A year to remember

2020 has certainly been some kind of year, which makes it easy to miss the latest scientific discoveries related to everyone's favourite (usually) four-fingered tetrapods. Last year I did a wrap on the entire decade, so there was a quite a lot, but in spite of the circumstances, 2020 has still produced some very exciting temnospondyl research! As with last year's wrap-up, this focuses on temnospondyl-centric research; there are obviously plenty of papers that make cursory mention of them or that might include a picture or two in an assemblage description, but those are not summarized here (I gotta be efficient with my time). As usual, links to everything are in the reference list as the end. Hopefully looking forward to getting back on a more regular track again in 2021 - stay tuned!

Leaving a mark

In lieu of body fossils, we often have evidence of temnospondyls in the form of trace fossils, which usually consist of a trackway. In contrast to amniotes, which basically all have pentadactyl (5-digit) hands, temnospondyl trackways have been considered to be historically easy to ID because of the tetradactyl (4-digit) hand that typifies temnospondyls and many modern amphibians (at least the ones that still have hands). Body and trace fossils rarely mix because the optimal conditions to preserve both the hard parts and the traces of activity will differ somewhat starkly, but this is good because it means where we might lack a body fossil record, perhaps we could supplement the total record with traces.

Bird et al. (J Geol Soc) reported a trackway from the early Carboniferous (Visean) of the U.K. (paper technically online in late December last year, but in print this year). This time period is a critical window of tetrapod evolution, when stem tetrapods really radiate; however, we still don't have most of the groups that directly lead to modern groups, and there is only one Visean temnospondyl, Balanerpeton woodi from Scotland (which is also debated about whether it might be a stem tetrapod and not a temnospondyl). The U.K. in general is pretty sparse on the Paleozoic tetrapods. As such, this is a good example where the trace fossil record pre-dates the main body fossil record and thus hints at a large gap in the body fossil record of the earliest stages of a group's evolution (temnospondyls in this case). The ichnotaxon (taxonomic framework for trace fossils) that they assigned the material to is Palaeosauropus, which is also known from the Blue Beach locality in Nova Scotia and Pennsylvania and which was previously interpreted to belong to an edopoid (the validity of this association is a little questionable). The oldest body fossil record of an edopoid is the Bashkirian, which begins around 323 million years ago, whereas the Visean ends at 330 million years, so at minimum, this possible edopoid record extends the group back by at least 7 million years, which is congruent with phylogenetic analyses that recover edopoids as a very early diverging group. 

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Herron et al. (Norwegian J Geol) reported slightly younger tracks from the Late Carboniferous (Moscovian-Kasimovian), when the temnospondyl body fossil record is much better established, from Svalbard. This archipelago is usually better known for its Triassic temnospondyl body fossil, which compares favourably with the closely situated Greenland assemblage, and the Scandinavian Arctic region has long been productive for some of the most important stem tetrapod fossils like Acanthostega. The new material that Herron et al. report is a massive block (almost 800 photos to produce a composite model) that captures a rare setting: the margin of a pond. As a result, the trackway captures the transition between the animal moving in the deeper part of the pond (with more scratch-like traces made through minimal contact with the pond bottom) and then moving out onto land (with more pronounced footprints). 

​The more distinct footprints compare well with one of the most common ichnotaxa, Limnopus, which is usually interpreted as belonging to eryopoids. Eryopoid body fossils don't appear until probably the latest Carboniferous (Ghezelian), so once again, there might be a slight backwards extension of the temporal range based on the trace fossils. More importantly, this is not only the oldest Limnopus fossil but also the highest latitude one; as it is today, Svalbard was much farther from the equator than places like the central U.S., where Limnopus was first described from. All previous Limnopus was confined to areas that were very close to the equator during the late Paleozoic, and we often assume that the more water-dependent temnospondyls would have had to stick to the more humid areas like today's near-equatorial tropical forests, but Svalbard was around 30°N at the time (and example of somewhere that's at 30°N in the modern day is around the U.S.-Mexico border), indicating that these amphibians might have extended much farther north than we thought (this can be compounded by decades of heavy sampling of areas around the paleoequator and light sampling of other areas).

