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Early mammalian radiations

Journal of Paleontology, Nov 2001 by Cifelli, Richard L

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Monotremes.-The monotremes, platypus and echidnas, have long and (almost) universally been regarded as comprising the outlier group among living mammals (Griffiths, 1978). They have fascinated evolutionary biologists for many reasons, not the least of which is the fact that they present a combination of reptilian and mammalian characteristics, together with their own striking specializations (Jenkins, 1990). While monotremes are generally regarded as an early-diverging clade relative to living therians, considerable uncertainty has long existed as to whether various extinct groups represent still more remote Glades, whether they are monotreme relatives, or whether they are more closely related to therians than are monotremes. These uncertainties persist to the present, despite important additions to knowledge through the 20th century. Part of the problem, at least, stems from the nature of the fossil record: until recently, Mesozoic mammals were known almost exclusively by teeth and jaws, and hence comparison to characteristics of the monotreme (and living therian) skull and skeleton could not be made. Conversely, the dental evidence for living monotremes is limited to ephemeral and highly modified vestigial teeth in the platypus. These are so dissimilar to those of other mammal groups that, lacking intermediate links, they are of little utility in assessing monotreme relationships (Simpson, 1929b).

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One argument developed in the mid-twentieth century is that monotremes are closely related to marsupials (Gregory, 1947; see also Kuhne, 1973). Molecular studies bearing on this hypothesis have yielded conflicting results (Penny and Hasegawa, 1997; Lee et al., 1999; Killian et al., 2001), and evaluation of the morphological evidence suggests that most of the touted marsupial-- monotreme similarities are plesiomorphic (Luo et al., in press). By the 1950s, the prevailing view was that monotremes represent a fundamentally different lineage from that leading to other living mammals (Simpson, 1945, 1959; see Fig. 1.1). As mammalian monophyly became generally accepted (Hopson and Crompton, 1969), studies including newly discovered fossils revealed similarities (particularly of the braincase) between monotremes and several groups of early mammals, including morganucodonts, triconodontids, and multituberculates (K. A. Kermack, 1963, 1967; Hopson and Crompton, 1969; K. A. Kermack and Kielan-Jaworowska, 1971; Kielan-Jaworowska, 1971). As a result, a fundamental prototherian-therian dichotomy in mammalian evolution was widely accepted (e.g., McKenna, 1975; see Fig. 1.2). Later cladistic studies, beginning with that of Kemp (1983), called into question the integrity of a broadly conceived "Prototheria." A result common to these works (Fig. 1.3) is the broad separation of morganucodonts (placed near the base of the tree) from monotremes (placed closer to Theria), but there remain fundamental points of disagreement as to relationships of various extinct groups (e.g., Rowe, 1988; Wible and Hopson, 1993; Rougier et al., 1996a).

Early mammalian radiations

Journal of Paleontology, Nov 2001 by Cifelli, Richard L

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