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Birds have paedomorphic dinosaur skulls

From: Ben Creisler

A new online article in Nature:

Bhart-Anjan S. Bhullar, Jesús Marugán-Lobón, Fernando Racimo,Gabe S. Bever,
Timothy B. Rowe, Mark A. Norell & Arhat Abzhanov (2012)
Birds have paedomorphic dinosaur skulls.
Nature (advance onlin publication)

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 birds combined with localized peramorphosis
(development 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.

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