids like Baryonyx and Suchomimus, as well as fragments of new Spinosaurus material, permitted more confident interpretations of Spinosaurus size and form as we approached the new millennium. By the 2010s, Spinosaurus was recognised as a gigantic, derived and perhaps semi-aquatic spinosaurid, adapted for feeding on large aquatic prey (above). Much of this interpretation relied on new Spinosaurus remains from multiple locations in northern Africa, including the famous Moroccan Kem Kem Beds, an expanse of Late Cretaceous rocks roughly contemporaneous with those Egyptian deposits yielding the original, destroyed Spinosaurus remains.

*For succinct overviews of Spinosaurus research prior to 2014, check out posts at Tetrapod Zoology and Laelaps.

Famously, last year saw Spinosaurus reinvented again, this time as a quadrupedal, knuckle-walking, long-bodied, tiny-legged dinosaurian take on a crocodile or early whale (below). The authors of this widely publicised study, Nizar Ibrahim and colleagues (2014), synthesised existing and new data on African spinosaurids to create this reconstruction, synonymising several taxa into S. aegyptiacus and presenting new Spinosaurus remains obtained from the Kem Kem beds. The most significant of these was a set of associated vertebrae, pelvic and hindlimb remains which were proposed as a neotype specimen for Spinosaurus (a specimen to hold the Spinosaurus name now that the original material is lost to science). That this neotype represents Spinosaurus was bolstered by it bearing similar hindlimb and vertebral proportions to 'Spinosaurus B', a collection of Egyptian spinosaurid specimens described by Stromer, considered referable to Sp. aegyptiacus by Ibrahim and colleagues. Spinosaurus B is also now lost, also being destroyed in WWII. The Ibrahim et al. study provided a lot of new data on Spinosaurus and has helped cement the concept of it being a semi-aquatic animal, but several aspects of the paper didn't meet the warmest reception from a number of academics. Specific issues were scaling of the skeletal components, how sensible it was to lump so much north African spinosaurid material into one species, and uncertainty about the provenance of the neotype specimen. Some of these concerns were diffused by the authors, but we await a promised monograph for answers to all the questions raised by their first paper. In the mean time, the 2014 Spinosaurus interpretation remains a debated topic among those interested in dinosaur palaeontology.

The Ibrahim et al. (2014) take on Spinosaurus aegyptiacus. Different colours represent different specimens: red is the neotype; brown is the original Spinosaurus material; yellow is referred, isolated Spinosaurus remains; green bones are borrowed from other spinosaurids, and blue bones are crafted to fit the skeleton based on neighbouring elements. Image borrowed from Smithsonian.com.

One year later...

This week, the Spinosaurus tale has taken another twist with publication of a mammoth (open access) paper penned by a team of European spinosaurid experts, led by Serjoscha Evers. Evers et al. have reappraised the affinities of Moroccan specimens seemingly related to Spinosaurus: Sigilmassasaurus brevicollis and Spinosaurus maroccanus. These animals, known only from vertebrae, were subsumed into Sp. aegyptiacus by Ibrahim et al. (2014) as part of their trans-African Spinosaurus concept, and that decision is a core focus of the Evers et al. paper. Their work contains extensive commentary on the detailed anatomy of Moroccan spinosaur material and what it might mean for recent interpretations of Spinosaurus form and lifestyle. Given the wide interest in Spinosaurus and the 2014 reconstruction, I thought it might be of interest to summarise some of what they outline here.

