- There is little, if any, direct fossil evidence of the cardiovascular, respiratory,
reproductive or digestive biology of dinosaurs. However, a variety of data can be used
to draw reasonable inferences about the physiology of the carnivorous theropod
dinosaurs (Archosauria: Theropoda).
Extant archosaurs, birds and crocodilians, possess regionally differentiated,
vascularized and avascular lungs, although the crocodilian lung is less specialized than
the avian lung air-sac system. Essential components of avian lungs include the
voluminous, thin-walled abdominal air-sacs which are ventilated by an expansive
sternum and specialized ribs. Inhalatory, paradoxical collapse of these air-sacs in birds
is prevented by the synsacrum, pubes and femoral-thigh complex. The present work
examines the theropod abdomen and reveals that it lacked sufficient space to have
housed similarly enlarged abdominal air-sacs as well as the skeleto-muscular
modifications requisite to have ventilated them. There is little evidence to indicate that
theropod dinosaurs possessed a specialized bird-like, air-sac lung and, by extension,
that theropod cardiovascular function was any more sophisticated than that of
Conventional wisdom holds that theropod visceral anatomy was similar to
that in birds. However, exceptional soft tissue preservation in Scipionyx samnitcus
(Theropoda) offers rare evidence of in situ theropod visceral anatomy. Using
computed tomography, close comparison of Scipionyx with gastrointestinal
morphology in crocodilians, birds and lizards indicates that theropod visceral
structure and “geography” was strikingly similar only to that in Alligator.
In modern birds, there exists a unique osseous tissue, medullary bone,
which forms only in adult females as a source of calcium for developing hardshelled,
calcified eggs. Although crocodilians also produce calcified eggshells, no
medullary bone is formed. Recent supposed identification of medullary bone in
Tyrannosaurus rex (Theropoda) suggests an avian style of reproduction was
present in dinosaurs and, if correct, would seem to support a close relationship
between dinosaurs and birds. However, scanning electron microscopy
demonstrates that claims of medullary bone in theropod dinosaurs should be
regarded with skepticism as the tissues recovered are not unique from those
recovered in growing Alligator.
These data provide evidence of significant but heretofore unrecognized
differences between avian and theropodan dinosaur internal structure and function.