New publication: Cold capitosaurs and polar plagiosaurs: new temnospondyl records from the upper Fremouw Formation (Middle Triassic) of Antarctica (Gee & Sidor, 2021; JVP)

Title: Cold capitosaurs and polar plagiosaurs: new temnospondyl records from the upper Fremouw Formation (Middle Triassic) of Antarctica
​Authors: B.M. Gee; C.A. Sidor
Journal: ​Journal of Vertebrate Paleontology
DOI:  10.1080/02724634.2021.1998086

General summary: The Middle Triassic captures a diverse global record of temnospondyls, which is also when we start to see the pinnacle of the evolution of large body size, with many taxa routinely exceeding skull lengths of half a meter and body lengths of probably 2m or greater. A variety of different groups are present at this time, all of which appeared in the Early Triassic and which would also continue through the Late Triassic, and many ecosystems were host to several different species of no close relatedness. However, the Antarctic record of Middle Triassic temnospondyls has only comprised members of a single clade, the capitosaurs. Dated and brief historical notes suggested the possible presence of another clade, the long-snouted crocodilian-like trematosaurs, but this was never substantiated, and thus the Antarctic record, despite preserving at least three different species, captures an overall much lower diversity of temnospondyls than found elsewhere around the world. In this study, we took a look at some of this more ambiguous historical material, combined with more recently collected material of some very large lower jaws. While every single one of these lower jaws belongs to a capitosaur, there is a partial interclavicle (part of the shoulder girdle) that is very clearly not that of a capitosaur but instead that of a plagiosaurid, a peculiar short-snouted clade that has hundreds of records from the northern hemisphere but a mere two others from the southern hemisphere (both of those are from the Early Triassic). We speculate on some of the reasons why the Antarctic record, which is undoubtedly undersampled, might reflect real patterns of differing ecologies among large-bodied temnospondyls (i.e. 'big crocodile analogue' is a gross oversimplication).

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We run this town

Globally, the Middle Triassic record of temnospondyls is quite diverse, with extensive records of many of the large-bodied taxa that people often associate with the Mesozoic. This includes the short-snouted brachyopids and plagiosaurids and the long-snouted capitosaurs and trematosaurs. Often, we find localities with multiple species, if not multiple families, of different temnospondyls, indicating that they were flourishing during this time period. The Antarctic record is therefore interesting because to date, there are only capitosaurs! Previous records mostly include brief reports of fragmentary material (e.g., Hammer, 1990), but more recently, three different genera, two of them entirely new, have been documented by Chris and colleagues (Sidor et al., 2007, 2008, 2014; see below). Some of these newer fossils are traveling around the U.S. right now as part of the traveling Antarctic Dinosaurs exhibit (currently in Buffalo at the Buffalo Museum of Science).

The holotype skull of the capitosaur Antarctosuchus from Antarctica (Sidor et al., 2014).

The holotype snout of the capitosaur Kryostega from Antarctica (Sidor et al., 2008)

One of the interesting historical notes was the report of a possible 'benthosuchid' from the Middle Triassic rocks of Antarctica. This naturally warranted some attention because today, benthosuchids (like Benthosuchus, see right) are classified as trematosaurs, which would provide evidence for the first non-capitosaur in Antarctica. Alas, revisiting these historical specimens, several fragmentary lower jaws, did not provide any evidence that one belonged to a trematosaur; many of the features cited originally by Hammer (1990) are generic features of higher stereospondyls, which underscores both the importance and essentiality of revisiting older identifications because previously diagnostic features may no longer be diagnostic (this was also an issue with our reanalysis of the putative Antarctic 'lydekkerinid' Cryobatrachus). One other point is that taxonomy often shifts. Today, Benthosuchus is considered to be a trematosaur, but in the 90's and into the 2000's, it was widely considered to be a capitosaur because it lacks many of the features found in more derived trematosaurs like the extreme elongation of the snout. Therefore, Hammer's concept of a 'benthosuchid,' when situated in the historical context of the time, should really be read as "here is a specific type of capitosaur" not "here is a non-capitosaur," which is how it would be read now if removed from that context.

Skull of the benthosuchid trematosaur Benthosuchus gusevae (Novikov, 2012).

We also looked at more recently collected material from approximately the same localities/horizons as the historical material. This material also only included lower jaws that appear to be capitosaur in nature. Some are also remarkably large - the scale bars below are 5 cm, and conservative estimates indicate that some of these lower jaws would have been more than 90 cm long, which ranks among the largest temnospondyl specimens known from the Triassic (most capitosaurs aren't known to exceed about 60 cm in skull length). So not only do we only have capitosaurs, we have only very mature individuals, which makes sense given that these remains come from a coarse sandstone that represents a high-energy setting that would have been less conducive to preserving the remains of juveniles.