Firstly, taxonomic revisions proposed by Evers et al. present a very different picture of what fossils we can identify as belonging to Spinosaurus. Their work on Si. brevicollis and Sp. maroccanus suggests these species are probably one and the same (the latter being sunk into the former), and that Sigilmassasaurus should be considered distinct from Sp. aegyptiacus. They go on to suggest that other Kem Kem vertebrae hint at a second spinosaurid species in the Kem Kem fauna, and outline several reasons why the Ibrahim et al. 'neotype' specimen cannot be referred to Spinosaurus. For one, the neotype is anatomically quite different from Stromer's Egyptian 'Spinosaurus B' specimen. Ibrahim et al. considered Spinosaurus B as representing Sp. aegyptiacus, but Evers and colleagues argue that Spinosaurus B is anatomically more similar to Sigilmassasaurus than Spinosaurus. Spinosaurus B therefore might have no use for linking any specimens specifically to Sp. aegyptiacus, including that all-important neotype.

In addition to these morphological objections, Evers et al, also raise palaeobiogeographic issues with the 'neotype' referral. Evidence for Egyptian dinosaur species being present in Morocco is scant at best, most data indicating little mixing of eastern and western African dinosaur species during the Late Cretaceous. It would be unusual, then, to find the Egyptian species Sp. aegyptiacus in Morocco. Palaeobiogeography is not a deal clincher for taxonomy of course - careful examination of the neotype and genuine Spinosaurus remains will be the deciding factor here - but it is another stick in the mud for the neotype proposal. Although the exact identity of the 'neotype' specimen is left in the air by Evers et al. - ongoing descriptive work on the specimen needs to be completed to truly assess this - they reject the proposal of the Kem Kem specimen as a Sp. aegyptiacus neotype, and leave Spinosaurus characterised by features in Stromer's illustrations. This is obviously quite a shake up of the suggestions made last year: Spinosaurus 2014 might be a mix of at least two named species, incorporate material of under-appreciated taxonomic importance, and substantial, newly published material might have little, if anything, to do with Spinosaurus.

The proposed Spinosaurus neotype. Image borrowed from Andrea Cau's excellent Theropoda blog.

Moving on, Evers et al. also raise concerns about interpretations of Spinosaurus in context of Kem Kem fossil collecting practises. Museum exhibitions and PR exercises suggest that the Kem Kem yields complete skeletons of dinosaurs and other fossil vertebrates, but the reality is quite the opposite. Kem Kem vertebrates are typically preserved as isolated, often broken bones in multitaxic bone beds (that is, bone beds comprising many species). Associated skeletons of single individuals do occur, but they're relatively rare and rely on precise collecting documentation to prove their authenticity. Unfortunately, historic and recent records of Spinosaurus occurrences and excavation are often poor. We might chalk a lack of historic documentation to the practises and technological limitations of bygone times, but recent issues are caused primarily by the commercial value of Kem Kem fossils. The greater majority of Kem Kem fossils, including dinosaurs, are collected without extensive documentation and then sold by private dealers. Even if localities are recorded, ambiguity often surrounds association of fossil material prior to excavation. Several alleged associated Spinosaurus specimens are meant to have come from single localities, but being from the same place is really only half the battle if they stemmed from multitaxic assemblages. Concordant size of bones might suggest genuine association, but this is not always certain either: Evers et al. report practises where collectors sort loose material from disparate locations into type and size categories before sale - nefarious individuals making fossil skeletons more substantial with unassociated elements is a real problem the world over. It's sad but true that the monetary value associated with substantial vertebrate fossils makes ascertaining their authenticity crucial for subsequent credible interpretation.

Unfortunately, Evers et al. report these factors as affecting virtually all associated Spinosaurus material, including the 'neotype' and the other specimen key to the 2014 reconstruction, Spinosaurus B. In the case of the latter, all we have to go on to establish association are Stromer's notes, which are not quite as detailed as we might like. For the neotype, we know some of the specimen was directly collected in the field, and that other bits were purchased from dealers by two academic institutions over a two year period - exact documentation of this remains to be presented (hopefully it will in the 'neotype' monograph). Without strict certainty over how many individuals these specimens might represent, Evers et al. suggest some of the odd proportions in recent Spinosaurus reconstructions may reflect the marrying of mismatched bones to one another. That's not a certainty, of course, but it's also something which shouldn't be casually ignored.

