New publication (First record of the amphibamiform Micropholis stowi from the lower Fremouw Formation (Lower Triassic) of Antarctica; Gee & Sidor, 2021, JVP)
Title: New publication (First record of the amphibamiform Micropholis stowi from the lower Fremouw Formation (Lower Triassic) of Antarctica
Authors: B.M. Gee, C.A. Sidor
Journal: Journal of Vertebrate Paleontology
General summary: Following the end-Permian mass extinction (252 mya), life on Earth hit a big reset button. Many major groups suffered substantial losses in biodiversity or went entirely extinct, being replaced by other groups that were rare or non-existent before the extinction. The temnospondyl amphibians are no exception - almost all of the characteristic groups found in the Permian are entirely wiped out, and that ones that survive it are found in very low diversity and are restricted to mostly a few localities that would have been at high latitude. Concurrent with this is the observation that a lot of the first temnospondyls that appear in the Early Triassic are rather small compared to their Permian predecessors or their later Triassic successors. One of these is a holdover from the Permian, the amphibamiform dissorophoids. There is only one definitive amphibamiform in the Triassic (there were over 30 in the Permian), Micropholis stowi, a species known from South Africa for over 150 years now. Our study reports the first record of M. stowi from outside of South Africa, in similarly aged Early Triassic rocks from Antarctica. This further solidifies the similarities between the Early Triassic of Antarctica and South Africa, but it still comes in the face of somewhat stark endemism otherwise - most of the temnospondyls known from the southern hemisphere at the time are found in only one spot, despite there being a rich record of this group in places like South Africa, Australia, India, and Madagascar. This is also the first paper from my postdoc and sets the stage for some of our in prep projects on other newly collected material from Antarctica!
Now species go extinct all the time naturally, but they are often replaced by something kind of similar and probably closely related. But with mass extinctions, not only do we get a lot of species going extinct and being replaced by new ones, but the structure of the ecosystem can change quite a bit. For example, after the dinosaurs go extinct, they were replaced at the top of the food chain by large reptiles and birds and then later by mammals, which is the type of food web we have today where mammals are the largest animals and are typically apex predators. Similarly, the end-Permian extinction turns everything over. At the end of the Permian, synapsids (the group encompassing mammals but well before the origin of mammals back then) were filling the major roles, like apex predators and large herbivores, while reptiles were pretty small. But the extinction wipes out many synapsids, especially the large carnivorous ones, which sets the stage for the "Age of Dinosaurs" (Mesozoic) in which archosaurs, particularly the bird/dinosaur line, takes off.
Stratigraphic ranges of various synapsids in the Karoo Basin of South Africa, which captures the end-Permian mass extinction. Here, the yellow is the late Permian, and the orange is the Early Triassic. As can be seen, some groups like Dinocephalia and Gorgonopsia don't survive the extinction, while others like Anomodontia are greatly reduced in number. Figure from Smith et al. (2020).
On the slimy side of things, temnospondyls make it through the end-Permian extinction too, but they are also totally different coming out the other side. Over the course of the Permian, increasing aridity on land and the radiation of more drought-tolerant synapsids and reptiles led to the decline of terrestrial temnospondyls, but there weren't too many other tetrapods in the water, so they flourished there (crocodiles and the kind didn't really show up until the Jurassic). The end-Permian extinction only accelerated their migration back to the water, where they once again began to flourish in the new Triassic world.
Phylogenetic biostratigraphy of temnospondyls in South Africa's Karoo Basin across the end-Permian mass extinction. At least in South Africa, the temnospondyl diversity is actually much higher post-extinction (in contrast to synapsids), but the overall point is that most of the groups that are found in the Early Triassic are new in this region. Figure from Tarailo (2018).
Changing of the guard
Most of the temnospondyls that show up in the Triassic are giants, larger than even today's giant salamanders from China and Japan (up to about 6 feet long), and many of them have no fossil record before the Triassic. Basically all of the Triassic temnospondyls belong to a group called Stereospondyli, which refers to a particular type of vertebral anatomy. Stereospondyls are quite rare in the Permian, with only really two groups documented in the southern hemisphere (Australia, Brazil, South Africa). However, when other groups of stereospondyls are first documented in the Triassic, they are already widely distributed, sometimes across essentially the entire globe, from Greenland to Antarctica, which suggests that they appeared earlier than the Triassic and just aren't documented in the fossil record for some reason. This could indicate that for one reason or another, these groups were not fossilized (e.g., living in habitats that didn't preserve many fossils), or their fossils have yet to be discovered (e.g., geographic regions that haven't been explored much).
