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!
New publication ( Description of the metoposaurid Anaschisma browni from the New Oxford Formation of Pennsylvania; Gee & Jasinski, 2021, Journal of Paleontology)
Title: Description of the metoposaurid Anaschisma browni from the New Oxford Formation of Pennsylvania
Authors: B.M. Gee, S.E. Jasinski
Journal: Journal of Paleontology
The amazing art above was done by Sergey Krasovskiy; you can find his DeviantArt profile here and his Twitter here! The number of metoposaurids is not random - we know that there were at least two individuals preserved at the site based on the number of duplicate elements from the site.
General summary: This study provides the first detailed description of metoposaurid temnospondyls from Late Triassic deposits in Pennsylvania. Metoposaurids are well-known for their widespread distribution in the Late Triassic, including in the southwestern U.S., but they have a much scarcer record west of Texas. The east coast in particular was geographically closer to other regions that preserve metoposaurids that have drifted apart over the millenia, like northern Africa and western Europe. The east coast is also the only region in North America where a large-bodied temnospondyl that isn't a metoposaurid is known from (this pattern of metoposaurid exclusivity is not observed anywhere else with decent sampling). While previous people have mentioned the Pennsylvania material and ID'd it, they never provided good figures or explicit justification for their ID, so essentially any reader was left to take them at their word. We formally describe the material, identifying it to Anaschisma browni (best known from Arizona, New Mexico, Texas, and Wyoming), which is not the same species that a metoposaurid from Nova Scotia belongs to, and discuss the importance of providing proper documentation and justification for taxonomic identifications (which are fluid hypotheses, not static facts). While the east coast is much more fossil-poor (this is definitely the best metoposaurid material from anywhere in North America west of Texas), it may also be the only region that preserves the transition from a diverse large-bodied temnospondyl assemblage to one that is entirely dominated by metoposaurids.
West Coast, Best Coast
If you follow me on any platform, you are well-acquainted with all of the metoposaurid content that I put out (and also that I'm from the west coast). The fact that I mostly share material from Arizona, New Mexico, and Texas isn't evidence of my west coast lean but rather the fact that the vast majority of metoposaurid specimens from N. America are from the western half of the continent. The quality is very different too - on the west side, we get entire bonebeds with dozens of complete skulls, like Lamy, NM (pic from the Museum of Comparative Zoology, below on left). On the east side...well we get things like the holotype of "Dictyocephalus elegans," a species so poorly known that it's not even considered valid anymore (pic from Colbert & Imbrie, 1956, below on right). Most of the metoposaurids on display in east coast museums are actually specimens from the west coast!
Now of course that doesn't mean that metoposaurids weren't abundant in what is now eastern North America - it just means they weren't preserved as frequently or that we haven't found their remains. The east coast Triassic deposits are very different from the west coast and much less common, and most of the metoposaurid specimens from the east coast were found during construction of tunnels and other infrastructure more than 150 years ago. But the fact remains that the record is really poor.
The "phytosaur hole"
Over a century ago, scientists collected a variety of mostly fragmentary vertebrate remains from southern Pennsylvania along the bank of the Little Conewago Creek near Zions View. The site isn't that far from a few other historical localities that have mostly produced isolated and very fragmentary tetrapod remains, but it initially yielded no amphibians. In the 70s, the curator of what was then the William Penn Memorial Museum (now the State Museum of Pennsylvania), Donald Hoff, resumed excavations at Zions View and recovered substantial metoposaurid material. Hoff alluded to this as the "phytosaur hole" locality in a 1971 article (the best material is that of phytosaurs, published by Doyle & Sues, 1995, who also mentioned the metoposaurids), and Donald Baird, one of the most prominent workers of the east coast's records of Paleozoic and early Mesozoic tetrapods, again mentioned this material in a 1986 article. Hoff's specimens are the focus of this study, including the very nice skull that can be seen below from our paper (scale bar = 5 cm). Our study confirms that these specimens belong to Anaschisma browni, the best known North American species that is also known from Wyoming, Arizona, Utah, Colorado, New Mexico, and Texas.
Can I see some ID, sir?
We aren't the first people to "describe" these specimens, and we don't claim to be; they were first noted in the mid 1980s by Don Baird and then subsequently figured by Lucas & Sullivan (1996), which you can see below on the left. The problem is that no previous publication figures them in much detail, certainly not enough to actually be sure of the anatomy, and therein, of the taxonomy. The material clearly belongs to a metoposaurid (the front-shifted eyes are the big giveaway), but that doesn't help to narrow down which species or genus it is since all metoposaurids look nearly identical. A slightly better photograph was published by Schoch & Milner (2000), below on the right, but it is still insufficient to assess the taxonomy, not to mention that it's in a very expensive book that only a temnospondyl geek would own (and that's pretty hard to come by in libraries as well).
Okay, so what is the problem here? After all, geography is informative - if you found a bear skeleton in Arizona today, you probably aren't going to consider the possibility that it's a panda or a polar bear. The problem is that the continents were in a completely different arrangement back then, and we have extremely poor understandings of biogeographic ranges of extinct tetrapods because we don't always have good data on the paleoenvironment that might allow us to say "it was too hot here, no X could live here" (like with polar bears in Arizona).
In the Triassic, pretty much all of the continents were smashed together, with North America glommed on to present-day South America, Africa, and a bit of Europe. This shrinks the distance between what are now widely separated geographic locations that preserve metoposaurid fossils, especially across the Atlantic Ocean. As it happens, the east coast of the U.S. was actually closer to Morocco and Spain than it was to other areas in the U.S. that also preserve metoposaurid fossils, like New Mexico and Texas (see the distribution of metoposaurids plotted on a reconstructed paleomap of the Late Triassic from Brusatte et al., 2015 below).
