New Morphological Studies

The past two decades also witnessed the development of important new venues for interpreting mammalian evolution. Among these are significant advances in understanding of mammalian ontogenesis and morphogenesis, owing to a resurgence of interest in the 1980s and 1990s. The integration of descriptive embryology with interpretation of soft tissue anatomy of the skull and skeleton has produced many successful case studies that are instructive for understanding mechanisms underlying character transformation (MacPhee, 1981; Kuhn and Zeller, 1987a; Zeller, 1989a,b, 1993; Smith, 1996,1997; Sánchez-Villagra et al., 2002; Sánchez-Villagra and Maier, 2002). Detailed studies on the ossification of the lateral wall from the embryonic precursor of the sphenoobturator membrane provided essential background for interpreting the ho-mologies of the braincase wall in Mesozoic mammals (Presley, 1981; Kuhn and Zeller, 1987b; Maier, 1987; Hop-son and Rougier, 1993). Observation on the relationships of soft tissues (such as vasculature and innervation) to skeletal features in extant mammals served as the basis for vascular reconstruction in stem taxa (Wible, 1984, 1987; Kielan-Jaworowska et al., 1986; Rougier et al., 1992; Wible and Hopson, 1993,1995; Wible and Rougier, 2000). Studies on the development of Meckel's cartilage and the middle ear bones have brought new perspective on the timing and possible epigenetic mechanism for development of the middle ear (Maier, 1990, 1993; Zeller, 1993; Rowe, 1996a,b; Sánchez-Villagra et al., 2002). Finally, studies of the pectoral girdles of monotremes and marsupials have contributed to the much-needed understanding of the ontogeny of some important mammalian features, such as the supraspinous fossa of the scapula (Klima, 1973,1987; McKenna, 1996; Sánchez-Villagra and Maier, 2002). Kielan-Jaworowska and Peter P. Gambaryan provided muscular reconstruction for Cretaceous multitubercu-lates and their masticatory and locomotory functions (Kielan-Jaworowska and Gambaryan, 1994; Gambaryan and Kielan-Jaworowska, 1995,1997).

Techniques of serial sectioning and computer tomography have made it possible to study the internal structures of the skull of early mammals, which would otherwise be difficult or impractical to access (e.g., Kielan-Jaworowska et al.,

1986; Graybeal et al., 1989; Luo and Ketten, 1991; Hurum, 1994,1998a; Rowe et al., 1994,1997; Luo et al., 1995; Cifelli et al., 1996; Rowe, 1996a,b;Luo,2001).Studies of the enamel microstructure of Mesozoic mammals started in the late 1980s (e.g., Grine and Vrba, 1980; Carlson and Krause, 1985; Fosse et al., 1985) and have been continued by numerous researchers (see Koenigswald and Sander, 1997a; Wood et al., 1999, for reviews).

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