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 DOI: 10.1017/jpa.2021.30 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 CoastIf 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). Drifting apart
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).
Chain reactionsScience 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. Refs
Nathan Parker
5/14/2021 09:05:46 am
This is very interesting! It's too bad there aren't more (any?) Late Triassic deposits in the mid-continent; it would be nice to know how these lowland critters got around the Appalachian cordillera to live on both sides of it.
Bryan Gee
5/17/2021 06:42:49 pm
Hi Nathan! Well it's nice to hear that someone thinks that this is getting simpler...we'll see if that holds up in the next few years. Yes, everything that was previously called Buettneria perfecta, Koskinonodon perfectus, or some permutation thereof should be Anaschisma browni! Comments are closed.
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