Zygochlamys Ihering, 1907
IHERING, H. VON. 1907. Les Mollusques fossiles du Tertiaire et du Crétacé supérieur de l’Argentine. Anales del Museo Nacional Buenos Aires [serie 3], 7: 1-611, pls. 1-18. [p. 250]
«Quant aux autres es peces de Chlamys de la formation patagonienne, elles sont presque toutes plus ou moins alliées entre elles, ce qui est prouvé surtout par l'inégalité de la sculpture des deux valves. Les cotes de la valve gauche sont simples, celles de la valve droite doubles, c'est-á-dire qu'elles sont divisées par un silIon dans toute leur longueur. Je propose le nom de Zygochlamys pour le sous-genre formó par cette espéce de la formation patagonienne, Ch. geminata Sow. étant le type du groupe. Ch. fissocostalis, nodosoplicatus, quemadensis et jorgensis de la formation patagonienne et Ch. actinodes, de la formation araucanienne, concordent avec cette derniére espéce. Entre les espéces vivantes, Ch. patagonica et, en partie aussi Ch. natans, montrent bien les mémes caracteres.»
HERMANN VON IHERING, 1907
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Pecten geminatus; G. B. Sowerby, 1846, Descriptions of the Tertiary Fossil Shells from South America. In C. Darwin: Geological Observations of South America, plate 2, figure 24.
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Zygochlamys geminata (Sowerby, 1846), 2019, Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina), figure 4.
Zygochlamys jorgensis Ihering, 1897, 2019, Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina), figure 6.
Zygochlamys sebastiani Morra, 1985, 2019, Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina), figure 7.
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«Subfamily Chlamydinae Teppner, 1922
Tribe Chlamydini Teppner, 1922 Genus Zygochlamys Ihering, 1907 Type species.— Zygochlamys geminata (Sowerby, 1846). Late Oligocene–early Miocene, San Julián and Monte León formations, by original designation.
Included species.— Zygochlamys geminata; Z. jorgensis Ihering, 1907 (early–middle Miocene, San Julián, Monte León, El Chacay, and Chenque formations), Z. sebastiani Morra, 1985 (early–middle Miocene, Monte León and Chenque Formations).
Diagnosis.— Adult shell subtriangular, prosocline or acline. Umbonal angle narrow throughout ontogeny. Byssal notch rectangular. Left anterior auricle with 11–19 radial ribs; posterior auricle with 5–11 radial ribs. Disc sculptured with 8–14 paired plicae on right valve and 7–23 simple plicae on left valve, with numerous ribs and riblets bearing prominent scales.
Occurrence.— Late Oligocene–middle Miocene of Austral and Golfo San Jorge basins.
Remarks.— As already discussed by Morra (1985) and del Río (2004a), when erecting the genus Zygochlamys, Ihering (1907) questioned the validity of the name Chlamys Röding, 1798 and replaced it with Myochlamys. He considered Zygochlamys to be a subgenus of Myochlamys, designated Pecten geminatus Sowerby, 1846 as its type species, and referred to Zygochlamys a large group of fossil and recent Patagonian species characterized by the development of simple plicae on left valves and paired plicae on the right valves: Z. fissocostalis (Ihering, 1899), Z. jorgensis Ihering, 1907, Z. quemadensis (Ihering, 1897) (late Oligocene–middle Miocene), and Pecten actinodes Sowerby, 1846 (late Miocene). Ihering (1907) also referred with doubts to Zygochlamys the Recent species Chlamys patagonica (King, 1832) and Chlamys natans (Philippi, 1845), both distributed along the southern South American coasts.
