Early development of the facial region in a non-avian theropod dinosaur

Abstract

An isolated maxilla of the theropod dinosaur Allosaurus from the Late Jurassic (the Kimmeridgian, 153 million years ago) of Portugal is the first cranial remain of a non-coelurosaurian theropod hatchling reported so far, and sheds new light on the early cranial development of non-avian theropods. Allosaurus hatchlings seem to have been one-seventh or less of the adult length and are thus comparable in relative size to hatchlings of large extant crocodile species, but are unlike the relatively larger hatchlings in coelurosaurs. The snout experienced considerable positive allometry and an increase in tooth count during early development. The element is especially noteworthy for the abundant and well-developed features associated with the paranasal pneumatic system. Pneumatic structures present include all those found in adult allosaurids and most are even more developed than in adult skulls. Together with evidence on the ontogeny of the tympanic pneumatic system in allosaurids, these findings demonstrate that cranial pneumaticity developed early in theropod ontogeny. The strong development of pneumatic features in early ontogenetic stages of non-avian theropods supports the hypothesis that pneumatization of cranial bones was opportunistic and indicates that heterochrony played an important role in the evolution of craniofacial pneumaticity in this group.

The Perinate Skull of Byronosaurus (Troodontidae) with Observations on the Cranial Ontogeny of Paravian Theropods

“…The size disparity in Byronosaurus supports the conclusions of Rauhut and Fechner (2005) that perinates of derived coelurosaurs were precocially large relative to more basal tetanurans (e.g., allosaurids)-even when the adult body size of the former is considerably smaller than the latter. One of the most striking features of the cranial morphology of the perinate Byronosaurus (most obvious in IGM 100/ 974) compared to that of the adult is the disparity in the relative size of the circular orbit.…”

“…The ascending process in both the perinate and adult of Byronosaurus narrows distally to a fingerlike terminus that fails to contact the rostral ramus of the lacrimal (a narrow contact occurs in Troodon formosus; Currie, 1985). The perinate ascending process differs from that of the adult by projecting caudodorsally along a curved rather than linear trajectory (i.e., the ascending process is vertical as in other embryonic or neonate theropods Dal Sasso and Signore, 1998;Rauhut and Fechner, 2005]). The alveolar ramus is laterally concave and narrows caudally into a longitudinal facet that lies medial to the jugal.…”

“…This generalized trend is less obvious within Theropoda, where the number of known specimens representing these relatively early stages of skeletal ontogeny-especially those with well-preserved cranial materialcontinues to be extremely limited (Rauhut and Fechner, 2005). Our understanding of skeletal development and its phylogenetic patterns in this important clade, which includes the origin of birds, is equally limited.…”

A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia

“…Instead, pneumaticity may be exceptionally variable among individuals (34) or may decrease throughout ontogeny. Indeed, an ontogenetic decrease is seen in Allosaurus (35), and juvenile Tarbosaurus specimens exhibit vertebral pneumatic features not seen in adults (11).…”

Postnatal Ontogeny of the Skull in the Extant North American Turtle Sternotherus odoratus (Cryptodira: Kinosternidae)

“…The number of studies explicitly describing postnatal transformations in cranial form in extinct nonavian reptile taxa (e.g., Sander, 1989;Carr, 1999;O'Keefe et al, 1999;Rauhut and Fechner, 2005;Horner and Goodwin, 2006;Goodwin et al, 2006) may actually be comparable to the number published for living species. The seemingly greater interest in studying late-stage ontogenetic transformations in extinct species might be understandable because of the array of unique skeletal features known only in extinct forms (e.g., Weishampel, 1981); however, it also is curious for at least two additional reasons.…”