Spondylus siccus Elder, 1996
ELDER, W. P. 1996. Bivalves and gastropods from the Middle Campanian Anacacho Limestone, south central Texas. Journal of Paleontology, 70 (2): 247-271, figs. 1-8. [p. 256, figs. 4.6-4.12]
1996 Spondylus (Spondylus) siccus Elder, 1996
W. P. Elder, 1996, figure 4.
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«Spondylus aff. guadalupae Roemer, 1849. ELDER, 1994, p. 5.
Diagnosis. — Moderately elongate Spondylus of moderate size with strongly inflated, subtrigonal right valve, and slightly to moderately inflated, subcircular left valve. Ornamentation on both valves of about 12 to 14 spinose primary radial costae between which are 3 to 4 secondary costae; costae about twice as wide as interspaces.
Types.— Holotype (USNM 485384) and paratypes (USNM 485385-USNM 485388).
Material.— Three right valves and two left valves. All from USGS Mesozoic locality M8690; marl interbed (bed "b" of Brown, 1965) in calcarenitic limestone (basal Anacacho Limestone?) in Seco Creek on Road 1796, ca. 3.2 km (2 mi) north of D'Hanis.
Description.— Shell of moderate size, strongly inequivalve. Right (attached) valve moderately elongate, subtrigonal in outline, slightly inequilateral, inflated with prolonged umbo and large attachment area; anterior and posterior slopes steep to vertical. Cardinal area deep and concave with subparallel margins that form an abrupt angulation or carination with anterior or posterior slopes. Anterior and posterior margins straight to broadly arcuate, rounding onto evenly rounded ventral margin. Left valve subcircular to moderately oval in outline, moderately convex, subequilateral with moderately opisthogyrous, slightly to moderately inflated umbo; anterior and posterior slopes moderately steep; cardinal area and auricles not preserved.
Ornamentation of both valves consists of about 12 to 14 squamose to spinose primary radial costae between which are 3 to 4 non-spinose secondary radials. Radial costae are typically slightly sinuous and are about twice as wide as interspaces. Internal features of shell are unknown except that radial ornamentation is present on inner surface at outer margin. Remarks.— Spondylus (S.) siccus is closely allied to S. (S.) guadalupae Roemer, 1849, which is the species typical of the Austin Group in Texas (compare with Figure 4.3-4.5). Although the general shape of these species is similar, S. (S.) siccus differs in having a greater number of primary radial costae (ca. 12 to 14 versus about 10) and, on average, fewer secondary costae intercalated between the primaries (3-4 versus 3-6), and in typically having a more inflated left valve. In addition, Roemer (1852) stated that the interspaces between the radial costae are twice as wide as the costae on S. (S.) guadalupae (the specimens at hand show the costae and interspaces to be about equal in width), versus the costate being about twice as wide as the interspaces on S. (S.) siccus.
Spondylus (S.) siccus differs from S. (S.) munitus Stephenson, 1941, from the Maastrichtian of Texas, and S. (S.) gregalis (Morton, 1833), from the late Campanian to Maastrichtian of New Jersey, in having a right valve with spines and in lacking strong concentric rugae. The degree of spine development on S. (S.) siccus is unknown, however, because all that is preserved on the present specimens are spine bases or projecting folds on the costae, which mostly are arranged in concentric rows 5 to 10 mm apart. Spondylus (S.) siccus can be differentiated from S. (S.) echinatus (Morton, 1835), the only other described Late Cretaceous Gulf or Atlantic Coast Spondylus species, by the latter having a broad, low right valve with very large attachment surface, and a greater number of secondary radials (4 to 5) between the primaries on the left valve. Dhondt and Dieni (1990,1993) have compared and discussed many of the numerous Late Cretaceous Spondylus species described from Europe. Spondylus (S.) siccus can be differentiated from European species that have both primary and secondary radial costae, such as S. fimbriatus Goldfuss 1835, S. arcuatus (Catullo, 1834), S. truncatus (Lamark, 1819), and S. latus (Sowerby, 1815), by its having a fewer secondary ribs intercalated between the primary costae. Entomology [sic].— Latin siccus, dry, for Seco Creek, the only location at which the species is known to occur.»
WILLIAM P. ELDER, 1996
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