O Fortuno-don

 A little-known name in the realm of dicynodonts—which makes it an especially unknown name outside that realm—is 'Fortunodon'. The name 'Fortunodon' has only ever been published twice in scientific literature, and one of those was the paper that coined the name in the first place, pretty well immediately fading into obscurity. It's only other appearance was in a list of dicynodont genera yet to be included in a phylogenetic analysis, as well as a brief appearance in at least one Russian book on Permian fauna.

'Fortunodon' was named as a replacement genus for 'Dicynodon' trautscholdi, a Russian dicynodont that was included in the massive over-stuffing of Dicynodon (the dicynodont dark ages). The name means "good-luck tooth", intended to refer to the very successful dicynodont skull design, and was named by Russian palaeontologist A. A. Kurkin in April of 2012. Kurkin also attributed another Russian Dicynodon species, 'Dicynodon' amalitzkii, to 'Fortunodon' as 'F.' amalitzkii. He considered them to be close relatives of Dicynodon, Delectosaurus and Vivaxosaurus and placed them together in the subfamily Dicynodontinae.

Keen readers who follow dicynodont taxonomy (there has to be at least one of you, surely) may be putting a few things together from these names and dates and know where this is going.

You see, only a few months prior in 2011, Christian Kammerer, Kenneth Angielczyk and Jörg Fröbisch published their landmark revision of the taxonomy of Dicynodon which finally cleaned up the taxonomic mess that had been building in that genus for centuries. In one fell swoop, Kammerer, Angielczyk and Fröbisch cut Dicynodon down from well over 30 accepted species to just two,* and resurrected numerous old forgotten names and coining as many new ones. Naturally, this revision included 'D.' trautscholdi and 'D.' amalitzkii.

(*Trivia: One of these later ended up in Daptocephalus, but another second species was named at the same time anyway. All's well that ends well for Dicynodon.)

After thorough comparisons, Kammerer and co. similarly concluded that 'D.' trautscholdi was distinct enough from Dicynodon to warrant separation. However, unlike Kurkin, they did not consider it distinct from Vivaxosaurus permirus, and chose to synonymise the two as Vivaxosaurus trautscholdi (the specific name 'D.' trautscholdi pre-dates the name Vivaxosaurus permirus and so has priority). 'D'. amalitzkii was also found to be distinct to warrant a separate genus, however it had no previous name to fall back on, nor could it confidently be referred to another genus, so Kammerer and co. erected the new name Peramodon to house it.

Restoration of Vivaxosaurus trautscholdi...or should that be 'Fortunodon'? By Dmitry Bogdanov, CC BY-SA 3.0

To recap the situation, in 2011 Kammerer synonymised 'Dicynodon' trautscholdi with Vivaxosaurus permirus, creating Vivaxosaurus trautscholdi, and split 'D'. amalitzkii into the new binomial Peramodon amalitzkii. In 2012, Kurkin also removed 'D.' trautscholdi from Dicynodon and named it the type species of a new genus, 'Fortunodon'. He also included 'D.' amalitzkii as a second species of 'Fortunodon', 'F.' amalitzkii. 

Where does this leave us? Well, seeing how 'Fortunodon' has been almost completely ignored since it was named, in and out of the literature, there isn't really any official word on the matter. However, under the current circumstances the rules of priority imply that 'Fortunodon' (2012) is a junior synonym of Vivaxosaurus, on account of the type species 'F.' trautscholdi currently being included under that genus. Consequently, 'F'. amalitzkii is simply just Peramodon.

But what if Kurkin is correct, and 'D.' trautscholdi is actually a separate genus after all? Unfortunately, that still wouldn't cleanly bring back 'Fortunodon' because the first novel genus name to be applied to any of its species is Kammerer and co.'s Peramodon. The only possible way to salvage 'Fortunodon' is for 'F.' trautscholdi to be distinct from V. permirus, while at the same time restricting 'Fortunodon' to the type species and keeping 'F'. amalitzkii separate in Peramodon, a scheme which so far has never been suggested by any dicynodont workers, to my knowledge.

