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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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