View - Kowalewski, M. - Virginia Tech

VAN VALKENBURGH AND JENKINS—HISTORY OF SYNAPSID PREDATORSDimetrodon during the Early Permian (Fig. 2.1).At that time, Dimetrodon alone had a craniodentaldesign that was sufficiently robust to disable andconsume large vertebrates; it gave rise to theanatomically progressive therapsids, which includethe saber-toothed genera (Kemp, 1982).So dominant were synapsids as terrestrial apexpredators that no member of any other amnioteclade took this role alongside synapsid killers. Theabundant, large, and formidable temnospondylamphibian Eryops of the Permo-Carboniferous wasperhaps the only possible instance of any nonsynapsidpredatory competition. Other gigantictemnospondyl amphibians were comparable tosynapsids in terms of their habitat dominance, butwere semi- or fully aquatic (Carroll, 1988; Benton,1998), as was Eryops. The history of nonmammaliansynapsids records five ecologicaldynasties of predator communities. Basaltherapsids form a possible sixth but are poorlyknown, and it is not yet feasible to reconstruct acommunity of these carnivores. An ecologicaldynasty of predators is here defined as a singleclade or small number of clades (relative to all thatare present) that fill the large-body-size, highlycarnivorous roles within a community for millionsof years. The five predator dynasties (Figs. 2, 3) ofthe non-mammalian Synapsida, in chronologicalorder, are sphenacodont ‘pelycosaurs’ (Permo-Carboniferous), anteosaur dinocephalians (middleLate Permian), therocephalians (Late Permian),gorgonopsids (Late Permian), and non-mammaliancynodonts (Permo-Triassic). These five dynastiesspan a total stratigraphic range extending from theLate Carboniferous (sphenacodonts) to the MiddleTriassic (chiniquodontid cynodonts).Permo-Carboniferous (approximately 300–270Ma): Pelycosaur-grade synapsids.—In pelycosaurgradesynapsids of the Permo-Carboniferous, theonly large terrestrial predators were Dimetrodon,Ophiacodon (Romer and Price, 1941; Reisz, 1986),and possibly Secodontosaurus (IJ, pers. obs.).Secodontosaurus (Reisz et al., 1992) was a longsnoutedform superficially similar to the muchlarger Ophiacodon. Secodontosaurus has beenconsidered to have been an insectivore that usedits elongate rostrum to reach the invertebrateoccupants of burrows (Reisz et al., 1992). However,its cranial size (280 mm in length) was comparableto that of a mature Komodo Monitor. Its cranialmorphology (narrow, deep, and long) shows a morerobust skull than that of Ophiacodon, a welldevelopedregion for the origin of the anteriorpterygoideus musculature, and stout, sharp teeth.These details reflect true carnivory, not insectivory,for which numerous simple teeth are more typical.Its rostrum was not sufficiently narrow to accessinvertebrate burrows but may have been able toenter those of small tetrapods. Ophiacodon had thelongest skull of any pelycosaur, and some remainssuggest an animal with a skull length in excess of500 mm (Romer and Price, 1941) and a total lengthof more than four meters. It is usually consideredto have been a semi-aquatic piscivore on the basisof its unossified ankle and wrist bones and its dentalarcade of many small conical teeth (Romer andPrice, 1941). Its skull size suggests that tetrapodprey was an option, but this is countered by thestructurally fragile nature of its lower jaws and arostrum that was not particularly strong in bendingand torsion (IJ, pers. obs.). Hence, the only largemega-carnivore in Permo-Carboniferous terrestrialecosystems was Dimetrodon.Dimetrodon species had very deep skulls witha strongly convex dorsal profile and well-developedtemporalis and anterior pterygoideus musculature.The reflected lamina of the angular bone in thelower jaw was the largest of any ‘pelycosaur’ andheralds the expansion of this structure in thetherapsids, where it becomes part of the hearingcomplex (Kemp, 1982). In Dimetrodon it supportedan enlarged anterior pterygoideus musculaturewhich operated a powerful KI bite. Dimetrodonhad a strong but not especially robust lower jawand a characteristic dentition (Romer and Price,1941; Reisz, 1986). The postcanine dentitionincluded fewer but larger teeth in the upper jaw.These teeth are recurved and very sharp withserrated edges; their cross section is ovoid to subrounded,suggesting considerable structuralstrength (Fig. 2.1). The pterygoid flanges in thepalate were huge and robustly designed; their edges271