The Missing Morph of Placerias

The Late Triassic dicynodont Placerias is probably one of the most famous dicynodonts, thanks largely to its prominent role in the first episode of the BBC television series Walking with Dinosaurs. In its onscreen debut, it was cast as an "endangered species", the last of its kind in the process of being usurped by the new, more advanced archosaurs.

Their final appearance quite literally had them walking out of the show in a desperate search for water, representing the last of the archaic "mammal-like reptiles" exiting the evolutionary stage while the plucky dinosaurs and mammal ancestors manage pulled through. The Placerias remain conspicuously absent when conditions improve again, and it's no coincidence that at the very end of the episode a herd of 'prosauropod' dinosaurs appear in their place, ushering in the age of the dinosaurs while the Placerias remain gone for good.

Placerias as it appeared in the 1999 BBC television series Walking with Dinosaurs, swaggering across the landscape.

We now know that these ideas about dicynodont decline are flawed, if not wrong. At the time, Placerias was one of the few Late Triassic dicynodonts known at all, the only one outside of South America, and was among the youngest. Their low abundance and geographic restriction fed into a notion that the last dicynodonts were evolutionary relicts, the last gasp of a clade that was already well on its way out.

We now know that Late Triassic dicynodonts, the stahleckeriids, were actually still fairly widespread (even if there was usually only one species in any given ecosystem at a time), ranging across Pangaea from Europe, through North and South America, Morocco and all the way to South Africa! They were also still diversifying, with two subfamilies showing continued dispersal right up until their extinction. To say nothing of the anatomical innovations going on with Lisowicia. But this is supposed to be a post about Placerias, not the state of Late Triassic dicynodonts, so I'll stop here.

A Placerias on display at the Rainbow Forest Museum in Petrified Forest National Park from 2009. Note the characteristic large "tusks". Photo by the NPS (CC BY-SA 2.0).

But this isn't the only thing Walking with Dinosaurs got wrong about Placerias. In the show, it is prominently depicted with a pair of large, forward-pointing tusks. This is a pretty persistent image for Placerias, and while Walking with Dinosaurs certainly wasn't the first to depict it like this, I do have to blame it for cementing this as the popular image for the animal (as well starting a trend of making it green, it's not just me seeing that right?).

In fact this is incorrect, the "tusks" on Placerias aren't teeth at all, but actually long horn-like extensions of bone growing off of the caniniform process, i.e. they're part of its skull. Savvy palaeoartists have gotten this right, covering the 'horns' (for a lack of better word) with, well, horn, much like the beak, as is suggested by their roughened bone texture. Placerias does still have true bona fide tusks, although they're nearly reduced to nubs and are completely hidden from view on the outside by the caniniform process itself. They're also only variably present, with some specimens lacking them altogether.

Regardless, the "bone-tusks" of Placerias are one of its defining characteristics, and every skeletal mount, television and book appearance depict it with them (including those that wrongly show the 'horns' as true tusks). Suffice to say it's easy to get the impression from pretty much every depiction ever that all Placerias had these 'horns'. But, according to what's actually known in the fossil record, this is not wholly representative of Placerias as it was in life.  

Placerias looking very flabbergasted by this revelation, while the phytosaur Redondasaurus gasps in astonishment. Photo by Lee Ruk (CC BY-SA 2.0).

Placerias was originally only known by, and was named from, a crushed humerus described in 1904. However, its big break came with the discovery of the aptly named Placerias Quarry in the Petrified National Forest of Arizona. The quarry is located in the well known Chinle Formation, which is spread over several southwestern states and spans many millions of years (it's a whole lotta sediment, in other words). The quarry contains over 1600 bones from at least 41 individual Placerias, and was meticulously described in 1965 by Charles L. Camp and Samuel P. Welles.

In their description, they noticed something unusual about the various maxillae (the upper jaw bone that housed the tusks) they were collecting. Of the maxillae they looked at, only around half of them bore the characteristic 'horn' we all know and love. The other half, however, either didn't have this 'horn' or only had a shorter, blunter 'horn' that was noticeably reduced by comparison.

This variation doesn't appear to correlate with size, so it's unlikely that this was due to ontogeny. Individual variation could be to blame, as occurs with the true tusks of some other dicynodonts, but the split appears so clear and evenly distributed that that doesn't seem to be the case. They concluded that this variation was perhaps sexual dimorphism, suggesting that the 'horn'-less maxillae may have belonged to females, while the males sported longer 'horns'. This suggestion has generally been upheld by later researchers as a reasonable explanation for the dimorphism.

