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VAN VALKENBURGH AND JENKINS—HISTORY OF SYNAPSID PREDATORSutilized. Broomicephalus probably preyed uponunusually large animals (Jenkins, 1998), and itsunusually flat sabers foreshadow the canines ofscimitar-toothed cats such as Homotherium.Despite the complexity and diversity in LatePermian carnivore guilds, all gorgonopsids becameextinct during the end-Permian event. To someextent, gorgonopsid ecological supremacy came atthe expense of saber-toothed carnivoroustherocephalians, which died out before the lastgorgonopsids and were to a large extent replacedby them as top predators (Jenkins, 1998, 2001a;Jenkins et al., in press).Late Permian (approximately 265–250 Ma):Therocephalians.—Therocephalians overlap the tenmillion-yearspan of the gorgonopsids and extendslightly further into the basal Triassic (Fig. 3). Earlytherocephalians include the families Lycosuchidaeand Scylacosauridae, and both were saber-toothed.Their saber canines are modest compared with thoseof large gorgonopsids, although in biggerscylacosaurid genera and in Lycosuchus they arevery pronounced (Fig. 2.3). The therocephalian SPbite is larger than that of gorgonopsids due toenlarged adductor chambers for the temporalismusculature (Kemp, 1982). The anteriorpterygoideus muscle fibers inserted relatively closeto the jaw joint, thus reducing the mechanicaladvantage of the KI system relative to that ofgorgonopsids but increasing the speed of closure(Jenkins, 1998). The palates of early therocephalianshave a large number of discretely located and welldevelopedscarf joints that reduce biting stresses(Jenkins, 1998; Jenkins et al., 2002). Lycosuchushas a shorter snout than most scylacosaurids suchas Glanosuchus, Scymnosaurus, and Ptomalestes,and its sub-rounded shape suggests strength intorsion in association with proportionally largesabers (Jenkins, 1998). The short snout also gives agreater mechanical advantage for bites at the canines.The therocephalian group Euchambersiidae(Hopson and Barghusen, 1986) contains the highlydistinctive genera Euchambersia and Moschorhinus.Both these forms are characterized by very short,robust rostra and canines that are rounded in crosssection.Euchambersia is known from only twopartial skulls; Moschorhinus is much better known.Moschorhinus skull dimensions range from thoseof a large monitor lizard to those of a lion. It is amost unusual therocephalian and shows a largetemporal fenestra, convex palate, and laterallycompressed incisors set in a shallowly curvingarcade, in addition to the short, strong snout(Fig. 5.2). The lower jaw has remarkable caninesand a symphysis that is expanded more than in anyother therocephalian (Jenkins, 2001b). The caninesare as long as the sabers of all except the mostderived gorgonopsids (Fig. 5.1), but are round incross-section, resembling the spikes used to securesome railway cross-ties. There is no modern analogfor these teeth, the closest perhaps being the hyperelongatecanines of the Clouded Leopard (Neofelisnebulosa) (Fig. 8.2). The combination of long, robustcanines and flattened incisors in a shallow arcadesuggests a felid-like mode of attack (sensuBiknevicius et al., 1996), in which the strong canineswould have sustained a long period embedded inprey (Jenkins, 2001b). This is the earliest knowninstance of this design. Moschorhinus went extinctafter gorgonopsids and was a therocephalianoccupying a gorgonopsid-like ecological niche(Jenkins, 2001b). The powerful symphysis, robustsnout, and extraordinary canines show Moschorhinusto have been a truly formidable predator; why it shouldhave survived when equally powerful and similarlydesigned gorgonopsids went extinct is probably dueto some as-yet-unknown feature of its physiology.Once Moschorhinus had disappeared, the roles of toppredators were taken up by cynodonts (Fig. 3).Permo-Triassic (approximately 255–225 Ma):Cynodonts.—Small cynodonts in the basal-mostTriassic (Lystrosaurus Zone) are represented byGalesaurus and Thrinaxodon (Broom, 1932). Theyshow expanded temporalis musculature and thefirst signs of the masseteric musculaturecharacteristic of modern mammals (Kemp, 1982;Hopson, 1991; Hopson and Barghusen, 1986). Thesnout is short and the canines dog-like; thepostcanine teeth are numerous and form a seriesof curving blades (Kemp, 1982), similar to thoseof the Komodo Monitor (Varanus komodoensis).In general skull proportions and size, they most275