This particular catch-22 with Peramodon raises the question of why even name 'Fortunodon' in the first place as Kurkin conceived it (i.e. including P. amalitzkii) after Kammerer and co. published their taxonomy earlier? The answer here seems to just come down to unfortunate timing. While Kurkin's paper may have been published in April of 2012, it was received by its journal all the way back in March, 2011. Kammerer et al. submitted their paper two months later in May of 2011, was accepted in September, but wasn't published until December. Kurkin's paper was later finally published less than 4 months later, but by then Kammerer et al. had simply beaten him to the punch and it was probably too late to change anything at that stage.

So that's the story of 'Fortunodon' as best as I can put together. A complicated taxonomic tale caught up in scientific bureaucracy that dealt poor unfortunate 'Fortunodon' a losing hand after losing hand. Of course, this mostly from a western perspective, I'm not sure what the thoughts on 'Fortunodon' are over in Russia. However, given I've yet to see it reappear in another paper, period, I get the impression that it's either flown mostly under the radar and/or people are just content to ignore it and let the name fade away. 

Perhaps 'Fortunodon' simply just wasn't quite that lucky, after all...

'Fortunodon' trautscholdi emerges from its hole on the rare occasion someone mentions it by name.
By Dmitry Bogdanov, CC BY-SA 3.0 (according to its license on DeviantART, at least, despite the watermark.)

(If people who know better notice that I've made any taxonomic mistakes in this, please do correct me.)

References

Kammerer, C.F.; Angielczyk, K.D.; Fröbisch, J. 2011. A comprehensive taxonomic revision of Dicynodon (Therapsida, Anomodontia) and its implications for dicynodont phylogeny, biogeography, and biostratigraphy. Journal of Vertebrate Paleontology. 31 (Suppl. 1): 1–158.

Kurkin, A. A. 2012. Dicynodontids of Eastern Europe. Paleontological Journal, 46(2), 187-198.

New taxon: Taoheodon baizhijuni

After lamenting the dearth of dicynodont research this year when I covered the publication of stable isotope analysis on Endothiodon, two dicynodont-focused papers have dropped in the interim! If that's how it's going to be, I should whinge about wanting papers more often.

I'm planning on covering each of these papers when I can, and first up is the description of a new genus and species published back in July by Liu (2020). Welcome Taoheodon baizhijuni, the first (and as of writing, the only) new dicynodont of 2020!

Taoheodon baizhijuni

The skull of Taoheodon baizhijuni (IVPP V 25335), from Liu (2020).

Etymology: Named after the Tao He stream, a nearby river that eroded the valley the fossil was discovered in. The species is named after the fossil hunter who discovered it, Bai Zhijun. The '-odon' (tooth) should be self explanatory.

Described by: Jun Liu, 2020

From: The Shanxi Province in northern China, in the lower Sunjiagou Formation.

Age: The Late Permian, probably around the latest Wuchiapingian to the Changhsingian approx. 254-253 mya.

Known from: A slightly squashed and eroded skull plus a partial lower jaw. 

As far as dicynodonts go, Taoheodon is fairly nondescript in appearance. It's not particularly large or small, and it's got your basic Dicynodon-esque shape—short snout, two tusks, a toothless beak, plus a slight boss on the snout—the standard affair for an animal in the Dicynodon area of the family tree. Taoheodon is uniquely set apart from the rest of the Dicynodon-nexus by a few minor details around the postorbital bones, braincase and the pterygoids. The fossils have been eroded a bit, so the skull's missing the tip of the snout and most of the zygomatic arches, while the jaw is missing the tip and everything behind the reflected lamina, but there's enough of them to get the gist of the shape.

IVPP V 25335 in dorsal view, from Liu (2020).

Taoheodon is the latest member of the ever growing collection of Dicynodon-like genera floating around in Dicynodontoidea. Most of these animals used to be stuffed into a severely over-bloated Dicynodon until it was finally blown apart by synapsid expert Christian Kammerer in 2011, resurrecting many old names and coining several new ones for the plethora of species crammed in there in the process. Permian dicynodontoids are one of the most unstable parts of the dicynodont family tree, so sorting out who's related to who is still an unresolved challenge. By itself, Taoheodon doesn't fix this conundrum, but it's still another piece of the puzzle and, perhaps, it might just hold some key biogeographical information.

