Spectember 2021 – Quadrupedal Pachycephalosaurs

Today’s #Spectember concept comes from an anonymous submitter:

A drawing of a speculative descendant of pachycephalosaur dinosaurs. It has a domed spiky head and a large quadrupedal body resembling a hadrosaur or ceratopsian. Behind it is a juvenile of the same species, with a flat spiky head, walking bipedally.
Mutatholus insularis

Mutatholus insularis is an unusual pachycephalosaur closely related to the eponymous Pachycephalosaurus.

Isolated on an island with little competition or predators, these dinosaurs have evolved to fill the ecological role of large herbivores, developing a more heavyset quadrupedal body plan with similarities to both hadrosaurs and ceratopsians.

This species is about 30% bigger than its largest mainland relatives in an example of island gigantism, growing up to about 6m long (19’8″) – more comparable in size to some ceratopsids. They have longer snouts with a hooked toothless beak at the front and batteries of shearing teeth further back, voluminous fermenting guts, and elaborate domes and spikes on their heads that are mainly used for visual display and flank-butting.

…Or, at least, that’s what they’re like as adults.

Juvenile Mutatholus almost seem like a different species entirely. They’re proportioned more like their ancestors, moving around much faster on just their hind legs – and they need to, being small enough to be susceptible to predation from azhdarchid pterosaurs. They lack the heavy bony domes, their coloration is more muted and orientated towards cryptic camouflage, their skin is covered in denser defensive spiky scales, and their diet is more generalist omnivorous.

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