Argopecten thetidis (G. B. Sowerby I, 1850)
SOWERBY I, G. B. 1850. Descriptions of new species of fossil shells found by J. S. Heniker [sic, Heniken] Esq.. In: On Some Tertiary Beds in the Island of San Domingo: From Notes by J. S. Heniker, Esq., with Remarks on the Fossils, by J. C. Moore. Quarterly Journal of the Geological Society of London, 6: 39-53. [p. 52].
1850 Pecten thetidis G. B. Sowerby I, 1850
1850 Pecten oxygonum G. B. Sowerby I, 1850
1873 Pecten (Aequipecten) augusticostatus [sic] Gabb, 1873
1898 Pecten (Pecten) eugrammatus Dall, 1898
1917 Pecten hatoviejonis Maury, 1917
1922 Pecten thetidis pelei Pilsbry, 1922
1850 Pecten oxygonum G. B. Sowerby I, 1850
1873 Pecten (Aequipecten) augusticostatus [sic] Gabb, 1873
1898 Pecten (Pecten) eugrammatus Dall, 1898
1917 Pecten hatoviejonis Maury, 1917
1922 Pecten thetidis pelei Pilsbry, 1922
Pecten thetidis Sowerby; H. A. Pilsbry, 1922, Revision of W. M. Gabb's Tertiary Mollusca of Santo Domingo, plate 44, figures 3, 6.
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«Testa orbicularis, tumida, radiatim costata, costarum marginibus quadratis, ad utrumque latus squamuliferis, squamulis minutissimis, interstitiis rugulosis; auribus inaequalibus, radiatim costatis, costellis squamulosis.»
GEORGE BRETTINHAM SOWERBY I, 1850
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«Remarks.— Past authors having only a limited amount of material were unaware of the great variation of external sculpture of Argopecten thetidis (Pl. 7, Figs 1-19) and introduced a number of unnecessary taxonomic names. The sharply trigonal ribs thought to characterize Pecten oxygonum and P. augusticostatus in fact intergrade with the more typical broader, medially grooved ribs of A. thetidis (Pl. 7, Figs 4-9). Such intergradation is evident even within single samples, as in TU 1410, where sharply trigonal ribs, trigonal ribs with crests too narrow to allow development of medial troughs, and broader ribs with medial troughs are all present (Pl. 7, Figs 4-7).
There has been substantial confusion about the meaning of the name Pecten oxygonum, resulting from the apparent mixing of labels and specimens in the Heneken collection that was sent by Heneken to London to be studied by Sowerby (Dall, 1898: 714; Pilsbry, 1922: 411). In his description of P. oxygonum as well as in his choice of the name, Sowerby (1850: 52) emphasized the acutely angular ribs, with interspaces crossed only by commarginal lamellae. Nevertheless, several authors have identified P. oxygonum with species similar to the extant species Lindapecten exasperatus, which have abundant scabrous secondary costae on both ribs and interspaces of the disk (e.g., Gabb, 1873: 256; Maury, 1917a: 185; Cooke, 1919: 138). This error apparently stems from a mix-up of type specimens. As noted by Dall (1898: 714), the two specimens bearing the name P. oxygonum in the Heneken collection are of two kinds, an articulated pair of valves having a "well-marked small rib in each furrow, coarsely squamose sculpture, and a height of forty-seven and a half millmetres," and "a single valve with the rib in the furrows obsolete or absent, the shell oblique, surface coarsely squamose." Because of the differences from the original description of P. oxygonum possessed by these specimens, Dall did not accept them as syntypes. Dall's opinion is confirmed by a cast of the articulated shell in the Heneken Collection reposited in the Smithsonian collections. The cast, which came to the Smithsonian by means of a transaction between L. R. Cox of the British Museum and W. P. Woodring of the U. S. National Museum in the 1920s, is catalogd as USNM 354159. The USNM label reads "cast of type, British Museum, Geological Society of London Collection." A second label with the cast is from the British Museum (now The Natural History Museum, London). It erroneously indicates that this articulated specimen is the holotype of Pecten oxygonum and gives the catalog number as "Geol. Soc. Coll. 12831." Dall (1898: 714) noted that this specimen resembles a new species that he named P. (Plagioctenium) gabbi further on in the same monograph (p. 717, pl. 29, fig. 3). In the present study, P. gabbi is placed in the synonymy of Interchlamys interlineata, described above. Dall (1898: 714) remarked that the Dominican Republic fossils that most closely correspond to Sowerby's description of Pecten oxygonum are those described