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Cisneros et al. (Palaios) reported two types of trackways from the middle Permian Tapinocephalus Assemblage Zone of South Africa's famed Karoo Basin. This region has produced extensive body fossil remains, but not too many Permian-aged trackways. The first (seen on the left) is referred to a known ichnotaxon, Batrachichnus salamandroides, which as the name's etymology implies, is interpreted to be an "amphibian" (in a broad sense). The trackmaker would have been quite small, about 20 cm in their estimate, so the authors didn't exclude 'microsaurs' (sometimes considered "amphibians" but also recently suggested to be reptiles) from consideration; the 'microsaurs' with known hands are also four-fingered, like the trackway and most temnospondyls. The Tapinocephalus AZ only preserves body fossils of two large-bodied temnospondyls (and no 'microsaurs,' whose fossil record essentially evaporates after the early Permian), so the authors proposed that the trackmaker is not a juvenile of the known temnospondyls but rather is some presently unknown "amphibian" (in a broad sense). It seems that the trackmaker was walking on a wet surface exposed to the air based on the tail trace ('tt'), which is nearly straight (we would expect undulating motion if it was swimming). A second trackway lacks distinct digits and a tail trace, which complicates things a bit (e.g., was the trackmaker a larval animal without fully formed digits or did the sediment just not capture the digits fully). The spacing of the prints suggests an animal with a different body type than trackmaker #1, so the authors suggest that this represents a second unknown "amphibian" present during this time in S. Africa.

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Slightly younger (okay, substantially younger) trackways were reported from the Middle Triassic of Germany by Mujal & Schoch (Palaeogeogr, Palaeoclimatol, Palaeoecol). Temnospondyl trackways become increasingly rare into the Mesozoic, which might be because a lot of the taxa are fully aquatic and thus swimming most of the time, rather than making distinctive footprints in the mud. Indeed, the shallow nature of the tracks and their spacing led the authors to propose the locomotory mode to the right, where the animal's rear end was suspended, with the tail playing a role in propulsion and steering (no tail trace), and the front end being used to push off from the bottom periodically. Such a mode would again indicate why these traces are so rare in the Mesozoic since there is less interaction with the substrate than an animal meandering around on land. Apparently, the particular mode of sort of walking on your hands underwater is employed by crocodiles in coastal settings today, so this is the parallel that the authors employ here as well.

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Now a lot of this ichnology work is founded in assumptions about the distribution of digits in the hand and the foot of different groups. The "four-fingers-in-temnospondyls" paradigm is a very old one, and it's stood up pretty well. However, there have been a few reports of specimens with five fingers, or at least enough bones to indicate five fingers. Because these were often older studies, the figures weren't too hot, and people assumed that the previous scientists had just gotten it wrong or accidentally combined material from two individuals to get five fingers. However, Konietzko-Meier et al. (J Anat) described an articulated five-fingered hand from the Polish metoposaurid Metoposaurus krasiejowensis that provides pretty clear evidence for a divergence from the paradigm. Isn't it possible that one or two groups acquired five fingers, you ask? Certainly, but another metoposaurid, Dutuitosaurus ouazzoui, is known from fully articulated skeletons, and these only have four fingers. This is bad news for anybody looking for a clean pattern because variation at that level suggests that maybe every clade could have both four- and five-fingered constituents that we just haven't recognized (realistically there are not many complete hands or feet in the temnospondyl body fossil record). It also suggests that pendactyl traces cannot be excluded from consideration as belonging to a temnospondyl like we have historically (but the four fingers = temnospondyl paradigm is still generally holding up). However, every report of five-fingered temnospondyls is only in Mesozoic taxa, so there may still be some saving grace for Paleozoic ichnologists!

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Of course in addition to the hard tissue body fossils and the trace fossils is the soft tissue record, which is usually associated with a hard tissue body fossil. There are a lot of soft tissue specimens of temnospondyls, mostly body silhouettes that show features like the presence of a tail fluke or the body contours. This year, my buddy Arjan Mann, another newly minted Ph.D. who's remotely working as a postdoc at the MCZ, and I (J Vert Paleontol) described the oldest record of toepads in an amphibian and the first in a late Carboniferous temnospondyl from Illinois. The close aquatic relatives of this group (micromelerpetids) appear to have narrow tapering toes without a pad, which makes sense for aquatic animals which typically don't need toepads for traction on really any kind of surface, vertical or horizontal. 

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There were a few other papers that briefly show some temnospondyl traces or trackways that might be temnospondyls:

• Farman, R.M. and Bell, P.R., 2020. Australia's earliest tetrapod swimming traces from the Hawkesbury Sandstone (Middle Triassic) of the Sydney Basin. Journal of Paleontology, 94(5), pp.966-978. DOI: 10.1017/jpa.2020.22
• Marchetti, L., Ceoloni, P., Leonardi, G., Massari, F., Mietto, P., Sacchi, E. and Valentini, M., 2020. The Lopingian tetrapod ichnoassociation from Italy, a key for the understanding of low-latitude faunas before the end-Permian crisis. Tetrapod ichnology in Italy: the state of the art. Journal of Mediterranean Earth Sciences, 12, pp.61-81. [link]
• Marchetti, L., Muscio, G., Petti, F.M., Pillola, G.L. and Zoboli, D., 2020. The Carboniferous tetrapod ichnoassociation from Italy. Journal of Mediterranean Earth Sciences, 12: 29-37. [link]

