The Therapsids

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Therocephalians  —  the 'Beast Heads'

The therocephalians quite probably originated in what is now Russia but,  like the dicynodonts, the beast heads spread throughout the Permian world. Most therocephalians were relatively small animals. Some were specialized plant-eaters, others were predators  –  the largest of which would rival their day's apex predator, the gorgonopsid.

Biting Down  —  the Predatory Therocephalians

Large predatory therocephalians, such as the Pristerognathus ('jaw sawn from the earth' - 25 cm skull — below, right)  faced the same challenges as did the gorgonopsids. A powerful set of jaw-closing muscles were needed to inflict the killing bite.

The major jaw muscle attachment area is in the skull opening behind the eyes — the temporal fenestra. Compared with a big gorgonopsid, the temporal fenestra of a therocephalian was proportionately shorter.  Nature must work with what it has.

Jaw Muscles — the Therocephalian Solution
The solution came in two parts. First, a new emphasis on the muscles inside the mouth (red) through their  forward expansion.  Second, the fenestra was expanded upward, forming a distinct 'sagittal crest' that provided added attachment area for the jaw-closing temporal muscles.

As with the gorgonopsids ,  a segment of  those jaw-closing  muscles
( medium blue ) attached  to the outside of  the lower jaw. The primary function of  this outside segment  (lighter blue) was to ensure that the jaw stayed hinged  –  vital when seizing large, struggling prey.  However, this segment of jaw muscle was also a limiting factor,  determining the therocephalian method of  feeding.

The mechanics of the therocephalian jaw hinge prevented them from using the sliding gorgonopsid-style nip-and-swallow technique. Instead, big therocephalians simply sank their front teeth into their kill, twisted their heads back and forth, while yanking away until a swallowable mouthful of flesh was torn free. Crude but it obviously worked. Canines would be too vulnerable to damage to play a role.

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