A revision of enantiornithine (Aves: Ornithothoraces) skull morphology

Abstract

Enantiornithines are the most speciose avian clade in the Mesozoic, with a fossil record that nearly spans the Cretaceous; however, with less than half of known taxa preserving skull material, our understanding of their cranial morphology remains incomplete. Here we present a comprehensive overview of the current knowledge of enantiornithine skull anatomy and discuss the range of morphologies known for each of the main cranial elements. The typical enantiornithine skull retains numerous ancestral features such as the absence of fusion among bones, the presence of a postorbital bone, a primitive quadrate with a single headed otic process, an unforked dentary, and teeth. The postorbital in at least one taxon is unreduced, suggesting the existence of a complete infratemporal fenestra and thus an unmodified diapsid skull as in confuciusornithids. The rostrum is well known and shows considerable variation, typical of theropods; however, in terms of rostral proportions, enantiornithines are extremely limited within the modern avian spectrum. Although Late Cretaceous skull material is extremely fragmentary, when compared to Early Cretaceous material it reveals a trend towards more specialized morphologies in younger taxa. The foramen magnum in all taxa points caudally, indicating that the 'flexed' type skull morphology may not have evolved in this group. Enantiornithine teeth show considerable diversity in numbers, size, morphology and placement, ranging from taxa with large teeth found throughout the jaws to taxa with small, rostrally restricted teeth, to the fully edentulous. Despite limited preservation of skull material, a number of trophic specializations can be deduced from the range of preserved morphologies, further hinting at the morphological and ecological diversity of the Cretaceous Enantiornithes.

Insight into the early evolution of the avian sternum from juvenile enantiornithines

“…4d). Enantiornithines are already known to have a developmental strategy different from most living birds in which they grew slowly for many years [20][21][22] , and spent extended periods of time as subadults, as evidenced from the large number of juvenile and subadult specimens uncovered (see Methods). These specimens reveal early ontogenetic stages that allow us to understand the skeletal development of the clade.…”

“…Although limited histological work has been conducted, what is apparent for the clade is that they grew rapidly during embryology and slowly after hatching. The large number of early juvenile enantiornithine specimens that have been collected (an ontogenetic period that is not recorded for any other clade of Mesozoic birds) leads to the interpretation that the postnatal period was prolonged in these birds 21,22 . After reaching adult morphology, enantiornithines continued to grow for an extended (but unknown) period of time, as evidenced from the preservation of several lines of arrested growth in adult specimens 37 .…”

“…However, for observable features (degree of feathering and ossification at hatching), enantiornithines are more comparable to living precocial birds (feathered and highly ossified) than to those that are altricial (naked, relatively less ossified). Subadult enantiornithines close to complete in their skeletal development are nearly adult in size (10% smaller) but the carpometacarpus and tarsometatarsus remain unfused (Longipteryx IVPP V12352, Shenqiornis DNHM D2950/1, Rapaxavis DNHM D2522) 22 . In the absence of histology, we can only estimate from the incomplete ossification of several important features that the specimens described here are relatively early in their postnatal development.…”

Cranial anatomy ofErlikosaurus andrewsi(Dinosauria, Therizinosauria): new insights based on digital reconstruction

“…This means that a rhamphotheca must have existed at an early stage, in which teeth were still present. Although this raises the question how two functionally equivalent structures can exist simultaneously, the presence of a rhamphotheca and teeth are not mutually exclusive (O'Connor and Chiappe, 2011;Hu et al, 2012). A functional shift from the (rostral) teeth to a rhamphotheca would relax the functional constraints on the teeth, permitting their reduction and increased cranial plasticity.…”

Insight into the growth pattern and bone fusion of basal birds from an Early Cretaceous enantiornithine bird

“…bone fusion | bird | Cretaceous | histology | development E nantiornithes, arguably the most diverse clade of Mesozoic birds, have been reported from every continent except Antarctica (1,2). More than half of the known global diversity of the Enantiornithes were recovered from the Early Cretaceous Jehol Biota, northeastern China (1), with the bird-bearing horizons spanning from 120 to 131 Mya (3,4).…”