Of course, there's no way to be sure what sex they each represent, if that's what it represents at all. Such suggestions can only be based on the assumption that male tetrapods are more likely to be the ones sporting exaggerated anatomical characteristics from sexual selection. Photos of these suggested "male" and "female" morphs by Christian Kammerer are shown below, also handily showing the real tusk that's usually hidden from view (below the thumb in the first image):

Sure. All Placerias material is fragmentary, but dimorphs recognized by Camp & Welles (1956) correspond to (L) ones with very elongate caniniform processes with pointed tips extending far beyond tusk and (R) blunter processes whose tip is close to tusk. pic.twitter.com/uHfEF4p0kM

— Christian Kammerer (@Synapsida) July 29, 2021

EDIT 12/2023: An archived version of the tweet, including photos, can be viewed here.

 

Regardless, the short-'horned' morphs exist and were seemingly present in half of the population. So why don't we see these "female" morph depicted more often, or rather, ever? To be clear, I'm not putting any blame on artists. This particular fact about Placerias biology isn't especially well known outside of specialists (or very heavily invested enthusiasts...ahem), and there's next to nothing available for artists to reference.

Indeed, the relevant literature on Placerias is difficult to access, including both Camp and Welles' monograph on the Placerias Quarry specimens and Timothy Rowe's 1979 publication on Placerias that went on to highlight this dimorphism and speculate on the function of the 'horns' (most of what I know was said in these publications comes from what has been repeated in later papers). It's not commonly reported outside of specialist literature either, so it's hardly a fact I'd expect people to have heard of in casual research.

It's also to be expected when palaeoartists have really only got one Placerias reference to go on, and that's the physical skeletal reconstructions. Because Placerias is mostly known from a bonebed, any complete skeletal reconstruction is a composite based on multiple individuals, including the skull. Most Placerias skulls are based on the composite reconstruction made by Camp and Welles in 1956—and later modified by Cox in 1965—which naturally was given the 'horns', as you would when you have an animal sporting a weird dimorphic feature. 

As a brief aside, Camp and Welles'/Cox's composite reconstruction was vindicated by a nearly complete and articulated skull described in 2002. This skull was originally interpreted upside down, but when flipped the right way around looks strikingly similar to the composite reconstruction! Goes to show the veracity of Camp, Welles and Cox's work, so fortunately it seems unlikely that the rest of their composite reconstruction would be far off. Alas, the near complete skull unfortunately isn't as well preserved around the snout, so it's not exactly clear whether this was a "male" or "female" morph.

As an artist, this speaks to me of the woes of flipping your canvas.
The original interpretation shown above, the flipped version compared with the reconstructed skull below. From Kammerer et al. (2013).

With all this in mind, it's not really surprising this isn't more commonly depicted in palaeoart. Although I've got to say, I struggled to find any palaeoart that actually does at all. I carefully re-watched the first episode of WWD to see if I missed any variation in the models—nothing, and scouring the web for palaeoart and any other depictions turned up similar results.

The one exception might be the work of the inimitable Douglas Henderson, who has illustrated two scenes—cropped versions shown below—depicting Placerias that look as if they might include "female" morphs with short 'horns'. I'm not entirely sure in honesty, it could be down to angle, and most are tucked into the background, but it does look like some of them have been depicted with genuinely shorter 'horns' than the others to me.

Crops of Doug Henderson's works featuring what look like both long and short-'horned' Placerias. Do these individuals deliberately reference the "female" morph? With apologies to Douglas Henderson for shamelessly reposting (part of) his art.

In any case, I wanted to contribute to the cause and drew up my own quick sketch the a few weeks back when #TriassicWeek was still going strong on Twitter. After a few kind pointers from Christian Kammerer, along with the photos posted above, I was able to make a few nips and tucks to land with the finalised sketch below:

Sketch of a "female" morph Placerias, front and centre.

I originally sketched it with the little nubbin' tusks sticking out of the bottom of the caniniform process, although as explained to me by Kammerer it's evident that the tusks would still be hidden by the process in these individuals and so I covered them up.

This was intended to be a short post, so I'll cap it here with a message to all the palaeoartists out there who are ever considering reconstructing Placerias: draw the "female" morphs! This goes especially if you're illustrating a herd, but even for a solo picture I say mix things up a bit and go for the "female" morph. And if you're not thinking about drawing Placerias, do it anyway. The world could always use more Placerias art.

References

Bandyopadhyay, S. (1988). A Kannemeyeriid Dicynodont from the Middle Triassic Yerrapalli Formation. Philosophical Transactions of the Royal Society B: Biological Sciences, 320 (1198), 185–233.

