A Specialized Cretaceous Pliosaurid and the Convoluted Pathways of Plesiosaur Evolution

Thalassophonean pliosaurs are some of the most iconic and long-lasting radiations of marine reptiles. Throughout their reign, a time span that lasted from the Callovian to the Turonian, almost all members of the clade were macrophagous carnivores. Some, such as Pliosaurus and Kronosaurus, were evidently apex predators with skulls and jaws exceeding 2 m in length and could have preyed on virtually any coexisting animal (Foffa et al. 2014, McHenry 2009, Taylor & Cruickshank 1993).

The newly described thalassophonean Luskhan itilensis Fischer et al. 2017 from the Hauterivian of western Russia notably deviates from this trend. Judging from a subcomplete skeleton preserving large parts of the appendicular and axial skeleton and a complete skull with associated mandible, its functional anatomy suggests a very different ecological niche from those held by other thalassophoneans. While the specimen is large, the lower jaw measuring 159  cm and total length estimated at 6.5 m, the morphology of its skull is much more reminiscent of polycotylid plesiosauromorphs or basal, plesiomorphic pliosaurids.

Morphospace analyses based on craniodental features confirm that Luskhan (as does Peloneustes) plots much more closely with non-thalassophonean pliosaurs and polycotylids than it does with macrophagous thalassophoneans. Several telling traits mark Luskhan as a mesopredatory piscivore and/or teuthivore rather than a generalist apex predator: First and foremost, the rostrum and lower jaw are considerably more slender than in other Thalassophoneans. Secondly, the dentition is relatively small and isodont, lacking the enlarged caniniform anterior teeth of other pliosaurs, and has a more limited posterior extent. Finally, Luskhan’s mandibular symphysis makes up 34% of mandible length, greater than in any other pliosaur but within the range seen in polycotylids (a long mandibular symphysis can be linked to reduced bending and torsional strength of the jaw, Walmsley 2013).


Life reconstruction of Luskhan itilensis with a scuba diver for scale.

In all cranial ecomorphological aspects, Luskhan hence converges with polycotylids, despite being a basal member of Brachaucheninae, a clade that otherwise famously includes gigantic, macropredaceous taxa such as Kronosaurus queenslandicus. The recent discovery of another basal Brachauchenine from the Hauterivian of Russia, Makhaira rossica, which does show pronounced macrophagous adaptions (Fischer et al. 2015) furthermore indicates that brachauchenines were not secondarily macrophagous, despite having lost some of the traits found in earlier, Jurassic thalassophoneans. Luskhan thus probably represents a unique side-branch of the Pliosauridae, that independently evolved many of the features Polycotylids would sport 40 Ma later after the extinction of pliosaurs.

This is a modified translation of an entry recently published on GeoHorizon.

–––References:
Fischer, V., Arkhangelsky, M. S., Stenshin, I. M., Uspensky, G. N., Zverkov, N. G. and Benson, R. B. J. 2015. Peculiar macrophagous adaptations in a new Cretaceous pliosaurid. Royal Society Open Science 2 (12): 150552.
Fischer, V., Benson, R. B. J., Zverkov, N. G., Soul, L. C., Arkhangelsky, M. S., Lambert, O., Stenshin, I. M., Uspensky, G. N. and Druckenmiller, P. S. In press. Plasticity and Convergence in the Evolution of Short-Necked Plesiosaurs. Current Biology.
Foffa, D., Cuff, A. R., Sassoon, J., Rayfield, E. J., Mavrogordato, M. N. and Benton, M. J. 2014. Functional anatomy and feeding biomechanics of a giant Upper Jurassic pliosaur (Reptilia: Sauropterygia) from Weymouth Bay, Dorset, UK. Journal of Anatomy 225 (2): 209–219.
McHenry, C. R. 2009. ‘Devourer of Gods’: The Palaeoecology of the Cretaceous Pliosaur Kronosaurus queenslandicus. University of Newcastle. Taylor, M. A. and Cruickshank, A. R. I. 1993. Cranial anatomy and functional morphology of Pliosaurus brachyspondylus (Reptilia: Plesiosauria) from the Upper Jurassic of Westbury, Wiltshire. Philosophical Transactions of the Royal Society of London B: Biological Sciences 341 (1298): 399–418. Walmsley, C. W., Smits, P. D., Quayle, M. R., McCurry, M. R., Richards, H. S., Oldfield, C. C., Wroe, S., Clausen, P. D. and McHenry, C. R. 2013. Why the Long Face? The Mechanics of Mandibular Symphysis Proportions in Crocodiles. PLoS ONE 8 (1).

