746
Journal of Paleontology 92(4):743–750
Figure 4. Comparison of (1) Platylithophycus (UNSM IP 16868) tesserae with cartilage tesserae of (2) extinct Libanopristis (AMNH FF 3705) and (3) extant Rhizoprionodon (AMNH FF 21652); (2) and (3) are modified from Maisey 2013; scale=1mm.
and flattened in shape, which may be a result of taphonomic distortion (Fig. 1). The exposed elements are considered to represent the actual gill-bearing parts of branchial arches (i.e., ceratobranchials and/or epibranchials). More ventral (basi-, hypo-) and dorsal (pharyngo-) elements could not be recog- nized. Gill rakers are present beneath the elements identified here as ceratobranchials, and are tessellated differently from the skeletal cartilage (Fig. 3).
Materials.—University of Nebraska State Museum IP 16868.
Remarks.—Platylithophycus cretaceus is founded on material that bears many similarities to the calcified cartilage of a chon- drichthyan fish. Its surface structure is remarkably similar to that of tessellated cartilage (Figs. 2, 3), and the overall morphology is that of large gill arches (probably epibranchials and cerato- branchials). Structures interpreted to be gill rakers are present, medial to the gill arches (Figs. 1.1, 3). The size of these elements indicates this was a large chondrichthyan.
Results
The tiled appearance of the surface of Platylithophycus is identical to the tessellated calcified cartilage of both extinct and extant chondrichthyans (Fig. 4). Tesserae are arranged with a sub-hexagonal close-packing arrangement, with individual tes- serae defined by an intertesseral joint system (Fig. 5), as well as local presence of intertesseral pores between tesserae (Fig. 5.1).
When previous descriptions of Platylithophycus are rein-
terpreted in the light of our interpretation of the structure and morphology of the holotype (Fig. 1, UNSM IP16868), several noteworthy features emerge. Johnson and Howell (1948) described a “plant consisting of many flat fronds about six inches long and half an inch wide.” The difference in texture between these structures and surrounding filamentous structures baffled these earlier workers, who wrote, “the filaments appear to have grown from both sides of the fronds and they were so numerous that, where the fronds lie flat on the bedding surface, as they do in our specimen, the filaments form an almost matted layer above and beneath them” (Johnson and Howell, 1948, p. 632). Based on our interpretation of Platylithophycus as having tessellated cartilage, the “filaments” described by Johnson and Howell probably represent differentially calcified cartilaginous gill arches (Fig. 2). The filaments resemble continuous strands at low magnification (Fig. 6.1), but when examined under an electron microscope, they are composed of semi-contiguous individual tesserae, separated from neighboring tesserae by faint traces of intertesseral joints (Fig. 6.2, 6.3). This filamentous structure might initially appear unusual, but similar coalesced strands of tesserae are sometimes present in the cartilage of extant chondrichthyans (e.g., Lamna;Fig.7). Johnson and Howell’s “fronds” are reinterpreted here
as the cartilage forming serial arrays of chondrichthyan gill rakers (Fig. 3). Comparison of their overall morphology with extant and fossil cartilaginous fishes indicates the fossil repre- sents only the gill arches of the animal. No teeth are visible on the holotype specimen, although some may be hidden inside
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