Taoheodon was found by Liu to be most closely related to two genera of Late Permian or Early Triassic* dicynodonts from Laos described just last year, Repelinosaurus and Counillonia. These two were originally thought to be unrelated, as the earliest kannemeyeriiform and another member of the Dicynodon-nexus (or 'Dicynodon-grade' as it's actually called), respectively. The addition of Taoheodon into the mix appears to have linked them instead as each other's closest relatives, dragging Repelinosaurus out of Kannemeyeriiformes in the process (admittedly not a shocking outcome, purported basal kannemeyeriiforms are known to jump around like that).

This makes some biogeographical sense, the two Laotian species are each other's closest relatives, and they are in turn closest to a species from northern China, very neat. But what's yet more interesting is that these three then grouped together with the two Russian genera Vivaxosaurus and Delectosaurus as successive outgroups, before all together forming a clade with Dicynodon itself at the base.

Liu identified this group as the 'core-Dicynodon' clade, and noted that they form a neat little series starting in South Africa (Dicynodon), up through Russia (Vivaxosaurus and Delectosaurus), North China (Taoheodon) and finally Laos (Counillonia and Repelinosaurus). It's tempting to see this as recording the geographic dispersal of a clade starting with Dicynodon in southern Gondwana, migrating and speciating northwards into Laurasia and finally ending up in Laos. Laos was mostly isolated from mainland Pangaea at the time on its own tectonic block, so the presence of these two dicynodonts there could mean that a land connection between the South China block and the Indochina block existed by the latest Permian.

Under this arrangement Taoheodon seems to be a sort of 'missing link' between the European and Laotian dicynodonts, both cladistically and geographically. It's a very neat and tidy—and thus, appealing—idea, but at the same time it's entirely possible that this is just a quirk of the existing data used in the phylogenetic analysis, and that the addition of more data in the future will shoot it down as nothing more than a red herring (surely Permian dicynodontoids wouldn't make it that easy for us). Whichever way it turns out, Taoheodon demonstrates that even an unassuming new species can potentially be key parts in making evolutionary relationships click.

My attempted decompression and life reconstruction of Taoheodon.

(*P.S. The Laotian dicynodonts were intriguingly suggested to date from the earliest Triassic based on radiometric dating of the rocks they were found in, which would make them the only Dicynodon-esque dicynodonts to survive into the Triassic (for Counillonia, at least). However, there's been some quibbles over the precision of this dating, and Liu further considered it unlikely that Laos was even habitable to dicynodonts in the earliest Triassic, and so combined with the close affinity to known Permian genera like Taoheodon, he suggested that the Laotian dicynodonts were more likely to be from the Late Permian.

However, another paper (Romano et al. 2020) has been published since then that reviewed the global distribution of Early Triassic tetrapods. From their observations, they concluded that the Early Triassic equatorial "death belt" was less extensive than previously thought, restricting it down from reaching 30° north to just 15°. The exact position of Laos during the Early Triassic is, to my knowledge, not entirely pinned down yet, so maybe, maybe, the Laotitan dicynodonts were just north enough to skirt the "death belt"...assuming they're even Triassic in age after all.)

References

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

Olivier, C.; Battail, B.; Bourquin, S.; Rossignol, S.; Steyer, J.-S.; Jalil, N.-E. 2019. New dicynodonts (Therapsida, Anomodontia) from near the Permo-Triassic boundary of Laos: implications for dicynodont survivorship across the Permo-Triassic mass extinction and the paleobiogeography of Southeast Asian blocks. Journal of Vertebrate Paleontology. 39(2): e1584745.

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, In press, p.103331.

Stable Isotopes and the Ecology of Endothiodon

It's been a pretty dry year for dicynodonts so far. There hasn't been much in the way of new dicynodont-centric research to write about, so I've been re-writing few post ideas scribbled drafted before Double Dog Teeth was made to compensate. But fortuitously, a new paper was published at the end of last month to help break up that dry spell! And what's more, it concerns the ecology and lifestyle of one of the strangest dicynodonts out there.