by Gabb (1873: 256) as P. augusticostatus. Pilsbry (1922: 412) agreed that the two taxa are closely related but thought that P. augusticostatus "should remain distinct until connecting specimens are found." He distinguished the holotype of P. augusticostatus, a "left valve" (ANSP 2866, actually a right valve) from P. oxygonum on the basis of differences in the number of ribs (said to be 18 or 19 in P. oxygonum, but 22 in P. augusticostatus), in rib shape ("far sharper" in P. augusticostatus), and in disk-flank costation (disk flanks without costae in P. oxygonum, but with low costae on the anterior disk flank in P. augusticostatus). These are but minor differences, allowing P. augusticostatus to be regarded as another junior synonym of Argopecten thetidis. Dall's (1898: 713) description of Pecten eugrammatus describes exactly the characters of Argopecten thetidis, with the exception of the smooth disk flanks, which are costellate in some but not all individuals of A. thetidis. Dall seems to have described P. eugrammatus only because he confounded P. thetidis with what is now known as Lindapecten, as evidenced by his statement (Dall, 1898: 714) that P. thetidis "is a shell much resembling the recent Florida shell which Conrad named fuscopurpureus ..." Dall's specimens of P. eugrammatus were collected by Guppy from Haiti and Santo Domingo. Two syntypes, both right valves, are cataloged under USNM 115533, although only a single specimen from Haiti is listed in the catalog for this number, and both Schuchert et al. (1905: 487) and Boss et al. (1968: 122) indicated only a single specimen. The larger of the two right valves, which is missing most of the auricles and dorsal margin, conforms to the measurements given by Dall (1898: 713) in his description; the smaller valve is more complete and conforms to the measurement given by Dall in the caption for his pl. 34, fig. 22. This latter valve (Pl. 7, Fig. 1) has "St Do" [Santo Domingo] pencilled on its inner surface and is presumably the specimen that Dall said is from the Dominican Republic. The larger valve, by default and in accord with the old USNM labels and the catalog entry for USNM 115533, is from Haiti. Both specimens have noncostate to weakly costate disk flanks and correspond to the present concept of A. thetidis. In describing Pecten hatoviejonis from the Río Amina area, Maury (1917a: 189) was particularly impressed by the shift in external sculpture following a growth interruption, after which the rib crests become suddenly medially grooved and secondary scabrous costellae intercalate on the rib flanks (Pl. 7, Figs 2-3). However, she based her description on only one individual (matching right and left valves). Other samples from the Río Amina area lacking the growth interruption show a more gradual change and intergrade with typical Argopecten thetidis. The sudden change in sculpture associated with a growth interruption is a common phenomenon in the Pectinidae. All of the characters used by Pilsbry (1922: 411) to distinguish Pecten thetidis pelei from the nominal subspecies vary widely within population samples. Although the presence of prominent scales on the edges of rib crests as well as distinct scabrous costellae on the disk flanks (Pl. 7, Figs 10-17) seem to be more common in samples from higher in the stratigraphic sections (in the upper Gurabo Formation), these features are still too variable and inconsistent to be of use in distinguishing chronological subspecies. Perhaps the increase in sculptural complexity in this case is ecophenotypic, occurring in deeper water deposits. Maury (1917a ) had earlier recognized the same variation and chose to distinguish it by the species name P. hatoviejonis. (Compare Pl. 7, Fig. 2 herein to pl. 44, fig. 4 of Pilsbry, 1922.) Comparisons.