Evolutionary trends and the origin of the mammalian lower jaw

Combining Methods.-Both stratigraphy- and phylogeny-based methods have been used to examine patterns of morphological change in fossil lineages (Gingerich 1976; Benton 1990; McShea 1994; Wagner 1996). Importantly, the potential weaknesses of either approach might be overcome by using both methods in a study. For example, if cladistic estimates of synapsid phylogeny have been led astray by rampant homoplasy, then the stratigraphic distribution of the taxa may yield a more informative measure of relatively primitive and derived taxa. Conversely, if the fossil record does not accurately portray the first appearances of synapsids because preservation rates vary widely, then phylogenetic measures might yield a more reliable sequence of branching events. The concordant results found in this study suggest that the synapsid fossil record is relatively well sampled and that the cladistic hypothesis of synapsid relationships presented here is in line with the distribution of fossil finds (Sidor and Hopson 1998).

Conclusions

The prevalence of homoplasy in synapsid evolution has been a hotly contested topic (Kemp 1988a; Rowe 1988; Hopson 1991a). Hopson (1994: p. 212) suggested that although "[t]he polyphyletic origin of mammals is no longer a tenable hypothesis. . . this is not to say that parallelism and convergence have not been significant aspects of pre-mammalian synapsid evolution."

The present study supports the following main conclusions:

1. The lack of a well-supported phylogeny has exaggerated previous estimates of morphological convergence or parallelism in the synapsid fossil record. The hypothesis of multiple therapsid groups arising independently from pelycosaur-grade ancestors (e.g., Olson 1962; Boonstra 1972) necessitated rampant homoplasy and are now considered untenable (Rubidge and Sidor 2001). Certain lower jaw characteristics and proportions are better viewed as broadly distributed synapomorphies indicative of common ancestry.

2. Despite the striking differences between the lower jaws of basal synapsids (i.e., "pelycosaur") and mammals, mandibular evolution within synapsids was predominantly conservative. Except for dicynodont anomodonts, most therapsid subclades do not acquire substantial morphological novelty in their lower jaw structure.

3. The area of the dentary and postdentary regions scales either isometrically or with slight positive allometry when compared with jaw length. This suggests that body-size trends are not sufficient to drive the reduction of the postdentary bones in synapsid evolution. Importantly, when compared with other synapsid subgroups, cynodonts are characterized by smaller-than-predicted postdentary areas.

4. Selection acting to decrease the size of the postdentary bones, and thereby improving high-frequency hearing, is still the most tenable mechanism for the evolution of the mammalian lower jaw (Allin 1975; Allin and Hopson 1992). However, this mechanism by itself has difficulty explaining the converse pattern in anomodont therapsids (i.e., decreasing the size of the dentary and increasing the size of the postdentary bones).