Outside of specialist literature, the diet and lifestyle of dicynodonts is often broadly simplified down to "herbivorous" and rarely elaborated upon. However, there is much more nuance to dicynodont diets than this, as evidenced by the variation in their jaws, teeth, and beaks which implies that many dicynodonts were often specialised in how they acquired and processed their food, and so were almost certainly feeding on different specific foodstuffs. One of the most puzzling in this regard is the large and tuskless-but-toothy Endothiodon bathystoma from the middle Permian of South Africa, Malawi, Mozambique, Tanzania, Zambia, India and Brazil (it got around).

The strange, strange skull of Endothiodon.
Ghedoghedo, public domain.

Endothiodon has been a bit of enigma regarding its diet because its jaws are so unlike any other known dicynodont. It flies in the face of conventional dicynodont anatomy by possessing prominent rows of post-canine teeth in both its upper and lower jaws, that were constantly and rapidly being replaced to boot (and in waves, no less!). Calling them post-canine teeth is a bit of misnomer though, since Endothiodon bathystoma lacks 'canines' (i.e. tusks) altogether,* and indeed the tooth-row in the upper jaw smoothly continues partly onto the back end of the premaxilla for an extra two teeth as if tusks were never there.

Endothiodon still has a beak at the tips of its jaws, but its lower beak is remarkably deep and hooks upwards into a very prominent point that slots into a vaulted palate. What's more, the teeth of Endothiodon seem to change shape as they grow up, going from simple, conical shapes (similar to the postcanines in other dicynodonts that have them) to being compressed and serrated, leading some authors to suggest that Endothiodon were more omnivorous as juveniles, possibly feeding on insects and other invertebrates before transitioning to full herbivory as adults.

(*I specify Endothiodon bathystoma on account of the recently described Tanzanian species Endothiodon tolani actually having (incipient) tusks. Who'd'a' thunk?)

Naturally, the peculiarities of Endothiodon has led to much speculation and analyses of its anatomy to try and figure out just what it was doing with itself. Suggestions for its diet have included using its strange hooked beak for grubbing up roots, vegetables and invertebrates from the soil like a garden hoe, a specialist browser of dense, high quality riparian vegetation, and even a highly specialised diet of conifer cone seeds that it extracted with its strangely shaped jaws and vaulted palate. All of these previous analyses have been derived from the animal's functional morphology alone. A newly released paper by Kévin Rey and colleagues (Rey et al., 2020) published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology tackles this conundrum with a novel technique: stable isotope analysis.

A quick rundown for the uninitiated: stable isotopes are variants of chemical elements that do not decay or alter into another isotope or element over time. These isotopes are present in the living tissues of organisms, and the ratio of isotopes for a particular element depend on the conditions that the organism lived in. Because they don't decay, these isotopes remain in these concentrations even after fossilisation, meaning that they record the isotope values from when the animal was alive. In analysing fossils, the values of oxygen-18 (¹⁸O) and carbon-13 (¹³C) relate to the water-content of animals and the vegetation consumed by them (or in the case of predators, the ¹³C signatures associated with their prey), respectively. These kinds of analyses have probably had the most publicity from their utility in determining the lifestyle and diet of dinosaurs, such as implying semi-aquatic habits for spinosaurs, and the recent identification of niche partitioning between juvenile and adult Deinonychus. As is often the case, this relatively new method for analysing fossils was pioneered with dinosaurs, and has since been trickling its way out into the study of other fossil groups, and here we are with dicynodonts.

Rey et al. (2020) studied the stable isotope values of six Endothiodon specimens spanning different age groups, nine specimens of the more standard dicynodont Tropidostoma, and an unidentified pareiasaur (a parareptile) for good measure. Curiously, Endothiodon was found to have lower δ¹⁸O values consistent with a higher water turnover in its body, meaning Endothiodon was either a frequent drinker or spent a lot of time around water, or was perhaps even semi-aquatic, similar to living hippos. Whichever way, it suggests Endothiodon was closely tied to water and had to occupy habitats where it was constantly available. The difference in oxygen isotope values compared to the pareiasaur is particularly interesting in light of other recent studies that found the stable isotopes of contemporary pareiasaurs, at times suggested to be semi-aquatic, to be more like those of terrestrial animals.

