What Is a Dicynodont? Tusked Herbivores That Ruled Before Anything Else
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For much of the Permian and Triassic the most abundant large land animals were beaked tusked herbivores on the branch leading to mammals. They survived the worst extinction in the record, recovered, spread worldwide and then disappeared, and almost nobody has heard of them.
The unusual mouth
The group's defining feature is a mouth combining a horny beak with a pair of tusks in the upper jaw and, in most species, no other teeth at all. The beak cropped vegetation, working against a lower beak in the way a tortoise does, and the jaw joint permitted a fore-and-aft sliding motion rather than only a hinge, so the beak sheared food as the jaw closed and slid, which is a mechanism no living animal reproduces exactly. The tusks grew continuously in at least some species and their function is debated, with digging, display, fighting and stripping bark all proposed, and the presence of tusks in some individuals and not others within a species has been read as a sexual difference. The skull is short and deep with large openings for jaw muscles, and the body is barrel-shaped with short sturdy limbs in most species, which is a straightforward herbivore build carrying a large fermenting gut.
How dominant they were
The group's abundance in the fossil record is striking and several features explain it:
- •In many Permian and Triassic assemblages they account for the great majority of large vertebrate specimens, which is a degree of dominance modern faunas rarely show
- •They ranged from burrowing animals the size of a rabbit to species approaching the bulk of a large modern cattle beast
- •They were distributed globally, with fossils on every continent including Antarctica, reflecting a connected supercontinent
- •One genus is a classic piece of evidence for continental drift, since Lystrosaurus fossils occur in Africa, India and Antarctica, which requires those landmasses to have been joined
- •They were among the very few large animals to survive the end-Permian extinction and briefly became overwhelmingly dominant afterwards
- •The last of them, discovered in Triassic rocks in Australia and elsewhere, overlapped in time with the earliest dinosaurs
Surviving the worst extinction
The end-Permian event removed the great majority of species on land and in the sea, and the terrestrial recovery is documented in unusual detail in the Karoo Basin of South Africa. In the immediately following beds, Lystrosaurus is so abundant that it dominates assemblages almost to the exclusion of anything else, which is a disaster taxon pattern, meaning a species that thrives in the depleted conditions following a catastrophe because competitors and predators are gone. Why it survived when so much did not has been argued over, with proposals including burrowing habits providing shelter from extreme conditions, a generalist diet, small body size in the surviving populations, and simple chance. The recovery that followed was slow, with ecosystems taking millions of years to rebuild complexity, and the world that emerged was dominated by different groups, since the archosaurs that became dinosaurs expanded through the Triassic while the dicynodonts declined.
Why nobody knows about them
Their obscurity relative to their importance is worth explaining. They are not dinosaurs, which removes them from the popular category that carries public interest. They lived before the periods that popular culture covers, and the Permian generally has almost no presence outside the sail-backed animals that are themselves frequently mislabelled as dinosaurs. They are not directly ancestral to mammals, sitting on a side branch of the same lineage, so they do not benefit from the interest in human ancestry. They are not dramatic, being herbivores rather than predators. And the material is concentrated in South Africa, Russia and China, which affects which museums display them. The result is a group that was globally dominant for tens of millions of years, supplies key evidence for continental drift and for extinction recovery, and is known chiefly to specialists, which is a reasonable illustration of how public awareness of the past is shaped by narrative rather than by significance.
The takeaway
A horny beak with two tusks and a sliding jaw joint that sheared as it closed is an arrangement nothing alive reproduces. The group dominated large vertebrate assemblages worldwide for tens of millions of years, and Lystrosaurus fossils in Africa, India and Antarctica are classic evidence that those continents were joined. One species became overwhelmingly abundant immediately after the end-Permian extinction.