Spectember #08: Giant Cavy-Ungulates

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Spectember 2020 #08 | nixillustration.com | alphynix.tumblr.com
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Giant Cavy-Ungulates

The present-day Patagonian mara (Dolichotis patagonum) is a long-legged caviid with almost hoof-like digits on its feet, somewhat resembling a rabbit-deer. These unusual rodents are diurnal fast-running grazers, already rather convergent with ungulates – and so it’s perhaps not too surprising that eventually this lineage produced the RODUNGULATES.

[Image: a modern mara, an animal that looks rather like a rabbit-headed deer.]
The ancestral Dolichotis patagonum

Starting out as relatively small animals similar to early horses, rodungulates quickly increased in body size thanks to some of the physiological quirks of their ancestors. Along with both their chisel-like incisors and grinding molars being ever-growing, they were hindgut fermenters specializing in eating very high-fiber diets of tough vegetation – including plants toxic to most ruminants – and breaking it all down in their large intestines. While less efficient than ruminant digestion this system allowed them to feed in bulk and process their food much more rapidly, which at larger body sizes became a significant advantage.

So, much like the proboscideans and indricotheres before them, some of the rodungulates were able to become especially big mammals.

Reaching at least 3m tall at the shoulder, the giant rodungulates range from heavily-built rhino-like forms to slender long-necked high browsers and tapir-like low-level browsers. Most of the largest species are ecosystem engineers similar to elephants, stripping bark, clearing trees, fertilizing soil, and creating areas of open grassland where other species can flourish in their wake.

[Image: a descendant of the mara that looks like a small horse or deer, with four hoof-like toes on its front feet and three on the back.]
Cavihippus velox, a basal rodungulate convergent with early horses


[Image: a heavily-built rodungulate that resembles a capybara with a rhino-like body. It has a thick bony dome on its forehead.]
Pachybara tholussa, a rhino-like rodungulate. The “dome” of thickened bone on its head is used for flank-butting.

[Image: a long-snouted rodungulate with a body plan somewhere between a camel and an indricothere, with long slender legs, a long neck, and a sloping back.]
Caviauchenia longinaris, a high-browsing rodungulate. Similar in body size to giraffes and mid-sized indricotheres, it uses a combination of a long neck and an unusually long snout to reach more vegetation.

[Image: a chunky vaguely tapir-like rodungulate with a tall blunt snout somewhat reminiscent of a moose or a ground sloth.]
The slightly smaller Theriosonax altirhinus is more specialized for browsing, with a semi-prehensile upper lip. Its large nose acts as a resonating chamber for loud calls.

[Image: a different view of Theriosonax‘s head with its mouth open, showing its large chisel-like incisor teeth.]
Like all rodungulates, its rodent ancestry is most obvious from its chisel-like incisor teeth.

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