Green, J. L. (2011). Bone and Dental Histology of Late Triassic Dicynodonts from North America. In: Chinsamy-Turan, A. (ed.), Forerunners of Mammals: Radiation Histology Biology. Bloomington: Indiana University Press, 178–196.

Green, J. L., Schweitzer, M. H., & Lamm, E.-T. (2010). Limb bone histology and growth in Placerias hesternus (Therapsida: Anomodontia) from the Upper Triassic of North America. Palaeontology, 53 (2), 347-364.

Kammerer, C. F. (2018). The first skeletal evidence of a dicynodont from the lower Elliot Formation of South Africa. Palaeontologia Africana, 52, 102–128. 

Kammerer, C. F.; Fröbisch, J. R., & Angielczyk, K. D. (2013). On the Validity and Phylogenetic Position of Eubrachiosaurus browni, a Kannemeyeriiform Dicynodont (Anomodontia) from Triassic North America. PLOS ONE, 8 (5), e64203. 

Sulej, T., & Niedźwiedzki, G. (2019). An elephant-sized Late Triassic synapsid with erect limbs. Science, 363(6422), 78-80.

Sort of referenced but not really:

Camp, C. L., & Welles, S. P. (1956). Triassic dicynodont reptiles. Part I. The North American genus Placerias. Memoirs of the University of California, 13, 255–304.

Rowe, T. (1979). Placerias: an unusual reptile from the Chinle Formation. Plateau, 51 (4), 30-32.

Making Junk of Dicynodont Trunks

Bill Munn's sculpture of a trunked Giraffatitan.
Classic imagery. Original photo featured on Cryptomundo, I think?

In the realms of the palaeo-blogosphere, one article I would consider to be essential reading is Tetrapod Zoology's Junk in the trunk: why sauropod dinosaurs did not possess trunks, which speaks for itself (and apologies to Darren for shamelessly riffing on his title).

As I'm sure some of you are familiar with, it’s become a fairly recurring pattern in vertebrate palaeontology for certain clades with peculiar nasal anatomy to be (often ultimately incorrectly) proposed to have had trunks. The idea of trunked sauropods has been decidedly debunked, dismissed and derided, but even such well established examples of purportedly trunked fossil tetrapods like Macrauchenia have recently been proposed to be stripped of their proboscises on account of incompatible musculature and other anatomical grounds.

But did you know that dicynodonts also got in on the trunk craze? Given that you're here and read the title, I'd imagine you'd already guessed that. But did you also know that they got in on the craze almost a whole century ahead of the game? Yes indeed, it may be seldom acknowledged nowadays but there really was one time when a dicynodont was proposed to have had a trunk. It wasn't just any dicynodont either, but arguably one of the more famous names in the dicynodont roster, Kannemeyeria, and in fact the original specimen from which it was named to boot.

Kannemeyeria was coined by British palaeontologist Harry Seeley (of Saurischia and Ornithischia fame) in September of 1908 at the 78th meeting of The British Association, published as a brief description just over half a page long. Seeley unfortunately died just some months later in January of 1909, and to my knowledge Kannemeyeria may be one the last taxa he ever named. However, it was not for the familiar K. simocephalus we know today. Indeed, that species had already been named twenty years earlier in 1888 by German palaeontologist Anton Weithofer, who regarded it as a species of Dicynodon (as of course was tradition for many dicynodont fossils in the 19th and early 20th century).

Seeley named Kannemeyeria from a peculiar partial skull discovered in the Karoo Basin of South Africa by fossil collector (among various other professions) Dr. Daniel Rossouw Kannemeyer in 1895, whose find would ultimately bear his name. The skull was, in Seeley's words, "not perfect" and pretty busted up and incomplete, but Seeley recognised it as a dicynodont for its toothless jaws and paired tusks (which I assume are broken, as I can't make them out much in the few images I could find of the fossil —scratch that, better photos provided by Dr. Christian Kammerer below more clearly show the tusks set in the jaw). However, one feature stood out to Seeley that compelled him to describe it as an entirely new genus and species that he named Kannemeyeria proboscoides. You can probably guess why.

Seeley's type specimen of Kannemeyeria proboscoides, viewing the left side from slightly above (a), underside (b), and the top side (c). The specimen's not very pretty, and neither are the pictures, but it's the best I could find (UPDATE: better quality photos have been provided at the bottom of the post!). From Cruickshank (1970).

