birds have skulls like baby non-avian dinosaurs

[Jason Brougham <jaseb@amnh.org>]

A new online paper:

http://www.nature.com/nature/journal/v487/n7406/full/nature11146.html

Birds have paedomorphic dinosaur skulls

Bhart-Anjan S. Bhullar,  Jesús Marugán-Lobón,    Fernando Racimo,        Gabe 
S. Bever,  Timothy B. Rowe, Mark A. Norell         & Arhat Abzhanov
AffiliationsContributionsCorresponding authors
Nature 487, 223–226 (12 July 2012) doi:10.1038/nature11146
Received 15 November 2011 Accepted 16 April 2012 Published online 27 May 2012

The interplay of evolution and development has been at the heart of 
evolutionary theory for more than a century1. Heterochrony—change in the timing 
or rate of developmental events—has been implicated in the evolution of major 
vertebrate lineages such as mammals2, including humans1. Birds are the most 
speciose land vertebrates, with more than 10,000 living species3 representing a 
bewildering array of ecologies. Their anatomy is radically different from that 
of other vertebrates. The unique bird skull houses two highly specialized 
systems: the sophisticated visual and neuromuscular coordination system4, 5 
allows flight coordination and exploitation of diverse visual landscapes, and 
the astonishing variations of the beak enable a wide range of avian lifestyles. 
Here we use a geometric morphometric approach integrating developmental, 
neontological and palaeontological data to show that the heterochronic process 
of paedomorphosis, by which descendants resemble the juveniles of their 
ancestors, is responsible for several major evolutionary transitions in the 
origin of birds. We analysed the variability of a series of landmarks on all 
known theropod dinosaur skull ontogenies as well as outgroups and birds. The 
first dimension of variability captured ontogeny, indicating a conserved 
ontogenetic trajectory. The second dimension accounted for phylogenetic change 
towards more bird-like dinosaurs. Basally branching eumaniraptorans and 
avialans clustered with embryos of other archosaurs, indicating paedomorphosis. 
Our results reveal at least four paedomorphic episodes in the history of
evelopment beyond the adult state of ancestors) in the beak. Paedomorphic 
enlargement of the eyes and associated brain regions parallels the enlargement 
of the nasal cavity and olfactory brain in mammals6. This study can be a model 
for investigations of heterochrony in evolutionary transitions, illuminating 
the origin of adaptive features and inspiring studies of developmental 
mechanisms.