Cossmann (1909) stated that Myochlamys is a homonym because it had been used previously for a group of coleopterans (Family Curculionidae; Fairmaire, 1876), and proposed its replacement by Chlamydina. Therefore, Chlamydina immediately became an objective synonym of Chlamys Röding, 1798 (Hertlein, 1969). However, Hennig (1911) continued to consider Myochlamys to be a valid name. Subsequently, Zygochlamys was widely accepted among paleontologists and neontologists who referred at least 15 species from the Cenozoic of the Southern Hemisphere to it. Fleming (1944) was the first to suggest that Z. geminata could be an ancestor of Chlamys patagonica and of the Pleistocene–Recent New Zealand species Chlamys delicatula (Hutton, 1873). Powell (1950) for the first time used Zygochlamys to locate C. delicatula. Later, Fleming (1957) proposed the inclusion in that genus of the Patagonian fossil species Pecten actinodes, as well as the Recent species C. patagonica and C. patriae Doello-Jurado, 1918 and the Neogene circum-Antarctic species C. anderssoni (Hennig, 1911), C. mawsoni Fletcher, 1938, C. (Zygochlamys) heardensis Fleming, 1957, C. seymouri (Marwick, 1928), and C. delicatula? subantarctica (Hedley, 1916). Grau (1959) was the pioneer author in claiming that Zygochlamys should not be considered a valid genus because its diagnostic characters had never been properly defined. He synonymized it with Chlamys Röding, 1798. Beu (1985) considered that all taxa placed in Zygochlamys by Fleming (1957) were related to each other but not to Z. geminata, rejected the use of Zygochlamys, included them in Chlamys sensu lato, and added the Neogene Chilean species C. tenuicostata (Hupé, 1854), C. vidali (Philippi, 1887), C. hupeana (Philippi, 1887), and a species assigned to C. geminata by Tavera Jerez (1979). At the same time, Morra (1985) included in Zygochlamys the Patagonian Miocene species Z. nicolasi Morra, 1985 and Z. sebastiani Morra, 1985 and carried out the first detailed description of the genus. He stated that Zygochlamys is a valid taxon with almost equiconvex (Z. geminata) or right-convex shells (Z. jorgensis), whereas in Chlamys s. str. the left valve is substantially more convex than the right valve, the byssal sinus is larger, and the cardinal teeth are less pronounced. Although Morra (1985) failed to provide a clear diagnosis, he mentioned for the first time the presence of shagreen and antimarginal (= ‘camptonectes’) microsculpture to characterize Zygochlamys. In disagreement with Beu (1985), Waller (1991) accepted the use of Zygochlamys for C. patagonica, C. delicatula, C. dichroa (Suter, 1909), and C. taiaroa (Powell, 1952) (the last considered to be a synonym of C. dichroa by Dijkstra and Marshall [2008]) and, like Fleming (1944, 1957), accepted a probable monophyletic origin for Zygochlamys. Del Río (1992) restricted the use of Zygochlamys to species older than the late Miocene in Patagonia, and for that reason, specifically excluded Pecten actinodes from it. Beu (1995) finally accepted the use of Zygochlamys for all the Recent and fossil circumpolar species, continued relating C. actinodes to Zygochlamys, and excluded C. dichroa, which he placed in Talochlamys Iredale, 1929. More recently, in his monograph of the high southern latitude pectinids, Jonkers (2003) placed the Neogene Antarctic species in the new genus Austrochlamys and assigned the circumpolar species Z. patagonica, Z. delicatula, and Z. seymouri to the new genus Psychrochlamys Jonkers (2003). Regarding the composition of Zygochlamys, he included the Patagonian fossil species (Z. geminata, Z. jorgensis, and P. actinodes) and the Chilean Neogene species Pecten vidali Philippi, 1887, P. coquimbensis Möricke, 1896, and P. hupeanus Philippi, 1887. Jonkers (2003) provided a long list of characters diagnostic of Zygochlamys that actually characterize the Chlamydini, such as the presence of shells with chlamydoid acline, prosocline, or opisthocline outlines, with nearly circular or asymmetrical discs, a high auricular asymmetry, a large and arcuate byssal notch with a functional ctenolium, an oblique resilifer, the presence of scaly plicae, smooth or sculptured interspaces, and the development of “shagreen microsculpture restricted to the umbonal region in ancient species, which would be replaced by antimarginal ridgelets in adults, whereas, in modern species, shagreen microsculpture would completely cover the disc” (Jonkers, 2003, p. 36). Morra (1985) proposed that Pecten fissicostalis Ihering, 1899 could have been based on specimens of Z. geminata, but P. fissicostalis is known only from fragmented casts where the number, disposition, and size of plicae just allow it to be considered a member of Zygochlamys. Of the remaining Patagonian species placed in Zygochlamys by Morra (1985), P. nodosoplicata belongs in Swiftopecten Hertlein, 1936 (del Río, 1995) and Pecten quemadensis, Z. nicolasi, and the highly debated Pecten actinodes are herein excluded from Zygochlamys because they belong in new endemic genera, as will be discussed. According to current knowledge, the presence of the genera Zygochlamys, Austrochlamys, and Psychrochlamys in the Neogene–Recent southern circumpolar regions is accepted here, and in this paper we restrict the use of