---posted 2100, 03/06/2017
Archive
The Trackmakers of Broome

The Broome Sandstone in Western Australia has long been known to those people inclined towards Dinosaur ichnology, but as it turns out, only snippets of its extraordinarily rich ichnofauna had actually been described up until recently. An intriguing new paper by Salisbury et al. recently provided a full scientific description based on the results of over 400 hours of field work on the Western-Australian Dampier Peninsula, where the authors documented what may be the most diverse dinosaurian ichnofauna currently known to science, with somewhere between 11 and 21 distinct track types recognized among 150 diagnosable tracks. Among the trace fossils found at the Lower Cretaceous (Valanginian-Barremian) locality are several Sauropods, Ornithopods, Theropods and Thyreophorans.

Deposition of the Broome Sandstone primarily took place in distal river- and deltaic environments. Its age and rich fauna make it a prime locality for studying the otherwise poorly known Gondwanan Dinosaur faunas of the earlier part of the Lower Cretaceous. The faunal composition at Broome is dominated by various sauropods, but also includes several different theropods, ornithopods and thyreophorans. In this regard it bears some resemblance to Laurasian assemblages of the Upper Jurassic, such as the Morrison and Lourinha formations. This hints at a comparatively smaller degree of faunal turnover at the Jurassic-Cretaceous boundary in Gondwanan ecosystems than happened in Laurasia.

While the presence of dinosaur ichnofossils is nothing new, with theropod footprints featuring prominently in aboriginal mythology and the Dampier Peninsula being the site of the first described sauropod tracks from Australia, the full scope of its fossil biodiversity turns out to be quite staggering, even having known about previous significant discoveries at the locality.

Apart from tracks assignable to two or three previously described ichnotaxa (Megalosauropus broomensis, Wintonopus latomorum and cf. Amblydactylus kortmeyeri), Salisbury et al. described a further six novel ichnotaxa: the theropod Yangtzepus clarkei , the sauropod Oobardjidama foulkesi (Sauropoda), ornithopods Wintonopus middletonae and Walmadanyichnus hunteri and the probable stegosaurs Garbina roeorum and Luluichnus mueckei. Apart from these taxa, a insufficiently diagnosable tracks are designated as sauropod morphotypes A-E, theropod morphotypes A-C and thyreophoran morphotypes A-B. Finally, some tracks don’t warrant a taxonomic assignment below the level of Dinosauria.

Especially noteworthy is that some of the Broome tracks are perplexingly enormous. It is "Broome Sauropod Morphotype A" that is most impressive of all. Its pes prints average 106cm wide and 137cm long, but the largest specimen is 140cm wide and at least 175cm long. What’s more, the authors specifically rule out these tracks being transmitted reliefs or having been enlarged through erosion. The bottomline seems to be that these gigantic dimensions appear to be legit, and that they are probably the biggest footprints in the world for which that can be said. By comparison, the feet of the holotype of Diplodocus carnegii are a mere 59cm wide, which would result in a width of 62-71cm with realistic amounts (5-20%) of soft tissue. By the same means, the feet of the subadult holotype of Giraffatitan can be estimated at 77-88cm wide. This means that conservatively, the largest broome footprints are almost twice the size of Diplodocus or 59% larger than Giraffatitan.


Comparative length and width measurements of all identified pes prints.

Sauropods are not the only cases of gigantism in this ichnofauna. One pes print tentatively assignable to Garbina roeorum measures 80 by 70cm, which would make it enourmous for a stegosaur. Some ornithopod footprints attain or approach 80cm in length, meaning that Australia was home to giant ornithopods remains of which have yet to be unearthed. And a large manual track 29cm wide may pertain to a very large ankylosaur.

This is an abbreviated translation of a blog entry recently published in German on GeoHorizon.

–––References:
Salisbury, S. W.; Romilio, A.; Herne, M. C.; Tucker, R. T.; Nair, J. P. (2017): The Dinosaurian Ichnofauna of the Lower Cretaceous (Valanginian–Barremian) Broome Sandstone of the Walmadany Area (James Price Point), Dampier Peninsula, Western Australia. Journal of Vertebrate Paleontology 36 (sup1) pp. 1-152, DOI: 10.1080/02724634.2016.1269539

Wedel, M. 2009. How big were the biggest sauropod trackmakers? Sauropod Vertebra Picture of the Week. Url: https://svpow.com/2009/10/13/how-big-were-the-biggest-sauropod-trackmakers last accessed 10/04/2017

---posted 1630, 10/04/2017
Are the teeth of Tyrannotitan less blade-like than those of other Carnosaurs?