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An abundance of skepticism

2020 has not been short on skepticism of all kinds, and the lissamphibian origins debate is no exception, trudging on. This year, we had two papers directly target Pardo et al.'s (2017) controversial hypothesis that batrachians (frogs + salamanders) come from one group of temnospondyls and that caecilians come from a totally different group of temnospondyls. Schoch et al. (PNAS) re-describe the enigmatic Triassurus sixtelae from Kyrgyzstan, a Middle-Late Triassic taxon that they consider to be a stem-salamander ("almost to modern salamander"). If you work on salamanders, there are some things in there for you, like the biogeography and dispersal of salamanders, possibly out of Eurasia, but the more hidden result is a return to the hypothesis of a single origin of Lissamphibia from within Amphibamiformes, the traditional version of the temnospondyl origins hypothesis. Note that while Chinlestegophis and Rileymillerus are still recovered as closely related to brachyopoids, as with Pardo et al., it would appear that just a handful of scoring changes (not based on new material, just on the new authors' opinion) led to the return to the traditional hypothesis (viz. a shift of caecilians), specifically related to whether these taxa have either a lacrimal or a lateral exposure of the palatine (LEP); the former is found in most temnospondyls, but the latter is quite rare. This underscores the fact that even when temnospondyl phylogenies are resolved, they usually lack good statistical support for most relationships, and relatively small changes to the matrix can produce some drastic shifts. 

The second study was a broader review of the fossil record of caecilians by Santos et al. (Biol J Linn Soc); the second author (Michel Laurin) is a longstanding proponent of a single origin of lissamphibians from lepospondyls. This paper repeatedly calls out various lines of argument used by Pardo et al. and even has a discussion section titled "Chinlestegophis: a true gymnophionomorphan?" In fact, they go so far as to say that every identified synapomorphy is questionable, and other shared features are plesiomorphies found in other temnospondyls. The issue of whether the element at the front of the eye is a lacrimal or a LEP is just one of those questioned features. No new phylogenetic analysis here, but a great summary of the available information on fossil caecilians (it is very poor) like the figure below on the left. In case you're wondering why the record is so bad, burrowing animals by virtue of where they live are not nearly as likely to end up near the types of environments that most frequently preserve in the fossil record, like ponds or floodplains. Their small size, like that of other lissamphibians, is another issue that is generally less favourable for preservation.

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The dissorophid dynasty

The temnospondyls long considered to be closely related to lissamphibians in one form or another, dissorophoids, continued to see major research interest, even outside of the context of lissamphibian origins. 2020 mostly brought re-descriptions, though we got a few other types of studies thrown in there as well. 

One important redescription that we got in 2020 was of the holotype of "Dissorophus" angusta, a species named by Bob Carroll in 1964. Basically everybody has recognized for decades that it is definitely not Dissorophus, lacking the large first osteoderm and having narrow osteoderms throughout the body, but it is only now, 56 years later, that its status is finally resolved as a new genus, Diploseira​, by Dilkes (J Vert Paleontol). The only known specimen is mostly the postcranial skeleton, which makes it hard to compare with many other dissorophids, but it definitely has distinctive features. David's usual impeccable attention to detail and illustrations really flush out every little nuance of the anatomy. 

This taxon is quite interesting in preserving a transitional series of osteoderms; there are two series at the front and only one at the back. As far as we know, this is the only dissorophid with this condition - other species either have a single continuous series or two continuous series (external and internal). Much of the osteoderm anatomy (though not the width) is shared with dissorophines (Broiliellus and Dissorophus), and Dilkes' phylogenetic analysis indeed recovered Diploseira within that group. This really mucks up how we use qualitative patterns of osteoderms to make taxonomic frameworks, continuing to highlight the issues with single-feature phenetic taxonomy (essentially arbitrary emphasis of certain observational data).

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Atkins et al. (J Vert Paleontol) re-described the only specimen of the amphibamiform Pasawioops mayi from outside of Oklahoma; this specimen was previously covered in a cursory fashion by Maddin et al. (2013), who removed it from Tersomius​, ​and is covered in greater detailed here. Using this specimen, which is much larger than the holotype, they discussed ontogenetic features at various taxonomic scales (within Pasawioops, within amphibamiforms, within dissorophoids, etc.). These data are usually rare for the terrestrial dissorophoids, which are usually lacking in large sample size, so additional data are always sorely needed.

