Crassadoma monroensis Waller, 1996
WALLER, T. R. 1996. Bridging the Gap between the Eastern Atlantic and Eastern Pacific: A New Species of Crassadoma (Bivalvia: Pectinidae) in the Pliocene of Florida. Journal of Paleontology, 70 (6): 941-946. [p. 943, figs. 2.1-2.6]
1996 Crassadoma monroensis Waller, 1996
T. R. Waller, 1996, figure 2.
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«Diagnosis.— Byssate, noncemented Crassadoma of moderate size with commarginal lirae in interspaces in early ontogeny slanting upward and ventrally on sides of radial ribs; ribs fine, numbering about 80 at margin, with number increasing on lefi valve by intercalation at the sides of interspaces throughout ontogeny.
Description.— Left-valve exterior: Height greater than length (Ht/L = 1.15) and anterior auricle much larger than posterior auricle (reconstructed AOL/POL = 1.18); anterior auricle with convex, rounded anterior margin and shallow byssal sinus; posterior auricle pointed, with posterior margin concave and forming an oblique angle with dorsal margin; dorsal margins of both auricles sloping ventrally away from beak. Prodissoconch not preserved; radial ribs on disk originating at shell height of less than 1 mm, with 22 ribs of varying sizes present on disk at height of 5 mm, 29 at 10 mm, about 60 at 30 mm, and about 80 along the ventral margin; major ribs high and rounded in cross section but without undercut sides to height of about 25 mm, thereafter lower and rounded with more sloping sides; ribs of all sizes bearing erect scales with concave side toward ventral margin; rib introduction almost entirely by intercalation near or at the sides of rib interspaces, the intercalations occumng throughout ontogeny. Commarginal lirae in interspaces to height of 25 mm, regularly spaced, about seven or eight per mm, with faint antimarginal microsculpture present in spaces between them; commarginal lirae extending up sides of ribs with distal inclination, then fading as they round rib crests; commarginally lirate phase ending at height of about 25 mm, thereafter replaced by scaly phase. Auricles densely costate, with costae increasing by intercalation, numbering about 18 at margin of anterior auricle and 12 at margin of posterior; disk flanks finely costate. Hinge line strongly inclined downward toward anterior in dorsal view (Figure 2.1). Left-valve interior: Muscle scars and pallial line not presserved, ligament system with evidence of substantial ventral migration during ontogeny, moderately deep hollow beneath resilifer, hinge dentition of two-element type as in Chlamys, auricular denticles absent, internal rib carinae absent; inner ostracum inside pallial line apparently originally aragonitic. Right valve: unknown. Comparison.— The new species differs from all known species in the tribe Crassadomini in having strong commarginallirae in an inclined pattern on the sides of its radial ribs in early ontogeny (Figure 2.5). The ribs of the left valve of the new species are less differentiated in size compared to those of Crassadoma gigantea of the eastern Pacific. In the latter the major ribs are substantially higher than secondary ribs throughout most of ontogeny. The scales of the new species are also more erect and less projecting than in C. gigantea. The foliated calcite transgression on the interior of C. gigantea comprises the entire inner ostracum inside the pallial line; in the new species such a layer is not preserved and was possibly absent, as in Crassadoma multistriata of the eastern Atlantic (Waller, 1993). Paleoecology.— The Ochopee Limestone appears to represent a slow rate of sedimentation in a highly productive warm or warm-temperate area, as evidenced by the paucity of terrigenous clastics, accumulation of lime, and abundance of mollusks, particularly bivalves. Some of the bivalves, notably Spondylus, reach very large sizes and yet show little evidence for extensive cementation, indicating that they were able to live under relatively undisturbed conditions. The left valve of Crassadoma monroensis has several generations of epifaunal or boring organisms on its exterior, including barnacles, serpulid worms, cheilostome bryozoans, and a trace of a lithophagid borehole. It is likely that the new species lived attached to the substrate by means of a strong byssus . Although its right (lower) valve is unknown, strong and persistent attachment is indicated by the extensive ventral migration of its ligament system (Yonge, l 951; Waller, 1972, 1993). That the attachment was by means of a byssus and not by cementation is indicated by the relatively undeformed morphology of the left valve and its hinge line, which slopes downward toward the byssal notch. In Crassadoma gigantea, the giant cementing "Hinnites" of the eastern Pacific (Waller, 1993), left valves of a comparable size to that of the new species show substantial deformation associated with the onset of cementation by the right valve. Etymology.— The species name refers to Monroe Station, Collier County, Florida. Material examined.— Holotype, a left valve, USNM 486328, height 66.5 mm, length 57.9 mm, USGS Locality 24947. The full entry in the Locality Register for this number, written by Druid Wilson, is as follows: "Tamiami" only original data with collection; according to Hughes' notes (p. 44-46), list of fauna, and map, the locality called "Trail Quarry" or Sunniland Rock Co. quarry at Monroe Station, Collier Co., Fla. 1958; 1969; 1960. For reasons discussed above, the specimen is inferred to be from the Ochopee Limestone Member of the Tamiami Formation at "Trail quarry" or Sunniland Rock Company quarry at Monroe Station, Collier County, Florida, collected by Thomas Hughes in the period 1958-1960. The extensive faunal collections of the National Museum of Natural History from the Neogene of the southeastern United States, the Caribbean region, and South America were searched for additional specimens but none were found. Examination of collections at the Florida Museum of Natural History, Gainesville, also failed to turn up additional material. Druid Wilson and John Waldrop, paleontologists of Lake Wales, Florida, who have collected extensively in southern Florida, were also unable to find additional specimens in their collections. Geographic and stratigraphic range.— The new species is thus far known only from the Ochopee Limestone Member of the Tamiami Formation at Monroe Station, Collier County, Florida, of early late Pliocene age (see preceding discussion of stratigraphic setting).» THOMAS RICHARD WALLER, 1996
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