Journal of Paleontology, 92(4), 2018, p. 743–750 Copyright © 2018, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2018.14
Taxonomic Note
Resolving the identity of Platylithophycus, an enigmatic fossil from the Niobrara Chalk (Upper Cretaceous, Coniacian–Campanian)
Allison W. Bronson,1 and John G. Maisey2 1American Museum of Natural History, Richard Gilder Graduate School, Central Park West at 79th Street, New York, NY 10024 ⟨
abronson@amnh.org⟩ 2American Museum of Natural History, Division of Paleontology, Central Park West at 79th Street, New York, NY 10024 ⟨
maisey@amnh.org⟩
Abstract.—Misidentified fossils are common in paleontology, but Platylithophycus has undergone a particularly proble- matic series of descriptions. The holotype of P. cretaceus comes from the Upper Cretaceous Niobrara Chalk of Kansas, and was first described as a calcareous green alga, based on the surface texture of the specimen. Later, Platylithophycus was re-identified as a sepiid cephalopod, based partly on a comparison of microstructure between P. cretaceus and the pen of modern squids. Platylithophycus then became part of the University of Nebraska teaching collection, where, according to paleontological legend, an undergraduate student suggested that the fossil’s tessellated surface looked a lot like shark cartilage. However, that interpretation has not been formally proposed until now. This work re-describes the holotype of Platylithophycus cretaceus as part of the branchial endoskeleton of an elasmobranch, based on both gross morphology and ultrastructure, including recognizable tessellated cartilage with intertesseral pores and joints.
Introduction
The Upper Cretaceous (Coniacian–Campanian) Niobrara Chalk of Kansas is a famed source of well-preserved vertebrate fossils, and is likely the “most-diverse and best-knownMesozoic fish assemblage in North America” (Wilson and Bruner, 2004, p. 583; Shimada and Fielitz, 2006). As the Farallon Plate was subducted under the North American Plate, in the channel known as theWestern Interior Sea- way, the Niobrara Chalk was deposited as part of the Niobrara cyclothem; it represents the farthest extent of Western Interior Seaway depositional events. TheWestern Interior Seaway served as a throughway for marine organisms, resulting in a diverse fossil fauna. Abundant macroinvertebrates (cephalopods, bivalves, ammonoids, and crinoids) are present in the Niobrara Chalk, as are invertebrate trace fossils (Frey, 1972). Vertebrate diversity of the Niobrara Chalk is comprised of bony fishes, (Stewart, 1999; Shimada and Fielitz, 2006) cartilaginous fishes (Stewart, 1978; Shimada,1996), and tetrapods (sea turtles [Matzke, 2007], plesio- saurs, mosasaurs [Everhart, 2001, 2002], pterosaurs [Bennett, 2000], and avian and non-avian dinosaurs [Carpenter et al., 1995]). The fish fauna includes isolated teeth, denticles, and body fossils of numerous taxa (Shimada and Fielitz, 2006), including holocephalans (Edaphodon, Callorhynchidae), elasmobranchs (Ptychodontidae, Mitsikurinidae, Odontaspidae, Cretoxyrhinidae, Anacoracidae), batoids (Cretomanta, Rhinobatidae), and bony fishes (actinopter- ygians [Pycnodontiformes, Semionotiformes, and many members of the teleost stem and crown], as well as sarcopterygians [Coelacanthiformes]). The depositional environment of the layers that yielded Platylithophycus may have been hyposaline, and likely
represented a circalittoral zone, as is the case with most marine chalks (Frey, 1972). The depositional environment of the Smoky Hill Member of theNiobraraChalk has beenwell reviewed, but the formation is otherwise not thoroughly catalogued or integrated, partly because it is mostly exposed as a series of discontinuities (Hattin, 1982). Hattin (1982) described a depositional environment with poorly oxygenated benthic zones, and a paleoenvironment in which epibenthic communities were highly diverse and nearly all benthic invertebrate taxa were suspension feeders. Platylithophycus has been ascribed to two different phyla
over the past 70 years, first deemed a green alga, and later re-described as a cephalopod. Johnson and Howell (1948) were the first to describe Platylithophycus and compared the texture of the slab with that of calcareous green algae, such as Codium. They described two parts of a “plant”: (1) surfaces covered with hexagonal plates, and (2) supposedly calcium carbonate-
covered thread-like filaments (Johnson and Howell, 1948, fig. 1). They struggled to determine how these two parts were related to one another—they proposed the hexagonal, tessel- lated structures might have been protoplasmic objects produced inside cells, rather than representing an external surface of the “plant.” They called the tessellated surfaces “fronds,” and described filaments so dense that they lay matted both beneath and on top of the fronds (Johnson and Howell, 1948). The focus of Miller and Walker’s (1968) work was to
describe two new teuthid cephalopods from the Niobrara Forma- tion, but they also included a revision of Platylithophycus. Their experience with cephalopod fossils led them to compare Platylithophycus with a sepiid (cuttlefish), based primarily on its textural similarities to cuttlebone. However, they did not confirm
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