Short answer: No
Theropod tooth data

Crown base ratios of Carnosaurs as an indicator of labiolingual compression, with Tyrannosaurus rex as an outgroup. Tooth data from Canale et al. 2015, Royo-Torres et al. 2009 and Smith et al. 2005



Frankly, I have got no idea where this myth originally came from, but being called Tyrannotitan does not imply being in any way more similar to Tyrannosaurus than its relatives were. T. rex does not have a monopoly on that part of its name, on the other hand it does seem to have one on its blunt, incrassate teeth.

–––References:
Canale, Juan I.; Novas, Fernando E.; Pol, Diego (2015): Osteology and phylogenetic relationships of Tyrannotitan chubutensis Novas, de Valais, Vickers-Rich and Rich, 2005 (Theropoda: Carcharodontosauridae) from the Lower Cretaceous of Patagonia, Argentina. Historical Biology: An International Journal of Paleobiology, 27 (1), pp. 1-32.
Royo-Torres, R.; Cobos A.; Alcalá, L. (2009): Diente de un gran dinosaurio terópodo (Allosauroidea) de la Formación Villar del Arzobispo (Titónico-Berriasiense) de Riodeva (España). Estudios Geológicos, 65 (1), pp. 91-99.
Smith, Joshua B.; Vann, David R.; Dodson, Peter (2005): Dental Morphology and Variation in Theropod Dinosaurs: Implications for the Taxonomic Identification of Isolated Teeth. The Anatomical Record, 285 (A), pp. 699-736.

---posted 192211/09/2015
The Myth and Truth of UCMP 118742

The Berkeley Tyrannosaurus rex specimen UCMP 118742. Anyone who has made experiences with T. rex fanatics has probably read of this specimen at some point, as evident from a quick google search most likely in the context of someone asserting some extreme body size for it. Why is it so famous? Because it is large, of course. How large exactly? That part of the equation always seems to get misrepresented.

There is only an isolated maxilla, which makes my job both easier and more difficult at the same time. More difficult, because as always when scaling up from a single bone there is quite a bit of uncertainty involved in the final estimate.
Easier because Larson (2008) luckily gave some measurements, and those represent pretty much everything relevant that you can measure in the specimen. So there is virtually nothing left that could suggest a size estimate other than what is indicated by these measurements.
So how do UCMP 118742’s measurements stack up against the size record holder and most complete known tyrannosaur, FMNH PR 2081?

UCMP 118742 FMNH PR 2081ratio
depth 390 4000.975
length 810 8550.947
diagonal length 690 7200.958
tooth row length 625 6450.969
Measurements of the Berkeley maxilla and Sue compared.

The obvious implication is that UCMP 118742 is on average just 96.2% the size of Sue, and no more than 97.5% based on the largest measurement. The geometric mean of its maxillary measurements is 607.5, compared to 631.3 for FMNH PR 2081, implying a total length of 11.84m, which is large, but not exceptional. To anyone with an elementary school level of math education this should suggest that Sue, at 12.3m, is the bigger specimen, shouldn’t it?

Messing with growth rates
Well, sadly, this is only the beginning of the argument. Granted, they somehow manage to make it look as if the specimen itself was already larger than sue, which is plain wrong, looking at the fact that every single one of its measurements is actually smaller. But there is an additional bit of information about the individual from which this maxilla came in Table S2 of the Supplement of Erickson et al. 2006: It is apparently 16 years of age. Now, as we know T. rex’ growth slowed down as it matured, until older adults (such as Sue) only experienced negligible growth, and a 16 year old would still be in a phase of fast growth (albeit not a juvenile, as fanboys like to claim), right?
This is Erickson et al.’s growth model for T. rex: Growth curve for Tyrannosaurus rex following Erickson et al. 2004
The weight figures are the typical underestimation based on obsolete femur-circumference regressions, but we can use them to compare between each other and estimate total lengths: Growth curve for Tyrannosaurus rex, modified from Erickson et al. 2004

Growth curve for Tyrannosaurus rex, modified from Erickson et al. 2004 so that the dependent variable is a length estimate scaled isometrically from FMNH PR 2081 instead of a weight estimate.


The first vertical line demarks the age of 16 years, the second that of 28 years, the age of the oldest known Tyrannosaurus.
This is technically what the fanboy claims are based on; the assumption that they can extrapolate the theoretical "fully grown", size of UCMP 118742 based on its actual size and age. That they did so using incorrect and biased premises is hopefully self-evident, but hypothetically, is this a sound method?
The model predicts total lengths of 9.62m and 12.22m at ages of 16 and 28 years respectively. So during this part of its life, the average T. rex is expected to grow 27%, or 2.6m. Applying these to UCMP 118742 results in hypothetical "adult" sizes of 15.05m and 14.45m respectively.