Science is built on propagating data and concepts - we don't try to keep reinventing the wheel, so to speak (although testing old ideas is an important part of reproducibility). The key is to be judicious in what is propagated - not everything proposed 200 years ago is correct, and some of it is very incorrect. For example, Branson & Mehl (1929) simultaneously proposed that metoposaurids were filter feeders based on their high tooth count (like baleen whales...) and were limbless because no limbs were known in North American species at the time. So just because something was published in a journal doesn't negate people's responsibility to critically think and fact check the work and obfuscates the fact that academic publishing has changed really dramatically overtime. And the way in which we've done science has changed a lot too!
This lengthy chain of thought is how we end up back on this study. The original photos that other people published aren't up to modern standards, and they aren't good enough to discern most of the sutures that we need to identify the skull to a particular species. Neither do previous authors clearly and explicitly justify why they believe it to be a particular species (what was called Buettneria perfecta at the time). Looking at it now, I'm not actually sure that Lucas & Sullivan (1996) was even peer-reviewed - there is at least no indication in the article (in Pennsylvania Geology, a magazine of the Pennsylvania Geological Society). Schoch & Milner (2000) is a book and also lacks explicit mention of formal review (less common in books because of their length). So basically any modern-day worker is left to take those people at their word and largely guess at what features might hint at a particular species. This is obviously not ideal - we would like, and should demand, concrete evidence and explicit rationale!
A case in point (or two)
Taxonomists are pretty familiar with constantly changing taxonomy and identification of specimens, but this isn't always apparent to non-scientists or non-taxonomists. In fact, of the 10 presently valid metoposaurid species, only two still have their original name (Apachesaurus gregorii and Metoposaurus algarvensis; this second one is also only six years old); all of the other ones have been renamed, synonymized, invalidated, etc. This is widely true across much of the tree of life and underscores the point that taxonomic identifications are hypotheses, even if we don't explicitly say "I hypothesize that specimen X belongs to species Y," and like all hypotheses, they are not set in stone and can be repeatedly tested (and eventually disproven) as new information comes to light. Below are two relevant "case studies" of sort that indicate why this study and others like it are important for validating unsubstantiated hypotheses of previous workers.
Above is the holotype of Calamops paludosus, a temnospondyl described from this partial lower jaw by Sinclair in 1917. Sinclair's description, like most of the time is very short - there are less than two full pages of text - and it doesn't really establish why he was so sure that it was a new genus and species (Sinclair was unsure what group of temnospondyls it might belong to). The only real hint is his comment that this specimen would have belonged to a much larger individual than the metoposaurids known at the time (and known now) - the holotype is 44.6 cm and would have been around twice as long when hypothetically complete.
For several decades, essentially no one discussed Calamops - there is not much to discuss after all! Then in 1956, Colbert & Imbrie's review of metoposaurids claimed that it was an indeterminate metoposaurid but probably synonymous with what they called "Eupelor durus." This was a putative species of east coast metoposaurid that is based on wholly undiagnostic material and that no one considers valid. Colbert & Imbrie didn't do a very good job of explaining why they even thought Calamops was a metoposaurid though - they didn't figure it at all, copied and pasted parts of Sinclair's description as a "diagnosis," and said it was too indeterminate to be a valid species. In other words, they never identified any metoposaurid features. But subsequent workers took them at their word essentially (e.g., Chowdhury, 1965). In the 80s, some additional preparation was done on the specimen that better exposed some anatomical features, and someone suggested that it was a capitosaur. However, that person's (R. William Selden) work was never actually published, and instead the capitosaur identification was propagated based on what we call "pers. comm." citations, which amounts to "I talked to this person on the side, and they said X." This appears first in Olsen (1980) and obviously lacks any evidence since presumably Olsen thought that Selden would publish his findings. This was then propagated forward for several decades, including in Schoch & Milner's (2000) book review of Stereospondyli, even though some metoposaurid workers (e.g., Hunt, 1993) considered it to only be an indeterminate temnospondyl. Either way, no one ever published new figures, descriptions, or explicit justification.
Okay, now let's take a look at a more recent paper. Barrett et al. (2020) report a new phytosaur specimen from the Late Triassic of Zimbabwe, which is a historically undersampled like most of Africa. This is the first sub-Saharan record of a phytosaur, which is pretty neat! In the abstract, the authors state that "the phytosaurs are associated with lungfish and metoposaurid amphibians, forming part of a terrestrial-aquatic dominated biota, a previously undocumented biome from the Late Triassic of southern Africa." So this is interesting because there are not many "amphibians" in the Late Triassic. The authors state that this is specifically a metoposaurid amphibian, which is very interesting because this is also outside of the previous range of metoposaurids. But there are no photos of the metoposaurid in the paper!
Okay, well maybe this record of metoposaurids isn't very important for this paper (hence why it's in the supplemental)? That seems not to be true - the authors state "the co-occurrence of fossil forests with phytosaurs, lungfish and metoposaurs thus provides the first Gondwanan analogue for various North American Late Triassic biomes, notably that recovered from various members within the Chinle Formation of Arizona and adjacent areas..." So the purported metoposaurid record is actually really important because it's 33% of the data being used to argue for biome similarity between very geographically disparate areas, but the taxonomic ID is totally without justification. I even went to the trouble of asking Paul Barrett what features led them to call it a metoposaurid, and he kindly replied to let me know that it was basically just what people in South Africa call most southern African Triassic material like this in the field (this is kind of odd because the only southern African metoposaurid records are in Madagascar, which isn't being worked by South Africans as far as I know).