Collectively, Evers et al. use these points to provide an alternative take on Spinosaurus to that presented in 2014. Ibrahim et al. argued that their new material helped simplify and integrate different interpretations of African spinosaurid material, but Evers et al. argue the opposite: they emphasise how poorly known Spinosaurus and kin are, and how interpreting fossils of north African spinosaurids is getting increasingly complex. Spinosaurus fossils remain very fragmentary to the point where most cannot be directly compared, they seem to hint at, but don't really crystalise, an apparent high species diversity, and are often of uncertain association or exact origin. At face value, that doesn't leave us with a lot to be confident about, although we'll have to see how this more despondent view goes down with other spinosaurid researchers. More complete and well documented discoveries will soon help smooth out bumps in our knowledge, but it seems likely that a lot of work and discussion remains to sort out what is really going on with north African, Late Cretaceous spinosaurids.

What does this mean for 'the Spinosaurus reboot'?

That's not quite the end of our discussion, however. It might be assumed that the points outlined above sound the death knell for the strangely proportioned 2014 Spinosaurus reconstruction, and that we should go back to our traditional interpretation of this animal. That might not be quite right, for two reasons. Firstly, given how distinctive many 'Spinosaurus' remains now seem to be, it's actually questionable what specimens should be considered Sp. aegyptiacus at all, other than the first specimen described by Stromer. A lot of referred isolated Spinosaurus specimens have been incorporated into our 'traditional' reconstructions in recent years, and we might need to think hard about their role in our interpretations of this animal. What we've become typically used to thinking of as Spinosaurus may not entirely be Spinosaurus!

Secondly, while some aspects of the 2014 interpretation of Spinosaurus have clearly been challenged by the Evers et al. paper, not all proportional aspects of the recent Spinosaurus reinvention are obviously erroneous. Last year, Ibrahim et al. noted that both Spinosaurus B and the 'neotype' have reduced hindlimbs with respect to their associated vertebrae, and used this fact as support for the diminutive legs in their reconstruction. Although arguing that there is no longer evidence for short hindlimbs in Spinosaurus itself, Evers et al. don't completely dismiss the notion of some African spinosaurids being short legged. The hindlimb proportions of those specimens is very similar despite the vagaries surrounding fossilisation and exhumation of ancient animal remains, maybe more similar than you'd expect from chance alone. If it is coincidence, it's certainly a startling one.

Stromer's 'Spinosaurus B' material: proportionally similar to the 'neotype' specimen, but does that tell us anything about spinosaurid proportions? Another image borrowed from Theropoda.

However, Evers et al. also attach some important caveats to this point. Stromer's notes clearly state that he did not consider the 'Spinosaurus B' material to represent one individual, and his testimony is the closest thing we have to a report on the excavation of the material. He specifically comments on the hindlimb being too small and slender to match the vertebrae, and thus interpreted them as representing a second individual. Other workers have agreed that this material must represent multiple animals or even several types of dinosaur (discussions about the possibly chimeric nature of Stromer's spinosaur specimens are not new - e.g. Rauhut 2003; Novas et al. 2005). Interpretation of the Spinosaurus B material as representing one animal is thus against some current thought and, of course, Stromer's original declaration. While the 'neotype' specimen might make a case for Stromer being mistaken, we really need to know more about the collection history to ascertain that. We're left with an intriguing set of measurements hinting at the reduced hindlimbs proposed by Ibrahim et al., but little in the way of objective information to explain their significance. The discovery of new specimens is needed to establish whether some spinosaurids were really short-legged, or if confusion of specimen inventories just made it look that way. In short, and no-doubt to the disdain of people who lose sleep about 'what science has done' to one of their favourite theropods, there's still something to play for with these short-legged spinosaurids.