Zygochlamys to the fossil species Z. geminata, Z. jorgensis, and Z. sebastiani of Patagonia. The presence of Zygochlamys in Chile is accepted here. It is represented by the early Miocene casts (SGO.PI 4329, SGO.PI 4459, SGO.PI 5178) assigned by Tavera Jerez (1979) and Frassinetti and Covacevich (1999) to Zygochlamys geminata. The poor preservation of the material precludes any specific assignment, but the arrangement of the paired plicae on right valves and the number of plicae on left valves allow their placement in the genus Zygochlamys. Zygochlamys is characterized by the presence of either right- or left-convex shells with umbonal angles between 80° and 98°. In contrast to what was proposed by Jonkers (2003), there is a remarkably strong ontogenetic variation from juvenile specimens with higher than long triangular discs to adults with more circular but not orbicular outlines. Valves are sculptured with plicae covered with scaly ribs, shagreen microsculpture is restricted on most specimens to a commarginal band between 3 mm and 30 mm from the beaks on both valves, and antimarginal microsculpture is well developed from at least the beginning of the radial stage down to the ventral margin. The commarginal lirae are well developed on the entire umbonal area and as scattered patches on the remaining surface of the disc. Zygochlamys differs from Chlamys Röding, 1798 (type species: Pecten islandicus Müller, 1776, North Atlantic and North Pacific Oceans, early Pleistocene–Recent; MacNeil, 1967, pl. 2, figs. a–d, pl. 18, fig. 8, pl. 19, figs. 2–5, pl. 20, figs. 7–9, pl. 22, figs. 7–8, pl. 24, figs.12, 13) in having larger and thicker shells, with the antero- and posterodorsal margins of the adult disc less inclined and a shallower byssal sinus. In Zygochlamys shells are sculptured with up to 14 paired plicae on right valves (bifurcated in early ontogeny) and up to 23 simple plicae on left valves, and interspaces on both valves are covered with two or three orders of scaly ribs, whereas discs of Chlamys lack plicae but have 30 ribs on left valves and 36 on right valves that commonly bifurcate in late ontogeny, and interspaces have one or two riblets on left valves but only one on right valves. Zygochlamys differs from Psychrochlamys (type species: Pecten patagonicus King, 1832, Magellanic and Argentine Malacological provinces, Holocene–Recent; Jonkers, 2003, pl. 1, fig. 1, pl. 8, figs. c–f, pl. 9, figs. e, f) in having larger, thicker, and less prosocline shells that are higher than long or as high as long in adult stages, a narrower umbonal angle, a longer hinge margin with more asymmetrical auricles, the free margin of left anterior auricles inclined anteriorly, a deeper byssal notch, and a byssal sinus. Sculpture also separates the two genera because Zygochlamys species have higher plicae, interspaces are covered with scaly riblets of two or three orders, and the commarginal microsculpture is almost restricted to the umbonal area or, as happens in Z. sebastiani Morra, 1985, to certain narrow bands of the shells, while in Psychrochlamys coarse commarginal lamellae extend over the entire surface and it lacks the shagreen microsculpture that characterizes Zygochlamys. Zygochlamys differs from Austrochlamys (type species: Pecten natans Philippi, 1845; Antarctica and the Southern Ocean, Miocene–Recent; Jonkers, 2003, pl. 12, fig. a–i, pl. 13, fig. a–g, pl. 14, fig. a–e, pl. 15, fig. a–h; Pirrie et al., 2011: fig. 3; Beu and Taviani, 2014, fig. 3d–f) in having subtriangular shells, more elongated posteriorly; a narrower umbonal angle; more asymmetrical auricles covered with strong riblets; a deeper, rectangular byssal notch; and a byssal sinus. Moreover, in Zygochlamys the plicae are covered with up to five riblets, the interspaces have a higher number of ribs, and shells are sculptured with shagreen microsculpture, while in Austrochlamys, when plicae are present, they are covered with only one or two fine ribs and coarse commarginal lamellae. Zygochlamys is distinguished from Talochlamys Iredale, 1929 (type species: Talochlamys pulleineana (Tate, 1887) (= Chlamys famigerator Iredale, 1925) (Recent, Australia; Beu and Darragh, 2001, fig. 29A–I; Dijkstra and Beu, 2018) (genus: Australia and New Zealand, Eocene–Recent; Beu and Darragh, 2001) by the presence of a much shorter preradial stage, larger and subtriangular discs with paired plicae on right valves that are covered with up to five riblets, and interspaces with more secondary riblets, which arise in early ontogeny. In Talochlamys, shells are ornamented with ribs or simple plicae on both valves, which are covered with only one or three ribs, and the interspaces are covered with fewer secondary riblets, which appear in late ontogeny. In addition, in Talochlamys shagreen microsculpture is infrequent and the entire surfface of the discs is covered with thick commarginal lamellae.» SANTELLI, M. B. & C. J. DEL RÍO. 2019. Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina): Zygochlamys Ihering, 1907 and three new genera. Journal of Paleontology, 93 (2): 312-336, figs. 1-11. [p. 316-318]
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Zygochlamys delicatula (Hutton); H. H. Dijkstra & A. G. Beu, 2018, Living scallops of Australia and adjacent waters, figure 82F.