Yes, Seeley was quite convinced that this was a hitherto unknown dicynodont with a trunk, and adamantly so at that. He explicitly described the area for the trunk insertion as being elephantine in structure, not like the short and stout proboscis of a tapir. Now usually in cases like these it’s at least understandable how a certain fossil's anatomy can invoke the possibility of a trunk: raised nostrils, large narial openings, abbreviated snouts, and so on. But how does the beaked skull of a dicynodont, which typically have small nostril openings I might add, even come remotely close to looking like it should have a trunk?

The simple answer is that the specimen Kannemeyer found was broken, and that the bean-shaped opening left behind by the broken-off snout was interpreted as the entire nasal aperture. In fact, most of the skull is missing, it's mostly a palate with part of the top of the snout and a left eye, the rest is all missing. In hindsight, the beak simply just being broken off sounds glaringly obvious, but let’s be kind to Seeley. The scope of dicynodont anatomy was still poorly understood at the time, and the tusk-bearing region was still roughly intact, so it's not inconceivable to have interpreted a large forward-opening hole in the skull as a natural feature. After all, by itself it's not that outrageous, especially when it recalls to mind another famously trunked synapsid with a large, vaguely bean-shaped opening on the front of its skull and a prominent pair of tusks. 

Synapsid? Check! Two tusks? Double check!
Vaguely bean-shaped narial opening? Check! Kannemeyeria proboscoides confirmed.
Photo byJimJones1971 (CC BY-SA 3.0).

The story gets even more interesting, though. Seeley went on to speculate on the lifestyle and habits of his Kannemeyeria proboscoides in surprising detail. He concluded that Kannemeyeria was something like an aardvark or giant anteater, using its tusks to rip open and tear down termite nests, a trait he ascribed to other dicynodonts as well. Quite how the trunk was involved with this is a bit of a puzzle to me, admittedly. He compares it to the elongated snouts of aardvarks and anteaters as a "soft flexible" extension of the head, but in these mammals the snout is stiff and tubular with the mouth situated at the very end, totally unlike the short jaws and trunk of elephants. Was it supposed to suck them up the way anteaters do in cartoons? Who knows.

Whatever Seeley exactly had in mind for Kannemeyeria, it didn't stick. Just four years later, David M.S. Watson recognised that Seeley's skull was an imperfect specimen of Weithofer's Dicynodon simocephalus, and while opinions on the internal taxonomy of Kannemeyeria differed in the years since, there was no question that Kannemeyeria had a beak like other dicynodonts. Seeley never illustrated his trunked dicynodont in the flesh (in fact he didn't even illustrate the fossil either), and as far as I can tell, no palaeoartists seem to have ever picked up on his suggestion either, before and after it was debunked. Which is a real shame, since this would be a fantastic subject for those goofy looking pieces of outdated palaeoart we all love to poke fun at. With that said, I’ve taken the liberty of doing it myself.

And so I present to you Seeley's trunked Kannemeyeria proboscoides in all its goofy glory:

My imagining of how Seeley may have imagined Kannemeyeria.

Now obviously this doesn't look much like Kannemeyeria at all, even ignoring the trunk, but I was trying to illustrate it through the lense of Seeley's interpretation in 1908. Admittedly, the type skull of 'D.' simocephalus was already known at the time, which preserved the characteristically crested parietal region we're so familiar with. However, Seeley made no connection between his proboscoides and 'D.' simocephalus, and there's no indication that he thought that his specimen would have been much different from most other dicynodonts in the rear of the skull. As such, I illustrated it with a more typical-looking skull (trunk notwithstanding). I kept the beak on the lower jaw, as Seeley also believed dicynodonts to be insectivores with a long, extendable tongue that slotted into a groove in their lower jaws and beaks, with the trunk of K. proboscoides presumably being a further specialisation of this lifestyle. The trunk itself is based on those of proboscideans, as Seeley described, with particular inspiration from gomphotheres and other early proboscideans given its more horizontally projecting arrangement.

Seeley’s trunked Kannemeyeria seems to have only ever made it into its brief, nominative publication before being dismissed and all but forgotten. At best, it has been relegated to a brief tidbit regarding Seeley’s misplaced taxonomy to explain the story behind the "proboscoides" name. To my knowledge, no one has ever made a similar suggestion since then either, although that's not surprising, putting a trunk on any dicynodont would be a pretty bold and brash claim to make (and would almost certainly belong in the trash). There's no controversy to be had, and it's incredibly unlikely the idea will ever be proposed again, but it's a quirky piece of history in dicynodont research and a fun idea to look back on.

Though, to be honest, if any non-mammalian synapsid was ever going to turn up one day with good evidence for a trunk of some sort, my money would be on it being an anomodont. It just sounds like it would be in their repertoire.