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The biggest major revision has been a longgggg time coming - the revision of the dissorophid Cacops aspidephorus by Anderson et al. (J Vert Paleontol). Cacops aspidephorus was the first species of Cacops to be named 110 years ago, but the material wasn't the best, and the preparation methods were a little rough, so we actually didn't know anything about the sutures of this animal, which are essential for taxonomic purposes. To further complicate things, the species is only known from one site, aptly named the Cacops Bone Bed, which is now underwater in an artificially dammed lake. The project dates back over 15 years when Jason Anderson discovered a specimen that had escaped the historic prep methods of the 1900's and was able to resolve some of the sutures on the jaw (Anderson, 2005). This study finally reports on the cranial sutures for the first time, completing the holy trinity of Cacops (perhaps just in time for someone to demonstrate that one species is not Cacops). 

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On the aquatic side of things, Werneburg (Semana) described some rather large specimens of the branchiosaurids Apateon​ and Melanerpeton, flushing out additional details of branchiosaurid ontogeny and doing some revisions on the diagnoses. Some of these include a narrowing or complete closure of the gap between the maxilla and the palatal bones, therein approaching the condition seen in terrestrial amphibamiforms, and the marked widening of the palatine and the ectopterygoid (these bones are very slender in some amphibamiforms compared to other dissorophoids). Branchiosaurid ontogeny remains very complicated, which is what happens when you have small, neotenic animals that not everyone agrees are taxonomically valid or distinct (i.e. are some of them juveniles of larger, already known taxa), and even if they are valid, are all the valid ones known from true adults? There's a lot of ongoing research directions with respect to branchiosaurids. Werneburg also reported gut contents in one specimen (not the one shown here) that seem to be a smaller branchiosaurid, but they're pretty fragmentary and disarticulated. 

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Mixed in with all these redescriptions was the description of a new taxon by ​Schoch et al. (J Paleontol) - Palodromeus bairdi from one of the classic late Carboniferous North American sites - Five Points in Ohio. While Palodromeus is represented by a small specimen, like numerous other small specimens of amphibamiforms from Five Points and other sites, it is not an amphibamiform but instead an olsoniform, the group of dissorophids + trematopids (=big terrestrial dissorophoids). Olsoniforms have a famously bad fossil record in the Carboniferous, even though we know they had to be around. Some of this is probably because they didn't live in the swampy kind of environments that the Carboniferous is well known for, so they wouldn't be preserved there either. Palodromeus is a generic olsoniform - it isn't a trematopid or a dissorophid but a form that precedes their split. That does not mean that this is when those groups split; it actually was earlier based on the oldest trematopid. However, we would expect to find lots of generic olsoniforms in this time range, and here is the first definitive one! Whether others are out there and waiting to be discovered or are already known but not recognized for what they are isn't yet apparent. We would expect early olsoniforms to lack distinctive features of trematopids (like the long nostril) or dissorophids (like the armour), so they may be hard to identify in the fragmentary fossil record.

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I continued doing a bunch of different dissorophoid stuff, all of which is previously summarized in longer blog posts for the relevant publications (re-linked here). Gee & Reisz (Anat Rec) presents a re-description of the Carboniferous trematopid Actiobates peabodyi, a taxon very briefly described in 1973 that is a classic "roadkill specimen," with the entire skeleton compressed into about a 1 cm thick layer [blog]. This is one of the few temnospondyls known from Kansas, where it was evidently washed into a tidal mudflat from somewhat upstream. The holotype is one of the most complete trematopids, but the postcranial skeleton had never been described or figured before. 

Gee et al. (Ecol Evol) presented the largest histological sample for any single Paleozoic tetrapod - 60 limbs in total (it was a lot of limbs to cut) from Doleserpeton, the diminutive amphibamiform. We showed that there is a poor correlation between size and inferred age (=developmental plasticity?) in that sample and then used it to show how sampling at much smaller size bins typical for paleo (<10) can produce some really wonky results. It was possible (albeit very unlikely) to sample a wide size range and yet to get increasingly younger age with larger size, a biological implausible result. This underscores the importance of being careful in deriving interpretations from small samples [blog].

Finally, Gee (Zool J Linn Soc) presented a detailed phylogenetic analysis of trematopids, seeking to reconcile differences between previous studies' results and to address the elephant in the room that several of us have hinted at recently - does major size disparity between trematopid species cause problems with phylogenetic inference? The short answer is yes, it does. The long answer is...yes too. I achieved some spectacular polytomies in this study, highlighting key points and considerations for how we incorporate and account for ontogeny in these types of analyses (it's tricky for the amphibians!). I also sank one of my PhD advisor's taxa... [blog].

