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Evolutionary trends and the origin of the mammalian lower jaw

Paleobiology, Fall 2003 by Sidor, Christian A

Abstract.-The single bony element forming the lower jaw of living mammals, the dentary, has been interpreted as representing the culmination of a long and gradual evolutionary trend. Numerous fossil nonmammalian synapsids ("mammal-like reptiles") show varying degrees of enlargement of the dentary and concomitant reduction in the postdentary bones. To quantitatively reexamine patterns of morphological change in the evolution of the mammalian lower jaw, measurement and discrete character data were collected from 322 fossil synapsid mandibles spanning Late Carboniferous through Jurassic time. Measurements confirm that the relative contribution of the dentary increased in theriodont (advanced therapsid) evolution with regard to both stratigraphic and phylogenetic position. However, dentary enlargement and postdentary reduction failed to typify all therapsid subclades. Qualitative characters of the mandible were used to quantify morphological similarity with regard to the early mammal Morganucodon. Analyses contrasting stratigraphic and phylogenetic position with mammalian similarity indicate that mandibular evolution was primarily conservative, with only anomodont therapsids evolving substantial morphological novelty. Scaling analyses comparing the area of the dentary and postdentary regions to jaw length uniformly show isometry or slight positive allometry, although cynodont therapsids have a smaller postdentary region than any other therapsid subgroup. These results suggest that body size decreases cannot fully explain the reduction of the postdentary bones. Finally, step size bias was tested as a mechanism for explaining long-term trends. Qualitative data reveal no significant difference in the magnitude of character changes occurring in mammalian and nonmammalian directions.

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Introduction

Mammals are unique among extant vertebrates in possessing a lower jaw (mandible) formed by a single bony element, the dentary. By contrast, the lower jaws of other vertebrates retain a host of postdentary bones (e.g., four to six in most lizards, five in crocodiles and many birds, and typically even greater numbers in fishes). Recorded from rocks dating from over 300 Ma, the mandibles of the earliest nonmammalian synapsids possessed up to seven postdentary bones (Fig. 1), whereas stratigraphically more recent taxa show various stages in the reduction and eventual loss of these bones (Fig. 2) (Romer and Price 1940; Crompton 1963; Allin 1975). The evolutionary fate of the mammalian postdentary bones has been well established; Reichert (1837) used embryological evidence to homologize the incus and malleus of the mammalian middle ear with the quadrate and articular, respectively, of nonmammalian vertebrates. The transformation of several postdentary jaw bones into sound-conducting middle ear bones within synapsids is one of the best-documented examples of a major evolutionary transformation in the vertebrate fossil record (Hopson 1966; Allin 1975; Allin and Hopson 1992; Luo and Crompton 1994). Indeed, synapsid mandibular evolution has come to be regarded as recording a gradual trend whereby enlargement of the dentary occurs at the expense of the postdentary bones (Crompton and Jenkins 1973; Kemp 1982; Hopson 1987). In this study, I use measurement and discrete character data to: (1) quantify the morphological changes that occurred in the evolution of the lower jaw between pelycosaur-grade synapsids and their mammalian descendants, and (2) address several previously proposed hypotheses concerning the nature and magnitude of morphological trends during the first ~100 Myr of synapsid history.

Background to Study Taxa.-Theories of synapsid evolution have traditionally been couched in terms of several adaptive radiations or grades of organization representing successive steps in the mammalian direction. However, a recent proliferation of numerical cladistic analyses has contributed greatly to our understanding of synapsid phylogeny (Fig. 3), and regions of broad consensus are gradually emerging (Rubidge and Sidor 2001).

The earliest occurring and phylogenetically most primitive synapsids are the "pelycosaurs" of traditional terminology. These taxa form a paraphyletic series and are primarily known from Upper Carboniferous to Lower Permian rocks in Europe and North America (Reisz 1986) although several taxa persisted into the Middle Permian in Russia and South Africa (Reisz et al. 1998; Modesto et al. 2001). Sphenacodontids, such as the familiar sailback Dimetrodon, are among the most advanced pelycosaur subgroups (Reisz et al. 1992). All more derived synapsids form the clade Therapsida.

All of the major therapsid clades first appear in the fossil record during the Middle and Late Permian (e.g., Biarmosuchia, Dinocephalia, Anomodontia, Gorgonopsia, Therocephalia, and Cynodontia) and-except for dicynodont anomodonts, some advanced therocephalians, and cynodonts-went extinct in this time interval as well. Therapsids taxonomically and ecologically dominated the end-Paleozoic Pangaean landscape and established the first herbivore-based food chains among vertebrates in the terrestrial realm (Olson 1962; King et al. 1989; Reisz and Sues 2000). The presence of several derived features recently led Laurin and Reisz (1990, 1996) to suggest that Tetraceratops insignis, from the Early Permian of Texas, is phylogenetically the most primitive therapsid (but see Conrad and Sidor 2001).

Evolutionary trends and the origin of the mammalian lower jaw

Paleobiology, Fall 2003 by Sidor, Christian A

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