An illustration of Pachydectes, an extinct relative of early mammals, standing with one foreleg raised against a teal background. It's a quadrupedal animal with a semi-upright posture, looking sort of like a lizard halfway into turning into a dog. Its boxy head sports multiple knobbly yellow bony protrusions. There are blue markings around its eyes and down the sides of a low crest along the top of its snout, and on each side of its snout is a large rounded bulge colored bright red. The rest of its body is dark brown, with a partial covering of greyish fur-like fuzz.

Modern mammals are the only living representatives of the synapsids, but back during the Permian there were numerous other evolutionary branches – first the pelycosaurs, and later their descendant the therapsids.

Some of the first non-mammalian therapsids were the biarmosuchians, mid-sized carnivores with a more upright posture than their pelycosaur ancestors. They had large canine teeth in their jaws and powerful bites, and some of them also developed elaborate ornamentation on their skulls, with various bony bumps and crests adorning their faces.

Pachydectes elsi was a 1.5m long (~5′) biarmosuchian living in what is now South Africa during the late Permian, about 265 million years ago. Bone texture indicates its head ornamentation was covered by either tough thickened skin or a keratinous sheath, and the large bulbous bosses on the sides of its snout had a particularly rich blood supply, suggesting these structures could have been continuously growing throughout its entire life.

But despite how well-protected it looked, Pachydectes’ skull was actually relatively fragile and wouldn’t have been able to withstand the impact forces of using its headgear for fighting or defense. Instead it may have been mostly used for visual display – and the blood supply to the snout bosses might even have given it the ability to “blush” them if they had a soft-tissue covering.


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