Addendum (16/07/21)

Update! Not long after posting, Dr. Christian Kammerer kindly provided some clear, high quality photos of Seeley's type specimen of Kannemeyera proboscoides! He also politely corrected me where I accidentally switched his name for Dr. Kannemeyer's...my bad.

Also, since the world seems to be devoid of decent pictures of this specimen: pic.twitter.com/2pdQsUJQLO

— Christian Kammerer (@Synapsida) July 16, 2021

Sources

Cruickshank, A. (1970). Taxonomy of the Triassic anomodont genus Kannemeyeria. Palaeontologia africana, 713, 47–55.

Haughton, S. H. (1915). On a Skull of the Genus Kannemeyeria. Annals of the South African Museum, 12, 91–97.

Seeley, H. (1909). On a fossil reptile with a trunk from the Upper Karroo rocks of Cape Colony. Report of the British Association for the Advancement of Science 1908, 78, 713.

Watson, D. M. S. (1912). On some reptilian low jaws. Annals and Magazine of Natural History, 10 (60), 573–587.

Weithofer, A. (1888). Ueber einen neuen Dicynodonten (Dicynodon simocephalus) aus der Karrooformation Südafrikas. Annalen des Naturhistorischen Museums in Wien, 3 (häft 1), 1–6.

Dicynodont Year-in-Review 2020

Took me long enough, but here it is: the review of dicynodont research published in 2020! It simultaneously feels like I started this blog just the other day and much, much longer than over 7 months ago. And yet, the time for the annual palaeoblog tradition of an end-of-the-year roundup of discoveries has come (arguably been and gone by now). So, how did dicynodonts fare last year? 

As you'd expect, given...circumstances, 2020 has been a good bit quieter than the last year with only one newly named taxon (compared to the rather exceptional seven new taxa of 2019, but read on). However, there has still been quite a few significant discoveries published this year on them, many of which I originally intended to dedicate a post to each during the year. How many did I manage to finish this year? Two?...Oh dear.

Well without further ado, let's see just how much I managed to miss!

Research

While this year was a bit of a wash discovering and collecting new specimens, it hasn't caused dicynodont research to stop in its tracks and there's plenty of new stuff to highlight.

The bizarre Endothiodon got some much needed attention this year, with two studies on it published. The first was published by Macungo et al. (2020) in March, who described a whopping 111 new specimens from both recent and historic expeditions in the Metangula Graben in Mozambique. Curiously, collection includes specimens attributable to both E. tolani and E. bathystoma, for which the authors put forward several explanations. One possibility is that that because these species appear stratigraphically separated in the Ruhuhu Basin of Tanzania, there may be a similar, as yet unrecognised, stratigraphic separation in the Metangula Graben. Another possibility is that the two species may have coexisted after all, at least in Mozambique. Perhaps their most intriguing suggestion is that there weren't even two distinct species at all, and that they instead represent the end members of a cline of variation from the Karoo in the southwest to Tanzania and Mozambique in the northeast. The answer isn't clear, and it likely won't be solved until we have more information on the stratigraphy of Endothiodon, determine how old each species is, and sort out its taxonomy once for all.

PPN2014-11, one of many new specimens of Endothiodon from Mozambique. The presence of tusks ('tsk' in the diagram) in this specimen implies it belongs to E. tolani. From Macungo et al. (2020).

Then in June, an isotopic study of Endothiodon from the Karoo Basin was published by Rey et al. (2020). Their findings suggest that Endothiodon was more reliant on water and riparian vegetation compared to more typical dicynodonts, which may relate to its unusual distribution geographically and over time. This was actually one publication I did manage to cover last year, so I'll not repeat myself here, but with this study and the questions raised by the new specimens from Mozambique, suffice to say there is still much ado about Endothiodon.

The natural habits and habitat of Endothiodon?

In July, a new skull of Dinodontosaurus was described from the Chañares Formation by Ordoñez et al. (2020) using 3D modelling from CT scans. Dinodontosaurus returned in October with the description of a monotypic bonebed of at least six juveniles published by Ugalde et al. (2020), further supporting group-living behaviour in this genus. The taphonomy of the specimens imply that the young animals were not simply transported together during or after death, but died close by each other and lay exposed on the surface for some time before burial. They may have died of malnutrition, disease, or perhaps abandoned by their parents.

The new Dinodontosaurus bonebed while it was still alive. By Márcio L. Castro, from Ugalde et al. (2020).

