Reticulochlamys del Río, 2004
DEL RÍO, C. J. 2004. Revision of the large neogene pectinids (Mollusca: Bivalvia) of Eastern Santa Cruz and Chubut Provinces (Patagonia: Argentina). Journal of Paleontology, 78 (4): 690-699, figs. 1-6. [p. 692]
«Genus RETICULOCHLAMYS new genus
Type species.— Reticulochlamys proximus (Ihering, 1897) (= Pecten centralis Sowerby in Ihering, 1897, pl. 8, fig. 49 [non Sowerby, 1846], = Pecten proximus Ihering, 1897, pl. 8, fig. 48).
Included species.-- R. borjasensis new species (Early Miocene Chenque Formation); R. zinsmeisteri new species (Early Miocene, Monte León Formation).
Diagnosis.— Shell of large size, weakly to moderately inflated, acline to weakly prosocline, equiconvex or with left valve flat or weakly concave. Auricles subequal, of large or medium size; very shallow byssal notch retained in adults with ctenolium absent or reduced to one tooth. Commarginal lirae well developed throughout ontogeny in most species, present only in young stages of right valves in some species. Shagreen microsculpture persistent throughout ontogeny on both valves. Interior without carinate edges on internal expression of radial plicae.
Etymology.— From the Latin word ‘‘reticulum,’’ in reference to the reticulated or network appearance of the shagreen microsculpture.
Occurrence.— Monte León Formation (southern Austral Basin) and basal horizons of the Chenque Formation (San Jorge Basin), Early Miocene, Chubut and Santa Cruz provinces.
Discussion.— Pecten proximus Ihering, 1897 was placed by Morra and Erdmann (1986) in the genus Mesopeplum Iredale, 1929 (type species: M. caroli Iredale, 1929 = Chlamys fenestrata Hedley, 1901), a Miocene–Recent Southwestern Pacific genus. Mesopeplum superficially resembles Patagonian pectinids in general outline and in the presence of broad plicae, but both features have proved to be convergent in different groups of scallops (Waller, 1991). Mesopeplum differs from Reticulochlamys in lacking shagreen microsculpture, and the details of the commarginal lirae in radial interspaces demonstrate its position in the Palliolum group (Subfamily Pectininae, Tribe Palliolini) (Waller, 1991).
Based on Waller’s (1991) paper, the Tribe Chlamydini includes several branches, among which are two generic groups that share the development of shagreen microsculpture throughout ontogeny and have few, prominent pliace. One of the groups is constituted, among other genera, by the Pacific genus Swiftopecten Hertlein, 1936, a taxon recognized in the Patagonian Tertiary faunas by del Río (1995). Swiftopecten differs from the large pectinid Reticulochlamys by its smaller size, its typically chlamydoid shape, its different pattern of plicae, and the development of prominent growth ledges. The other group includes the Australian genera Equichlamys Iredale, 1929 (type species: Pecten bifrons Lamarck, 1819) and Notochlamys Cotton, 1930 (type species: Chlamys anguineus Finlay, 1927 = Pecten hexactes Lamarck, 1819). According to Waller (1991), these genera are closely related and share a common ancestry that ‘‘may lie with a group of large chlamydoid scallops with persistent or widespread shagreen microsculpture found in the Eocene? to Early Miocene of Patagonia, such as Pecten centralis Sowerby, 1846, P. praenuncius Ihering, 1897, and P. proximus Ihering, 1897’’ (Waller, 1991: 31). Notochlamys is set apart from Reticulochlamys by its smaller size, by being almost equivalve, by having longer anterior auricles than posterior ones, by the deep byssal notch retained in the adult, by having a functional ctenolium throughout life, and by the presence of internal rib carinae. Notochlamys is much more similar to Swiftopecten than it is to Reticulochlamys. More recently, Beu and Darragh (2001) returned to Waller’s ideas and proposed including Pecten centralis Sowerby in Equichlamys, considering the Argentinian species to be the immediate ancestor of Equichlamys bifrons (Lamarck, 1819). Moroever, they synonymized ‘‘all later named species’’ from the Tertiary of Patagonia with P. centralis, arguing that this species has a wide range of sculptural variation. Beu and Darragh (2001) did not mention, describe, or illustrate the Argentinian species they were referring to, and so we are unable to confirm their systematic reassignments. We infer that ‘‘the later named species’’ they proposed to include in P. centralis are Jorgechlamys juliana (Ihering, 1907) and Reticulochlamys proximus, which we show here to be distinct species belonging in different genera. In any case, if there is an Argentinian species that could be compared with Equichlamys bifrons, that species is Reticulochlamys proximus (Ihering, 1897), which shares its equidimensional acline shape, subequal auricles, a similar pattern of plicae, and persistent shagreen microsculpture throughout ontogeny with strongly sculptured specimens of E. bifrons. Despite these similarities, we consider that Reticulochlamys proximus belongs in a genus distinct from Equichlamys because of its more strongly inequivalve shell, with no byssal notch or ctenolium retained in adult specimens, and because it lacks internal rib carinae, which are present in E. bifrons. A. G. Beu (personal commun., 2003) has confirmed that the material examined by Beu and Darragh (2001) was inadequate for separating the two genera Reticulochlamys and Jorgechlamys n. gen., and they did not realize the diversity of Argentinian scallops. Reticulochlamys appears in the Early Miocene and, apart from its type species R. proximus, it includes R. zinsmeisteri. Both taxa are restricted to the southern Austral Basin, and are found only in the Monte León Formation. Reticulochlamys also includes R. borjasensis, which occurs in the San Jorge Basin, where it characterizes and is restricted to the base of the Chenque Formation.» CLAUDIA JULIA DEL RÍO, 2004
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Reticulochlamys proximus (Ihering, 1897); C. J. del Río, 2004, Revision of the large neogene pectinids (Mollusca: Bivalvia) of Eastern Santa Cruz and Chubut Provinces (Patagonia: Argentina), figures 4.1-4.4.
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