Caribachlamys paucirama Waller, 1993
WALLER, T. R. 1993. The evolution of Chlamys (Mollusca: Bivalvia: Pectinidae) in the tropical western Atlantic and eastern Pacific. American Malacological Bulletin, 10(2): 195-249. [p. 223, figs. 9m-s]
1993 Caribachlamys paucirama Waller, 1993
T. R. Waller, 1993, figure 9.
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«Etymology.— From the Latin, paucus, meaning few, combined with ramus, meaning branch, with reference to the relatively few rib introductions in the late ontogeny of the shell.
Types.— Holotype: USNM(P) 474637, 1 left valve (Fig. 9n), from rock pit at east edge of Ft. Denaud, Hendry County, Florida, probably from the Caloosahatchee Formation of early Pleistocene age. Ht. = 29.6 mm, L. = 25.7 mm, umbonal angle = 92º, AOL = 9.4 mm, POL = 4.3 mm. Paratypes: USNM(P) 474638 (Figs. 9r,s), 1 left valve, USGS 26543 (TU 1175) from spoil banks along canals south of Florida Highway 858, 3.2km (2 miles) east of junction with Florida Highway 846 (SE¼, section 24, T48S, R27E), northeast of Naples and southwest of Immokalee, Collier Co., Florida. Pinecrest Beds. Collected by S. E. and R. E. Hoerle. USNM(P) 474639, 1 juvenile right valve and 2 fragments, rock pit 5.6 km (3.5 miles) west of LaBelle, off Florida Highway 78 on north side of Caloosahatchee River, Glades County, Florida. Caloosahatchee Formation. USNM(P) 474640, 1 right valve, 3 left valves, and 1 fragment, Cochran Shell Pit, NE¼ sec. 23, T43S, R28E, Hendry County, Florida. Collected from spoil, Caloosahatchee Formation. UF 24479, 1 right valve (Figs. 9p,q) and 2 left valves, same data as preceding. UF 24894 (Figs. 9m,o), 1 pair of valves, 3 right valves, and 5 left valves, same data as preceding. UF 24986, 1 left valve, same data as preceding. UF 41906, 1 right valve, same data as preceding. UF 55665, 3 right valves and 2 left valves, same data as preceding. UF 9492, 3 left valves, Caloosahatchee River, Florida, precise locality not specified, presumed to be from the Calosahatchee Formation. Diagnosis.— Caribachlamys with valves about equally convex; well-spaced, fine, continuously scaly, rounded ribs equal to or narrower than interspaces, with new rib introductions uncommon after mid-ontogeny (shell height of about 15 mm); about 40 to 50 ribs and riblets at margin of mature individuals (shell height of about 30 mm); scales fine, closely spaced, distally concave, and open; commarginal lirae absent; microsculpture in rib interspaces consisting of antimarginal striae of irregular trend in early ontogeny, regularly diverging antimarginal striae in later ontogeny; crests of ribs without antimarginal striae. Description of holotype.— Exterior: Left valve with height somewhat greater than length (Ht/L = 1.15) and anterior auricle much larger than posterior one (AOL/POL = 2.19); anterior auricle pointed, with deep byssal sinus; posterior auricle oblique, forming an angle with hingeline of about 120°; posterior margin of posterior auricle nearly straight, slightly convex, or slightly concave. Prodissoconch not preserved; radial ribs originating at shell height of 1 mm, initially 10 in number, increasing to 27 at shell height of 5 mm, 37 at 10 mm, and 42 at distal margin at shell height of 30 mm. Ribs high and rounded in cross-section, bearing erect, distally concave, open, closely spaced scales throughout ontogeny; ribs remaining narrower than or equal to interspaces in width. Rib introduction initially mainly by intercalation submedially in interspaces of earlier ribs; microsculpture of pre-radial stage of beak pitted or with short discontinuous antimarginal striae; microsculpture in rib interspaces consisting of striae of irregular, mainly antimarginal, trends; commarginal lirae obscure or absent in early ontogeny. Interior: Hinge dentition of two-element type as in Chlamys; umbonal region aragonitic, without foliated-calcite transgression; adductor and pedal retractor muscle scars not preserved; ribs expressed internally near margin as simple corrugations without internal carinae. Morphological variation.