Taphonomy was also the focus of another study by Kammerer et al. (2020) (open access!), who quantified the effects taphonomic deformation using the many, many known skulls of Diictodon as a case study. They found that deformation tends to overprint any sort of biological signals in Diictodon skulls, such as those attributed to sexual dimorphism or changes during growth. They also found that all the deformed Diictodon skulls cover a frighteningly broad range of the total dicynodont skull morphospace, which is bad news if you're trying to identify real variation within a species. There is some good news though, a large sample of deformed skulls can still be used to approximate the mean, undistorted morphology for higher clade-level analyses, so their utility is not all lost!

Diictodon skulls come in a variety of flavours, including but not limited to normal, squashed side-to-side, squished top-to-bottom, and many more. From Kammerer et al. (2020).

Other notable publications include a histological study of the preparietal bone in Diictodon and Lystrosaurus by Marilao et al. (2020) from July. The preparietal is an extra bone in the skull roof unique to therapsids, and seemingly independently evolved in dicynodonts, gorgonopsians and biarmosuchians, and so is about as well understood as you'd expect for a bone with no known analogue. Their findings imply that the preparietal grew rapidly to the back along the border of the pineal foramen, at least in dicynodonts, and had unusually complex sutures with the surrounding skull bones. Whether this holds true for other dicynodonts, and indeed other therapsids, is yet to be seen, but this is still an important first step in figuring out the preparietal and dicynodonts are leading the way.

Last but not least, in December, a new record referred to cf. Jachaleria candelariensis from Brazil was reported by Martinelli et al. (2020). It admittedly isn't much, just four vertebrae, but it could be the first J. candelariensis remains discovered outside of the type quarry. These remains also help to further establish correlations of the Riograndia Assemblage Zone throughout Brazil.

New 'donts on the block

In July, we saw the first and only new dicynodont named in 2020: Taoheodon baizhijuni. This was another publication I covered last year, so again I won't repeat myself much. It was a dicynodontoid, seemingly closely related to Dicynodon itself, from Northern China, part of a growing record of dicynodonts from the Late Permian of East Asia. Taoheodon may be of particular importance as it may provide a sort of link between the dicynodonts known from South Africa and Europe and those from Laos that have been known since the end of the 19th century (recently described as their own genera by Olivier et al. in 2019).

Reconstruction of Taoheodon baizhijuni.

Tangentially, Taoheodon may also be the first dicynodont known to have coexisted with an embolomere (not counting Triassic chroniosuchians). Seroherpeton was also described this year from the lower Sunjiagou Formation in China, which would place it as roughly contemporaneous with Taoheodon. While itself not a dicynodont-related discovery, it's still a bit of a novelty as to my knowledge dicynodonts and embolomeres have never been found in coeval deposits before now.

Taoheodon may be the only new name, but it may not be the only new taxon identified this year. All the way back in January, Yi and Liu (2020) described another partial dicynodont skull from the same strata as Taoheodon. They referred the specimen to Cryptodontia, which would be a first for China. Additionally, two other fossils were also described this year that may hint at even more as yet unrecognised dicynodonts. 

IVPP V 26043, also known as 'Cryptodontia indet.'. From Yi and Liu (2020).

The first of these was published by Maisch (2020) in October, who re-evaluated a partial skull roof of a large dicynodont from the Triassic Santa Maria Formation in Brazil. The specimen, GPIT RE/09622, was discovered by von Huene and Stahlecker all the way back in 1928, but remained ignored and undescribed in the near century since then. Despite its incomplete nature, Maisch concluded that the specimen is not from the two known dicynodonts of the Dinodontosaurus Assemblage Zone, Stahleckeria and the eponymous Dinodontosaurus, nor any other kannemeyeriiform for that matter. 

Maisch also raised the possibility that Stahleckeria ("Barysoma") lenzii, which has typically been lumped into S. potens, may itself also be valid after all, reiterating previous arguments for its distinction. If correct, this would raise the total number of dicynodonts in the Dinodontosaurus AZ to not just three, but four taxa.

However, and this just shows how late I am with this post, these suggestions have already been met with scepticism in a paper by Kammerer and Ordoñez (2021) published in January (I swear I'll get to this one in a timely fashion). In a review and reassessment of South American dicynodont taxa, they also evaluated the proposals made by Maisch (2020). They suggest that GPIT RE/09622 may fall within the realm of variability for Dinodontosaurus after all, with preservational complications accounting for some of the purported differences between it and the compared specimens of Dinodontosaurus. Nonetheless, they acknowledged that it is possible that there are still unrecognised dicynodonts in the Dinodontosaurus AZ, perhaps even within the many incompletely prepared specimens of "Dinodontosaurus" out there.