Before jumping to conclusions: This is not to say that this T. rex would ever have grown that big, and far less even to say that this means T. rex was 14-15m long (it is not, more on that in a future post).
There are several problems with this method. Perhaps the most obvious is sample size. The growth model bases on just a handful of specimens, so the uncertainty involved is very high, and so is the potential impact of adding even a single additional specimen.
Sample of Tyrannosaurus rex length estimates based on femur lengths in Larson (2008) added to data and growth model from Erickson et al. 2004

Sample of Tyrannosaurus rex length estimates based on femur lengths in Larson (2008) added to data and growth model from Erickson et al. 2004 to demonstrate variability of T. rex sizes with respect to the crowth curve.



Attentive readers might have noticed that UCMP 118742 was not part of the original dataset (not surprising considering the data set bases on femur measurements), and adding it could have a rather significant impact, meaning these estimates are already biased because they exaggerate the difference between the average 16-year old and a fully grown T. rex.

Also, the method assumes that its growth would have continued just as fast as that of any other T. rex (14.45m estimate), or even at an accellerated rate (15.05m estimate). But this was probably not the case, after all, had it grown just like any other T. rex, it would not have been an estimated 11.8m long at the age of 16 in the first place
Many people become oblivious of individual variation as soon as it comes to the tyrant king, but not all specimens of a species grow the same way, and being larger than expected for one’s age during adolescence does not automatically mean being larger later in life (think back to your time at school!). One T. rex might already have been close to full-sized at a given age, while others weren’t.
And indeed, that is what seems to be the case with T. rex specimens that apparently stopped growing at a much earlier age than predicted by this model, around the age of UCMP 118742 to be exact. Horner & Padian (2004) document this in MOR 1125, which they state to have "effectively stopped growing at 16±2 years". This could plausibly apply here too; since the animal had already achieved a large size it could also have grown more slowly from then on. The tyrannosaurine analogue of a human teenager who is very tall at 15, but doesn’t get any larger from then on.
Considering it had already reached a size consistent with those of large adults (e.g. CM 9380), is there any reason to assume this individual would have grown much bigger? Probably not.
And finally, a note to all those people who were eager to see T. rex as the unchallenged biggest theropod due to just this specimen: In all probability, similar cases existed among other giant theropods, their ontogeny just hasn’t been studied yet and/or too few specimens have been found so far. This is a single individual, even if it were the abnormal giant of your wildest dreams, it would not have a profound impact on the size of T. rex as a species.

In a nutshell, it is pointless to estimate a hypothetical adult size for such a specimen, and it is dishonest to refer to it as if it were an actual size recorded for the species. We will never know how big it would have grown, but it is most likely that it is just a precocious teenager, big for its age, but not necessarily extraordinarily large had it grown into senescence. and in the end, keep in mind that it didn’t. It is better to stick to real specimens.

–––References:
Larson, Peter (2008): Variation and Sexual Dimorphism in Tyrannosaurus rex. In: Larson, Peter; Carpenter, Kenneth: Tyrannosaurus rex the Tyrant King. Bloomington, pp. 103-128.
Erickson, Gregory M.; Currie, Philip J.; Inouye, Brian D.; Winn, Alice A. (2006): Tyrannosaur Life Tables: An Example of Nonavian Dinosaur Population Biology. Science, 313 (5784), pp. 213-217.
Erickson, Gregory M.; Makovicky, Peter J.; Currie, Philip J.; Norell, Mark A.; Yerby, Scott A.; Brochu, Christopher A. (2004): Gigantism and comparative life-history parameters of tyrannosaurid dinosaurs. Nature, 430 (7001), pp. 772-775.
Horner, John R.; Padian, Kevin (2004): Age and growth dynamics of Tyrannosaurus rex. Proceedings of the Royal Society B, Vol. 271 (1551), pp. 1875-1880.

---posted 1720, 20/08/2015
Carcharocles subauriculatus–bigger yet smaller than we think?

Well first of all, C. subauriculatus is actually the older, and hence the valid name, not the inexplicably more popular C. chubutensis (PalaeoDB ONLINE).

This shark species is not nearly as well known, or indeed as common (at least based on ocurrence in museum displays) as its bigger relative, C. megalodon, but it, too, is among the largest chondrichthyans in earth’s history.

