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.