Likewise, they concluded that the skull characteristics used to support S./"B". lenzii  can be chalked up to taphonomy and simply the range of variation seen in other kannemeyeriiforms known from lots of specimens, and so it is likely still S. potens. So, back to square one? Maybe, but who knows, there might still be a new dicynodont taxon hidden in the Dinodontosaurus AZ yet.

The skull roof known as GPIT/RE/09622, new taxon or more Dinodontosaurus? From Maisch (2020).

The second proposed new taxon was published by Smith et al. (2020) just the next month in November, and they also described a partial skull. This time it's a chunk of the back of the skull from a kannemeyeriiform discovered in the Triassic upper Fremouw Formation in Antarctica. The Fremouw is better known for its abundant Antarctic Lystrosaurus material in its lower member, but the younger upper Fremouw has spat out fossils of kannemeyeriiforms too, albeit only fragments of maxilla, tusks and a squamosal. The partial skull described by Smith et al. is by far the most complete kannemeyeriiform remains yet recovered from the Fremouw.

Intriguingly, the specimen has a combination of characteristics unlike any known dicynodont, and may even have at least one entirely unique trait, which could mean that this Antarctic kannemeyeriiform may represent a new taxon endemic to Antarctica. Still, the specimen is too incomplete to be certain, and so it remains nameless, but the presence of a unique Antarctic Triassic dicynodont is tantalising. The specimen itself is comparable in size to Kannemeyeria, although it's less clear what type of kannemeyeriiform it belongs to.

The presence of tusks from the Fremouw would be consistent with a kannemeyeriid-esque animal, but the aforementioned squamosal shows signs of a distinctly stahleckeriid affinity. This suggests there are two kannemeyeriiforms from the upper Fremouw, and theoretically this new specimen could come from either (although it may have affinities with kannemeyeriid types). Only time will tell just what kind of dicynodont the new skull-chunk belongs too, but it's a tantalising clue of what awaits us from Antarctic dicynodonts.

LACM-DI 160748, the Antarctic kannemeyeriiform, in occipital view (from behind, in other words). From Smith et al. (2020).

Lystosaurus Galore

To cap the roundup off, special mention goes to good old Lystrosaurus, which received a fair bit of attention this year! Three studies focused on it were published this year, two from within the last two months of the year, with significant implications for its lifestyle and just how we think of this (relatively) famous dicynodont. And to top it off, they're all open access!

The first of these was a study by Whitney and Sidor (2020) on the growth of Lystrosaurus tusks from Antarctica. Compared with tusks from South African Lystrosaurus, the Antarctic specimens showed clear signs of periodic slowed growth rates, signs of stress and interrupted growth. These dark zones are consistent with these specimens entering a period of torpor or hibernation, slowing their metabolism and growth rate to sit out environmental stresses not experienced by their relatives in South Africa. Given that Antarctica was below the Antarctic circle at this time, it seems probable that polar populations of Lystrosaurus were capable of going into torpor to sit out the worst of the dark polar night, when food would have been scarce. Perhaps such metabolic flexibility shouldn't be unexpected from an animal that survived the Permo-Triassic mass extinction, but it's still a cool discovery.

Cross sections of Lystrosaurus tusks from Antarctica (a) and South Africa (b), highlighting the dark 'hiernation lines' (or lack thereof). Modified from Whitney and Sidor (2020).

The other two papers re-examine some of the stereotypical claims surrounding the genus and its survivorship of the Permo-Triassic mass extinction.

The first of these, Botha (2020), examined the growth of all four Lystrosaurus species in South Africa to test the claimed "Lilliput effect" whereby the two Triassic Lystrosaurus species were dwarfed compared to their Permo-Triassic predecessors. She found fast bone-growth in all four species, but only in L. maccaigi, the largest species, did any ever reach the stage of slower, mature growth. The case seems to be then that the smaller Triassic species could very well have reached the same large sizes as L. maccaigi and L. curvatus, but many died before reaching this size. Inconsistent growth marks in the Triassic species further imply that environmental stress was affecting their growth patterns. This builds upon previous work that proposed Triassic Lystrosaurus were breeding and dying young all while they were still growing, and weren't dwarfed adults at all (Botha-Brink et al. 2016).

Finally in December, a paper by Modesto (2020) revisited the claim that Lystrosaurus was a "disaster taxon", and concluded that whichever way you define it, Lystrosaurus probably was not a disaster taxon. Shocking, I know.

