Evolutionary trends and the origin of the mammalian lower jaw

Paleobiology, Fall 2003 by Sidor, Christian A

The hypothesis that disparate groups of synapsids independently acquired mammallike characteristics has a long pedigree (Olson 1944, 1959, 1962; Romer 1965; Simpson 1959). However, some examples of "convergence" probably arose from the taxonomic framework accepted at that time-one that recognized paraphyletic and polyphyletic grades of organization (Hopson 1994). The application of cladistic methods to synapsid systematics has dispelled some cases of morphologic homoplasy as unnecessary when viewed from the standpoint of total character congruence (Hopson and Barghusen 1986; Rowe 1986; Kemp 1988b). Here, I have readdressed the oft-noted observation that the size of the dentary increased during the course of synapsid evolution. Both quantitative and discrete data indicate that a lower jaw of increasingly mammalian cast was a prevalent feature of pre-mammalian synapsid evolution (Fig. 10), but finer scales of phylogenetic resolution yield more complex patterns.

In Theriodontia and its subordinate clades that encompass mammals, the pattern of both measurements and similarity values is suggestive of a driven trend (in that both the minimum and maximum values steadily increase). Thus, the measurement results accord well with Allin's (1975) hypothesis that reduction of the postdentary bones improved high-frequency hearing in these taxa and was therefore selectively advantageous. However, corresponding directionality is not apparent within the "side-branch" clades (Tables 1-4), which suggests that a common driving force is doubtful. In the most extreme case, anomodonts show the exact opposite trend: decreasing dentary size and increasing their lower jaw's distinctiveness from that of mammals. This suggests either that high-frequency hearing was not important to anomodonts or that selection for this feature was not exclusively molding mandibular evolution in this group.

The specialized structure of the anomodont mandible is an interesting exception to another result of these analyses-the relative scarcity of divergent lower-jaw morphologies among synapsid side-branches. Although there are certainly some features that are autapomorphic for the clades that do not encompass mammals (e.g., the extremely slender dentary of varanopseids, the near-vertical ridge on the reflected lamina of gorgonopsians, or the elongate angular process of the dentary in some advanced cynognathians), no one synapsid subgroup amasses more than a few such specializations, except for the anomodonts. Importantly, this lack of mandibular autapomorphy indicates that the acquisition of only a few mammalian characters would be sufficient to drive an apparent trend toward a mammal-like jaw.

Disruptive Patterns.-The Y-shaped pattern of dentary size and mammalian similarity (Figs. 6, 7) that emerged from several analyses is strikingly similar to that of disruptive selection within modern populations (i.e., when selection acts against intermediates and favors morphological extremes). Foote (1993) showed that blastoids exhibit a similar disruptive pattern, but he suggested that if a bias against intermediates were present, then its explanation would require investigation at finer scales. In the case of synapsids, the lack of intermediates is due to the early extinction of gorgonopsians and therocephalians, compared with the relatively long-lived anomodont and cynodont clades.