Now if I had to guess, people are making the assumption that because a big temnospondyl is found with a phytosaur, that it has to be a metoposaurid. Phytosaurs and metoposaurids have an interesting very similar geographic congruence (i.e. where there are phytosaurs, they are usually metoposaurids). This is not a 1-to-1, but it is very tight, more than other tetrapod pairs, and is probably because both of these were aquatic tetrapods. Below is a comparison of maps showing the distribution of both groups (helpful that both were made by teams led by Steve Brusatte). Phytosaurs on top, metoposaurids on bottom.
But there are a bunch of other red flags that suggest that the Zimbabwe material might not be a metoposaurid. For one, there are a number of places where phytosaurs occur in the absence of metoposaurids, and these include the higher latitudes like Brazil, Greenland, and Latvia. Zimbabwe would have been high latitude back then as well. Phytosaurs can also occur with other large temnospondyls, like with plagiosaurids in Thailand. There is also the time; radioistopic dating of the Zimbabwe locality suggests an age around 209 mya (+/- 4.5). This is pretty close to the end of the Late Triassic, by which point metoposaurids are unknown from anywhere else in the world except in North America in which essentially all of the specimens are of very small animals (Apachesaurus gregorii). Elsewhere, metoposaurids have been replaced by other large temnospondyls, like by capitosaurs in Poland and by chigutisaurids in India. All of the occurrences of metoposaurids from the southern hemisphere are considered to be Carnian in age (a particular stage of the Late Triassic), which crucially ends at 227 mya, i.e. at least 13.5 million years before the Zimbabwe record. Now on one hand, you could argue that this is actually evidence for the hypothesis that Zimbabwe had a similar biome to N. America (two isolated similar habitats)...or it might just argue more strongly that whatever temnospondyl is down there is not a metoposaurid.
A modified time-calibrated phylogeny of metoposaurids from Buffa et al. (2019). Black bars are the definitive range of the taxon, and dashed lines are more ambiguous. I added the red line at 209 mya to show where the Zimbabwe locality may fall by relation. The Carnian (when most metoposaurids occur) is indicated by the first large gray rectangle and the abbreviation (Carn.)
Just to be clear, I obviously love metoposaurids, so I am happy if there are new records from anywhere. Range extensions are fun! But there is no evidence suggesting that this is more likely to be a metoposaurid over any other large temnospondyl clade known from the Late Triassic. There may not be many species, but there are definitely other clades, including chigutisaurids, which we already know occur in the Late Triassic and into the Jurassic in South Africa! There is also a concern because two other studies have already propagated this purported record as if it is well-supported...but to me it just isn't. Like we note in the paper, I am not saying that the Zimbabwe material is definitely not a metoposaurid, but it can't be said that it is even more likely than not to be a metoposaurid. At least collect it so someone else can study it... The bottomline is that I would just urge people to be more careful and critical in propagating what could be considered "outlier" or "fringe" occurrence points or those only based on circumstantial evidence (or without any justification). This is a good reminder of the import of good anatomical and taxonomic work!
Comparisons of large temnospondyl distribution over the Late Triassic in major temnospondyl-producing regions. Colour key: Orange = Capitosauria; yellow = Brachyopoidea (inclusive of plagiosaurids); green = Metoposauridae; Blue = non-metoposaurid Trematosauria. Grey indicates a possible occurrence that is not certain due to a lack of resolution of the age of the fossil-bearing unit (e.g., the Economy Member of the Wolfville Formation in the eastern U.S.). Key takeaways are that metoposaurids are not found above latitudes at about 30 degrees (the Zimbabwe locality was around 40 degrees), and most metoposaurids are restricted to the Carnian. North America is the only region where metoposaurids occur beyond the early Norian (the Zimbabwe locality is no older than late Norian), and there are no other large temnospondyls in this region whereas capitosaurs and brachyopoids persist elsewhere.
New publication ( New information on the dissorophid Conjunctio (Temnospondyli) based on a specimen from the Cutler Formation of Colorado, U.S.A.; Gee et al., 2021, JVP)
Title: New information on the dissorophid Conjunctio (Temnospondyli) based on a specimen from the Cutler Formation of Colorado, U.S.A.
Authors: B.M. Gee, D. S Berman, A. C. Henrici, J.D. Pardo, A. K. Huttenlocker
Journal: Journal of Vertebrate Paleontology
The Red River
The Red River is a mostly east-west oriented, eastward-flowing indirect tributary of the Mississippi River, stretching from the Texas Panhandle all the way to Louisiana (over 1,350 miles). It forms the border between Texas and Oklahoma (hence where the "Red River Rivalry" in collegiate football between University of Texas and the University of Oklahoma derives from), two of the most historically productive states for early Permian tetrapod fossils (especially the famed Texas redbeds). Much of our record of North American temnospondyls from this time comes from these two states, with more minor contributions from New Mexico, Utah, and Ohio. Dissorophids, the armoured dissorophoids, are among the best known taxa from these redbed deposits, and in many instances, the entire record of a given species is restricted to one or two states. Whether this is an artifact of the fossil record or an accurate representation of a truly restricted geographic range is often unclear; many taxa are in fact known from only one specimen or one site (the two species of Cacops below are examples; figures from Gee & Reisz, 2018 and Anderson et al., 2020).
