ORBITAL BULLA RETICULARIS: A NEW ORBITAL STRUCTURE OF A CRETACEOUS CRAB

Reticulate hemispherical bullae, 150 ,um in diameter, are present on the outer wall of the inner orbital fossa of Dakoticancer overanus Rathbun, 1917. The surface of this structure is covered by a netlike hexagonal pattern of furrows, bisected by shallower secondary furrows. The function of this structure is unknown, and it is undescribed in extant crabs. Although crabs are scarce in the fossil record, five localities separated temporally and geographically in the Late Cretaceous Pierre Shale in South Dakota have yielded thousands of specimens of small decapod crustaceans. These assemblages have a remarkably similar: (1) mode of distribution through restricted (approximately 3 m) intervals of rock over large areas (1,500 km2), (2) mode of preservation as apatite concretions, and (3) faunal composition. Because of these similarities, these repetitive crab accumulations were named by Bishop (1981) the Dakoticancer Assemblage. The Dakoticancer Assemblage is dominated by the fossil crab Dakoticancer overanus Rathbun, 1917, but includes numerous other crabs. They are, in decreasing abundance, Necrocarcinus pierrensis (Rathbun, 1917); Homolopsis punctata Rathbun, 1917; Raninella oaheensis Bishop, 1978; Sodakus tatankayotankaensis Bishop, 1978; Zygastrocarcinus mendryki (Bishop, 1982); and other decapods including Callianassa cheyennensis Rathbun, 1930; Palaeonephrops browni (Whitfield, 1907); and Linuparus sp. The Assemblage also includes ammonoid cephalopods and inoceramid bivalves. The fossil decapods are exquisitely preserved by apatite replacement in small concretions. Evidence in the form of fecal pellets (Bishop, 1977) and open burrows around and in the crab fossils indicate rapid burial after death. As decomposition of the crabs begins, the thin articulating membranes between somites disintegrated, allowing mud to squeeze into the interiors of the crabs. Decaying organic matter in this mud that was ingested by deposit feeding worms was fossilized as burrows and fecal pellets. High concentration of phosphate in the fecal pellets (Rhodes and Bloxam, 1971) in combination with the enclosed microenvironment caused precipitation of apatite. For unknown reasons (time, phosphate supply, or microenvironment), phosphatization occurred within the crab bodies forming steinkers (molds of the interior), replaced the exoskeleton, and in most cases spread beyond the exoskeleton to form the concretions which encase most specimens. This mode of preservation preserves ultrafine structures (e.g., setal pits) as well as gross structure but not usually the distal parts of appendages. From such finely preserved, abundant material, I am attempting to describe completely the morphology of Dakoticancer overanus. It is hypothesized that the orbital region of Dakoticancer overanus varies from one stratigraphic level to another. An orbital feature, herein called the orbital bulla reticularis, was discovered within orbital regions of the crabs examined with an ISI Model II scanning electron microscope. The orbital region (Fig. 1A) of Dakoticancer overanus occupies 43 per cent of the anterior face of the carapace, bounded dorsally and laterally by a prominent orbital margin, and ventrally by the underfolded lower edge of the carapace and the mouth frame. The orbits form receptacles into which eyestalks (e) fold for