Bronson and Maisey—Re-identification of Platylithophycus
a reticulated mesh structure that was not observed in Platylithophycus. Nevertheless, large, filter-feeding rays such as Manta and Mobula have densely packed, cartilaginous gill rakers (Paig-Tran and Summers, 2013, figs. 5, 6), somewhat like those observed in Platylithophycus. A thorough review of the gill structures in filter-feeding chondrichthyans, including ima- ges to which we compared Platylithophycus, was presented by Paig-Tran and Summers (2013). Paig-Tran et al. (2013) provided a detailed comparison of filter pads in devil rays, at both microscopic and macroscopic scales. Rare teeth from a putative manta ray relative, Cretomanta,
have been documented from the Upper Cretaceous of northern Africa and North America, as well as from the Niobrara For- mation of Saskatchewan, Canada (as Cretomanta canadensis, Case et al., 1990). No skeletal remains of this enigmatic taxon have been found, so it is unknown whether its branchial skeleton is like that of Platylithophycus.
Conclusions
Based on the tessellated structure of Platylithophycus,in combination with the gross morphology of the structures that we interpret as gill arches and gill rakers, P. cretaceus is here interpreted as a large cartilaginous fish, possibly related to extant filter-feeding rays such as Manta and Mobula. This identification potentially expands the range of morpholo- gical diversity in the Niobrara elasmobranch fauna (which includes a form that possessed Manta-like teeth; Case et al., 1990). However, we cannot definitively identify P. cretaceus beyond the level of Elasmobranchii incertae sedis because no name-bearing teeth or other identifiable elements such as denticles or fin spines are associated with the holotype specimen. While we disagree with identifications of Platylithophycus
as a plant or invertebrate, we recognize that those earlier com- parisons with an alga or squid were based upon reasonable arguments given the interpretations presented. Prior researchers may simply have lacked expertise in vertebrate hard tissue ultrastructure (particularly the unique form of chondrichthyan calcified cartilage). According to paleontological legend, an undergraduate was the first to identify this fossil’s chon- drichthyan affinity (a student in a University of Nebraska paleontology class is said to have suggested Platylithophycus looked like fossil cartilage). Paleontology is home to myriad temporarily misplaced
taxa, including medullosan ferns that were once regarded as sponges (Dunn et al., 2003) and lungfish teeth misattributed to polypore fungi (Brown, 1938). Accurate identification and classification of fossils are obviously paramount criteria for estimates of clade age as well as for meaningful reconstructions of paleodiversity and paleoecology. Wilson and Bruner (2004) recommended a thorough review of Niobrara Chalk fish sys- tematics and stratigraphy, citing uncertainties and outstanding issues with taxonomy as possible obstacles for new workers. The Niobrara Chalk is so diverse and its fossils so abundant that there may be other taxonomic puzzles besides Platylithophycus stowed away in teaching collections and museums around the United States, patiently awaiting fresh eyes and renewed interest.
Acknowledgments
We thank M. Gottfried, J. Kriwet, and a third anonymous reviewer for their helpful comments, which improved this manuscript. Thanks also to A. Smith and M. Hill for their management of the Microscopy and Imaging Facility (American Museum of Natural History). A. Summers (University of Washington) and M. Paig-Tran (California State University, Fullerton) provided helpful suggestions regarding the gill structure of mobulid rays. M. Eklund (ThinklabZ) produced exceptional photographs of the holotype. We are particularly grateful to G. Corner and R. Secord (University of Nebraska State Museum) and A. Gishlick (AMNH) for specimen loans, and especially to A. Prybyla (Columbia University), who flew from Nebraska to New York with this large specimen as her carry-on luggage.
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