Land of Enchantment
Conjunctio multidens (reconstruction from Schoch & Sues, 2013) is one such dissorophid, known only from two sites (but the same county) in New Mexico. Although New Mexico and Texas are adjacent states, their faunal assemblages were somewhat different, a result of regional geography no longer present that would have created some degree of spatial segregation. Other examples can be found in the trematopids Anconastes and Ecolsonia, only found in New Mexico, or the dissorophid Broiliellus reiszi, which differs starkly from other members of the genus that are known from Texas. Because the Texas redbeds have been so extensively sampled, with at least a dozen other dissorophids known, it can be reasoned that indeed Conjunctio also did not occur in Texas. But does that mean that it was only found in New Mexico? Not necessarily. The rest of the Four Corners region and the midcontinent is less well-explored, probably because the early Permian deposits that yield tetrapod fossils are less extensive. One of the well-known productive localities in the Four Corners region is in the Placerville area in Colorado, where the Cutler Formation (also found in New Mexico) is several hundred meters thick. First reported by Lewis & Vaughn (1965), the assemblage preserves one of the few sails of the dissorophid Platyhystrix (below on right), the holotype of Diadectes sanmiguelensis, a diadectomorph stem amniote (below on left), and a number of other tetrapods also known from other redbeds deposits. Synapsid fans may also know this as the type locality for the sphenacodontid Cutleria.
Terminology: While 'transitional form' or 'transitional fossil' is a commonly used term that even the general public knows (e.g., Archaeopteryx), scientists are moving away from it because it's misleading, suggesting that one animal directly turns into another (one of the most common misconceptions about evolution). In fact, this very rarely occurs (a process called anagenesis), and it does not capture major transitions like the theropod-bird transition or the fish-tetrapod transition. Instead, continual splitting of new species that go extinct is what gradually leads to a shift in a group's anatomy. A given species' suite of features may be transitional insofar as it captures part of this shift, but the species or individual itself is not.
Agree to disagree
As one might expect from a taxon with a weird mixture of features, there is no agreement on the position of Conjunctio in phylogenetic analyses of dissorophids, as summarized on the right. Historically, the phylogeny did not support that they were conspecific (expected recovery as exclusive sister taxa), as they were frequently scored separately due to uncertainty over Carroll's referral (the referred specimen was called the "Rio Arriba Taxon"). It doesn't really help that the holotype (C-D) and the much smaller referred specimen (A-B) differ from each other in cranial proportions, somewhat questioning whether they are actually the same taxon (since Bob Carroll created Conjunctio in 1964). You don't have to be a paleontologist to look at the two specimens above and kinda wonder whether they are actually the same (or whether the bottom one can be said to be much of anything really). Sometimes both specimens were recovered as eucacopines (Holmes et al., 2013), sometimes only one was recovered as a eucacopine (Schoch, 2012), sometimes Eucacopinae didn't exist (Maddin et al., 2013), sometimes the composite was recovered as a eucacopine (Schoch & Sues, 2013), and sometimes the composite was recovered as a dissorophine (Liu, 2018). So basically every possible result short of it being recovered as a non-dissorophid!
The Centennial State
The new specimen that we report here was collected from what's known as the Placerville locality by my coauthors, Adam Huttenlocker and Jason Pardo, a few years back. It is not nearly as complete as the other two specimens, but it is well-preserved, enough to show some distinctive features not found in most or all of the other dissorophids. For example, the two bones called the postorbital (right behind the eye) and the supratemporal (a little farther back) usually touch, but here they do not (this is found in two of the species of Cacops as well). The jaw articulation (marked by the quadrate) also sits in front of the level of the back of the skull (marked by the postparietal); this is something found only in Dissorophus and Scapanops. So there is actually quite a lot of information in this little skull despite it being at best 25% complete!
Our phylogenetic analysis, which combines two previous matrices (it was originally intended for a larger sample), finally recovers not just the original two specimens, but also this third one as a proper clade (i.e. what you expect if they really do all belong to the same species)! The statistical support is not very good though, and it's quite possible that a future study could find a different result. Dissorophid phylogenetics remains very much in flux outside of just Conjunctio (something that I'm working on right now).
COVID addendum: Lest someone think that all of us are superhumans who pumped out this paper in the pandemic, it was mostly completed before the pandemic really set in (we submitted the first version in May 2020). Jason and I in particular as current and recently graduated PhD students have really slowed down in putting new work into the pipeline during the pandemic, which I think is important to note since a lot of people are really struggling and should not be concerned with their perceived relative productivity!
Temnospondyls are a weird bunch of animals, which naturally means that the people who work on them are also a weird bunch. I've been kicking around some ideas for a blog post looking at the demographics of temnospondyl research for a while, both out of a general interest and with an eye towards diversity (or the lack thereof) in early tetrapod research, which is particularly niche among vertebrate paleontology. Meant to do it in 2020, but a lot of things obviously happened that delayed it to now...
Where to find the temnos (Part 1)
Where do temnospondyls "live" today? It's probably not that surprising that my semi-arbitrary list of the institutions with the most temnospondyl material are in predominantly western, northern hemisphere countries.
This is obviously not a full list; there are many other institutions in the U.S. with decent temnospondyl holdings (KUVP - Lawrence, KS; OMNH - Norman, OK), especially ones with large quantities of material from single localities (TTU - Lubbock, TX; PPHM, Canyon, TX), as well as a few international museums like BPI (Johannesburg), MGUH (Copenhagen), and GSI (Kolkata), but the ones mapped above hold a lot of the world's temnospondyl holotypes and best specimens.
Where to find the temnos (Part 2)
Now how does this stack up to where temnospondyls are actually found (i.e. where they actually lived)?
The most obvious takeaway is that temnospondyls are all over the world (they're also known from Antarctica, but that's not mapped on here since it's not a sovereign country). If you sort of compare the two maps, there's pretty good alignment between the top temnospondyl-holding institutions and the countries with the most temnospondyl fossils. However, there is one small (geographically speaking) outlier: the U.K. This brings me to the next part of this pseudo-study - are temnospondyls frequently the subject of "helicopter research?"
