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Enantiornithine skull morphology

From: Ben Creisler

In case this new paper has not been mentioned yet:

Jingmai K. O'Connor and Luis M. Chiappe (2011)
A revision of enantiornithine (Aves: Ornithothoraces) 
skull morphology.  
Journal of Systematic Palaeontology (advance online 
DOI: 10.1080/14772019.2010.526639 

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.