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A slice of life

In addition to my Doleserpeton paper, there were a few other studies that sliced and diced up some temno bones. The first one was technically published online last year (in print this year), and I very briefly mentioned it in my year-end summary last year, but it's worth discussing here in greater detail. Mukherjee et al. (Papers Palaeontol) describe the histology of a few Middle Triassic capitosaurs from India, which is a follow-up to another paper led by Muhkerjee a decade ago on a more preliminary sample of Indian temnos. Despite the material of the two taxa (Cherninia and Paracyclotosaurus​) coming from the same small locality, they show distinctive differences in their bone histology. Cherninia shows a lot of what temnospondyl workers term 'incipient fibrolamellar bone,' which generally meets the criteria of fibrolamellar bone in amniotes. Note that contrary to some oversimplifications, fibrolamellar bone is not an unequivocal hallmark of endothermy - it only indicates rapid growth, which can be accomplished in ectotherms living in harsh conditions that require fast growth (for example, taxa with larval forms that need to get out of water before it dries up).

Comparison of histology of the humerus of Cherninia (on the left) and Paracyclotosaurus (on the right).

In Cherninia, this fibrolamellar bone tissue is found in sub-adults too (it would be more likely to be found in juveniles that are still growing), but it is not found in any growth stage of Paracyclotosaurus, which also lacks the woven-fibered bone of the smallest and most rapidly growing individuals of Cherninia.  So Cherninia = fast grower, Paracyclotosaurus = slow grower. The histological / microanatomical differences can be correlated with distinct differences in the proportion of limb features and the torsion of the humerus and femur, suggesting biomechanical differences in their locomotion. The authors proposed that Cherninia was a classic obligately aquatic taxon, like what we think of most stereospondyls, whereas Paracyclotosaurus was more capable of moving around on land, not like what we typically think of large stereospondyls. It's debatable in my opinion whether the latter "spent a considerable amount of time on land," but it definitely seems likely that being able to at least move between ponds would have been advantageous, and it suggests that there may be a lot of cryptic information to be derived from the histology and microanatomy that is obscured by more conserved external anatomy.  

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Comparison of sections of the humerus of Panthasaurus (on the left) with the femur, ulna, and tibia (on the right; A-H = femur; I-J = ulna; K-M = tibia).

2020 was the first year that I didn't have any new metoposaurid papers published (there's stuff in the pipeline, don't worry), but the metoposaurid histology train rolls on. Teschner et al. (PeerJ) reported histology and microanatomy of the Indian metoposaurid, Panthasaurus. This taxon shows pretty normal features for obligately aquatic stereospondyls, like parallel-fibered bone with remodelling that's accompanied by lamellar bone tissue (these reflect slower rates of growth), a fairly poorly vascularized cortex, and distinct growth marks (but not LAGs).  This study is really nice because it adds to the growing body of literature helping us to understand how closely related taxa (within the same family) might differ in ecology or response to local climate, things we might not be able to nuance out from the external anatomy alone. Years ago, there was a nice comparative study by Dorota Konietzko-Meier (who's on this paper) and Nicole Klein comparing the signals in Dutuitosaurus from Morocco and Metoposaurus from Poland, which showed that the Polish paleoenvironment was milder and led to slow-downs or short stagnations but not long cessations in growth. Panthasaurus shows a similar signal to Metoposaurus (growth zones and annuli but no LAGs), indicating the Indian climate was also not too harsh. The authors also sampled a bunch of different elements, which is a key step forward to expanding the utility of non-limbs for histological studies by providing a reference point that isn't a humerus or a femur; if you wonder why nobody has done any of this work on North American taxa, it's because there are very few limb bones that are basically off-limits to destructive sampling right now (a number of people other than me have looked around, and this is why I keep cutting intercentra instead). 

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Finally, Uliakhin et al. (Paleontol J) reported what is probably the highest age estimation for any Paleozoic vertebrate and definitely the highest one for any temnospondyl - a whopping 57 years! This was reported from the late Permian dvinosaur Dvinosaurus campbelli from Russia; dvinosaurs are one of the few uncontroversially fully aquatic Paleozoic temnospondyls, and there remains fringe speculation that one of the stereospondyl clades might actually be tied to this group. We see a lot of the same features that typify stereospondyls (e.g., parallel-fibered bone, lots of secondary remodeling from the medullary cavity, calcified cartilage), which is, at minimum, a reflection of what happens when you're neotenic and end up permanently living in the water. 

