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Notes: Based on molecular sequences of nuclear genes (Kumar and Hedges, 1998) and both nuclear and mitochondrial genes (Springer et al., 2003; middle two columns). The last column shows the approximate age of the oldest known fossils for each clade. Fossil occurrences are discussed in later chapters.

Notes: Based on molecular sequences of nuclear genes (Kumar and Hedges, 1998) and both nuclear and mitochondrial genes (Springer et al., 2003; middle two columns). The last column shows the approximate age of the oldest known fossils for each clade. Fossil occurrences are discussed in later chapters.

a125 Ma estimate based on Eomaia, a basal eutherian; oldest plausible placentals are zalambdalestids and zhelestids from 85 Ma, but even their placental status is controversial. bBatodon; could be much older if Paranyctoides or Otlestes are eulipotyphlans. cOlder estimate based on plesiadapiforms; younger estimate based on euprimates.

sheltered organisms directly" (Robertson et al., 2004: 760). They further speculated that a small number of Cretaceous mammal lineages found shelter in subterranean burrows or in the water and survived the heat pulse. In their scenario, it was these lineages that ultimately gave rise to the Ceno-zoic mammalian radiation. This scenario supports the long-fuse model.

Several other possible explanations for the absence of modern orders in the Cretaceous have been advanced (Foote et al., 1999). Some researchers have claimed that the Cretaceous fossil record is too incomplete to reveal whether the mammalian radiation occurred during the Cretaceous or subsequently (e.g., Easteal, 1999; Smith and Peterson, 2002). Alternatively, it has been argued that Cretaceous fossils of modern orders might actually exist but are unrecognized because they lack any distinguishing characters. In other words, genetic divergence may have preceded morphological divergence (Cooper and Fortey, 1998; Tavare et al., 2002). Neither argument is very convincing. The possibility that mammals were diversifying somewhere with a poor fossil record, such as Africa or Antarctica (dubbed the "Garden of Eden" hypothesis by Foote et al., 1999), of course cannot be ruled out. Our knowledge of Cretaceous faunas remains limited both geographically and temporally, and the possibility exists that none of the explorations to date has sampled the locations or habitats where the antecedents of modern orders were evolving (see Clemens, 2002, for a recent discussion). Nevertheless, it is also notable that the fossil record of Cretaceous mammals has increased exponentially in recent years, extending into areas and continents where the record was formerly blank; yet no new evidence of the presence of extant orders has materialized. Instead, an array of mostly archaic Mesozoic clades has emerged. Therefore, it is reasonable to conclude that fossils of extant orders have not been discovered in the Cretaceous because they had not yet evolved (Benton, 1999; Foote et al., 1999; Novacek, 1999).

It is also true that if molecular and morphological evolution were decoupled, it might be impossible to recognize early ordinal representatives (in analogy with the genetic but not morphological separation of sibling species). However, no precedent is known for such a lengthy period of significant genetic evolution without concomitant anatomical change, and the fossil record argues against it. Although gaps remain in our knowledge of the origin of many orders, the past decade or so has seen the discovery of many remarkable fossils that appear to document post-Cretaceous transitional stages in the origin of orders, including Rodentia, Lago-morpha, Proboscidea, Sirenia, Cetacea, and Macroscelidea.

Both fossil and molecular evidence are pertinent to resolving the timing of the therian radiation. Better understanding of both are necessary to resolve remaining conflicts. It will also be important to understand the actual effects on the mammalian fauna of physical events, such as the terminal Cretaceous bolide impact.

MAMMALIAN PHYLOGENY, INTERRELATIONSHIPS, AND CLASSIFICATION

There is only one true phylogeny of mammals, and deciphering it is the challenge of mammalian systematics. All phylogenetic studies are works in progress, based on the evidence at hand or, more often, subsets of the available evidence. They should be regarded as hypotheses based on that evidence. Some are better (and presumably more reliable) than others, but none is likely to be the last word on the subject. Each hypothesis is only as good as the evidence it is based on, the characters chosen, how carefully those characters have been examined, and the phylogenetic methods and assumptions employed.

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