Morphological Clocks in Paleontology, and a Mid-Cretaceous Origin of Crown Aves

Birds are among the most diverse and intensively studied vertebrate groups, but many aspects of their higher-level phylogeny and evolution still remain controversial. One contentious issue concerns the antiquity of modern birds (=crown Aves): the age of the most recent common ancestor of all living birds (Gauthier 1986). Very few Mesozoic fossils are attributable to modern birds (e.g., Clarke et al. 2005; Dyke and Kaiser 2011; Brocklehurst et al. 2012; Ksepka and Boyd 2012) suggesting that they diversified largely or entirely in the early Paleogene, perhaps in the ecological vacuum created by the extinction of non-avian dinosaurs, pterosaurs, and many archaic (stem) birds (e.g., Longrich et al. 2011). In contrast, molecular studies indicate that modern birds commenced radiating deep within the Mesozoic, for example ∼130 Ma (Cooper and Penny 1997; Haddrath and Baker 2012) or ∼113 Ma (Jetz et al. 2012), with ratites, galliforms, anseriforms, shorebirds, and even passerines surviving across the KPg boundary (∼66 Ma). The oldest molecular dates further imply an extraordinarily rapid early bird evolution, with the modern birds appearing only 20 myr after Archaeopteryx (∼150 Ma). However, both approaches entail considerable uncertainties: for example, nonpreservation of fossils always underestimates the antiquity of lineages, whereas rate heterogeneity, saturation, and calibration uncertainty can strongly bias molecular divergence dating.