How about the 57 years - is that out of bounds? We know that slow-growing animals tend to live longer (why grow slow if you'll die in a year), and some modern amphibians top 60 years (and it's not one of those weird one-offs that far exceeds the norm for the species). Like the authors point out, neotenic individuals tend to live longer than metamorphosing ones. Of course, this assumes that the interpretation of the LAGs is correct; could it be double LAGs (two cessations in growth per year) for example (thus halving the estimated age)? Hard to say. The giveaway for double LAGs is their spacing - two closely spaced lines with a gap smaller than that from the line on either side. That doesn't mean you couldn't get double LAGs that are spatially indistinguishable from normal LAGs if the process that formed them was timed differently.
​      For example, the way that we define seasons usually sets the start of winter and the start of summer as exactly 6 months apart. But as most people experience them, the "peak conditions" of each season aren't on Day 1, and the peaks may not be 6 months apart. Growing up in SoCal, I would say the hottest month of the year can be as late as September, and having lived in Canada, February or March can be the coldest. That asymmetry is likely what produces the characteristic spacing of double LAGs. But of course there's variation - who really knows the finer nuances of the climate in Russia over 250 million years ago, after all. So it's possible that climate peaks really could have been nearly 6 months apart and thus produce a pattern resembling single LAGs when the animal actually stopped growing twice a year.  But we can't really prove that either. The best way to interpret this would be to cautiously accept the author's interpretation (single LAGs = 57 years) but not to make too much of it. Comparing maximum ages in extinct taxa is hard because how do you know that you got one that died around its maximum age? A lot of animals (especially those lower on the food chain) frequently die well before their maximum age, often because they're eaten, so the fact that other temnospondyls rarely exceed estimates of 15 years only says something about the sample, not necessarily the species as a whole.

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Piling up​

Rakshit & Ray (Hist Biol) reported a new metoposaurid-dominated locality from India with more than 1,500 distinct specimens (of the 727 identifiable ones, 655 are metoposaurid). The locality is near the type locality of the Tiki Formation, which has produced material of Panthasaurus, but not in the abundance known from some localities in North America, Poland, and Morocco. While it seems likely that the new material would also be Panthasaurus, the Tiki Formation is pretty thick, up to 400 meters in some places, so it's possible that there could be more than one metoposaurid if there's a decent section of time captured from top to bottom like in the Timezgadouine Formation of Morocco; the authors punted on the taxonomic identity. The minimum number of individuals (MNI) for the site is marked at 27 based on the presence of 27 axes (the first vertebral position), which is pretty good, though the material is largely disarticulated, and it seems like there isn't any complete skulls.

Based on a slew of taphonomic analyses, there appears to be disparate pre-burial conditions between the different tetrapods found at the site that explains the differences in their representation (the complex graph on the top left). There are mixed age classes for the metoposaurids, whereas the phytosaurs and rhynchosaurs are skewed towards either juveniles or adults. Obviously some of this has to do with which animals lived in the immediate environment versus which might have been carried in and the cause of death. This is not a mass death assemblage like that of Dutuitosaurus or the Lamy quarry insofar as it appears to be time-averaged for metoposaurids (but not rhynchosaurs, which may have mostly gotten swept in by a single event), and instead seems to be accumulation of their remains through periodic flooding and a build-up of remains in low-energy spots; the model is on the top right. The authors went so far as to argue that the metoposaurid remains might have been left out on the banks, exposed for years and constantly weathering, which might account for the total lack of complete skulls. Really great prospects based on the sheer volume of material that's come out of the site - hope to see more in the future!

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That's gotta hurt

With advances in technology and application of modern methods to paleontological specimens, paleopathology (studying diseases in fossils) has come a long way - we're seeing more and more reports of specific diagnoses and maladies beyond "that is clearly not right." Novikov et al. (Paleontol J) describe a bone lesion in the Early Triassic trematosaur Benthosuchus. This is one of the most common Early Triassic tetrapods found around the world (but mostly in Russia in this case), and the authors report a bone lesion on the lower jaw, which would be immediately identifiable to even non-scientists as a very odd-looking round protrusion from the side. 

Based on features such as denser structure compared to surrounding bone, localized presence of a smooth feature, and no apparent connection to the teeth, the authors propose that this represents a non-odontogenic osteoma (non-tooth-related tumour). Because they couldn't do histology on the jaw, their results are based only on the external examination and the CT data (not the sharpest), so there are a few other possibilities (like a cyst or specifically a bone cancer). This remains the oldest example of a tumour forming in a tetrapod, although other reports like my friend Yara Haridy's 240-million-year-old turtle tumour are not that much later.

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Welcome to the club

The year was somewhat light on new taxa. Dilkes' work naming Diploseira is really a renaming of an already known species that didn't belong in its originally assigned genus. Other than Palodromeus, there was only one other new temnospondyl named, Rastosuchus hammeri from the Permian of Brazil. Dias et al. (​Revista Brasileira de Paleontologia) has an odd history because it was sort of named before, but not properly, so this is the first official naming of it as such. A mention in 1980 indicated a full description that would act as the official naming, but that never happened, yet the name kept getting used by other authors. The best material is from the lower jaw and the postcrania; a nearly complete skull is badly flattened and with the skull roof being very poorly preserved. Despite this, a number of primitive features for tetrapods, like three tooth-bearing coronoids and a bordering of the Meckelian foramen on the lower jaw by three elements (rather than two), combined with some stereospondyl features, led the authors to propose rhinesuchid affinities. There is no phylogenetic analysis in this study, but the taxon was included in a 2016 analysis where it came out as a rhinesuchid.