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A southern sojourner

Probably the most exciting new specimen is the partial interclavicle shown below, a large unpaired element of the shoulder girdle that would have been positioned where the sternum sits in humans (see figure to the right as well). While temnospondyls tend to have similarly shaped interclavicles that differ largely in proportions, this specimen has two distinctive features, pustulated ornamentation rather than the typical grooves and pits found in most temnospondyls; and a contour indicating a 'starfish-like' outline. These features are only found in plagiosaurids, a clade never reported from any deposits in Antarctica!

The skull and shoulder girdle of the plagiosaurid Gerrothorax from Greenland (Jenkins et al., 2008).

This specimen isn't just notable for being the first plagiosaurid from Antarctica, but also for being the first plagiosaurid from anywhere in the southern hemisphere in the Middle Triassic. One of the ideas that we advance here is the notion that plagiosaurids were pioneer species, something previously postulated based on bone histology (e.g., Witzmann & Soler-Gijón, 2010; Sanchez & Schoch, 2013). In this scenario, plagiosaurids would have been well-adapted for more unstable environments that could have seen fluctuations in oxygen content, salinity, and other aspects of water chemistry, and these habitats may not have been so great for other temnospondyls. Conversely, plagiosaurids may thus not have been as common in more stable habitats that were frequented by other temnospondyls, like floodplains or high-energy rivers. There is some evidence for this in the temnospondyl-rich Late Triassic deposits of Germany, where plagiosaurids tend to be common in oxygen-poor environments where remains of other temnospondyls are rare but are rare in more stable environments where there is a greater diversity of other temnospondyls.

This still leaves the question of where are the trematosaurs? This clade is found throughout the world, including in the Arctic Circle, yet there is no trace of them in the Fremouw Formation, either the Early or the Middle Triassic exposures. One idea is that trematosaurs have often been cited as being euryhaline, as they are often (but not always) found in marine deposits. If so, it's possible that trematosaurs didn't inhabit the freshwater ecosystems in Antarctica (as they do in Germany, for example), and that the marine settings that they did inhabit around the south pole weren't preserved or haven't been discovered yet. If this were true, it would indicate that the Middle Triassic clades had settled into different niches, not only with respect to what they ate, but also what type of aquatic habitats they preferred.

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References

• Hammer, W. R. 1990. Triassic terrestrial vertebrate faunas of Antarctica; pp. 42–50 in T. N. Taylor and E. L. Taylor (eds.), Antarctic Paleobiology: Its Role in the Reconstruction of Gondwana. Springer Verlag, New York.
  
• Jenkins Jr., F. A., N. H. Shubin, G. M. Gatesy, and A. Warren. 2008. Gerrothorax pulcherrimus from the Upper Triassic Fleming Fjord Formation of East Greenland and a reassessment of head lifting in temnospondyl feeding. Journal of Vertebrate Paleontology 28:935–950. DOI: 10.1671/0272-4634-28.4.935
  
• Novikov, I. V. 2012. New data on trematosauroid labyrinthodonts of Eastern Europe: 4. Genus Benthosuchus Efremov, 1937. Paleontological Journal 46:400–411. DOI: 10.1134/S0031030112040089
  
• Sanchez, S., and R. R. Schoch. 2013. Bone histology reveals a high environmental and metabolic plasticity as a successful evolutionary strategy in a long-lived homeostatic Triassic temnospondyl. Evolutionary Biology 40:627–647.
  
• Sidor, C. A., R. Damiani, and W. R. Hammer. 2008. A new Triassic temnospondyl from Antarctica and a review of Fremouw Formation biostratigraphy. Journal of Vertebrate Paleontology 28:656–663. DOI: 10.1671/0272-4634(2008)28[656:ANTTFA]2.0.CO;2
  
• Sidor, C. A., J.-S. Steyer, and W. R. Hammer. 2014. A new capitosauroid temnospondyl from the Middle Triassic upper Fremouw Formation of Antarctica. Journal of Vertebrate Paleontology 34:539–548. DOI: 10.1080/02724634.2013.808205
  
• Witzmann, F. and Soler‐Gijón, R., 2010. The bone histology of osteoderms in temnospondyl amphibians and in the chroniosuchian Bystrowiella. Acta Zoologica, 91(1), pp.96-114. DOI: 10.1111/j.1463-6395.2008.00385.x