Db_abstract

The sclerotic ring of the Late Triassic theropod dinosaur Coelophysis

Describes the ring of small bones that supported the eye of Coelophysis.

Item Details

Abstract

We report the first complete sclerotic ring of the Triassic Dinosaur Coelophysis bauri from the New Mexico Museum of Natural History's Whitaker Quarry block, C-8-82. The block is from the Apachean Rock Point formation of the Chinle Group in north-central New Mexico. Specimen P-42200 is a gracile morph of Coelophysis with a skull length of 123mm, which places it in the large juvenile or small gracile adult range. The anterior orbit and sclerotic ring show no deformation, but the posterior portions show slight elongation in the caudal direction. The orbit has a mean diameter of 31mm and contains ~20 articulated ossicles preserved in a ring measuring ~21 mm outside diameter, and ~12 mm inside diameter. As preserved, the ring occludes 63% of the orbit. Individual ossicles are sub-trapezoidal to triangular, except for the nasal and temporal ossicles, which are more elongate oval in shape. The ossicles show a sigmoid radial cross section. The ossicles typically measure ~7mm wide (circumferentially) and ~5mm high (radially), with the wider bases of the elements bounding the corneal aperture. The elongate nasal and temporal ossicles are ~10 to 12 mm long and ~5 mm high.


Colbert (1989, Museum of Northern Arizona Bull. 59) observed four or five partially articulated sclerotic ossicles in a Harvard University specimen of Coelophysis (MCZ 4327) and extrapolated that there would be a total of 20. He estimated that they were arranged with the dorsal and ventral elements being outermost in a series of four sequentially overlapping plates that proceeded anteriorly and posteriorly and ultimately overlapped the nasal and temporal ossicles from both the top and bottom. This arrangement has mirror-symmetry antero-posteriorly and dorso-ventrally. Our discovery demonstrates that Colbert's extrapolations were essentially correct.


The ring anatomy revealed by this specimen offers rare insight into visual acuity and accommodation in the eye of Coelophysis. Based on analysis of the ring and orbit morphology and comparison to extant reptile and bird outgroups, we conclude that Coelophysis was a diurnal, visually oriented predator.