View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002filled roles analogous to those of modern small,long-snouted canids such as jackals and foxes thatprey on both arthropods and small vertebrates.Larger gorgonopsids such as Leontocephalus,Scylacops, Aelurognathus, and Gorgonops (Fig.4.2) display fairly long rostra and skulls with a flatdorsal profile, large canine teeth, and up to fourpostcanine teeth (Kemp, 1969; Sigogneau, 1970;Sigogneau-Russell, 1989). In these genera, theadductor chamber shows the characteristic ‘flaring’to produce expanded areas of origin for thetemporalis musculature. The pterygoid flanges aresituated far anteriorly, allowing for long anteriorpterygoideus fibers that powered a strong KI bite(Jenkins, 1998). A number of even largergorgonopsid genera, such as Arctops (Fig. 4.3),display shorter skulls with a very strongly convexdorsal profile that is reminiscent of the sphenacodontDimetrodon. The convex profile is especiallyefficient for accommodating the stresses imposedduring powerful canine biting (Jenkins, 1998). Inespecially large (skull length about 300 mm),convex-skulled genera such as Arctops, the reflectedlamina of the angular bone is expanded and henceprovided an increased insertion area for acorrespondingly enlarged anterior pterygoideusmusculature. This indicates a very powerful KI biteand the capacity to accelerate the jaws shut from avery wide gape. The saber teeth in some specimensare exceptionally long, and would have necessitateda wide gape to clear the tips of the huge teeth(Jenkins, 1998). Another gorgonopsid ecotype isdefined by the short-snouted genus Arctognathus(Fig. 4.4). This small to medium-sized gorgonopsidhas a box-like robust skull with a deep rostrum anda rather flat dorsal profile; the canines are large,areas of bone associated with the anteriorpterygoideus musculature are expanded, and thesymphysis of the lower jaw is enlarged. Thesedetails suggest a rostrum that was strong in bendingand torsion, powerful KI and SP constituents ofthe bite cycle, and a stable lower jaw (Jenkins,1998, 2001a, 2002b; Jenkins et al., 2002).Gorgonopsids attain their most impressivemanifestation in their most derived members, theRubidgeinae. These were the latest-survivinggorgonopsids and are found solely (so far) in SouthAfrica just below the horizon beds of the end-Permian extinction event (Rubidge, 1995).Rubidgeine gorgonopsids are characterized by asuite of cranial features that evolved for anexceedingly predaceous way of life. The skulls areoften huge, exceeding 400 mm in length. Skullbuttressing is extremely pronounced. The rostrumis broad, pterygoid flanges are situated far anteriorly,the lower jaw is massive, and the symphysis isexpanded more than in other gorgonopsid genera.The incisors are very large, postcanine teeth areabsent, and the saber canines are huge—those ofRubidgea atrox (Fig. 4.5) are longer than the teethof the theropod dinosaur Tyrannosaurus rex(Jenkins, 1998). Rubidgeine gorgonopsids aredefined taxonomically by a posterior ventral flangeon the zygomatic arch (Sigogneau-Russell, 1989),which appears to reflect an enlarged zygomaticomandibularismuscle (Kemp, 1969) that stabilizedthe lower jaw and jaw joint during use of the largesaber teeth (Jenkins, 1998). Within the Rubidgeinae,distinct ecotypes are defined by the huge buttressskulledRubidgea, the very wide-snoutedClelandina, and the convex-skulled, long-sabertoothedDinogorgon and Prorubidgea (figured inSigogneau, 1970; Sigogneau-Russell, 1989). A finalgorgonopsid ecomorph is represented by one ofthe rarest rubidgeines, Broomicephalus (Fig.4.6).Broomicephalus is the only known gorgonopsid inwhich maximum skull width exceeds total skulllength (Sigogneau, 1970; Sigogneau-Russell, 1989;Jenkins, 1998; Jenkins and Dyke, in press). It hasan unusually broad rostrum, short skull, especiallyflat blade-like sabers, and the largest jawsymphyseal mass of any gorgonopsid. The widerostrum includes an expanded origin area for theanterior pterygoideus musculature, indicating moreof an emphasis on the KI component of its bitecycle than in some other rubidgeines. Associatedwith this are rather small adductor chambers,suggesting a reduced SP constituent. Wide jawsare very stable and robust (Covey and Greaves,1994); large jaw symphyses increase thepenetration potential of the sabers and allow bothsides of the jaw adductor muscle force to be274