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Evolutionary trends and the origin of the mammalian lower jaw
Paleobiology, Fall 2003 by Sidor, Christian A
Cynodonts are first recorded from Upper Permian strata in southern Africa and Russia and represent the therapsid subclade that includes mammals. Numerous derived features associated with obtaining food and its mastication characterize Cynodontia, including the presence of a fossa for the neomorphic masseter muscle on the lateral surface of the dentary, postcanine teeth with accessory cusps and lingual cingula, and a complete sagittal crest for the origin of temporalis musculature. According to the phylogenetic hypothesis proposed by Hopson (1991b, 1994; Hopson and Kitching 2001), a key dichotomy in cynodont phylogeny occurred with the Triassic divergence of the cynognathian and probainognathian lineages. Terminal cynognathians (tritylodontids) range stratigraphically upwards into the Lower Cretaceous (Tatarinov and Matchenko 1999), whereas terminal probainognathians (mammals) first appear in Upper Triassic or Lower Jurassic rocks and survive until the Recent (Lucas and Luo 1993; Luo 1994).
Vertebrate paleontologists have traditionally defined mammals as possessing a well-formed dentary-squamosal jaw joint (Simpson 1960). Taxa included under this (apomorphy-based) definition include Morganucodon and Kuehneotherium, although these and other early Mesozoic forms (e.g., Sinoconodon) probably possessed a functional quadrate-articular jaw joint as well (Hopson 1991b; Luo and Crompton 1994). More recently, Rowe (1988) and Rowe and Gauthier (1992) have advocated using a crown-group definition for Mammalia, and they have termed the larger clade-including traditional mammals that lie phylogenetically outside the clade bounded by extant forms-Mammaliaformes. My use of Mammalia and of the terms "mammal" and "mammalian" correspond to traditional usage (see also Luo et al. 2002).
Data Collection
Taxon Sampling.-Fossil synapsids included in this study range from the earliest-appearing (Late Carboniferous) pelycosaur-grade taxa through some of the most primitive mammals, such as the Early Jurassic genera Morganucodon and Sinoconodon. In total, 19 "pelycosaurs," six basal therapsids, 13 dinocephalians (including five anteosaurians and eight tapinocephalians), 25 anomodonts, ten gorgonopsians, ten therocephalians, and 25 cynodonts were included. The cynodonts include six non-eucynodonts, 11 cynognathians (including six tritylodontids), and eight probainognathians (including two Mesozoic mammals). All taxa were at the genus level or, in several instances, below.
The stratigraphic range of each taxon was collected from original museum locality information or the literature (e.g., Kitching 1977; Rubidge 1995; Ivachnenko et al. 1997) and then binned into one or more of 18 age ranks (AR) for the purpose of analysis. ARs are non-overlapping stratigraphic bins in an ordered sequence (Gauthier et al. 1988). Importantly, ARs are not necessarily of equal duration; some ARs are equivalent to a single geological formation, whereas others encompass several formations or groups. The goal of this type of binning is a single, resolved sequence of the synapsid fossil record despite its derivation from a variety of widely separated continental deposits (see also Sidor 2001). One major drawback to the AR approach is that gaps in the synapsid record are effectively ignored; time periods lacking synapsid fossils are not represented in the analysis. For example, a major hiatus in the synapsid record occurs between the youngest continental deposits in North America (e.g., the San Angelo and Flowerpot Formations) and the oldest in Russia and South Africa (e.g., Mezen and the Eodicynodon Assemblage Zone, respectively) (Lucas and Heckert 2001). This approximately 2-Myr hiatus encompasses much of Roadian time, but is not evident between ARs 6 and 7. Appendix 4 reports the geological formations and vertebrate biozones making up each AR.
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