Following our traditional concept of Mammalia, and thus including a number of forms known only by fossils of the Mesozoic, the earliest mammals are from the Late Triassic, a bit over 220 Ma old (figure 2.1). The Meso-zoic record thus spans about 155 Ma of mammalian history—more than twice the duration of the entire Ceno-zoic. Yet the Mesozoic record is frustratingly sparse.1 Reasonably complete specimens—skulls and skeletons— are rare and are known for only a few taxa, while for many groups the discovery of a partial jaw is a notable occurrence. The record is also discontinuous, being punctuated by large gaps in time for which little or nothing is known. A third deficiency of the existing fossil data is that diversity is often sampled poorly or unevenly. As a result, our understanding of Mesozoic mammals in their faunal context and of the roles they played in their faunas is sketchy. Finally, the geographic representation of Mesozoic mammals is rather checkered. For some significant landmasses, particularly the southern continents, the record is blank, or nearly so, during much or all of the Mesozoic. Obviously, these deficiencies in the fossil record place limitations on what we can reasonably infer about the paleo-

1 The map in the logo above shows symbolically the distribution of Mesozoic mammals among continents, marked by shading of those countries with a varying record of Mesozoic mammal fossils. The shading is not an accurate representation of the actual Mesozoic mammal sites, which are always sparse, far smaller, and more localized in geographic area than the countries where they are known.

biology and relationships of Mesozoic mammals. But it must also be appreciated that the deficiencies serve as factors that influence interpretation.

We return to further discussion of these factors in the summary at the end of this chapter. However, we must point out here that there is another side to this coin. Many major advances in knowledge of Mesozoic mammals have been made over the past several decades. In our judgment, most of these advances have come about because of new fossil discoveries. Indeed, the limited nature of the existing fossil record provides an outstanding and, to us, inspirational opportunity for contributing to knowledge. In few other fields of investigation can a single new specimen provide such significant insight into age-old controversies or reveal such unimagined diversity.

Our purpose here is to review the record of Mesozoic mammals—their fossil representation and their geographic, geologic, and paleoenvironmental occurrences. The nature and amount of information for each occurrence varies considerably, depending on what is available in published accounts and on the significance of the occurrence. Although we have relied on colleagues and our own personal knowledge in making faunal lists as up-to-date as possible, we have otherwise relied strictly on published literature and theses in assembling this compendium. In many instances, a mammalian fauna or a local fauna (an assemblage reasonably interpreted as being made up of species that lived at more or less the same time and in more or less the same place) is known by specimens from one site only. In such cases, each species' occurrence is identical with its presence at that site. In other instances, however, species have been collected from a number of sites, closely spaced stratigraphically and geographically,

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