— Argopecten thetidis differs from A. eccentricus s. l. in having more elaborate rib crests that can be narrow and sharp, flattened with knife-like laterally projecting edges, or deeply grooved with upright edges containing looped lamellae or projecting scales. The posterior auricular margins of A thetidis are deeply sigmoidal and overall form an acute angle with the dorsal margin, whereas the corresponding margins of A. eccentricus s. l. and in particular those of A. e. eccentricus and A. e. caimiticus, are straighter, forming an obtuse angle with the dorsal margins. The adductor scars and associated foliated-calcite re-entry of the two species are markedly different. In A. thetidis, the striate and nonstriate adductor scars of the right valve are completely separated beginning in early ontogeny and have foliated-calcite extending between the scars as well as below the ventral margin of the nonstriate scar. In the three subspecies of A. eccentricus, the striate and nonstriate adductor scars of the right valve are conjoined except for a small anterior notch. Th e foliated-calcite re-entry might or might not fill the notch and seldom extends below the ventral margin of the nonstriate adductor scars of either valve. Argopecten parathetidis n. sp. is smaller in size than A. thetidis and retains the angularity and central keel of its parabolic ribs throughout ontogeny. Argopecten uselmae of the Mao Formation of the Dominican Republic shares extensive foliated-calcite re-entry with A. thetidis s. l. but differs in rib profiles, the ribs of the right valve having broad, smooth, gently rounded crests and those of the left valve narrow and rarely with a faint central keel on rib crests. These features also separate the extant species Argopecten noronhensis from A. thetidis s. l. Evolution.— Because Argopecten thetidis is apparently endemic to Hispaniola and is not found elsewhere, it is assumed that it originated on this island through adaptation to the shifting sedimentary conditions on the continental shelf due to abundant terrestrial sediment input from a tectonically active source. Although sharply trigonal ribs can occur throughout the stratigraphic range of A. thetidis, they appear to be more common in samples from the lower part of the range, suggesting that such ribs could be ecophenotypic and associated with shallower, sandier sediments, whereas the variants with broader rib crests, more prominent scales along the edges of rib crests, and more distinct radial costellae on disk fl anks could be more common in deeper, siltier sediments. From a functional point of view, the narrow, trigonal ribs might facilitate anchoring of the shell in coarser sediment, and the broader crested ribs might facilitate avoidance of sinking into finer sediments (the snowshoe effect). The oldest occurrences of Argopecten thetidis in the Cibao Valley are in the upper Cercado Formation in the Río Gurabo beginning at 122-126 m above the base of the section (Table 11) in nannofossil Zone NN11 and in pectinid zone EL-II, of late Miocene age. These first appearances, however, are based on only a few specimens and are facies controlled, because stratigraphically lower strata represent depths of deposition that were probably too shallow for this species. Possibly A. thetidis originated by allopatric speciation from A. eccentricus lacabrensis n. ssp., with which it shares acute posterior auricles with deep posterior sinuses and a tendency to possess well developed commarginal lamellae. Occurrence.— Argopecten thetidis is most abundant in the shallower parts of the Gurabo Formation (Table 11). It was probably excluded from most of the lower Cercado Formation by conditions that were too shallow and from the uppermost Gurabo by conditions that were too deep. The species is unknown in the Mao Formation. In terms of the pectinid biozones based on A. eccentricus developed in the preceding section, the oldest occurrences of A. thetidis are in Biozone EL-II in the Cercado Formation beginning at 122-126 m above the base of the section on the Río Gurabo. These are in Zone NN11, of late Miocene age. Th e youngest occurrences are above the Miocene/Pliocene boundary on the Río Cana, Río Gurabo, probably on the Río Mao downstream from Bluff 1, and in the Santiago area (Table 11). Th e species is reported here for the first time from the southern Dominican Republic in the Río Yaque group of the Río Yaque del Sur, Azua Province (USGS 8590; see Vaughan et al., 1921: 102). Distribution.— Argopecten thetidis is unknown outside the Dominican Republic». WALLER, T. R. 2011. Neogene Paleontology of the Northern Dominican Republic. 24. Propeamussiidae and Pectinidae (Mollusca: Bivalvia: Pectinoidea) of the Cibao Valley. Bulletins of American Paleontology, 381: 1-197, pls. 1-18. [p. 58-64]
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Argopecten thetidis (G. B. Sowerby I, 1850); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic, plate 7, figures 1-19.
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