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A few other temnospondyl adjacent studies:

• Dickson, B.V., Clack, J.A., Smithson, T.R. and Pierce, S.E., 2020. Functional adaptive landscapes predict terrestrial capacity at the origin of limbs. Nature, pp.1-4. DOI: 10.1038/s41586-020-2974-5
• Dunne, E.M., Farnsworth, A., Greene, S.E., Lunt, D.J. and Butler, R.J., 2020. Climatic drivers of latitudinal variation in Late Triassic tetrapod diversity. Palaeontology. DOI: 10.1111/pala.12514
• Novikov, I.V., Sennikov, A.G. and Ivanov, A.V., 2020. Rare and Endemic Elements in Triassic Tetrapod Assemblages of Obshchii Syrt Highland (Eastern Europe). Paleontological Journal, 54(6), pp.640-651. DOI: 10.1134/S0031030120050111
• Pardo, J.D., Lennie, K. and Anderson, J.S., 2020. Can we reliably calibrate deep nodes in the tetrapod tree? Case studies in deep tetrapod divergences. Frontiers in genetics, 11, p.1159. DOI: 10.3389/fgene.2020.506749
• Romano, M., Bernardi, M., Petti, F.M., Rubidge, B., Hancox, J. and Benton, M.J., 2020. Early Triassic terrestrial tetrapod fauna: a review. Earth-Science Reviews, p.103331. DOI: 10.1016/j.earscirev.2020.103331
• Schultz, C.L., Martinelli, A.G., Soares, M.B., Pinheiro, F.L., Kerber, L., Horn, B.L., Pretto, F.A., Müller, R.T. and Melo, T.P., 2020. Triassic faunal successions of the Paraná Basin, southern Brazil. Journal of South American Earth Sciences, 104, p.102846. DOI: 10.1016/j.jsames.2020.102846

And the recent special issue on Karoo biozonation: ​https://pubs.geoscienceworld.org/sajg/issue/123/2 (note there are a few included taxa that are probably junior synonyms in some articles).

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In closing

All things considered, an excellent year for temnospondyl research! I reviewed a bunch of these papers as well, which is always neat to see what people are working on ahead of press. It's been a few months since I had anything come out (a coincidental pile-up of my 2019 productivity in the first half of this year), but I've got a few things working their way through the pipeline and will hopefully be getting back to some semblance of regular blogging in the new year! Thanks for reading, and best wishes for the new year!