So that's the latest chapter of research in Spinosaurus, then: I don't doubt that it's going to cause a lot of discussion in popular and academic circles. My personal take-home is that we seem to know less about Spinosaurus than might have been recently suggested, or at least that some issues need to be ironed out before we can develop a clear picture of what Spinosaurus is, and what sort of lifestyle it led. I don't know that any recent proposals about this animal have been shot down entirely yet, although clear gauntlets have been established for some of the more extreme ideas suggested in the last few years. It's going to be very interesting to see how others interpret these latest developments in the ongoing Spinosaurus saga, and where our understanding of this animal moves to next.

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References

• Evers, S. W., Rauhut, O. W. M, Milner, A. C, McFeeters, B, & Allain R. (2015) A reappraisal of the morphology and systematic position of the theropod dinosaur Sigilmassasaurus from the “middle” Cretaceous of Morocco. PeerJ 3:e1323
• Ibrahim, N., Sereno, P. C., Dal Sasso, C., Maganuco, S., Fabbri, M., Martill, D. M., & Zouhri, S., Myhrvold, N. and Iurino, D. A. (2014). Semiaquatic adaptations in a giant predatory dinosaur. Science, 345(6204), 1613-1616.
• Novas, F., Dalla Vecchia, F., & Pais, D. (2005). Theropod pedal unguals from the Late Cretaceous (Cenomanian) of Morocco, Africa. Revista del Museo Argentino de Ciencias Naturales nueva serie, 7(2), 167-175.
• Rauhut, O. W. M. (2003). Special Papers in Palaeontology, The Interrelationships and Evolution of Basal Theropod Dinosaurs (No. 69). Blackwell Publishing.

The Spinosaurus hindlimb controversy: a detailed response from the authors

No-one with an interest in Mesozoic reptiles will have missed the week of controversy following Ibrahim et al.'s (2014) new reconstruction of Spinosaurus. The most important debate has focused on the allegedly reduced Spinosaurus hindlimbs, which are integral to the proposed locomotor and lifestyle hypotheses proposed for the 'new look' animal, but also difficult to reconcile with presented data. Scott Hartman, who's no stranger to producing high-quality skeletal reconstructions, blew this whistle first when he found the reconstructed proportions of the Spinosaurus neotype specimen - a series of vertebrae and hindlimb elements - were questionably scaled against measurements of the bones themselves. Lead author of the Spinosaurus study, Nizar Ibrahim, publicly responded and suggested that the measuring landmarks Scott used in comparing vertebral and hindlimb elements may be wrong. When reviewing the controversy before the weekend, I attempted my own scaling effort, using Nizar's suggested landmarks, but ended up replicating Scott's results almost exactly. I concluded "[s]omething - the original measurements of the specimen or the reconstruction - just doesn't add up, and I suspect the latter, as I figure someone would have owned up to and corrected simple numerical errors in the paper by now."

It turns out that I've got to eat a few of those words. Following my post, Nizar opened a chain of correspondence where I directly asked about these scaling issues. Nizar's response was bringing his coauthor Simone Maganuco into our chat, who had taken the time to demonstrate and describe how the restored vertebral and hindlimb lengths match the dimensions reported in the paper. In his screenshot and email, Simone provided an enlarged view of the restored Spinosaurus trunk and took the time to explain where he thought the alleged scaling errors came from. Appreciating their interest to a wide audience, Simone has kindly allowed me to reproduce his screengrab and email here.

Image courtesy Nizar Ibrahim and Simone Maganuco, used with permission.

Dear Mark,

It is nice to be in touch with you. I am writing to comment briefly on my photoshop image, forwarded by Nizar a couple of hours ago.

I hope it is the key to understand the misunderstanding about the measurements, so I would be really glad to know your opinion about it.

I have tried to replicate the coefficients for scaling obtained by you and Scott Hartman and here is my line of reasoning.

Look at the vertebra D8 in my photoshop image. For convenience, we can focus our attention on the D8 on the left.

