Evolutionary trends and the origin of the mammalian lower jaw

Paleobiology, Fall 2003 by Sidor, Christian A

Character-based Analyses

The six measurements used above can provide only a limited view of morphological changes occurring within synapsid mandibular evolution. Potentially more informative is quantifying morphological similarity with reference to an exemplar primitive mammal (Morganucodon, in this case) using discrete characters (Appendix 4). Phenetic similarity is an appropriate metric to use in this case because the convergent acquisition of a certain phenotype pertains to net, rather than total, morphological change (Foote 1996). As with the measurement-based analyses, significantly positive correlations between the degree of similarity to mammals and stratigraphic or phylogenetic position would support the hypothesis of a morphological trend toward gaining mammal-like features, whereas non-significant correlations would refute it. Furthermore, negative correlations correspond to increasing dissimilarity; i.e., the morphological modifications experienced by a clade's lower jaw consistently distance it from the mammalian position in morphospace.

Stratigraphic and Phylogenetic Results.-Figure 7 and Table 4 contain the principal results of the discrete character-based analyses, which are remarkably similar to those based on measurements (compare with Fig. 6). This similarity implies that both data sets are capturing a common signal from synapsid evolution. When compared with stratigraphic position (Fig. 7A), Late Carboniferous and Early Permian pelycosaur-grade synapsids begin with approximately 60% of their (comparable) lower-jaw characters matching the condition in Morganucodon (AR 1-6). By the middle of Late Permian times (AR 9), however, therapsid diversification expanded this range of values, with anomodonts becoming increasingly dissimilar to mammals, and theriodonts becoming increasingly similar (presumably through synapomorphy). The "increase in variance" pattern continued until the demise of anomodonts in the Late Triassic (AR 17), whereby only the advanced cynodonts (i.e., the right tail of the distribution) remained. As with the measurement-based results, subclades encompassing mammals typically show significant correlations between AR and the degree of similarity to mammals, whereas side-branches often do not (Table 4).

Figure 7B compares the degree of mammal mandibular similarity against each taxon's CR, with the inferred primitive condition at each point highlighted. An increasingly mammal-like lower jaw is expected to correlate with higher CRs, given that some of the features used in this analysis have been proposed as synapomorphies diagnosing higher-level synapsid clades. An interesting result is the relatively low degree of divergence (i.e., range of values) from the presumed ancestral condition at each CR (filled circles). Only anomodonts, and in particular their derived dicynodont subclade (e.g., Fig. 2D), show substantial morphological divergence. Although I attempted to be as exhaustive as possible in my character selection, doubtless additional characters could be discovered and affect this low degree of subclade morphological divergence.