Glorified dinosaurs: the origin and early evolution of birds

For all their current popularity as objects of research interest, fossil birds remain chronically under-represented in the popular literature. Only one book aims to cover all of avian palaeontology: Alan Feduccia’s The Origin and Evolution of Birds (1996, and republished 1999; itself an expanded version of Feduccia’s 1980 The Age of Birds). However, Feduccia’s views on such topics as avian origins, the phylogeny and biology of Mesozoic birds, and the affinities of the major modern bird groups are idiosyncratic and do not accurately represent the status of palaeornithological knowledge. A significant gap in the market therefore remains. Until now perhaps, for Luis Chiappe’s new book – Glorified Dinosaurs: The Origin and Early Evolution of Birds – fills that gap for Mesozoic birds, at least. As I hope to show here, the book is beautiful, superbly written, authoritative and comprehensive. Two things are emphasised from the beginning: a strong reliance on cladistics and the dinosaurian origin of birds. Chiappe spends considerable time (103 of the book’s 263 pages) covering these subjects. Keen to get to the book’s extensive coverage of Mesozoic birds, I found the discussion of geological time, reptile evolution and non-avian dinosaur diversity over-long and unnecessary. Authors of palaeontological texts do of course have to set the main focus of their attention within the broader context of geological and evolutionary history, but if you’ve read a lot of popular texts on vertebrate evolution you end up feeling that you’ve heard the same story all too many times before. As Chiappe shows, the evidence linking birds to other maniraptoran theropods is outstandingly good. He uses many of the recently discovered Cretaceous fossils from Liaoning Province and Mongolia to show how features once thought unique to birds all evolved earlier on in other maniraptorans. These features include morphological attributes such as complex feathers, a particular type of wrist folding and distinctive eggshell microstructure, as well as aspects of brooding behaviour and the habit of laying egg clutches over many days. The best evidence supporting the theropod origin of birds remains the detailed anatomy of maniraptorans and other coelurosaurs, and since the 1990s our knowledge and appreciation of coelurosaur anatomy has increased exponentially. New specimens of long-known taxa like Velociraptor have been described in detail, but we also have such new fossils as the oviraptorosaurs Caudipteryx, Protarchaeopteryx and Citipati, the troodontid Mei and the dromaeosaurids Sinornithosaurus and Microraptor. Nonmaniraptoran coelurosaurs like Sinosauropteryx and Dilong have shown that a simple, filamentous integument appeared before true feathers did, therefore providing fossil support for the model of feather origins proposed by Prum (1999). This discovery adds to the evidence indicating that feathers did not evolve from scales, but rather represent evolutionary novelties (Brush 1996). An origin of birds from within coelurosaurian theropods would seem to suggest that bird flight originated on the ground, rather than among the trees. But the possibility that such maniraptorans as small dromaeosaurids and early birds were capable of at least some climbing still appears reasonable to some workers. Chiappe is well known for advocating the origin of flight in ground-running maniraptorans (Burgers and Chiappe 1999), and he includes discussion here of the wing-assisted running hypothesis. He is also highly critical of the idea that Archaeopteryx and other maniraptorans might have been scansorial or arboreal. Claims that Archaeopteryx possesses a claw geometry indicating an arboreal lifestyle (Feduccia 1993) are contradicted by newer analyses (Hopson 2001; Glen and Bennett 2007), and virtually all non-avian maniraptorans lack features indicative of a climbing lifestyle. Despite these arguments, it is likely that we will never be able to disprove the possibility that maniraptorans such as early birds, microraptorines and scansoriopterygids climbed at least occasionally; more importantly, such behaviour remains plausible based on our knowledge of living tetrapods that climb (some of which definitely lack climbing adaptations). Indeed, despite his initial argument, Chiappe does not dismiss the possibility that bird ancestry did involve the ascent of trees and other high places, and he ends this section of the book by discussing Dial’s wing-assisted incline running hypothesis in favourable terms. I found the discussion of another controversial topic – manual digit homology – quite satisfying. The argument from embryology (that the tridactyl avian hand must represent digits II–IV) has never been sound, relying on the identification of ambiguous embryonic structures as putative digits, and on the conservation of the embryonic fourth digit as the primary axis of condensation in the developing hand. New data from Hox genes indicate that the digit that looks like a thumb in bird embryos really is a thumb (Vargas and Fallon 2005), in which case the primary axis of condensation has shifted to another digit.