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.