748


Journal of Paleontology 92(4):743–750


Figure 7. Surface of (1, 3) UNSM IP 16868, showing threadlike aggregations of tesserae on the surface of a gill arch in Platylithophycus, compared with (2, 4) a similar threadlike appearance of cartilage on the surface of the cranium in the extant mackerel shark (Lamna, AMNH FF 20426), scale=2mm.


endoskeleton varies among taxa and even across different parts of the skeleton in a single individual (Dean, 2011; Maisey, 2013). Anecdotal evidence exists for a diversity of tesseral configurations, but these have been poorly documented in the literature, and a review of these is beyond the scope of this paper. Our observations nevertheless demonstrate that Platylithophycus is founded on parts of a chondrichthyan whose cartilage exhibits at least two types of tesseral morphology that can be recognized in modern elasmobranchs. Only recently has much attention been devoted to resolving the development of tesserae in extant chondrichthyans (Seidel et al., 2016), the regulation of the mineralization process, and the effects of environment on calcification (Dean et al., 2015), but it seems likely that the tessellated calcification in Platylithophycus developed in identical manner to extant elasmobranchs. While it is possible to diagnose Platylithophycus cretaceus on


the basis of its unusual and potentially apomorphic gill raker mor- phology, the lack of features such as teeth in the holotype specimen mean that this taxon can only be classified as Elasmobranchii incertae sedis, pending the future discovery of more complete remains, which might provide clues as to its identity or possible synonymy with another large Niobrara Chalk elasmobranch.


Several large, predatory lamniform sharks have been


described from the Niobrara Chalk, including Cretoxyrhina (Shimada, 1997a, b), Scapanorhynchus (Hamm and Shimada, 2002), and Cretolamna (Shimada, 2007), and it is possible that Platylithophycus belongs to one of these previously described taxa, although modern predatory lamniform sharks do not have densely arranged cartilaginous gill rakers like those of Platylithophycus. Dense arrays of gill rakers are found in mod- ern filter-feeding elasmobranchs, including the whale shark (Rhincodon typus, order Orectolobiformes), basking shark (Cetorhinus maximus, order Lamniformes), megamouth shark (Megachasma pelagios, order Lamniformes), and rays (Manta, Mobula, order Myliobatiformes). However, extant filter feeding sharks do not have the type of gill raker structure observed in Platylithophycus; instead, the rakers consist either of elongate modified denticles as in Megachasma (Paig-Tran and Summers, 2013, fig. 14), have denticles covering their surface of cartilage as in Cetorhinus (Paig-Tran and Summers, 2013, fig. 15), or are entirely cartilaginous elements arranged into filtering pads as in Rhincodon (Matthews, 1950; Motta et al., 2010; Paig-Tran and Summers, 2013, fig. 13). Addition- ally, the filtering pads of Rhincodon are characterized by


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