A reconstruction of the Usili Formation in Tanzania by Emilio López-Rolandi, featuring Endothiodon, a gorgonopsian, and the archosauromorph Aenigmastropheus in the foreground. It's a gorgeous painting, but perhaps doesn't represent the kind of habitat where you'd likely find Endothiodon. CC BY 2.5

As for the carbon isotopes, they also record a significantly lower percentage of ¹³C than in Tropidostoma. Now, there is the possibility that such a difference was due to changes in vegetation over time, as Tropidostoma is known from the younger rock layers above those of the studied Endothiodon. However, ¹³C isotope analyses of the little burrowing dicynodont Diictodon from both rock units were available to act as a sort of control to compare them to, and assuming the diet of Diictodon didn't drastically change over that time, they record a decreasing trend in the ¹³C value of plants, meaning that the disparity between Endothiodon and Tropidostoma was likely even greater!

So what does that mean? For one, it could rule out the hypotheses that Endothiodon specialised in conifer cones, roots, tubers, or any other heterotrophic (non-photosynthesising) parts of plants (i.e. the roots, tubers, seeds, and fruits), as these regions are enriched in ¹³C compared to the leaves. Instead, these values may support the hypothesis that Endothiodon was specialised for feeding on soft riparian vegetation, which have lower ¹³C values compared to vegetation found in drier environments away from rivers and other sources of water. This would explain the disparity between Endothiodon and the more typical, browsing Tropidostoma, and is also consistent with the oxygen signatures suggesting a close affinity with water. Altogether, it paints a picture of Endothiodon as a water-dependent herbivore with a preference for soft, probably riparian vegetation that presumably spent much of its time around rivers and lakes, perhaps even in them.

Admittedly, this still doesn't answer the question of why Endothiodon was so weird. Was there a particular kind of riparian vegetation that it was feeding on that led to its peculiar jaws and teeth? Did this require it to feed in a particular way? The prospect of Endothiodon being semi-aquatic and feeding in or under water is tempting, although I have to admit that it doesn't look particularly specialised for doing so. Its nostrils and eyes aren't set particularly high up on the skull, and there's not much in the way of its skeleton to suggest it was a good swimmer, or even designed to be negatively buoyant. That being said, those deep lower jaws and vaulted palate are both features associated with large tongues and suction feeding in other extinct and extant tetrapods that are known or are suggested to feed in water, including some without any aquatic adaptions at all (Deinocheirus, anyone?), so maybe there's something to this after all...?

Of course, that's all frivolous speculation, and I don't want to turn this into a place for pushing unorthodox ideas about dicynodonts, but hey, maybe?

Endothiodon bathystoma, reconstructed as a water-loving browser of riverside vegetation (apologies to any palaeobotanists for whatever vegetative abomination I conjured up here, I am very naive to Permian vegetation).

There's clearly much more to learn about Endothiodon, and a lot more worth writing about as well (I didn't even cover what its inferred jaw mechanics, inner ear, and postcrania suggest about its lifestyle), so Endothiodon will no doubt show up again on the blog some time in the future. There's also still much to learn from stable isotope analyses on dicynodonts. Relatively few species of them have been studied in this way so far, with even fewer having been used to look into their detailed ecologies, and even preliminary studies like this one can turn up some intriguing possibilities.

References

Amiot, R., Buffetaut, E., Lécuyer, C., Wang, X., Boudad, L., Ding, Z., Fourel, F., Hutt, S., Martineau, F., Medeiros, M. A., Mo, J., Simon, L., Suteethorn, V., Sweetman, S., Tong, H., Zhang, F. & Zhou, Z. 2010. Oxygen isotope evidence for semi-aquatic habits among spinosaurid theropods. Geology, 38(2), 139-142.

Canoville, A., Thomas, D. B., & Chinsamy, A. 2014. Insights into the habitat of Middle Permian pareiasaurs (Parareptilia) from preliminary isotopic analyses. Lethaia, 47(2), 266-274.

Cox, C. B. 1964. On the palate, dentition, and classification of the fossil reptile Endothiodon and related genera. American Museum of Natural History 2171.

Cox, C. B. 1998. The jaw function and adaptive radiation of the dicynodont mammal-like reptiles of the Karoo basin of South Africa. Zoological Journal of the Linnean Society 122, 349–384.

