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Pseudosuchian origin of the crocodylian palpebral bone

From: Ben Creisler

A new online paper:

Sterling J. Nesbitt, Alan H. Turner and Jonathan C. Weinbaum (2013)
A survey of skeletal elements in the orbit of Pseudosuchia and the
origin of the crocodylian palpebral.
Earth and Environmental Science Transactions of the Royal Society of
Edinburgh (advance online publication)
DOI: http://dx.doi.org/10.1017/S1755691013000224

Ossified skeletal elements within the orbit, such as scleral ossicles,
palpebrals, supraorbitals and sesamoids, are widespread across
Reptilia, including extant members of Crocodylia, Aves, Squamata and
Chelonia. Extant crocodylians lack scleral ossicles, but have a unique
palpebral that has recently been shown, through developmental studies,
to be an osteoderm in the upper eyelid. Here, we examine the diversity
and disparity of the crocodylian palpebral in extant members of
Crocodylia and, through the fossil record, trace the origin and
evolutionary history of the element throughout Pseudosuchia (all
archosaurs more closely related to crocodylians than avians). We show
that the crocodylian palpebral originated outside of Crocodylomorpha
in early pseudosuchian groups (e.g. Aetosauria, Loricata) and that
scleral ossicles are lost in nearly all groups of pseudosuchians, but
appear in poposauroids, at least one early crocodylomorph, and some
partially or fully marine Crocodyliformes (e.g. thalattosuchians). The
morphology and number of palpebrals differs across Crocodyliformes;
the presence of two palpebrals is plesiomorphic, but this is reduced
to one by Crocodylia. We further recommend the restriction of the term
palpebral to the structure in crocodylians and their homologues, but
not to the structure in ornithischian dinosaurs.