— The oldest specimen, USNM(P) 474638, from the Pinecrest Beds (Figs. 9r,s), differs from most of the other specimens, all from the Caloosahatchee Formation, in having a slightly greater number of ribs at shell heights of 10 mm and 20 mm, and it exceeds most but not all of the Caloosahatchee specimens in number of ribs present at the distal margin of its disk. Comparison.— Caribachlamys paucirama resembles C. sentis in overall aspect but differs in having most of its rib introductions limited to early ontogeny, thereby producing by late ontogeny a far more regular ribbing pattern and a lower total number of ribs at maturity. By the time a shell height of 20 mm is reached, C. paucirama rarely has as many as 50 ribs, whereas C. sentis has between 50 and 60. Specimens of C. sentis that are 30 mm in height or greater have at least 60 radial ribs present along the margins of their disks; in contrast C. paucirama generally has fewer than 50. In microsculpture the two species differ in their first 10 mm of growth. Distinct commarginal lirae of fairly regular trend are present in C. sentis, whereas C. paucirama has irregular antimarginal striae. Caribachlamys paucirama also resembles C. mildredae. The microsculpture pattern of the two species in early ontogeny is very similar but ribbing patterns differ. C. mildredae generally has fewer than 30 ribs at a shell height of 10 mm, whereas C. paucirama generally has more than 30. The ribs of C. mildredae are distinctly clustered on the right valve and distinctly ordered on the left; those of C. paucirama tend to be more uniform in spacing and height. Lastly, the scales on the ribs of C. mildredae, particularly those on its left valve, are more widely spaced than are the scales of C. paucirama, with the scales of some specimens of C. mildredae approaching the closed knobby condition that is common in C. imbricata. Paleoecology.— Caribachlamys paucirama appears to be most abundant in the Caloosahatchee Formation at sites where there are abundant scleractinian corals. Presumably the living habit and habitat of the new species were similar to, if not the same as, that of extant reef-dwelling members of the genus. Geographic range.— Thus far known only from southern Florida. Stratigraphic range.— Middle and Upper Pliocene (Pinecrest Beds) and Lower Pleistocene (Caloosahatchee Formation). The age of the Pinecrest Beds has been somewhat controversial, depending on how the boundaries for this formation are drawn. The Middle to Late Pliocene age is based on studies by Akers (1974), Waldrop and Wilson (1990: 220), and Jones et al. (1991). Discussion.— Caribachlamys paucirama is quite clearly a member of the same clade that contains C. mildredae and C. imbricata, the shared derived characters being the reduction of early commarginal lirae in rib interspaces, the irregularity of early antimarginal microsculpture, and the reduction in rib number (see preceding section on genus Caribachlamys and Figure 8). In its lack of distinct rib clustering, the new species is the most primitive known member of this clade. Although there is only a single specimen of the new species available from the Pinecrest beds, the ribbing pattern of this specimen suggests that between Pinecrest and Caloosahatchee time, rib introductions decreased and rib spacing became more regular. This is just the opposite of a trend that produced C. mildredae and C. imbricata, in which the ribs are strongly clustered and ordered, with a common trend toward distant spacing of scales on major ribs and toward cusping of scales. This trend in addition to the peculiar microsculpture of C. paucirama, with highly irregular antimarginal striae and a lack of commarginal lirae, suggests that the new species is a sister group of a clade that contains both C. mildredae and C. imbricata (Fig. 8). Material examined.— USNM: the 10 specimens and fragments listed above as types. UF: the 22 specimens listed above as paratypes.» THOMAS RICHARD WALLER, 1993
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