The concept of "helicopter science," where researchers from predominantly western countries in N. America and western Europe figuratively (but sometimes literally) helicopter into an underdeveloped country without comparable research or academic infrastructure, collect fossils, and then publish them without local collaboration in the authorship team, is fairly recognizable today. This can realistically happen in almost any discipline, and there are plenty of good articles out there on the internet (see here, here, and here), but in paleontology, it's particularly common in Africa (excluding South Africa to some degree) and South America. Helicopter research is hardly ahistorical - it is a barely disguised arm of the long history of colonialism, rooted in the exploitation of human and natural resources of underdeveloped regions and populaces, and it remains a huge problem in paleontology because many historical collections were made by European countries in places like Africa and now continue to benefit from the research and public interest valuations of those fossils. Repatriation and other means of reconciliation remain sticky subjects (e.g., Germany's holding of the sauropod Brachiosaurus from Tanzania; source and source). Note that this is different from repatriation of explicitly illegally held fossils, like those smuggled out of Brazil (source) or Mongolia (source); many of the historical collections were made through legal, museum-sponsored expeditions (admittedly at a time when fossil regulations were near non-existent in most countries, perhaps a convenient thing).
If I put the previous map of major temnospondyl hotspots side-by-side with a tiered map showing major repositories of temnospondyl fossils (in the blues; white only indicates very few to no fossils, not absolutely nothing), the U.K. should pop out as best a small set of islands can. France punches a little above its weight, but nothing like the U.K. So let's talk about the U.K.
The sun never sets on the British empire
The U.K. has obviously punched way above its geographic weight in global politics for centuries, and vertebrate paleontology is no exception; other than maybe Germany, it produces more vert paleontology research output than any other European country. Now unsurprisingly, there are not very many fossils in general on the isles; islands are not usually major sources of fossils. But the U.K. holds a remarkable number of not just fossils, but holotypes, from other countries, including a number of ones with fairly well-developed paleontology infrastructure.
Basically all of these exports are housed at the Natural History Museum (NHMUK, formerly the British Museum of Natural History [BMNH]). The list of holotypes housed there is fairly impressive (left) compared to the domestic holdings (right):
Anyone with a fuzzy sense of world history will probably notice that most of these exported holotypes come from countries that were major colonies of the British empire (evidently American independence warded off any prospective future attempts to take most of our fossils). In fact, the only continent that the Brits don't have temnospondyl fossils from is Antarctica, and there are barely any from there to begin with. This changes the discussion a little bit though - the wide-ranging holdings of the U.K. is really just straight-up colonialism rather than helicopter research, which is a term mostly used with respect to contemporary research. Helicopters didn't even exist when most of this material was collected (mainly 18th and 19th centuries). As far as I know, no one in the U.K. is globetrotting to collect more temnospondyls (or has for several decades), although Andrew Milner made an entire career out of working on the extensive holdings of non-domestic, European temnospondyls (especially from the Czech Republic) at the NHMUK. I won't even pretend to know anything about the history of Czech paleontology, but there is a massive amount in the Narodní Muzuem in Prague, so it's not like the country was exclusively pillaged by other countries.
What the heck, let's do France too:
France of course had a much smaller colonial reach by the time that paleontology had taken off as a field. However, they cornered the market on northern Africa (politically and paleontologically), and the MNHN has extensive material from Morocco in particular (four holotypes). The French also outperformed the Brits in Madagascar, holding a number of trematosaur holotypes.
I'll map the U.S. as well, but I'm not even going to begin listing holotypes; there are more than a dozen dissorophid holotypes alone, both from and housed in the U.S. If I had to make a crude estimate, there's probably close to 100 holotypes from the U.S. The main American institutions with foreign holdings are the AMNH and the UCMP. The U.S. probably never developed major forays into other countries with the intent of bringing back fossils since there are already so many in the U.S. I'd be that there are some loan swaps with other countries that aren't described in the literature, but no need to get greedy. South Africa clearly got farmed by literally everyone though.
Now in all of this, I want to point out that non-holotype holdings from another country could be swaps - you give me something I don't have, and I'll give you something that you don't have. This happens a lot between museums and is a great way to diversify their holdings and exhibits. For example, there's one Lydekkerina specimen at the Royal Ontario Museum in Toronto, but I'm pretty sure nobody from that museum went down to South Africa just to collect one Lydekkerina... The Natural History Museum in London notes on their website that a bunch of their German material was donated by Herman Credner.
Let's come back to the original question of helicopter research. Most of the historical work is direct colonialism, but what about nowadays? To be frank, there isn't that much opportunity simply because nobody's getting a million-dollar grant just to go into a relatively unexplored (paleontologically speaking) region and dig up specifically temnospondyls. Brazil remains largely worked by Brazilians, or at least other Latin Americans (e.g., Eltink et al., 2016; Pacheco et al., 2017; Azevedo et al., 2019; Dias et al., 2020), with minimal involvement from North America or Europe. Asia outside of India has never produced many temnospondyl fossils, most of which are published by local researchers (e.g., Liu, 2016, 2018; Chakravorti & Sengupta, 2018; Mukherjee et al., 2019; Rakshit & Ray, 2020). So Africa is the only other region with the potential for helicopter research, and even that hasn't produced a lot of new material.
The track record there is pretty good for local collaboration. There is a single Carboniferous temnospondyl occurrence on the entire continent - the micromelerpetid Branchierpeton saberi from Morocco (Werneburg et al., 2019); the last author on that paper is affiliated with Chouaib Doukkali University (Morocco). Two very fragmentary bones were also reported from the mid-late Permian of Morocco by Steyer & Jalil (2009); Jalil is at the Muséum national d'Histoire naturelle in Paris, one of the major repositories of material from Morocco, but he remains affiliated with Université Cadi Ayyad. South Africa was mostly worked by South Africans historically, and they haven't excavated substantial new material in a while. The only other substantial contemporary study of African temnospondyls is the set of papers reported Nigerpeton and Saharastega from Niger; three of the four papers include local collaborators from Niger (Sidor et al., 2005; Damiani et al., 2006; Steyer et al., 2006), and the fourth is a single-authored festschrift contribution (Sidor, 2013). Perhaps most crucially, the material is reposited at local institutions.