The yellow line is 18 "units" (and matches our measurements in the table) but if you include the posteriormost margin of the slanted posterior face and the condyle you have nearly 23 units.

23:18=X:71 where 18 and 71 are also the measurements in cm in the table of the Science paper; 23 units is the length of the whole vertebra in the drawing; and X should be the length of the ilium to match the length of the vertebra in the drawing, if one assumes that the whole vertebra - and not the yellow line - is 18 units, i.e., if one thinks  we used different landmarks and measured the maximum length of the centrum.

The value of X is 90.72  units.

90.72 /71  = 1.27 that is exactly the coefficient for pelvic girdle and hindlimb scaling suggested by Scott @ skeletaldrawing.com to resize the pelvis and the legs to match the size of the D8 vertebra measured with different landmarks (i.e., if 18 is considered the maximum length).

I can see that your coefficient is slightly lower, and I wonder if you have taken slightly lower measurements (it seems to be the case looking at the white lines in your test).

Do you think that this could be the explanation of  what happened?

In the paper, we thought it was better to measure the vertebrae from rim to rim (the rounded margins of the faces), excluding the condyle, and at the same dorsoventral height (because some vertebrae are like parallelograms). It is easier to compare anterior dorsals and posterior dorsals in this way, and it is easier also to compare the centra with those of some specimens not prepared three-dimensionally but preserving well-articulated vertebrae, i.e. specimens in which it is difficult to look at the anterior condyle.

As what concerns the femur, it must be taken into account that there is also a slight perspective effect, because in the digital model it points a bit laterally. i.e., it is not 100% parallel to the sagittal plane.

The misunderstandings generated by the comparison between the figure and the table clearly indicate that we had to indicate our landmarks in one extra figure, or dedicate a couple of lines to this into the text to satisfy the need to compare figure and measurements by people who want to test our skeletal reconstruction.

When I work with palaeoartists to prepare illustrations and flesh-models I also compare figures and measurements, so I can understand this need.

Sometimes there are figures that are not 100% in the view indicated in the caption (also because it is not easy to put a bone in plane!) and sometimes it is difficult to understand the landmarks used to take measurements. What if I were in your shoes? Who knows... but I can understand that the new look of Spinosaurus has unexpected proportions that leads to think that there is something wrong.

In the monograph everything will be more clear because the detailed figures will report measurements directly on the bones, permitting everybody to see the landmarks.

In the meantime, however, I think it is useful to clarify this aspect.

Best wishes,

Simone

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So there we have it: the measurements, landmarks and an image where they can be measured accurately. The latter is especially important because dorsal vertebra 8 in the full restoration is rather small, and thus prone to measuring errors even when measuring landmarks are known. A slip of a few pixels may not seem like much but, because the bone is a tiny component of a huge reconstruction, such minor errors can throw a scaling calibration right off. These risks were identified in Scott's original posts, and it seems they have been borne out. Nevertheless, it is interesting that Scott and I - and others, according to some Facebook chat - found such similar results: this could be coincidence, or it might be that the published reconstruction lends itself to a erroneous interpretation. Either way, there is plenty of food for thought here as goes presentation and reading of reconstruction data. For the record, when attempting to replicate the scaling again, this time on the screenshot, I found my results matched measured values given in Ibrahim et al. (2014) within a few percent. My confidence in the published proportions is thus fully restored.

Hopefully this helps resolve the scaling controversy with the 'Spinosaurus reboot', and the result is much more confidence about the downright weird and remarkable anatomy of this genuinely unusual animal. Thanks to Nizar and Simone for taking the time to explain their work, and allowing me to post their response here.

Reference

• Ibrahim, N., Sereno, P. C., Dal Sasso, C., Maganuco, S., Fabbri, M., Martill, D. M., Zouhri, S. Myhrvold, N. & Iurino, D. A. (2014). Semiaquatic adaptations in a giant predatory dinosaur. Science, 1258750.