Systematic Issues

We employ either stem-based or node-based definitions, together with diagnostic characters, for higher-rank monophyletic groups comprised solely of extinct genera. We also used stem-based definitions for the three major clades that include living mammals—Monotremata, Metatheria, and Eutheria. For example, eutherians are defined as all mammals that are more closely related to the common ancestor of crown Placentalia, such as humans, than to crown Marsupialia, such as kangaroos. In this book, we restrict usage of formal taxonomic names to those higher-rank groups for which monophyly can be demonstrated on the basis of evidence now available (even in cases where such evidence is weak). If several names are in use for a monophyletic taxon, we have given preference to the name that, in our judgment, is most widely understood and meaningful.

A fully resolved phylogeny with a large number of terminal taxa can greatly proliferate the resultant hierarchical categories or cladistic ranks (figure 1.1). Each well-supported node represents a cladistic hierarchy; ideally, each node would be defined formally and supported by an attendant diagnosis based on synapomorphies. However, this becomes impractical and undesirable in the case of a phylogeny comprised of numerous terminal taxa (some of which are obscure, poorly represented, or both) resolved into many dichotomous nodes. Our goal herein is to summarize current knowledge of Mesozoic mammals within the context of a general systematic review and framework. For the purposes of this synopsis, it would be cumbersome if not outright impractical to formally name and diagnose each node of the mammalian phylogenetic tree. Indeed, given the instability of a number of nodes and the diversity of tree topologies in extant phylogenetic hypotheses, we are skeptical that such an exercise would be a useful endeavor.

In compiling this book, we quickly learned that it is both unrealistic and linguistically inconvenient to entirely avoid all uses of names for some traditional groupings that are now recognized as paraphyletic; in many cases, no simple, explicit nomen based on monophyly exists or can be defensibly proposed. Moreover, mention of some paraphyletic groupings is unavoidable because they have been historically associated with certain familiar, yet symplesiomorphic features (such as "triconodonts," named after a molar pattern shared by what are now considered to be separate clades). In such cases, we follow the convention of placing these historical and widely understood (but nonmonophyletic) names in quotes (such as "symmetrodontans"). Finally, for the practical table 1.2. Simplified Traditional Linnaean Classification of the Class Mammalia Linnaeus, 1758, above the Suborder Level(1)

Subclass and order incertae sedis

Family Sinoconodontidae Mills, 1971 (chapter 4)

Subclass incertae sedis

Order Morganucodonta Kermack et al., 1973 (chapter 4) Order Docodonta Kretzoi, 1946 (chapter 5) Subclass Australosphenida Luo, Kielan-Jaworowska, and Cifelli, 2001, new rank (chapter 6) Order Ausktribosphenida Rich et al., 1997 Order Monotremata C.L. Bonaparte, 1837 Subclass incertae sedis

Order Shuotheridia Chow and Rich, 1982 (chapter 6) Subclass incertae sedis

Order Eutriconodonta Kermack et al., 1973 (chapter 7) Subclass Allotheria Marsh, 1880 (chapter 8)

Order Haramiyida Hahn et al., 1989 Order Multituberculata Cope, 1884 Subclass and order incertae sedis ("Symmetrodontans") (chapter 9)

Family Kuehneotheriidae Kermack et al., 1968 (Comment: In addition to Kuehneotheriidae, which might not be related to any other "symmetrodontans" and which probably are not closely related to at least Tinodontidae and Spalacotheriidae [see chapter 15], there are seven poorly known "symmetrodontan" families, of which only one [Spalacotheriidae(2)] can be supported as monophyletic.) Subclass and order incertae sedis ("eupantotherians") (chapter 10) Superorder Dryolestoidea Butler, 1939, emended(3) Order Dryolestida Prothero, 1981 Order Amphitheriida Prothero, 1981 Superorder Zatheria McKenna, 1975

Stem-lineage Zatheria Martin, 2002 "Peramurans" (former order Peramura McKenna, 1975) Subclass Boreosphenida Luo, Kielan-Jaworowska, and Cifelli, 2001, new rank ("Tribotherians," chapter 11) Order Aegialodontia Butler, 1978 Order incertae sedis Infraclass Metatheria Huxley, 1880 (chapter 12) Cohort Deltatheroida Kielan-Jaworowska, 1982 Cohort Marsupialia Illiger, 1811

Superorder "Ameridelphia" Szalay, 1982, new rank Order Asiadelphia Trofimov and Szalay, 1994 Order "Didelphimorphia" Gill, 1872 Order Paucituberculata Ameghino, 1894 Infraclass Eutheria Gill, 1872 (chapter 13) Magnorder Epitheria McKenna, 1975 Superorder and order incertae sedis Superorder Asioryctitheria Novacek et al., 1997 Superorder Anagalida Szalay and McKenna, 1971 Superorder Archonta Gregory, 1910, new rank Superorder Insectivora Bowdich, 1821 Order Leptictida McKenna, 1975 Order Lipotyphla Haeckel, 1866 Superorder Ferae Linnaeus, 1758

Order Cimolesta McKenna, 1975 Superorder Ungulatomorpha Archibald, 1996b Order "Condylarthra" Cope, 1881 ?Order Notoungulata Roth, 1903 Subclass incertae sedis

Suborder Gondwanatheria Mones, 1987 (chapter 14)

1 The suborders and orders incertae sedis are omitted. For full lists of all the genera and incertae sedis high-rank taxa, see tables 4.1-14.1.

2 McKenna and Bell (1997) assigned a part of "symmetrodonts," including Spalacotheriidae (but not Kuehneotheriidae), to the superlegion Trech-notheria McKenna, 1975.

3 Superorder Dryolestoidea has been assigned by McKenna and Bell (1997) to the legion Cladotheria McKenna, 1975.

purpose of organizing this book into a manageable number of chapters, some chapters must cover multiple hierarchical levels in the mammalian tree—that is, portions of the tree that we now know to be made up of para-phyletic groups, or grades. These include chapter 4 (earliest stem mammals), chapter 9 ("symmetrodontans" or stem trechnotherians), chapter 10 ("eupantotherians" or stem cladotherians), and chapter 11 ("tribotherians" or stem boreosphenidans).

Taxonomic classification is essential to a compendium such as this, and here we faced the dilemma of which scheme to adopt. There is a large body of literature on the pros and cons of traditional Linnaean taxonomy versus a strictly cladistic hierarchy that closely mirrors genealogy (see the recent review of the history of classification by McKenna and Bell, 1997). It is beyond the scope of this book to resolve these long-standing problems in biological classification. Rather, we decided to offer two alternative schemes of Meso-zoic mammal classification (tables 1.1 and 1.2).

Table 1.1 presents a cladistically based classification of mammals that uses indentation of successively lower ranks to reflect the intended scheme of phyletic sequencing (Wiley, 1979, 1981; Nelson and Platnick, 1981; McKenna and Bell, 1997). Because the main purpose of the table is to provide a broad outline, it includes mainly higher-rank taxa; however, some stem genera are included for their importance in representing the successive hierarchies of phylogeny. The sequence of the classification in table 1.1 is illustrated in figure 1.1 (see also Luo et al.,2002 and figure 15.1 herein).

Table 1.2 presents a simplified Linnaean taxonomy of Mesozoic mammals. The taxonomic arrangement mainly follows the phylogenetic sequence resulting from our par simony analyses, presented in chapter 15 and figures 15.1 and 15.2. Reference to the chapters in which descriptions of corresponding taxa appear are given in parentheses following each taxon name. The complete systematic lists, including genera and species, are presented as individual tables in systematic chapters 4-14.

0 0

Post a comment