This isn't to say that Lystrosaurus wasn't an especially abundant animal in the wake of the P-Tr extinction event, it certainly was. But according to Modesto Lystrosaurus just doesn't fit the criteria for any of the definitions of disaster taxa in palaeontology. It lived about as long as could be expected of any fossil tetrapod genus, it evolved prior to the extinction, not during and/or immediately after, and its great abundance is not associated with the immediate aftermath either. Rather than a disaster taxon in the strict sense that flourished in the wake of an environmental crisis, Lystrosaurus was a genus that simply managed to whether the P-Tr extinction and was able to recover afterwards, exploiting a vacuum left behind by the extinction of most other large herbivores. I suspect that this is what some of you picture as a disaster taxon anyway, but hey, that's semantics.

Summary

Overall, I think it's fair to say it's been a busy year for Lystrosaurus for one, but there's certainly been no shortage of other dicynodont research across the board either. One new named species isn't bad for a group like dicynodonts, but raising the prospect of others definitely counts for something too. The phylogenetic scope hasn't been that bad either, though there was a clear bias to the dicynodontoids. Endothiodon takes the cake for non-dicynodontoids, although only Diictodon of the pylaecephalids provides much competition, with little to say for the cryptodonts and let alone anything for the emydopoids. To say nothing of other amomodonts, which were conspicuously lacking this year.

That wraps it up for 2020, hopefully 2021 will deliver just as many exciting new discoveries! The year's already off to a strong start with a review of South American dicynodonts (Kammerer and Ordoñez), including coining two new taxa, so let's hope the momentum keeps up.

And if not, it means I've got plenty of time to play catch-up.

A smiley Galeops wishes you all a long overdue happy 2021.

References

Botha, J. (2020). The paleobiology and paleoecology of South African Lystrosaurus. PeerJ, 8, e10408.

Botha-Brink, J., Huttenlocker, A., Angielczyk, K. D., Codron, D., Ruta, M. (2016). Breeding young as a survival strategy during Earth’s greatest mass extinction. Scientific Reports 6, 24053.

Kammerer, C. F., Deutsch, M., Lungmus, J. K., & Angielczyk, K. D. (2020). Effects of taphonomic deformation on geometric morphometric analysis of fossils: a study using the dicynodont Diictodon feliceps (Therapsida, Anomodontia). PeerJ, 8, e9925.

Kammerer, C. F., Ordoñez, M. D. (2021). Dicynodonts (Therapsida: Anomodontia) of South America. Journal of South American Earth Sciences. In press.

Liu, J. (2020). Taoheodon baizhijuni, gen. et sp. nov. (Anomodontia, Dicynodontoidea), from the upper Permian Sunjiagou Formation of China and its implications. Journal of Vertebrate Paleontology, 40 (1), e1762088.

Maisch, M. W. (2020). An unusual historic dicynodont specimen (Therapsida: Dicynodontia) from the Dinodontosaurus Assemblage Zone of the Santa Maria Formation (Middle Triassic) of Rio Grande do Sul, Brazil. PalZ, 1-16.

Marilao, L. M., Kulik, Z. T., & Sidor, C. A. (2020). Histology of the preparietal: a neomorphic cranial element in dicynodont therapsids. Journal of Vertebrate Paleontology, 40 (2), e1770775.

Modesto, S. P. (2020). The disaster taxon Lystrosaurus: a paleontological myth. Frontiers in Earth Science, 8, 617.

Ordoñez, M., Marsicano, C. A., & Mancuso, A. C. (2020). New specimen of Dinodontosaurus (Therapsida, Anomodontia) from west-central Argentina (Chañares Formation) and a reassessment of the Triassic Dinodontosaurus Assemblage Zone of southern South America. Journal of South American Earth Sciences, 102597.

Smith, N. D., Makovicky, P. J., Sidor, C. A., & Hammer, W. R. (2020). A kannemeyeriiform (Synapsida: Dicynodontia) occipital plate from the Middle Triassic upper Fremouw Formation of Antarctica. Journal of Vertebrate Paleontology, e1829634.

Ugalde, G. D., Müller, R. T., de Araújo-Júnior, H. I., Dias-da-Silva, S., & Pinheiro, F. L. (2020). A peculiar bonebed reinforces gregarious behaviour for the Triassic dicynodont Dinodontosaurus. Historical Biology, 32 (6), 764-772.

Whitney, M. R., & Sidor, C. A. (2020). Evidence of torpor in the tusks of Lystrosaurus from the Early Triassic of Antarctica. Communications biology, 3 (1), 1-6.

Yi, J. & Liu, J. (2020). Pareiasaur and dicynodont fossils from upper Permian of Shouyang, Shanxi, China. Vertebrata PalAsiatica. 51 (8): 16–23.