A secret life abroad
What about trafficking of temnospondyls? Laws regarding fossil ownership are famously variable between countries and usually lacking in those without established infrastructure for collections and research. Brazil is the most recent country re-emerge in the news over the dinosaur Ubirajara, a fossil exported to Germany under rather dubious circumstances that suggest a fair bit of impropriety at the bare minimum (Science, NatGeo). Eagle-eyed readers might have noticed that Brazil is one of the countries with specimens housed in the Natural History Museum in London - this material is of Prionosuchus plummeri, the largest known temnospondyl and in fact includes the specific material used to get the approximately 9 m length estimate. The paper describing it, Cox & Hutchinson (1991), indicates the material was collected in collaboration with Llewellyn Price, one of the most famous Brazilian paleontologists, in 1970 and 1972. This falls within the gray area between 1942, when Brazil requires governmental approval for export, and 1990, when the government bans permanent export; obviously at the time, this was not considered an ethical issue, and the paper makes no mention of the export process.
Behind the scenes
Now that we've looked at things from a fairly coarse scale, what about the individuals who actually do the research? To get an idea of this, I built out a dataset of publications in two time intervals, 2000-2009 and 2010-2020 (both inclusive). This is mostly grabbed off of Google Scholar, although I got ones that aren't indexed there if I knew about them.
Some general guidelines I was using:
Let's start by looking at the most productive temnospondyl workers. Note that to keep these graphs somewhat concise (there's several dozen unique first authors from 1999 to 2009 and almost 100 from 2010 to 2020), I'm using an arbitrary minimum cutoff of 6 publications (authorship position irrelevant). I'll mostly show graphics related to this particular subset, but it isn't a referendum on what I think about either publications or authors that aren't captured here because of the cutoff, and it does not mean that everyone who is listed here is a temnospondyl "expert," either as self-defined or as perceived by others. That being said, many of the names on here are the first people that the average paleontologist would think of for temno-related matters. The point of these graphs is just to show numerical production. Citation count or other measures of impact take longer to data-mine and get complicated when you can't use a single search engine to get measures for every study, not to mention that 11-year sampling bins are not as good.
I think the plots are pretty self-explanatory, and they show some interesting patterns. Firstly, two German dudes, Rainer Schoch and Florian Witzmann, have dominated the temnospondyl literature (note that their true net contribution is overestimated by these plots because they frequently co-author papers, at least 10 on a quick count). Country trends will be discussed next, so I won't get into them too much here, but there's a few other areas to comment on that aren't clear from the plots themselves.
As an aside, it might look weird that the 1999-2009 interval has more publications, but what this really reflects is a declining "market share" of the entire set of publications by this arbitrarily defined subset that I'm using here. Both of these subsets represent a majority of the total output of each time interval, but it's much more substantial for the 1999-2009 interval (63.5% for 1999-2009; 52.1% for 2010-2020).Between 1999 and 2009, there were only 66 unique first authors; that number went up by essentially 50% to 97 unique first authors between 2010 and 2020, and as a result, a smaller proportion of the total papers were produced by the subset of the most productive workers. Whether this increase in distinct workers reflects increased interest or just increased access can't be addressed here.
Now let's back out and check the entire dataset - is it possible that women are overrepresented below the cutoff of >5 publications that I defined here (i.e. that women are either less likely to be first authors or that they produce fewer first-authored publications than men; not mutually exclusive)? Indeed, the proportion of women as first-authors is higher in the overall dataset for both time intervals (17.3% and 25.2%, respectively) than in the subset of the most productive workers (12.6% and 16.8%).
Indeed, between 1999 and 2009, there were 16 unique women who first-authored a paper, but only three of those published more than five papers in that time (women were 24.2% of all unique first-authors); between 2010 and 2020, there were 28 unique women first-authoring a paper, and also only three of those published more than five papers in that time (women were 28.8% of unique first-authors).
By country of affiliation of lead author
Now let's back out and take a look at how things break down by where the primary listed affiliation of the first author is physically based for all of these studies.
'The single genius'
Science is often fixated on this concept of single geniuses (coincidentally mostly men) where we elevate these really well-known figures like Einstein, Hawking, Darwin, Newton, etc. as incredible geniuses who essentially accomplished their major findings on their own. This has drawn a lot of criticism in recent years because it often totally obfuscates the actual history of discovery, whether in Crick & Watson's overshadowing of Rosalind Franklin, or in the more everyday aspects of scientific research where PIs claim credit for student work. Some reads here (BBC), here (New Republic), and here (Vox). People don't have to be scientists or Nobel laureates to have this title figuratively bestowed upon them (or to place it upon themselves); we are all cultured to think of the people at the top as the 'masterminds' or the 'big brains' behind a complex machine, whether in film (directors, producers) or in science (PIs).
The TLDR is that there is a definite trend toward low authorship teams, though I obviously don't have comparative data to assess how that stacks up against other groups of tetrapods. Now there's plenty of ways to speculate on why temnospondyl research is skewed towards small research teams. One, of course, is that certain workers do perceive themselves in this 'genius' vein and either feel that additional collaborators have nothing to contribute or will in fact slow them down. That's sort of pessimistic, but it's inherently true that there is only one limiting actor in a single-authored project, which could be good or bad depending on how you work and what you're working on. Re: the latter, there might be less of a perceived need for collaboration in descriptive papers, which remain the bulk of temnospondyl analyses - all you need is one well-traveled person who has seen a lot of the relevant material in theory (this is not a personal advocacy for avoiding collaboration in descriptive papers). A third factor is that some of the most productive workers are based at museums with good temnospondyl holdings (e.g., Fröbisch, Lucas, Schoch, Steyer, Witzmann) - that means that not only may they not have to go anywhere to look at specimens, there's also a smaller chance that authorship associated with another curator granting access to collections is involved.