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References

• Anderson, J.S., Scott, D. and Reisz, R.R. 2020. The anatomy of the dermatocranium and mandible of Cacops aspidephorus Williston, 1910 (Temnospondyli: Dissorophidae), from the Lower Permian of Texas. Journal of Vertebrate Paleontology, 40: e1776720. DOI: 10.1080/02724634.2020.1776720
• Atkins, J.B., Sourges, P., Fröbisch, N.B., Reisz, R.R. and Maddin, H.C. 2020. Late ontogeny in the small Early Permian amphibamiform dissorophoid Pasawioops mayi. Journal of Vertebrate Paleontology, 40: e1772800. DOI: 10.1080/02724634.2020.1772800​
• Bird, H.C., Milner, A.C., Shillito, A.P. and Butler, R.J. 2020. A lower Carboniferous (Visean) tetrapod trackway represents the earliest record of an edopoid amphibian from the UK. Journal of the Geological Society, 177: 276-282. DOI: 10.1144/jgs2019-149
• ​Cisneros, J.C., Day, M.O., Groenewald, J. and Rubidge, B.S. 2020. Small footprints expand middle Permian amphibian diversity in the South African Karoo. Palaios, 35: 1-11. DOI: 10.2110/palo.2018.098
• Dias, E.V., Dias-da-Silva, S. and Schultz, C.L. 2020. A new short-snouted rhinesuchid from the Permian of southern Brazil. Revista Brasileira de Paleontologia, 23: 98-122. [link]
• Dilkes, D.W. 2020. Revision of the Early Permian Dissorophid ‘Dissorophus’ angustus (Temnospondyli: Dissorophoidea). Journal of Vertebrate Paleontology, p.e1801704. DOI: ​10.1080/02724634.2020.1801704
• Gee, B.M., 2020. Size matters: the effects of ontogenetic disparity on the phylogeny of Trematopidae (Amphibia: Temnospondyli). Zoological Journal of the Linnean Society 190: 79-113. DOI: 10.1093/zoolinnean/zlz170
• Gee, B.M. and Reisz, R.R., 2020. A redescription of the late Carboniferous trematopid Actiobates peabodyi from Garnett, Kansas. The Anatomical Record 303: 2821-2838. DOI: 10.1002/ar.24381
• Gee, B.M., Haridy, Y. and Reisz, R.R., 2020. Histological skeletochronology indicates developmental plasticity in the early Permian stem lissamphibian Doleserpeton annectens. Ecology and Evolution, 10: 2153-2169. DOI: 10.1002/ece3.6054
• Herron, S.T., Fleming, E.J. and Flowerdew, M.J., 2020. Transition from swimming to walking preserved in tetrapod trackways from the Late Carboniferous of Bjørnøya, Svalbard. Norwegian Journal of Geology, 100: 202012 [link]
• Konietzko‐Meier, D., Teschner, E.M., Bodzioch, A. and Sander, P.M., 2020. Pentadactyl manus of the Metoposaurus krasiejowensis from the Late Triassic of Poland, the first record of pentadactyly among Temnospondyli. Journal of Anatomy, 237: 1151-1161. DOI: 10.1111/joa.13276
• Maddin, H.C., Fröbisch, N.B., Evans, D.C. and Milner, A.R. 2013. Reappraisal of the Early Permian amphibamid Tersomius texensis and some referred material. Comptes Rendus Palevol, 12: 447-461. DOI: 10.1016/j.crpv.2013.06.007
• Mann, A. and Gee, B.M. 2019. Lissamphibian-like toepads in an exceptionally preserved amphibamiform from Mazon Creek. Journal of Vertebrate Paleontology, 39: e1727490. DOI: ​10.1080/02724634.2019.1727490
• Mujal, E. and Schoch, R.R. 2020. Middle Triassic (Ladinian) amphibian tracks from the Lower Keuper succession of southern Germany: Implications for temnospondyl locomotion and track preservation. Palaeogeography, Palaeoclimatology, Palaeoecology, 543: 109625. DOI: 10.1016/j.palaeo.2020.109625
• Mukherjee, D., Sengupta, D.P. and Rakshit, N. 2020. New biological insights into the Middle Triassic capitosaurs from India as deduced from limb bone anatomy and histology. Papers in Palaeontology, 6: 93-142. DOI: 10.1002/spp2.1263
• Novikov, I.V., Haiduk, P.A., Gribanov, A.V., Ivanov, A.N., Novikov, A.V. and Starodubtseva, I.A. 2020. The earliest case of neoplastic bone lesion in tetrapods. Paleontological Journal, 54(1), pp.68-72. DOI: 10.1134/S0031030120010074
• Pardo, J.D., Small, B.J. and Huttenlocker, A.K. 2017. Stem caecilian from the Triassic of Colorado sheds light on the origins of Lissamphibia. Proceedings of the National Academy of Sciences, 114: E5389-E5395. DOI: 10.1073/pnas.1706752114
• Rakshit, N. and Ray, S. 2020. Mortality dynamics and fossilisation pathways of a new metoposaurid-dominated multitaxic bonebed from India: a window into the Late Triassic vertebrate palaeoecosystem. Historical Biology, 1-23. DOI: 10.1080/08912963.2020.1777550
• Santos, R.O., Laurin, M. and Zaher, H. 2020. A review of the fossil record of caecilians (Lissamphibia: Gymnophionomorpha) with comments on its use to calibrate molecular timetrees. Biological Journal of the Linnean Society, 131: 737-755. DOI: 10.1093/biolinnean/blaa148
• Schoch, R.R., Henrici, A.C. and Hook, R.W. A new dissorophoid temnospondyl from the Allegheny Group (late Carboniferous) of Five Points, Mahoning County, Ohio (USA). Journal of Paleontology, 1-14. DOI: 10.1017/jpa.2020.101
• Schoch, R.R., Werneburg, R. and Voigt, S. 2020. A Triassic stem-salamander from Kyrgyzstan and the origin of salamanders. Proceedings of the National Academy of Sciences, 117: 11584-11588. DOI: 10.1073/pnas.2001424117
• Teschner, E.M., Chakravorti, S., Sengupta, D.P. and Konietzko-Meier, D. 2020. Climatic influence on the growth pattern of Panthasaurus maleriensis from the Late Triassic of India deduced from paleohistology. PeerJ, 8: e9868. DOI: 10.7717/peerj.9868
• Uliakhin, A.V., Skutschas, P.P. and Saburov, P.G., 2020. Histology of Dvinosaurus campbelli (Temnospondyli, Dvinosauria) from the Late Permian Locality Gorokhovets, Vladimir Region. Paleontological Journal, 54: 632-639. DOI: 10.1134/S0031030120060106​
• Werneburg, R. 2020. On the morphology of large branchiosaurids (Amphibamiformes) from the Rotliegend (Lower Permian) of the Saar-Nahe basin, Germany. Semana​, 35: 39-54.