A shift in ontogenetic timing produced the unique sauropod skull

Sauropod dinosaurs include the largest terrestrial vertebrates that have
ever lived. Virtually every part of the sauropod body is heavily modified
in association with gigantic size and associated physiological
alterations. Sauropod skulls are no exception: they feature elongated,
telescoped facial regions connected to tilted neurocrania and reoriented
jaw adductor muscles. Several of these cranial features have been
suggested to be adaptations for feeding on the one hand and the result of
paedomorphic transformation near the base of Sauropoda on the other.
However, the scarcity of sauropodomorph ontogenetic series has impeded
further investigation of these hypotheses. We re-evaluated the cranial
material attributed to the early sauropodomorph Anchisaurus, which our
phylogenetic analyses confirm to be closely related to sauropods. Digital
assembly of μCT scanned skulls of the two known specimens, a juvenile and
an adult, permitted us to examine the detailed ontogeny of cranial
elements. The skull anatomy of Anchisaurus is distinguished by a mosaic of
ancestral saurischian and sauropod-like characters. Sauropod-like
characters of the braincase and adductor chamber appear late in ontogeny,
suggesting that these features first evolved by the developmental
mechanism of terminal addition. Shape analyses and investigation of
allometric evolution demonstrate that cranial characters that appear late
in the ontogeny of sauropodomorphs closely related to sauropods are
already present in the embryos and juveniles of sauropods, suggesting a
predisplacement-type shift in developmental timing from the ancestral
anchisaurian condition. We propose that this developmental shift relaxed
prior constraints on skull morphology, allowing sauropods to explore a
novel range of phenotypes and enabling specializations of the feeding
apparatus. The shift in timing occurred in concert with the evolution of
gigantism and physiological and locomotory innovations.