Cicimurri et al.—Late Paleocene armored lizard fossils from South Carolina, USA
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Figure 2. Late Paleocene glyptosaur osteoderms from Berkeley County, South Carolina. (1) Body osteoderm in (1a) external, (1b) internal, and (1c) profile views, SC 2014.30.1. (2) Body osteoderm in (2a) external, (2b) internal, and (2c) profile views, SC 2014.30.2. (3) Cephalic (?) osteoderm in external view, SC 2014.30.3. (4) Body osteoderm in external view, SC 2014.30.4. In (1a, 1b, 2a, 2b, 4) anterior margin of osteoderms at top. In (1c, 2c) anterior margin of osteoderms at left. In (3), scale bar = 0.5mm; in (1, 2, 4) scale bars = 1mm.
the specimen shown in Figure 2.1. The interior surfaces are smooth and pierced by several foramina, and the broken posterior end of one osteoderm has a lamellar appearance (Fig. 2.2b). An incomplete polygonal osteoderm shows the effects of postmortem abrasion (Fig. 2.3). This specimen is slightly thicker than the body osteoderms previously described (1 mm), and lateral surfaces are rugose and vertical, not beveled. Eleven randomly located tubercles are preserved, whereas the interior surface is smooth.
Remarks.—Osteoderms are common to all anguid lizards, but there are distinctive differences among the various subfamilies (Klembara and Green, 2010). Two tribes are recognized within Glyptosaurinae, Melanosaurini and Glyptosaurini (Sullivan, 1979), and synapomorphies characteristic of osteoderms include tubercular dorsal ornamentation and rugose lateral bevels (Gauthier, 1982), both of which are present on the body osteoderms in our sample. Although glyptosaur osteoderms are not considered to be diagnostic below the tribe level (Sullivan, 1979; Gauthier, 1982; Sullivan and Augé, 2006), certain morphological features allow us to meaningfully compare our sample to osteoderms of described species. For example, within Glyptosaurini, Eoglyptosaurus Sullivan, 1979, Paraglyptosaurus Sullivan, 1979, and Glyptosaurus Marsh, 1871 have keeled osteoderms, but tuberculation occurs in a concentric pattern (Sullivan, 1979; Lucas et al., 1983), a feature not seen on any of the Jamestown material. Helodermoides Douglass, 1903 osteoderms lack concentrism and may be keeled, but they are subconical and the tuberculation is much more robust than on the South Carolina specimens (Lucas et al., 1983; Sullivan and Augé, 2006). Within Melanosaurini, Proxestops Gauthier, 1982 and Placosauriops Kuhn, 1940 lack keeled osteoderms, and those of Proxestops have a vermiculate- tuberculate patterning. Xestops Cope, 1873, Arpadosaurus Meszoely, 1970, and Melanosaurus Gilmore, 1928, have keeled osteoderms that lack concentrism as in the South Carolina taxon (Meszoely et al., 1978; Sullivan, 1979; Gauthier, 1982; Gunnell et al., 1992). The greater thickness and vertical, rugose lateral surfaces suggest that SC 2014.30.3 (Fig. 2.3) articulated with adjacent osteoderms, possibly forming more of a rigid outer shell around the head (Sullivan, 1986). The lack of ventral rugosity indicates that it was not fused to an underlying bone
and instead was embedded in the dermis, and its apparent hexa- gonal original outline suggests taxonomic placement within Glyptosaurini (Sullivan, 1979, Gauthier, 1982). The body osteoderms have an anterior gliding surface, and
the posterior end of the preceding osteoderm overlapped this area. Beveled margins of SC 2014.30.1 and SC 2014.30.2 (Fig. 2.1b, 2.2a, respectively) indicate that these osteoderms partially overlapped (and were overlapped by) adjacent
in-column osteoderms and that there was at least some degree of flexibility to the armor covering (see Meszoely, 1970; Gauthier, 1982). The keeled osteoderm, SC2014.30.1 (Fig. 2.1), occupied a position on the dorsal surface of the body (Sullivan, 1979; Lucas et al., 1983), and although it could have come from one of a number of locations (Meszoely et al., 1978, fig. 4), the curvature of the beveled margins indicates it is from the left side. At this point, there is no reason to assume that our osteoderm sample represents anything other than normal morphological variation in the body armor elements of a single taxon.
Discussion
Age and stratigraphic occurrence of fossils.—The elasmo- branch species we have identified include, among others, Coupatezia woutersi Cappetta, 1982, Jacquhermania duponti (Winkler, 1874), Heterotorpedo fowleriWard, 1983, Rhinobatos bruxelliensis Jaekel, 1894, and Otodus obliquus Agassiz, 1843. These are well-known species from upper Paleocene and lower Eocene strata elsewhere in Europe and theUnited States (Leriche, 1905, 1908, 1922;Casier, 1946, 1950, 1966, 1967;Case, 1994a, b; Kent, 1999a, b; Weems, 1999). The close similarity between the fossil fish assemblages of the United States, England, France, and Belgium has led some to conclude that the Jamestown, South Carolina, deposit was of early Ypresian age, possibly as old as NP10 (Knight et al., 2007; Cicimurri and Knight, 2009; Cicimurri, 2010). Case et al. (2011) identified the Jamestown deposit as being part of the Fishburne Formation (Ypresian, NP11), probably on the basis of previously published age estimates. However, analysis of calcareous nannofossils from the sediment we collected has revealed that the fossilifer- ous deposit is slightly older and of latest Paleocene age (Thanetian Stage, subzone NP9a, equivalent to part of the Clarkforkian NALMA). This determination is based on the
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