Crassadoma Bernard, 1986
BERNARD, F. R. 1986. Crassadoma gen. nov. for "Hinnites" giganteus (Gray, 1825) from the northeastern Pacific Ocean (Bivalvia: Pectinidae). -. Venus [Journal of the Malacological Society of Japan], 45 (1): 70-74. [p. 72]
«Juvenile a typical Chlamys, equivalve, biconvex. Right valve ornamented with bifurcating weakly imbricated riblets. Anterior auricle long, with imbricated radial sculpture. Byssal notch deep, ctenolium with six teeth. Posterior auricle small, wide. Left valve with 10 to 15 spinose ribs, separated by three small weakly imbricate riblets. Free or byssiferous. Adult cemented to substrate by right valve. Shell ponderously thickened, irregular, auricles and byssal notch obsolete. Right valve idiomorphic flat or deeply cupped, ornamented with concentric lamellae or with radial rows of imbricated scales. Left valve flattened, with radial rows of imbricated ribs. Hinge line distorted, displaced resulting in large cardinal area.»
FRANK REINHOLD BERNARD, 1986
[From T. R. Waller, 1993, The evolution of Chlamys (Mollusca: Bivalvia: Pectinidae) in the tropical western Atlantic and eastern Pacific, p. 210] |
Pecten (Hinnites) giganteus Gray; R. Arnold, 1906, The Tertiary and Quaternary pectens of California, plate 29, figures 1, 2, 2a.
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«Genus CRASSADOMA Bernard, 1986
[emend. Waller, 19931 Remarks.— Waller (1993) discussed the morphology and phylogenetic relationships of four tribes in the Chlamydinae on the basis of differences in shell ribbing patterns and the ontogeny of microsculpture. The tribe Chlamydini is distinguished from the other three tribes by the presence of shagreen microsculpture at least in early ontogeny and in the early phylogeny of lineages. The Mimachlamydini and Aequipectinini share ribbing patterns that are dominated by the presence of simple ribs that begin on the early dissoconch and remain as the dominant ribs throughout ontogeny. The tribe Crassadomini was viewed as the paraphyletic stem group for the Mimachlamydini and Aequipectinini, distinguished from the latter two groups by the retention of a plesiomorphic sculpture wherein the ribs branch and/or intercalate but do not develop internal carinae. Although members of the Chlamydini and Crassadomini resemble one another in their Chlamys-like form and complex ribbing patterns, they differ in the ontogeny of microsculpture. Members of the Chlamydini have continuous antimarginal striae preceding the development of radial ribs, generally develop shagreen microsculpture on part of the shell, and lack commarginal lirae. The Crassadomini, in contrast, have discontinuous antimarginal striae preceding the development of radial ribs, lack any trace of shagreen microsculpture, and instead have distinct commarginal lirae in rib interspaces at least in early ontogeny (Waller, 1993).
The tribe Crassadomini comprises two extant genera distinguished primarily on the basis of mode of larval development. The plesiomorphic genus Crassadoma is inferred to have normal planktotrophic larval development based on its larval shell, which displays a low PI/PII ratio. The derived genus Caribachlamys Waller, 1993, in contrast, has a high PI/PII ratio, implying that its mode of larval development is lecithotrophic (Waller, 1993; see also Waller, 198 1, 199 1 for discussions of stages I and I1 of the prodissoconch and PI/PII ratios). The two genera also differ in the configuration of their commarginal lirae, those of Crassadoma being regularly commarginal, those of Caribachlamys exhibiting distorted patterns. They also have different phyletic histories (Waller, 1993). Crassadoma originated in the Tethyan (proto-Mediterranean) region, probably in the early Miocene, with descendant species still living in the eastern Atlantic region. One of these is a byssate species, Crassadoma multistriata, that is largely confined to the Mediterranean; the other is a cemented species, C. pusio, which lives mainly in the northeastern Atlantic. The giant cementing pectinid of the eastern Pacific was shown by Waller (1993) to be another descendant species which probably reached the eastern Pacific via seaway connections from the tropical western Atlantic. Caribachlamys, in contrast, is not known before the late Pliocene and is restricted to the Caribbean and tropical western Atlantic. It originated after the closure of seaways to the eastern Pacific and has diversified rapidly on the eastern side of the Americas.» 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]
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Crassadoma gigantea; T. R. Waller, 1993, The evolution of Chlamys (Mollusca: Bivalvia: Pectinidae) in the tropical western Atlantic and eastern Pacific, figures 6a, 6b.