Cox, C.B. and Angielczyk, K.D. 2015. A new endothiodont dicynodont (Therapsida, Anomodontia) from the Permian Ruhuhu Formation (Songea Group) of Tanzania and its feeding system. Journal of Vertebrate Paleontology, 35(4), p.e935388.

Frederickson, J. A., Engel, M. H., & Cifelli, R. L. 2020. Ontogenetic dietary shifts in Deinonychus antirrhopus (Theropoda; Dromaeosauridae): Insights into the ecology and social behavior of raptorial dinosaurs through stable isotope analysis. Palaeogeography, Palaeoclimatology, Palaeoecology, 109780.

Latimer, E. M., Gow, C. E., Rubidge, B. S. 1995. Dentition and feeding niche of Endothiodon (Synapsida;Anomodontia). Palaeontologia Africana 32, 75-82.

Lee, Y. N., Barsbold, R., Currie, P. J., Kobayashi, Y., Lee, H. J., Godefroit, P., Escuillié, F., Chinzorig, T. 2014. Resolving the long-standing enigmas of a giant ornithomimosaur Deinocheirus mirificus. Nature, 515(7526), 257-260.

Rey, K., Amiot, R., Fourel, F., Rigaudier, T., Abdala, F., Day, M.O., Fernandez, V., Fluteau, F., France-Lanord, C., Rubidge, B.S., Smith, R.M. 2016. Global climate perturbations during the Permo-Triassic mass extinctions recorded by continental tetrapods from South Africa. Gondwana Research, 37, 384-396.

Rey, K., Day, M.O., Amiot, R., Goedert, J., Lécuyer, C., Sealy, J. and Rubidge, B.S., 2018. Stable isotope record implicates aridification without warming during the late Capitanian mass extinction. Gondwana Research, 59, 1-8.

Rey, K., Day, M. O., Amiot, R., Fourel, F., Luyt, J., Van den Brandt, M. J., Lécuyer, C., Rubidge, B. S. 2019. Oxygen isotopes and ecological inferences of Permian (Guadalupian) tetrapods from the main Karoo Basin of South Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 538.

Rey, K., Day, M. O., Amiot, R., Fourel, F., Luyt, J., Lécuyer, C., Rubidge, B. S. 2020. Stable isotopes (δ¹⁸O and δ¹³C) give new perspective on the ecology and diet of Endothiodon bathystoma (Therapsida, Dicynodontia) from the late Permian of the South African Karoo Basin. Palaeogeography, Palaeoclimatology, Palaeoecology, In press.

The Dos and 'Donts of Dicynodonts

The skeleton of a bog standard dicynodont, some obscure thing called Lystrosaurus I think...
By Ghedoghedo, CC-BY-SA 3.0

Or: A less-than-brief primer on dicynodonts.

The much delayed first proper post on this blog, partly through a combination of dreaded other obligations and having to cut down and rewrite the thing to keep it down to size, but it's out at last. If you're reading this blog in the first place, I'm presuming you already know a thing or two about dicynodonts. But for those of you who may be less familiar (and for the sake of completeness), here's what was intended to be a quick and rough rundown that outlines some of the basics of what dicynodonts are, their shape and appearance, and their evolutionary history across time.

The quintessential dicynodont Dicynodon lacerticeps
By Nobu Tamura, CC BY SA 3.0

Dicynodonts are synapsids, the group of animals that includes the modern mammals and their extinct relatives that branched apart some 312 million years ago from the line that would lead to living reptiles (birds included). Dicynodonts are among these (sadly) extinct relatives, and it used to be quite common to see dicynodonts and the other non-mammal synapsids be called "mammal-like reptiles" due to their apparent transitional nature. However, this term has fallen out of favour, since early synapsids aren't really reptiles as we define them anymore, either. Nowadays, the collection of non-mammalian synapsids is typically referred to as 'stem-mammals', which means that they are more closely related to the living group of mammals (crown-mammals) than to any other living thing alive today. Short, sweet and simple. As a part of this group, dicynodonts are also stem-mammals.

More specifically, dicynodonts are therapsids, a clade of synapsids more derived (and so, quote—unquote, "more mammal-like") than the earlier pelycosaurs such as the sail-backed Dimetrodon. Dicynodonts are one of the major groups of therapsids, along with the biarmosuchians, dinocephalians, gorgonopsians, therocephalians, and the cynodonts (←mammals go here).