The size figure one sees cited commonly is a little over 12m, based on the regression between tooth height and total length in Gottfried et al. (1996) and the largest teeth, which reportedly approach 13cm in diagonal, or slant length. However, this is fallacious on more than one level, because the method was not supposed to be used with the diagonally measured length, but with vertical tooth height, and because it assumes the same proportions within the dentition as in Great White Sharks, which do not actually appear to be present in C. subauriculatus.

So, how large was this taxon really?
Purdy et al. 2001 figured, described and listed measurements for two sets of associated teeth, each representing the partial dentition of a single animal.
Better still, the fact that the lateral cusplets (a paedomorphic feature) are absent or very weakly developed is a strong indication that the specimens were adults at the time of death.
The smaller of the two, USNM 411881, has one upper quadrant almost completely preserved. Based on the relative widths of the overlapping teeth, the second one, USNM 299832, is 15.8% bigger.
Completing the dentition by extrapolating the missing anteriormost tooth from the larger specimen (and the diminuitive posteriormost tooth, which is not preserved in either specimen, from C. megalodon), the result is an estimated summed tooth width of ~600m, resulting in a total of ~1200mm in both sides of the upper jaw. Adding 15% of interdental spacing (admittedly a little liberal), this results in the overall length of the upper toothrow being ~1380mm, that of the larger individual is thus an estimated 1598mm long.

How does this help us? Much it turns out, since estimating the size of a shark from a complete or nearly complete dentition is much more reliable than using a single tooth. Lowry et al. 2009 examined the relationship between the length of the tooth row (or bite circumference) and the total length of the shark, and found a strong correlation. Since the best overall analogue in terms of size, ecology and morphology is probably the Great White, the formula relevant here is that for C. carcharias, which we can transform and solve for total length:

LOG(tooth row length)=1.007*LOG(total length)-0.8
LOG(tooth row length)=LOG(10^-0.8*total length^1.007)
tooth row length=10^-0.8*total length^1.007
(tooth row length/10^-0.8)^(1/1.007)=total length
Length estimates based on Lowry et al. 2009

So these specimens were ~8.2 and ~9.5m long, and based on published length-weight-regressions (Casey & Pratt 1985, Kohler et al. 1995) for C. carcharias they probably massed ~5.0-6.0 and ~7.8-9.4t respectively.
But that is not the end of the story. Purdy et al. mention a first lower lateral tooth that is 9.5cm tall, which is 79% and 56% bigger than the equivalent teeth in the aforementioned dentitions, suggesting a tooth-row length of ~2474-2489mm and a total length of 14.5-14.7m.
To put this into perspective, that is well within the territory of C. megalodon, and actually above its average size (which, for adults, is about 14m based on data from Pimiento & Balk 2015).
Obviously every estimate that just bases on a single tooth is prone to huge margins of error, so this should be taken with a grain of salt. All this shows is that what is popularized about this species’ size is not founded on facts all that firmly.

–––References:
  Casey, John G.; Pratt, Harold L. (1985) Distribution of the White Shark, Carcharodon carcharias, in the Western North Atlantic. Memoirs of the Southern California Academy of Sciences, 9 (Biology of the White Shark, a Symposium.) pp. 2-14
  Kohler, Nancy E.; Casey, John G.; Turner, Patricia A. (1995): Length-Length and Length-Weight Relationships for 13 Shark Species from the Western North Atlantic. Fishery Bulletin, 93 pp. 412-418
  Lowry, Dayv; Castro, Andrey L. F. de; Mara, Kyle; Whitenack, Lisa B.; Delius, Bryan; Burgess, George H.; Motta, Philip: (2009): Determining shark size from forensic analysis of bite damage. Marine Biology, 156 pp. 2483-2492
  Pimiento, Catalina; Balk, Meghan A. (2015): Body-size trends of the extinct giant shark Carcharocles megalodon: a deep-time perspective on marine apex predators. Paleobiology, 41 (3) pp. 479-490
  Purdy, Robert W.; Schneider, Vincent P.; Applegate, Shelton P.; McLellan, Jack H.; Meyer, Robert L.; Slaughter, Bob H. (2001): The Neogene Sharks, Rays, and Bony Fishes from Lee Creek Mine, Aurora, North Carolina. In: Ray, Clayton E.; Bohaska, David J.: Geology and Paleontology, of the Lee Creek Mine, North Carolina, III. Smithsonian Contributions to Paleobiology, 90 pp. 71-202
  PaleoDB: Fossilworks: Carcharodon subauriculatus. http://fossilworks.org/bridge.pl?a=taxonInfo&taxon_no=83172 (accessed 21 July 2015)

---posted 1911, 22/07/2015
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