A field guide to mesozoic birds and other winged dinosaurs

Bird Ancestry
Since the 1970s, a consensus has emerged among scientists that
birds evolved directly from a certain branch of specialized bipedal, carnivorous
or omnivorous dinosaurs, the Coelurosauria. In fact, the anatomical
thread of transition from a lizard-like early diapsid reptile to a modern bird
can be followed throughout the entire dinosaurian lineage. While a few dissenters
to this theory remain, they have so far failed to propose alternative
models of bird origins and to support them with rigorous, repeatable studies.
All researchers agree that the closest living relatives of birds are crocodylians;
this view has been overwhelmingly supported by both fossil evidence
and genetic evidence showing crocodiles to be more closely related to
birds than to lizards. The last shared common ancestor of birds and crocodylians
(i.e. the ancestral archosaur) probably lived during the early Triassic
period at the dawn of the Mesozoic era. Shortly thereafter, the archosaurian
lineage split into the crocodile line (Pseudosuchia, the inappropriately
named “false crocodiles”) on one hand, and the bird line (Ornithosuchia,
or “bird crocodiles”) on the other. While it’s difficult to imagine two types
of animals more different in terms of anatomy, the ancestor of birds and
crocodiles did share some important characteristics of both. For example,
both birds and crocodiles have four-chambered hearts, and so it is likely
that their Triassic common ancestor did as well. This common ancestor,
or “concestor”, also probably built nests out of vegetation rather than burying
eggs in soil, and exhibited some degree of parental care--even modern
crocodiles will look after and protect their young from predators for a short
time after they hatch. However, this concestor was still more crocodile-like
in appearance than bird-like. Though the sprawling, dragging gait of modern
crocodylians is partly a result of their aquatic specializations, the bird/
crocodile concestor was still probably a sprawling quadruped.
The next major branching event in the bird lineage was the split
between the ancestor of birds and pterosaurs, the highly diverse flying vertebrates
of the Mesozoic which include the pteordactyls. This concestor, of
the group Ornithodira, was likely also a semi-sprawling quadruped, and
still laid eggs with leathery shells rather than the rigid, fragile eggshells of
birds. Interestingly, all known pterosaurs were covered in dense coats of
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