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«Zygochlamys Ihering, 1907
Zygochlamys Ihering, 1907: 250 (proposed as a subgenus of Chlamys). Type species (by original designation): Pecten geminatus G. B. Sowerby I, 1846. San Julian Formation (late Oligocene or early Miocene), near Puerto Madryn, Argentina.
Psychrochlamys Jonkers, 2003: 43. Type species (by original designation): Pecten patagonicus King, 1832. (Pliocene?) Pleistocene & Recent, southern South America and the Falkland Islands. Diagnosis. Medium to large Pedini with antimarginal microsculpture throughout ontogeny, some (Argentinian Eocene–early Miocene) species with shagreen microsculpture throughout or early in ontogeny; weak interstitial commarginal sculpture in early radial stage; scaly primary, secondary and tertiary radial macrosculpture in a characteristic fasciculate pattern, forming wide, triangular-section primary plicae on lv; dorso-ventrally elongate, with byssal notch and ctenolium prominent when young, becoming subcircular or length exceeding height when adult; byssal notch shallow and ctenolium weak in adults. Internal radial furrows present, representing internal expressions of external plicae. Hinge with moderately weak to prominent resilial and dorsal teeth.
Distribution. Eocene–Recent. Southern Hemisphere (southern Australia, New Zealand, southern Chile and southern Argentina; fossil in the subantarctic islands; Jonkers, 2003), living in the littoral to bathyal zones in colonies on soft sediment, free-lying as adults.
Discussion. Hertlein (1969: N355) treated Zygochlamys as a junior synonym of Chlamys Röding, 1798, placed in the Chlamys group.
Waller (1991: 28) considered Zygochlamys to be an extant genus from the Southern Hemisphere, including the following species: Z. patagonica (King, 1832), Z. delicatula (Hutton, 1873) and the “closely related” New Zealand species Pecten dichrous Suter, 1909 [sic], and Chlamys (Mimachlamys) taiaroa Powell, 1952, and placed it in the suprageneric Chlamys group. Iredale (1925: 253) and Beu (1995: 18) pointed out that the type material of Talochlamys pulleineana is closely similar to Pecten (Chlamys) dichrous and that Chlamys (Mimachlamys) taiaroa is a subjective junior synonym of the former. According to Beu (1995) and Beu & Darragh (2001), Zygochlamys and Talochlamys have different origins, and C. dichroa should be placed in Talochlamys. According to Jonkers (2003) circum-subantarctic Recent species, formerly placed in Zygochlamys, belong in a distinct genus Psychrochlamys, lacking the shagreen microsculpture present on some early species of Zygochlamys. However, in our opinion middle Eocene specimens observed (AGB, with M. Griffin, Museo de La Plata, Argentina) in the Rio Turbio Formation, southern Argentina, are typical of Zygochlamys as well as very similar to Recent specimens of Z. patagonica and Z. delicatula in all characters. This group merely lost shagreen microsculpture during its evolution, and these taxa all form one clade. We follow the opinion of Beu & Darragh (2001: 118) and Dijkstra & Marshall (2008: 63) and treat Psychrochlamys as a synonym of Zygochlamys.» DIJKSTRA, H. H. & A. G. BEU. 2018. Living scallops of Australia and adjacent waters (Mollusca: Bivalvia: Pectinoidea: Propeamussiidae, Cyclochlamydidae and Pectinidae). Records of the Australian Museum, 70 (2): 113-330, figs. 1-102. [p. 273]
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