The evolutionary relationships of therapsids is often portrayed like in the cladogram below, but it isn't always like this. Quite how dicynodonts are related to other therapsids is, perhaps surprisingly, still not fully resolved. Do dicynodonts group together with the dinocephalians? Are dicynodonts closer to mammals than the gorgonopsians are, or vice versa? Wherever they go, it is undisputed that dicynodonts are a real, natural group (monophyletic), and aren't just a hodge podge of unrelated animals that all just look similar. As such, they represent one of the 'core' therapsid groups, and were certainly major players in therapsid evolution.

Your typical cladogram of synapsid relationships (except when it's not).

Admittedly, I'm cheating a bit here and have deliberately left out the other anomodonts for simplicity, but I'm saving them for another post, or posts. Promise. Briefly, anomodonts are the wider clade that includes dicynodonts and their closest relatives, of which there are several and all just as peculiar as the dicynodonts are. If you can picture a few branches coming off the dicynodont main branch on that tree there, then that group all together would be anomodonts.

The crazy jaw muscles of dicynodonts,
demonstrated here by the tuskless
Sangusaurus parringtonii.
By Ali Nabavizadeh, from Angielczyk et al. 2017.

In a heavy handed generalisation of their appearance, all dicynodonts have a roughly similar body plan. They were all stout and dumpy quadrupeds, with broad, barrel-shaped bodies, and tails reduced to nubs. Their posture was somewhat functionally intermediary between those of early 'reptilian' amniotes and the later mammals, and many had upright hindlimbs paired with sprawling forelimbs, but variations and exceptions existed—and certainly not to suggest that dicynodont posture was 'halfway' between early amniotes and mammals (it's much more complex then that).

The skull is the most characteristic body part, with unusually short snouts and eyes set far forward on the skull to make room for truly massive jaw muscles behind them. As synapsids, dicynodonts only have one opening behind each eye (the temporal fenestra) to house these jaw muscles, but you'd be forgiven for thinking dicynodonts had more than one hole there at a glance. In dicynodonts, the zygomatic arch (a bridge of bone rimming the temporal fenestra) is often massively flared out from the skull and arched high over the cheeks, creating multiple areas of expansive muscle attachments. To go with these exceptionally powerful jaw muscles, most dicynodonts had some form of horny, keratinous beak, much like (to use that oft-repeated comparison) a tortoise's for cropping, slicing and chewing vegetation. And of course, their upper jaws often sported a single pair of prominent, tusk-like teeth.*

(*Of course, they don't all just have tusks, some have additional 'normal' teeth in their jaws, others have no tusks at all, and one or two have only 'normal' teeth but no tusks! 'Anomodonts' indeed.)

The skull of Dicynodon lacerticeps drawn
by Richard Owen in 1845, showing off the
original double dog teeth.

It was these tusks that got them their name. 'Dicynodont' means 'two canine-tooth', or more literally 'two dog-teeth'*, in reference to those characteristic tusks.** They were named in 1859 by this boffin called Richard Owen—you may have heard of him for that "Dinosauria" thing he did beforehand—based upon the eponymous Dicynodon itself, which he had named some years before in 1845.

Dicynodon would go on to become a bit of a taxonomic monster over the next century and a half (another tale worth chronicling another time), but at the same time Dicynodontia as a whole proliferated with an abundance of new taxa being described since Owen named them, branching off into numerous different lineages and families of varying sorts, even with the rampant lumping at hand. Suffice to say, from their fossil record it's clear that dicynodonts were an enormously successful group, highly diverse and impressively disparate for a group of herbivores with a reasonably conserved body plan.

(*Wink)

(**While I'm at it, the 'canines' of dicynodonts aren't really canines at all, in the sense that they're not homologous with the canines of other therapsids, including ours. Technically they're called caniniforms, but the name dicynoniformodonts is a lot less catchy.)

