Premammalian Synapsids

The synapsid lineage that includes mammals split from other amniote vertebrates over 300 Ma ago, during the Late

Carboniferous (Pennsylvanian) in the Paleozoic. Synapsids are characterized by the presence of a lower temporal fen-estra in the skull, among other apomorphies. Their stem taxa, or pelycosaurs, thrived in the Late Pennsylvanian through the Early Permian (Kemp, 1982; Carroll, 1988; Hopson, 1994). Therapsids, a derived subgroup of synapsids, are characterized by enlargement of the temporal fen-estra to accommodate enlarged jaw muscles, a distinctive notch in the angular bone of the lower jaw, several derived features in the limb girdles, and a more anteroventral (near parasagittal) posture of the hind limbs than in the prether-apsid synapsids. Therapsids first appeared at the beginning of the Late Permian and promptly split into several stem groups, which dominated the terrestrial biota in the Late Permian. Cynodonts, a derived subgroup of therapsids, appeared in the Late Permian. Cynodonts are distinguished from precynodont therapsids in having a better-developed secondary bony palate, to separate the nasal passage for breathing from the mouth cavity for feeding, and a larger coronoid process on the dentary, with a definitive masseteric fossa for strengthened jaw adductor muscles, among many other derived features (Kemp, 1982; Hopson, 1994). The advanced cynodonts, such as probainognathids, trity-lodontids, and tritheledontids, achieved many mammallike features in both the skull and postcranial skeleton (see chapter 3). Mammalia (see box, next page) are nested within the advanced cynodonts and are defined here as all descendants of the common ancestor of Sinoconodon and extant monotremes, marsupials, and placentals. Mammals so defined are diagnosed by the dentary condyle to the squamosal glenoid in the jaw hinge and the presence of the petrosal promontorium. The earliest-known and transitional taxa bearing these and other derived features appeared about 220 Ma ago, in the Norian age of the Triassic, possibly as early as the Carnian (Fraser et al., 1985; Lucas and Luo, 1993; Sigogneau-Russell and Hahn, 1994; Datta and Das, 1996; Jenkins et al., 1997; see also figure 1.1). The poorly known Adelobasileus Lucas and Hunt, 1990, is also tentatively included within the Mammalia.

A substantial body of evidence supports the hypothesis that therapsids achieved a higher level of overall activity and more elevated growth rates than pretherapsid synapsids. Brink (1956, 1980) speculated on the basis of

Mammalia are a clade defined by the shared common ancestor of Sinoconodon, morganucodontans, docodontans, Monotremata, Marsupialia, and Pla-centalia, plus any extinct taxa that are shown to be nested with this clade by parsimony analyses. This stem-based definition excludes from Mammalia such advanced cynodonts as chiniquodontids, probainog-nathids, tritylodontids, tritheledontids, or any taxa nested within a subset of the latter cynodont groups. The Mammalia concept here is equivalent to the Mammaliaformes of Rowe (1988: figure 4, but not that of Rowe,1993: figure 10.2).

In their impressive book, Classification of Mammals above the Species Level, McKenna and Bell (1997) adopted a crown-based concept of Mammalia, originally proposed by Rowe (1988) and subsequently accepted by some other authors. Following the definition by Rowe (1988) and McKenna and Bell (1997), several groups regarded here to be mammals (e.g., Morganucodonta, Docodonta, Haramiyoidea) would be excluded from the Mammalia, instead being regarded as nonmammalian mammaliaforms. The status of several other groups—including the best known and most diverse of Mesozoic mammals, the Multituberculata—would be ambiguous under this definition, as their phylogenetic position relative to crown Mammalia has yet to be settled decisively (see chapter 15).

There is a massive body of literature dealing with the merits and problems associated with crown-, stem-, or character-based definitions of Mammalia (see Lucas,1992; Rowe and Gauthier,1992; Bryant, 1994). The definition adopted by us is one of several alternatives. We prefer this inclusive definition of mammals because (1) it is consistent with widespread, traditional usage (e.g., Hopson, 1994); (2) it has the virtue of being relatively stable in its membership contents of both living and fossil taxa (Luo et al., 2002); and (3) it happens to be diagnosed by characters that we view as being biologically significant (see chapter 3). The first two of these criteria, at least, address a primary purpose of taxonomic nomenclature: to promote stability and commonality of usage.

skeletal evidence that some of the nonmammalian cynodonts (advanced "mammal-like reptiles") acquired hair and a diaphragm, which he suggested are indicative of an evolutionary trend toward endothermy. Bakker (1971, 1975) argued that predator-prey ratios of Late Permian-Early Triassic therapsid communities were closer to those of extant endothermic communities than to ectothermic communities. Based on a study of bone histology, Ricqles (1974,1976) concluded that therapsids had a high level of overall activity and high growth rates in comparison to pe-lycosaurs. He argued that nonmammalian therapsids probably had high metabolic rates and were likely to have achieved some kind of endothermy. In comparison to the pretherapsid pelycosaurs, advanced nonmammalian therapsids had a more upright posture, indicating a more active locomotory pattern (Kemp, 1982; Hopson, 1994). Several studies (e.g., Bennett and Ruben, 1986; Hillenius, 1992, 1994) have documented the presence of a maxillary ridge in the nasal cavity in nonmammalian therapsids. This provides indirect evidence for the development of maxillary turbinates, which in turn are related to thermoregulation in extant mammals (Moore, 1981). Hillenius (1994) proposed that thermoregulatory adaptation was developed among cynodonts and even in precynodont therapsids. Although there is broad consensus that therap-sids had elevated levels of overall activity and growth rates, it remains unclear as to whether any nonmammalian therapsids (including cynodonts) developed the same homeo-thermic (constant body temperature) physiological functions as those seen in modern mammalian insectivores (Crompton et al., 1978).

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