One other possibility is trying to avoid COI (conflict of interest) for a certain period of time. I don't know how it works in Europe, but here in North America, frequent collaborators are usually banned from reviewing your grants ('frequent' for NSF is defined as any collaboration within the last year, graduate supervision, and a few other activities). The same thing was a problem for me when trying to find an external examiner for my PhD defence (most universities require that you have someone not affiliated with the university) - Toronto requires someone who has themselves graduated a PhD student and who has not collaborated with you or your advisor in the last 5 years! I can certainly appreciate that it is already hard enough getting temnospondyl reviewers for papers (which have far looser COI rules, which are really more like "suggestions"), and I think that this may also influence how people go about forming research teams.
If I had to guess though, the main reason why temnospondyl teams are small is because the temnospondyl community is also small. To pass muster, you gotta find at at least two and maybe three or four reviewers to green light the paper. That's a lot (I have problems getting more than two a lot of the time), so any potential collaborator is also a potential reviewer - put them on the paper and now you've knocked down your reviewer pool. I have definitely had non-temnospondyl workers review some of my papers, and I don't think that's they were the top choices either. Mathematically, needing at least two reviewers per paper means some people get asked to review a lot, and for any given reason, someone might not be able to do it. You might also just really like somebody as a reviewer - I have certain people who I find to be very good reviewers (=/= short review necessarily) - so regardless of whether that person is also a great scientist, you might leave them off the team unless you think that they are essential (hard to argue for essentiality in most descriptive work).
The average person is probably most accustomed to thinking of 'cusp' along the lines of "on the edge" or "on the verge." As in, "we're on the cusp of returning to a normal life with this vaccine." But 'cusp' actually has a different simple meaning, which is derived from the Latin word for 'point' or 'apex.' Architects use it to refer to a feature that became particularly popular in the Gothic period, a small projection between small arches. Mathematicians use it to refer to the point where a curve drastically changes directions. And biologists use it to refer to either a tissue fold that contributes to a valve to prevent blood backflow or to a pointed protrusion on the surface of the tooth. (There's also usage in astrology, but I'm not getting into that). I'm not an architect or a mathematician, nor do I work on blood vessels, so here we are talking about teeth!
A decidedly non-model system
In the old days, a lot of what we inferred about other "complex" animals was based on what we knew about humans, since we knew ourselves best (hypothetically), which is why a lot of anatomical features were first named based on humans. Of course, if you go to a nice nature area and look around, it's pretty easy to see that humans are not really a great "model" for other animals beyond primates. Ignoring things like complex societies and technology, we walk on two legs but can't fly, have reduced hair, and have opposable thumbs, features that are mostly shared (through common ancestry) only with other primates.
Mammals are generally bad models for other animals, which is why when scientists refer to "model organisms" that are used for their combination of adaptability to lab settings and applicability to a wide range of biological systems, we don't use many mammals. Anyone who's been through a high school or college biology class probably knows a little bit about the humble common fruit fly (Drosophila melanogaster), or perhaps the microscopic nematode Caenorhabditis elegans (usually just called 'C elegans' to dodge the wordy genus name), or maybe even the quietly invasive African clawed frog (usually called Xenopus by scientists and not to be confused with the African dwarf frog that Petsmart sells). These are considered model organisms for many reasons, none the least of which is that they're very hardy in lab settings, but also because their anatomy/physiology makes them reasonable proxies for something that's more complex and harder (or impossible) to work with in labs. At least some readers of the blog may be surprised to know that in the 50's, the frog I mentioned was used as a living pregnancy test for women with shocking accuracy (details in article; frogs not really harmed in process). I'm pretty sure it would be illegal to do the test in reverse to test whether a frog was pregnant by experimenting on a human. Which is one reason why Xenopus continues to be a model organism despite being discontinued as a pregnancy test.
Coincidentally, Xenopus is also a good model for the topic of today's blog post - the primitive tooth condition in tetrapods. Teeth come in a wide variety of shapes, none more evident than in our own mouth, in which the various tooth types are named as such because they look different. Most animals don't have this kind of differentiation though (called 'heterodonty'); we call animals without differentiated teeth 'homodont.' Look in the mouth of the fish on your cutting board, and you're likely to see a lot of teeth that all look the same. They probably look quite simple too - simple cones with a single point. We call this tooth 'monocuspid' because it has one cusp. Teeth with two cusps are 'bicuspid,' teeth with three cusps are 'tricuspid,' and so on (though usually 4+ is just called 'multicuspid'). Monocuspid teeth are found in Xenopus, as well as most early tetrapods, including most temnospondyls.
But Xenopus is an outlier among its froggy friends. Most frogs actually have bicuspid teeth (although toads, which are frogs, have no teeth), and a handful are reported to have tricuspid teeth (e.g., Greven & Ritz, 2009), but none have more than three. If you want an example of a tooth with more than three cusps, just look at your own molars - there are 4-5 in humans. Bicuspid teeth are typical for salamanders and caecilians, the other two groups of living amphibians, but they are not super common in animals with homodont dentition or are only found in part of the mouth (our premolars are bicuspid, which you maybe heard your dentist say when looking at your x-rays). Amphibians usually have homodont dentition (same size and shape throughout the mouth), so naturally, this has put an emphasis on searching for bicuspid teeth in the fossil record as one means of elucidating the origins of modern amphibians.
