Evolutionary trends and the origin of the mammalian lower jaw

The pattern depicted in Figure 6A confirms that the earliest-occurring, pelycosaur-grade taxa had the relatively smallest dentaries and largest complement of postdentary bones, and that the latest-occurring synapsids (e.g., tritylodontids and mammals) had mandibles that were almost exclusively formed by the dentary. Importantly, both the maximum and average dentary size increased over time in this study sample. However, it is interesting to note that several late-occurring synapsids retained relatively small dentaries.

Table 1 displays the results of a series of analyses that examine evolutionary patterns within several synapsid subclades. These analyses show that a significant positive correlation between DI and AR is nearly uniformly present in those clades encompassing mammals (e.g., Synapsida, Therapsida, Theriodontia, Cynodontia). Probainognathia is the exception to this pattern, but this may be due to the relatively few intervals that this clade spans. In contrast, clades not encompassing mammals (i.e., side branches such as anomodonts) generally have nonsignificant correlations. This crucial disagreement suggests that only the ancestral lineage leading to mammals (i.e., along the backbone of the cladogram) shows a consistent dentary enlargement (see below), and that clades budded off from this line retained their ancestral proportions but did not systematically continue the trend. It is worth noting that pelycosaur-grade synapsids show little indication of directionality, even though they span seven long intervals (ARs 1-7; Late Carboniferous to early Middle Permian, or approximately 35 Myr) and represent the primitive morphotype from which all subsequent change was derived.

Phylogenetic Results.-Figure 6B plots the relationship between DI and each taxon's cladogram position, as measured by CR (see Table 2 for complete results). It is clear from this graph that the degree to which a synapsid clade shares ancestry with mammals has a strong, positive relationship with that clade's average dentary size. However, when the inferred primitive condition for each consecutive clade is highlighted (filled circles), this point does not consistently reside in the left tail of that clade's range of DI values. This position suggests that diversification within each synapsid subclade expanded the range of DI values but did not uniformly increase the relative contribution of the dentary.

Directionality within subclades is more fully considered in Figure 6C (and Table 3), which contrasts the number of branch points separating each taxon from the root of the cladogram (patristic distance; PD) with its DI. Because stratigraphic and phylogenetic position show a strong relationship in synapsids (Sidor and Hopson 1998), Figure 6C is very similar to 6A. Taxa diverging relatively early (i.e., with low PDs) tend to have small dentaries, whereas phylogenetically more derived taxa show a wider range of values. The expanding range of values observed at high PDs can be attributed to the persistence of small-dentaried anomodonts (plus signs) with theriodonts (filled circles) that consistently enlarge the dentary. However, just as with the stratigraphic analyses, the individual theriodont subclades that lack mammals as a subgroup lack a corresponding trend (Table 3). Again, this suggests that increasing the dentary size was not a universal feature of synapsid evolution.