View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002FIGURE 5—Diagram illustrating the geometricdifferences in skull, jaw and tooth morphologyamong: 1, a very derived gorgonopsid,Dinogorgon; 2, the unusual therocephalianMoschorhinus, which represents a convergencewith the gorgonopsid cranial design; and 3, thelarge Triassic cynodont Cynognathus. Thisillustration charts the change from a largely KI bite(with some SP component)—Dinogorgon—to anSP bite with very little KI component—Cynognathus. Ventral views of the anterior palatesare shown to the right. Note the parabolic incisorarcade of gorgonopsids, and the peculiarly felid-likestraight incisor arcade of Moschorhinus along withits palatal fenestrae (for muscular attachments). InCynognathus an almost complete osseoussecondary palate has formed. The flat sabers ofthe gorgonopsid can be contrasted with the roundelongate ‘spikes’ of Moschorhinus. See text fordetails. Scale bar = 100 mm. Skulls not to scale.resemble modern-day mustelids (weasels) andviverrids (civets), and probably behaved more likemodern hypocarnivorous/omnivorous viverridsthan specialized hypercarnivorous mustelids.The largest carnivorous cynodont,Cynognathus, defines the Cynognathus Zone of theLower-Middle Triassic (succeeding the LystrosaurusZone), and has a disproportionately large, robustskull (Broom, 1932; Kemp, 1982). More than 400mm long, its proportions are similar to those ofextant canids, characterized by a long rostrum. Thedentition is broadly similar to that of Galesaurusand Thrinaxodon, with pointed incisors, welldevelopedcanines, and a long row of sharp, recurvedpostcanine teeth. There are two striking features ofthe Cynognathus skull—both the lower jaw andzygomatic arch are extraordinarily deep vertically(Fig. 5.3). The zygomatic arch is so deep that theorbit appears as a simple hole ‘punched’ throughthe ‘plate’ formed by the zygomatic arch (Kemp,1982). Application of lever mechanics to theCynognathus skull reveals a very powerfultemporalis musculature and an expanded massetericcomponent. There is little KI component to the bite,but the SP constituent must have been extremelypowerful. This and the overall robustness of the skullsuggest a hyena- or bear-like design. However,Cynognathus did not have the bone-crackingpremolars or carnassial blades of modern hyaenids.Its trenchant, recurved postcanine teeth, in concertwith this strong SP bite and huge lower jaw, indicatethat Cynognathus used its postcanine teeth to a muchgreater extent than perhaps all previous synapsidpredators (IJ, pers. obs.).Later cynodont predators are thechiniquodontids of the Middle and Late Triassic.Their skull dimensions (Romer, 1969a, 1969b) rangefrom those of a modern coyote (Probelesodon, Fig.2.4) to those of a large hyena (Chiniquodon andBelesodon). Analysis of jaw muscle biomechanicsindicates that the horizontal component of the jawmuscle vectors in cynodonts increased throughoutthe lineage and served to reduce reaction forces atthe jaw joint (Crompton, 1963, 1972; Bramble,1978). At this stage, the jaws of carnivorouschiniquodontids were functioning in an almostidentical manner to those of modern carnivores.276