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«Emended diagnosis.— Byssate or cemented Crassadomini with normal prodissoconch (small PI stage and large PII); antimarginal striae absent or weakly developed between commarginal lirae in rib interspaces in early ontogeny; medial intercalation of secondary riblets in rib interspaces recurrent throughout ontogeny of left valve.
Type species.— Lima gigantea Gray, 1825, by original designation (Bernard, 1986).
Other species.— Ostrea pusio Linnaeus, 1758, extant, eastern Atlantic; 0. multistriata Poli, 1795, extant, eastern Atlantic; Chlamys harmeri Regteren Altena, 1937, Plio-Pleistocene, Europe.
Geographic range.— Eastern Pacific and eastern Atlantic.
Stratigraphic range.— Early? Miocene, Middle Miocene to present. (See following section on the type species, Crassadoma gigantea.)
Discussion.— Bernard (1986) included only one species in his new genus, the extant "Hinnites" giganteus of the eastern Pacific, and asserted that shell attachment to the substrate in this species evolved independently of the same habit among other extant cementing species (see above discussion of tribe Chlamydini). He also contended that Crassadoma is an obligatory cementer, whereas Hinnites is only a facultative one, its right valve merely appressed against the substrate, not cemented. Unfortunately, Bernard (1986: 71) misstated the type species of Hinnites to be Hinnites distortus (DaCosta, 1778) (= H. pusio) and gave no information on H. crispus, the correct type species. Bernard's (1986) diagnosis of Crassadoma, quoted above, is merely a description of the type species and does not serve to differentiate this from other genera that may or may not assume a cemented life habit. Harper (1991: 193) found that although some specimens of "Hinnites" pusio may be uncemented and merely wedged into the substrate by their irregular growth, the cemented forms are cemented in the same manner as Crassadoma and other "Hinnites".
In the present study, the concept of Crassadoma is expanded to encompass both non-cemented and cemented Crassadomini that have a plesiomorphic normal prodissoconch (small PI and large PII stage, Fig. 5). This prodissoconch morphology distinguishes Crassadoma from the contribal new genus Caribachlamys, all species of which share a derived state of the prodissoconch (large PI and small PII stage, Fig. 7). The two genera also differ in the development of antimarginal striae between the commarginals. In Crassadoma these striae are obscure or absent (Fig. 5); in Caribachlamys they are strong (Figs. 7i, j) and in the more derived species cause the commarginal lirae to assume irregular trends (Fig. 71). Cemented species of Crassadoma are well separated on the basis of the macro- and microsculpture of the Chlamys stage from other "Hinnites", most of which fall within the tribe Chlamydini (see preceding discussion of the tribe Chlamydini). Evidence from the fossil record suggests that cementation in the Crassadomini has evolved independently and at different times in Crassadoma gigantea of the eastern Pacific and in C. pusio of the eastern Atlantic (see following sections on species). Like all cemented Chlamydinae, these species display changes in morphology resulting from cementation and growth in a confined space, including ventral migration of the ligament system and increased distance between the pallial line and the shell margin (Yonge, 1951; Waller, 1972, 1991; Harper, 1991). Like many species adapted to cooler waters, whether cemented or not, these cemented species have a prominent transgression of foliated calcite ventrally across the originally aragonitic region of the umbonal interior. These are features that occur in each independent origin of "Hinnites", including the two examples of cemented species in the Chlamydini described above. A foliated-calcite transgression has also evolved in many species in other clades both within and outside of the subfamily Chlamydinae (Waller, 1991; Waller and Marincovich, 1992).» 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, figs. 1-14. [p. 210]
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