Conventions

As discussed in chapters 1 and 3, we have adopted a traditional, broad-based definition of Mammalia that includes some wholly extinct groups that lie outside the crown clade formed by the three major living groups—monotremes, marsupials, and placentals. Under an alternative definition that restricts Mammalia to the mammalian crown group (see Rowe, 1988; McKenna and Bell, 1997), several groups included in this book, such as sinoconodontids, morganu-codontans, docodontans, and Hadrocodium, would be considered "nonmammalian" mammaliaforms. The difference between these definitions is, of course, arbitrary. Irrespective of the varying opinions as to where the line between mammals and nonmammalian synapsids should be drawn, students of early mammals unanimously agree on two fundamental premises: sinoconodontids, morganu-codontans, and docodontans are exceptionally well known, and they are critical to interpreting the history of early mammals. Regardless of what we call them, they are an indispensable part of our understanding of early mammalian evolution and must be included in any book on the subject.

With a few very minor exceptions, all data presented in this book are based on published references. We do not describe any new taxa, nor do we erect new higher systematic units. Along the same lines, we have adopted the traditional usage of Linnaean taxa, which we feel is more appropriate and accessible for present purposes (see discussions by Benton, 2000b; Dyke, 2002), in lieu of developing a new, Phylocode-based scheme. Linnaean classifications of the systematic chapters (4-14) list full taxon names (including author[s] and date) and taxa down to the species level. Chapters 1 (all mammals), 4, 9, and 10 each cover a nested hierarchy of several clades. For each of these chapters, we also provide a cladistic classification of the taxa covered in respective chapters, side-by-side with the traditional Linnaean classification table. The hier archical scheme for the cladistic classifications given in chapters 1, 4, 9, and 10 is denoted by the established method of successive indentations; we refer readers to the description and example of McKenna and Bell (1997: 30) for an explanation of this procedure. For higher-rank taxa, we simply use the term "clade" with its attendant taxic definition, instead of the explicit ranks used by other authors, for example, Prothero (1981) and McKenna and Bell (1997). For lower taxonomic ranks, we use the traditionally accepted Linnaean units such as superorders, orders, families, and subfamilies. In a cladistic phylogeny it is also unavoidable that some taxa (often stem taxa) will cluster in polytomies in various types of consensus trees. These taxa are listed as sedis mutabilis, following the procedures recommended by Wiley et al. (1991).

Collective taxa are referred to in the plural, following Mayr and Ashlock (1991: 394-395), for example,"Euthe-ria are." In some cases the name for a higher taxonomic category, such as an order, bears the same root as a family and genus (e.g., Morganucodonta, Morganucodontidae, Morganucodon). In order to avoid confusion in vernacular usage (using the same example, "morganucodonts," which could be interpreted in significantly different ways), we refer to the higher category by adding the suffix "ans" ("morganucodontans").

We made an attempt to follow the anatomical nomenclature of Schaller's (1992) Illustrated Veterinary Anatomical Nomenclature (which is based on Nomina Anatomica Veterinaria, 1983) and Miller's Anatomy ofthe Dog (Evans, 1995). However, in some cases strict adherence to the terminology of the Nomina Anatomica Veterinaria proved to be impractical. Most problematic in this respect are the bones of the wrist (carpus) and ankle (tarsus). Pertinent literature, both paleontological and anatomical, almost universally employs the terminology of human anatomy with respect to these elements, and we have followed this custom.

Unfortunately, there is an important exception to this rule as well: the medial bone of the proximal row in the ankle. Here we have followed the long-standing and widespread practice of referring to this element as the astragalus (e.g., Jollie, 1962; Romer and Parsons, 1977) rather than the talus, as used in both the Nomina Anatomica Veterinaria and human anatomy. For structures that do not occur in domestic mammals, we follow the terminology of widely used paleontological and anatomical texts (Jollie, 1962; Romer, 1966; Romer and Parsons, 1986; Lillegraven et al., 1979; Kuhn-Schnyder and Rieber, 1986; Carroll, 1988), together with specialized monographs and articles, cited in particular chapters as appropriate. For the dentition, we use the standard abbreviations of I, C, P, and M for incisors, canines, premolars, and molars, respectively;

upper and lower case letters designate the upper and lower dentitions, respectively. Where possible, dental formulae have been abbreviated by sequentially listing the number of incisors, canines, premolars, and molars in each quadrant, separated by periods; the formula for the upper dentition is given first and is separated from that of the lower dentition by a slash. Hence, the human dentition, which normally has two incisors, a canine, two premolars, and two to three molars in each upper and lower quadrant, would be abbreviated 2.1.2.2-3/2.1.2.2-3.

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