Time-calibrated phylogeny of temnospondyls showing which groups survived the extinction, which did not, and which appeared after the extinction. Most temnospondyls in the Early Triassic and onward belong to the subgroup called Stereospondyli (blue circle). The end-Permian extinction is marked in the red dashed line.
At the edge of the world
If temnospondyls were already having a tough time because of more gradual climate harshness in the Permian, how'd they make it through the extinction? The answer might really be as simple as sheer dumb luck. One of the longstanding ideas is that the high latitudes in the southern hemisphere, which would have been cooler and more temperate, were a refugium as the world got really hot. Most of these high latitude areas are still at high latitude today, like Antarctica, Australia, and South Africa. It seems likely that temnospondyls were found in other places at the end of the Permian but those closer to the equator didn't make it (rather than all of the temnospondyls migrating as far south as possible as the extinction event got underway in some sort of Noah's ark kind of tale). So then we are left with these survivors in high latitudes that start going back out to repopulate the world. One of the early proponents of this idea, Yates & Warren (2000) even suggested that perhaps the south coast of Antarctica specifically was the so-called 'safe haven.' We don't really have enough data or refined time constraints to assess that specific idea yet, but it is certainly possible.
Part of the recent excavations in Antarctica led by my postdoc advisor, Chris Sidor, recovered a bunch of new temnospondyl fossils from the Early Triassic exposures in the lower Fremouw Formation. Some of this is a sizeable block, seen below (scale bar = 5 cm), that preserves four individuals of Micropholis stowi, the first record of this species from Antarctica / outside of South Africa. Interestingly, some of the South African material is preserved similarly, with multiple individuals of semi-articulated nature found next to each other.
Micropholis is super distinct, especially in the Early Triassic because no other amphibamiforms are around. One of the main features that sets it apart is its ornamentation - rather than pits and grooves that you usually see in temnospondyls (see an old blog post on that here), Micropholis instead has raised bumps (pustules). It's sort of like flipping the skull inside out to produce a negative relief of the typical pitting pattern. These pustules follow the same spatial distribution and pattern as the pits in other temnospondyls, and it's not clear whether there is some functional significance to having pustules rather than pits. Micropholis was more terrestrial than most other Triassic temnospondyls, but there are also plenty of other terrestrial taxa, including other amphibamiforms, that have pits, not pustules. HUGE props to the illustrator, Crystal Shin (Instagram portfolio) for her artwork (see part C below); all temnospondyl ornamentation is really labour-intensive and hard to draw, but pustules are especially hard, and this is frankly the best illustration of pustules that I have seen in the literature!
A world apart
On one hand, it might not be that surprising to find Micropholis in Antarctica; Africa and Antarctica were connected for much of their geologic history, including in the Triassic. But we actually don't see very many shared temnospondyls between any geographic regions in the Early Triassic. There are more than 30 different species of Early Triassic temnospondyls between South Africa, Australia, India, Madagascar, and Antarctica alone, but only two of those are found in more than one region (Lydekkerina huxleyi and Micropholis stowi); this holds true even if you bump it up to the genus level. In fact, even if you go up to the family level, there is still a lot of disparity between these southern Pangea hotspots. These patterns are suggestive of what we call 'endemism,' where species are locally restricted in their distribution, rather than 'cosmopolitanism,' where species are widely distributed. Some of this might be artificial due to limited sampling efforts, especially in Antarctica considering the logistical complexity with getting down there, but some of it may reflect legitimate variation in environmental conditions and subsequent physiological differences between clades. Rhinesuchids, for example, are known almost entirely from South Africa, where they are exclusively found after the mass extinction; they may have become particularly adapted for some local condition there (and perhaps in South America).
Comparison of temnospondyl assemblages of major fossil-bearing formations in southern Pangea. The Lystrosaurus declivis Assemblage Zone (LAZ) is in South Africa; the Sakamena Formation is in Madagascar; the Panchet Formation is in India; and the Arcadia Formation is in Australia (no relation to Arcadia, CA, near where I grew up). Black = present; grey = disputed; white = no record.
We hope that future work will help to fill in at least some of the possible gaps in Antarctica and therein better inform our understanding of the paleobiogeography of Early Triassic temnospondyls - stay tuned!
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A blog on all things temnospondyl written by someone who spends too much time thinking about them. Covers all aspects of temnospondyl paleobiology and ongoing research (not just mine).