Tales of the teeth
Like I mentioned before, monocuspid teeth are the base model for tetrapods and temnospondyls. This isn't really surprising; adding points is complex and requires more nuanced spatial control over the tooth's development rather than just forming one point. There is only one example of a temnospondyl with tricuspid teeth, the branchiosaurid Tungussogyrinus from Siberia (Werneburg, 2009). The broad acquisition of more complex teeth, as well as heterodont dentition, tracks the expansion of tetrapods into different ecological roles, more specifically their dietary preferences. Scraping algae off of rocks, for example, is apparently a key driver of tricuspid teeth in marine iguanas (e.g., Melstrom, 2017); this idea has been extended down to some extinct tetrapods with tricuspid teeth as well (e.g., Mann & Maddin, 2019). Temnospondyls, like amphibians today, aren't picky, but they aren't generalists either. Amphibians can't really eat plant matter, for example, whereas the capacity for herbivory was an acquisition that facilitates the diversification of amniotes on land. But temnospondyls did eat other animals, and for that, monocuspid teeth work just fine, whether for grabbing, piercing, or stabbing.
There are several examples of temnospondyls with bicuspid teeth, or to be a little safer, several taxa in which bicuspid teeth are reported. Probably the most unequivocal of them is Doleserpeton annectens, which John Bolt keyed in on in the 1960s as a "proto-lissamphibian" based on his observation of this feature. With a lot of material and excellent preservation, subsequent research has confirmed the presence of bicuspid teeth in this taxon (e.g., Sigurdsen & Bolt, 2010).
With the recognition of this feature in one amphibamiform, workers began looking for more evidence of bicuspid teeth in what was then called 'amphibamids.' Amphibamus, a close relative of Doleserpeton, also has bicuspid teeth (Bolt, 1979). Platyrhinops, which is somewhat debated over whether it is the same as Amphibamus, also has bicuspid teeth (Clack & Milner, 2009). Most amphibamiforms do not have bicuspid teeth however, including Gerobatrachus, the 'frogamanader' ('batrachian' is the technical term for the grouping of frogs and salamanders; Anderson et al., 2008). There is also some controversy about whether any species of Tersomius has bicuspid teeth. Bolt (1977) indicated some specimens of Tersomius texensis (but not the holotype) have bicuspid teeth, but he only figured one of these bicuspid teeth (it was hard to get clear photographs or microphotographs in the 70's). The specimen that he did figure does appear to have two cusps, but they are not nearly as well-developed as in Doleserpeton or in most living amphibians. Probably important to bear in mind here is that we're talking about teeth that are less than one-tenth of a millimeter in width, so the precision in the plane of sectioning could be a factor. Notable too is that all of the Tersomius material with bicuspid teeth is from the South Grandfield site in Oklahoma; possibly, it is not actually T. texensis (or Tersomius at all). The original assignment was provisional (cf. designation by Daly, 1973) and made no comments on the teeth. That material has not been redescribed. This is further complicated by Anderson & Bolt's (2013) description of Tersomius dolesensis. Not only does it have the peculiar combination of monocuspid marginal teeth with at least one bicuspid palatal tooth, but there is disagreement about whether this is actually properly placed in Tersomius (their phylogenetic analysis excludes other species). In Maddin et al.'s (2013) analysis with the type and one good referred specimen of T. texensis, T. dolesensis does not group with what are now termed micropholids (Pasawioops + Micropholis + Tersomius). Suffice it to say that some amphibamiforms have bicuspid teeth and most do not.
Unsurprisingly, paleontologists have looked to other possible lissamphibian candidates, primarily 'microsaurs,' to see whether they too might have bicuspid teeth (diminishing the value of bicuspidity in amphibamiforms). And indeed, there are numerous reports of bicuspidity among 'microsaurs,' including Carrolla craddocki (Langston & Olson, 1986; Maddin et al., 2011), the now defunct 'Bolterpeton carrolli' (Anderson & Reisz, 2003; Bolterpeton is synonymous with the parareptile Delorhynchus), and Cardiocephalus peabodyi (Anderson & Reisz). The description of two cusps in C. craddocki still stands, whereas that of C. peabodyi seems to have been quietly abandoned. Haridy et al. (2017) suggest that in taxa with cutting edges on the teeth, like in many parareptiles, these edges may be misinterpreted as the ridges leaving to two cusps; they specifically suggest that what used to be 'B. carrolli' appeared to have two cusps only because of uneven tooth wear. It's also worth noting that the putative stem caecilian Chinlestegophis, a stereospondyl temnospondyl, also lacks bicuspid teeth (Pardo et al., 2017), as does the more highly nested amphibamiform Gerobatrachus (Anderson et al., 2008).
It's just a phase
One of the key caveats in bicuspidity in modern amphibians is that it is known to change in the development of the animal. Teeth are monocuspid early in development and only later do they become bicuspid. This is one of the reasons why John Bolt proposed that some amphibamiforms might in fact be juvenile dissorophids, and he proposed the inverse scenario in which the adult stage was a monocuspid tooth that transitioned from a bicuspid tooth.
Features that are transient throughout an animal's life become difficult to work with in the fossil record because it means that absence (of bicuspidity in this case) is not necessarily attributable to a legitimate absence of homologous features - instead, the material under study might be immature and not yet have achieved the derived condition that is being sought but would have at a more advanced stage of life. Bolt wasn't the first to suggest that some terrestrial amphibamiforms might actually be juveniles of larger animals (same problem with the mostly aquatic branchiosaurids), but the data collected since his work in the 70s and 80s has not supported this conjecture.
A higher purpose
About the blog
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).