Dicynodonts first appear inconspicuously in the fossil record during the Middle Permian in South Africa, and then promptly exploded across Pangaea from the Middle Permian all the way through the Late Permian. During the Middle Permian, dicynodonts were fairly abundant but still smaller bit-players in their ecosystems, with big tapinocephalian dinocephalians occupying the role of large herbivores (though tell that to Endothiodon). Once the dinocephalians kicked the bucket, however, dicynodonts rapidly became the dominant herbivores on the scene, and by the Late Permian there were some very big dicynoconts indeed—some Permian giants such as the geikiid Rhachiocephalus had skulls that exceeded 1 metre in length! Permian dicynodonts came in all shapes and sizes (while adhering to the standard dumpy shape, I mean), and had adapted their teeth and beaks for a surprising manner of different lifestyles and diets. They even included some very small, mole-like forms that were evidently fossorial—adapted for digging and living in underground tunnels.

Deep in the dark, cramped tunnels of its burrow, Cistecephalus is coming for you...
...if you happen to be a worm. By Fabio Alejandro (Dragonthunders) (used with permission).

Then, like pretty much all of everything at the time, dicynodonts take a hard hit during the end-Permian mass extinction, (a.k.a. "The Great Dying", "The Big Die", and various other names) but quite impressively manage to make a comeback and ride a second wave of diversification during the Triassic. This was in direct contrast to almost all other therapsids, which died out during or shortly after the extinction, apart from the mammal-antecedent cynodonts (some dog-teeth solidarity there).

In fact, no less than five distinct lineages of dicynodonts would survive the extinction, but only one would go on to re-diversify during the rest of the Triassic, the kannemeyeriiforms. While not as ecologically diverse as their fore-bearers, the kannemeyeriiforms would nonetheless prosper as abundant large herbivores across the globe through the during Middle Triassic, diversifying into various distinct lineages. Alas, by the Late Triassic only one family was left, and while they remained geographically widespread, their diversity was gradually waning. Ultimately, they would die out just a few million years short of the end of the Triassic with little fuss or fanfare. A somewhat anticlimactic end for a lineage that survived the mother of all extinctions just 50 million years earlier.

A highly rigorous and authentic reconstruction of the end-Permian mass extinction through the eyes of Lystrosaurus (there's that name again), experience the horror for yourself here!

The tragic extinction of the dicynodonts during the end of the Triassic is itself ultimately a bit of a mystery, but they may simply have been a victim of the tumultuous climatic changes going on during the Late Triassic that reshuffled and reshuffled again the dynamics of Late Triassic ecosystems. Alternatively, it's been suggested that competition with herbivorous archosaurs was to blame. Whatever the cause, I think we can all completely agree that it's really unfair they didn't make it.

Regardless of how they met their demise, the dicynodonts left an impressive legacy behind; a hugely abundant fossil record spanning two eras and a mass extinction or two, a highly modified and specialised anatomy that made them one of the most common and successful herbivorous amniotes ever known, and presenting a staggering diversity of forms and function.

There's so much more I could talk about for dicynodonts, all the innovative specialisations in their jaws, beaks and teeth, what studies suggest about their metabolism, what their external appearance may have looked like in life. But this post is already long enough as it is, and I think this is a good spot to end it at. That and I wouldn't want to spoil the rest of the blog, after all.

References

Kemp, T.S. (1982). "Anomodonts". Mammal-like reptiles and the origin of mammals. Academic Press. London.

King, G.M. (1990) The Dicynodonts: A Study in Palaeobiology. Chapman and Hall. London and New York.

Two Blog-Teeth

Welcome to Double Dog Teeth, a blog dedicated to any and all things dicynodont (and that includes their other anomodont cousins)! Now, I am by no means a professional of any sort (yet...), so this is entirely a passion project for a group of fossil animals I happen to have found myself (obsessed with) awfully fond of. And for, in my opinion, good reason! Dicynodonts and their closest kin are an impressively numerous, diverse and relatively long-lived group of animals, and have a long, rich taxonomic history to boot. The idea for a blog was almost inevitable, and as far as I can tell, the niche was open.

By and large, Double Dog Teeth will be for talking about any new and past dicynodont focused (or at least dicynodont-adjacent) papers published, tracing the history of dicynodont research, and for digging up any interesting and unusual ideas about dicynodonts from the past odd century and a half.

The recently described mid Triassic stahleckeriid Ufudocyclops mukanelai Kammerer et al., 2019.

Plus I'll be drawing them too sometimes, so stick around if you like what you see.