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Progress and future directions in archosaur phylogenetics
Journal of Paleontology, Nov 2001 by Brochu, Christopher A
It thus seems that both sources of data support pre-Tertiary divergences among major avian lineages, provided both are examined in a phylogenetic light. One could even argue that both models are "correct," in that the Glades that diverged in the Cretaceous might have maintained low levels of diversity until after the Cretaceous-Tertiary boundary (Marshall, 1999; Cracraft, 2001; Fig. 6). The explosive-radiation model is an expression of diversification, not of divergence-we have more land birds in the Tertiary than in the Cretaceous, but this does not address the question of when these land bird groups diverged.
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But we are still left with a significant gap between fossil first appearances in the Maastrichtian and molecular divergence estimates. Some molecular estimates put the neognath-paleognath and galloanserian-neavian splits prior to 100 m.y. and the division between oscine and suboscine passeriforms in the Cretaceous (Cooper and Penny, 1996; Kumar and Hedges, 1998; van Tuinen and Hedges, 2001). This implies that, for many avian Glades, the known fossil range is much less than half of the group's actual longevity (van Tuinen and Hedges, 2001).
The problem with this controversy is that the explanations put forth are largely ad hoc. We might as well assume that early avian Glades were equipped with cloaking devices. Claims that the fossil record is simply incomplete make solid predictions of what should be discovered on the basis of a particular model, but do not directly address the problem at hand (Benton, 1999b). Arguments that fossil crown-group birds and mammals are known but not recognized because they lack diagnostic traits are also problematic. The fossils we have (and can identify) usually lack diagnostic traits for extant lineages and have diagnostic traits for extinct, unrelated lineages (Padian and Chiappe, 1998).
But conversely, claims that all modern bird Glades experienced equivalent mutational speedups after the Cretaceous-Tertiary boundary, inflating genetic distances between species and causing molecular approaches to overestimate divergence timing, are also ad hoc. Most tests of rate heterogeneity cannot detect such phenomena. Those that can (e.g., Kumar and Hedges, 1998; Easteal, 1999) are based on other fossil-based calibrations that may be even more subject to biased sampling (for example, the bird-- mammal split in the Carboniferous) than those being investigated.
Whatever the strengths of the different sources of data, they all have weaknesses. The fossil record for birds has exploded in recent years, but we are still working with extremely fragile skeletons; indeed, the fact that the record has expanded as much as it has should give pause to anyone believing the record to be well-- sampled. That we usually cannot confidently identify fragmentary bird remains also implies the obverse-that we cannot rule out identity with one of the extant bird "orders." Arguments that avian fossil sampling in the Cenozoic contradicts lineage extensions into the Mesozoic (Bleiweiss, 1998) assume Cenozoic levels of diversity within the Glades being investigated (Marshall, 1999). Moreover, whatever arguments can be made about the randomness of fossil occurrence distributions over time, these occurrences are clearly not unbiased. As acknowledged by Benton (1999b), historical biogeographic studies suggest a Gondwanan (or even Antarctic) origin for bird "orders" (see also Cracraft, 2001). Benton (1999b) cited spectacular localities with breathtakingly preserved small animals-none of them members of extant avian "orders"-as evidence for sufficient sampling; but these localities are all Laurasian. The effect of Lagerstatten on avian stratigraphic ranges should not be understated-many Glades first appear in the Eocene and Oligocene, but they more specifically tend to occur in places like the Green River Formation, the phosphorites of Quercy, or the Messel locality of Germany (Olson, 1985; Feduccia, 1996). And although the Late Cretaceous crown-group-free Malagasy site mentioned by Benton (1999b) shares faunal elements with other Gondwanan areas, it is also very endemic (Krause et al., 1999) and may not reflect vertebrate diversity elsewhere at that time.
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