The question of when the therian radiation took place is a contentious issue, whose answer depends on the kind of data employed—paleontological (morphological) or molecular. There are three principal models of the timing of origin and diversification of placental mammals (Archibald and Deutschman, 2001), which also apply generally to the therian radiation (Fig. 1.2):
1. The explosive model, in which mammalian orders both originated and diversified in a short period of about 10 million years after the K/T boundary (see also Alroy, 1999; Benton, 1999; Foote et al., 1999);
2. The long-fuse model, in which mammalian intraordinal diversification was mostly post-Cretaceous, but interordinal divergence took place in the Cretaceous, when stem taxa of the orders existed (Douady and Douzery 2003; Springer et al., 2003); and
3. The short-fuse model, in which ordinal origin and diversification occurred well back in the Cretaceous (e.g., Springer, 1997; Kumar and Hedges, 1998).
Paleontological evidence generally supports either the explosive model or the long-fuse model, whereas molecular evidence generally supports the short-fuse model.
Let us consider the molecular evidence first. Although this book is about the fossil record, the impact of recent molecular studies on our understanding of mammalian interrelationships and divergence times has been substantial and cannot be ignored. It is chiefly molecular evidence (genetic distance, as measured by differences in nucleotide sequences
of mitochondrial and nuclear genes) that has been used to suggest that many therian mammal orders originated and diversified during the Cretaceous, some of them more than 100 million years ago (e.g., Hedges et al., 1996; Springer, 1997; Kumar and Hedges, 1998; Easteal, 1999; Adkins et al., 2003). According to this hypothesis, it was the break-up of land masses, not invasion of vacated niches following K/T extinctions, that accounts for the mammalian radiation (Hedges et al., 1996; Eizirik et al., 2001). Other recent molecular studies, however, have produced later divergence times, much closer to the K/T boundary or even early in the Cenozoic, which are more consistent with the fossil record (Table 1.1; Huchon et al., 2002; Springer et al., 2003).
It is often claimed that molecular evidence is more reliable (if not infallible) for assessing divergence times and relationships than is the fossil record, leading some molecular systematists to dismiss fossil evidence entirely. But discordant divergence estimates in different studies—and their variance with the fossil record or with anatomical evidence— raise questions about their dependability. The literature contains many examples of molecular divergence times and phylogenetic conclusions that have subsequently been discredited. Discrepancies in divergence estimates may result from various factors, including the choice of molecular sequences and taxa used, calibration dates, phylogenetic methods applied, and the assumption of a constant rate of molecular change (Bromham et al., 1999; Smith and Peter son, 2002; Springer et al., 2003; Graur and Martin, 2004). It is now known that rates of molecular evolution are heterogeneous both between and within lineages, and at different gene loci (e.g., Ayala, 1997; Smith and Peterson, 2002). Moreover, it appears that molecular clock-based estimates consistently overestimate divergence times (Rodriguez-Trelles et al., 2002). In view of these potential problems, divergence estimates based on molecular data should be viewed with caution.
The fossil record provides the only direct evidence of the occurrence of mammalian orders in the past. But fossils merely indicate the minimum age of a clade, which is likely to be younger than its origin (i.e., its divergence from a sister group or ancestor). Nearly all "modern" orders—those with living representatives—are first seen in the fossil record after the K/T boundary, apparently supporting the explosive model, or possibly the long-fuse model. Indeed, only four extant orders of mammals are potentially known from the Cretaceous, and the ordinal assignments of the relevant fossils are far from secure. They include the monotreme order Platypoda and two living orders of marsupials, Di-delphimorphia and Paucituberculata (McKenna and Bell, 1997). Among placental mammals, only a single extant order, Lipotyphla, has so far been tentatively identified in the Late Cretaceous of the northern continents. There is a possible Early Cretaceous record of Lipotyphla from Australia, but it is highly controversial.
Several other Cretaceous fossils might be related to the Cenozoic radiation, but all are too distant morphologically and phylogenetically to be assigned to modern orders. Notable among them are zalambdalestids and zhelestids, the oldest of which are about 85 million years old. Zalamb-dalestids are considered by some experts to be stem members of the superordinal clade (Anagalida) that includes rodents, lagomorphs, and possibly elephant-shrews (Macro-scelidea), whereas zhelestids have been considered to be basal ungulatomorphs (at the base of the ungulate radiation). But recent phylogenetic analyses based on new morphological evidence have challenged these hypotheses. Even if the original assessments were correct, they would at best place a minimum age of 85 million years on some superordinal divergences, which would be consistent with the long-fuse model. Other therians of similar age can be identified as metatherians or eutherians, but they are so primitive that they are not assignable to extant orders or even superordinal clades. It is not until the latest Cretaceous (Maas-trichtian or Lancian), the last 5 million years or so before the K/T boundary, that a small number of lineages are present that could represent "modern" clades or stem taxa of extant orders. Thus, taken at face value, the fossil record seems to provide overwhelming evidence that most modern orders did not evolve until the Early Cenozoic.
Robertson et al. (2004) proposed an intriguing scenario that could explain the "explosive" appearance of the early Cenozoic mammalian radiation. They postulated that the terminal Cretaceous bolide impact resulted in a short-term (hours-long) global heat pulse that "would have killed un
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