Evolutionary trends and the origin of the mammalian lower jaw

Paleobiology, Fall 2003 by Sidor, Christian A

Presuming that the characters used herein are an unbiased sample from the total pool of possible lower-jaw characters, an interesting pattern emerges: except for caseasaurs (CR 1), the inferred primitive condition at each CR lies at the mammal-like (right-hand) tail for non-theriodonts (CRs 2-10), shifts to an intermediate value within gorgonopsians and therocephalians (CRs 11 and 12), and then lies on the non-mammal-like (left-hand) end for cynodonts onward (CRs 13-22). This implies that morphological change within each subclade went from being primarily divergent, within non-cynodonts, to convergent, within cynodonts (but see below). It is also worth noting that discontinuities between the inferred ancestral condition between adjacent CRs could indicate gaps in the fossil record, if synapsid evolution was predominantly monotonic (Sidor and Hopson 1998), or variation in the rate of character acquisition, if taxon sampling probabilities were relatively constant (Sereno et al. 1999).

Figure 7C plots the number of inferred branch points from the root of the cladogram to each terminal taxon (PD) against the degree to which each taxon's lower jaw is similar to that of Morganucodon. Taxa positioned near the base of the tree (with low PDs) hover around a mammal mandibular similarity of 60%, whereas anomodonts and nonmammalian theriodonts expand this range by roughly 20% in negative and positive directions, respectively. Thus, prior to the early mammal Sinoconodon, the lower jaws of advanced nonmammalian cynodonts such as Probainognathus maintained substantial differences from those of early mammals. This plot most clearly depicts the Y-shaped pattern hinted at in several other graphs (compare Figs. 6A,C and 7A,C), where anomodonts and theriodonts morphologically diverge from one another after an early phase of nondirectionality within more basal synapsids. The gap between the branches of the Y is due to the relatively low diversity and short temporal duration of the clades with intermediate similarity values (viz. gorgonopsians and therocephalians).

Scaling Patterns

Synapsids diversified into an impressive array of body sizes and presumed ecologies during the Permian and Triassic. However, the possibility that changes in body size were important factors in the reduction of the postdentary bones has received scant attention in the literature, instead, most analyses have focused on the detailed morphology of several exemplar taxa assumed to be phylogenetically close to the line leading toward mammals (Allin 1975).

Figure 8A compares dentary area and total jaw length for the 160 synapsids with both measurements (Appendix 5). A line with a slope of two indicates isometry in this case because an area is being plotted against a linear measurement. A reduced major axis regression (RMA) indicates that synapsids as a whole conform to this expectation (slope = 2.031 or - 0.101). Furthermore, various synapsid subgroups show either near isometry or slight positive allometry (Table 5).