Evolutionary trends and the origin of the mammalian lower jaw

Paleobiology, Fall 2003 by Sidor, Christian A

From a recent compilation of synapsid cladistic analyses, I also collected phylogenetic inference data, which consisted of each taxon's clade rank (CR) (Gauthier et al. 1988) and the number of branch points from the root of the cladogram (i.e., patristic distance, PD) (Figs. 3, 4). A rationale for this specific arrangement of synapsid relationships is provided in Appendix 1. CR equals the number of branching points a taxon is positioned up the phylogenetic trajectory from Synapsida to Mammalia (Fig. 3). Branching within a terminal taxon on this pectinate tree is not considered. For example, every species within Gorgonopsia has a CR of 11. In contrast, PD measures the total number of nodes passed from the root of the cladogram to the taxon in question because branching within side-branches is taken into account. Only when a singleton attaches directly to the primary spine of the cladogram (e.g., Tetraceratops or Dvinia) are CR and PD equal.

Data were collected from study of fossil specimens at the following institutions: Albany Museum, Grahamstown, South Africa; American Museum of Natural History, New York; Field Museum of Natural History, Chicago; Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts; National Museum of Natural History, Washington, D.C.; University of California Museum of Paleontology, Berkeley; Bernard Price Institute for Palaeontological Research, University of the Witwatersrand, Johannesburg; National Museum, Bloemfontein; South African Museum, Cape Town; Transvaal Museum, Pretoria; The Natural History Museum, London; Museum of Zoology, University of Cambridge, Cambridge, United Kingdom; Oxford University Museum, Oxford; Paleontological Institute, Moscow; Bayerische Staatssammlung fur Palaontologic und Historische Geologie, Munich; Humboldt Museum fur Naturkunde, Berlin; and Institut und Museum fur Geologie und Palaontologie, Tubingen. Only four taxa were coded exclusively from the literature: Bienotheroides wanhsienensis, Ecteninion lunensis, Probelesodon sanjuanensis, and Sinoconodon sp. (Sun 1984; Crompton and Luo 1993; Martinez and Forster 1996; Martinez et al. 1996).

Qualitative Data.-I used 82 qualitative characters to describe morphological variation within the synapsid mandible. These characters included 22 pertaining to the dentary and 41 to the postdentary bones, six general shape features, and 13 dentition-related features. The last set specifically did not include characters describing the morphology of the dentition itself (such as cusp patterns). Rather, these characters focused on dental features manifesting themselves on the form of the lower jaw itself (e.g., whether or not the teeth were set in sockets). Many of the characters and character states were taken from previous cladistic analyses of synapsid relationships. Characters, character state descriptions, and literature references are presented in Appendix 2. The corresponding taxon/character data matrix is in Appendix 3.

The data matrix in Appendix 3 was converted to a taxon/taxon similarity matrix using the Simple Matching Coefficient (S^sub SM^) (Sokal and Sneath 1963), which is equal to the number of characters for which two taxa match divided by the number of characters for which they could possibly match (in order to accommodate missing or inapplicable characters). Appendix 4 contains the final line in the similarity matrix, which compares the early mammal Morganucodon with every other taxon.