Limopsis squiresi, Hickman, 2023

Hickman, Carole S., 2023, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs, PaleoBios 40 (5), pp. 1-51 : 18-21

LIMOPSIS SQUIRESI N. SP.

FIG. 10C, E

ZooBank LSID — urn:lsid:zoobank.org:act:8D7171A4-268E-4C19-AA5F-7264CDAF1E94

Diagnosis —Shell small (<10 mm), ovate, nearly equilateral, thin, somewhat compressed, with impressions in matrix of numerous (>40) fine radial ribs and thatched periostracum. Hinge line straight, with central trapezoidal amphidetic alivincular ligament and resilifer. Remnant of fibrous ligament preserved but beaks and hinge teeth missing. Inner aragonitic layer glossy; altered outer layer preserving radial ribs, but lacking fine detail preserved in external mold.

Discussion —Description of a new species based on a single specimen is seldom justified unless it is extraordinarily well preserved. The specimen described and figured here is remarkable because it does preserve evidence of the characteristic dense exterior periostracum, the ligament pit with altered remnant ligament, and features of both outer and inner shell layers ( Fig. 10 C, E ). The only previously described Limopsis species from the Cenozoic of the Pacific Northwest are based on type specimens that are less well preserved. The primary justification for proposing a new name here is to provide a search image for collectors who are eager to include the potentially valuable specimens and fragments that typically are left behind in the field.

The shell is larger, more compressed, and more obliquely ovate than Limopsis marysvillensis Dickerson (1913) ( Fig. 10D ) from the Paleocene and early Eocene Meganos and Capay Stages (Merriam and Turner 1937) of northern and southern California (E.J. Moore 1983). Dickerson’s species is based on a minute (ht. 5 mm, wd. 5 mm) shell from a tropical Eocene fauna in northern California and was subsequently recognized by Givens (1974) from coeval strata in southern California. The species was described originally under Glycymeris da Costa (1778) with two line drawings. The holotype was damaged and ineptly glued together but is illustrated photographically here in hinge view ( Fig. 10D ) because it shows the hinge teeth and trapezoidal ligament and is more informative than Dickerson’s drawings. Limopsis nitens ( Conrad, 1849) from a bathyal methane seep horizon in the uppermost Lincoln Creek Formation (early Oligocene) also has a smaller, thicker, and more inflated shell that is distinctly crenulate on the inner margin. It is abundant at the type locality (Knappton, Washington) and well documented by E.J. Moore (1963) and Kiel (2010). Limopsis carmanahensis Clark (1925) from the upper Oligocene Sooke Formation on Vancouver Island, was described from a single specimen. Because it lacks a straight dorsal margin and exposed hinge it cannot be confirmed as a limopsid. Limopsis phrear Woodring (1938) , known only from lower Pliocene well cores in the Los Angeles Basin, is more obliquely ovate and more closely allied with living Eastern Pacific deep-water limopsids. A specimen of the living L. panamensis Dall, 1902 from 1,719 m (reported as 940 fms.) is illustrated here ( Fig. 10A, B ). Epibenthic dissolved oxygen concentrations associated with live-collected individuals indicate that it lives under moderately to severe hypoxic conditions (Suárez-Mozo et al. 2019).

Limopsid history in the Northeastern Pacific has four distinct phases: (1) Late Cretaceous diversification and prominent representation in shallow-water communities (Squires 2012) followed by (2) Paleogene decline in diversity, shell size, and abundance in the tropical Eocene fauna, (3) Eo-Oligocene disappearance from shallow water at the onset of global cooling, and (4) appearance in deep-water turnover communities ( Hickman 2003) in association with chemosymbiotic bivalves. Prominence in deep- and cold-water environments in the Northeastern Pacific persists in the modern fauna.

Limopsid history in the southern hemisphere is characterized by a similar Cenozoic transition at the onset of global cooling. The event corresponds with the isolation of Antarctica and establishment of the circum-Antarctic Current and resulted in an evolutionary radiation of cold-water limopsids (Whittle et al. 2011).

Etymology —Named for Richard L. Squires in recognition of his many contributions to systematic paleontology of Paleogene mollusks of the Northeastern Pacific and his documentation of Late Cretaceous shallow-water limopsids.

Material examined — one specimen. The new name is proposed following focused examination of fossil and living limopsids in an effort to encourage more careful attention to poorly preserved and fragmental material in Paleogene bathyal facies, including unique detail in external molds in fine-grained rocks.

Holotype — UCMP 110728 , length 9 mm, height 9.5 mm (a nearly complete disarticulated left valve).

Type locality — UCMP IP1600 (= USGS 15315 ) . Upper member, Keasey Formation. The specimen was collected and donated by Casey Burns. The fauna at the type locality includes basal root tufts of hexactinellid sponges and a unique assemblage of bivalves suggestive of methane seepage, hypoxic conditions, and chemosynthetically fixed carbon.

Comparative figured material — Limopsis marysvillensis , holotype, UCMP 11766 and Limopsis panamensis , hypotype, SBMNH 474078.

PTERIIDA NEWELL, 1965

PTERIOIDEA J.E. GRAY, 1847

ISOGNOMONIDAE WOODRING, 1925

Isognomonids are an ancient cosmopolitan group of shallow water, epifaunal and semi-infaunal, byssally attached bivalves that first appear in the Permian. Shells are edentulous, with an internal multivincular ligament. Shell shape varies from rounded to narrowly elongated. Taxonomy and phylogenetic relationships are poorly resolved and contentious because isognomonids are only one of five pterioid families with multivincular ligaments, variation in shell shape, and variation in inferred life habit. Fossils in these families are often poorly preserved. Independent origins of the multivincular ligament have been proposed by many authors (e.g., Cox 1954, Kauffman and Runnegar 1975, Fischer-Pietté 1976, Crampton 1988, Knight and Morris 2009). A brief comparative summary of each family follows.

Isognomonidae and Pulvinitidae Stephenson (1941) are the only two families with living representatives and known anatomy and are the most easily distinguished. Living and fossil Pulvinitids are distinguished by few, closely spaced ligament grooves with narrow interspaces and a circular foramen in the right valve. The byssus emerges from the foramen in the deep-water monotypic genus Foramelina Hedley (1914) . The life habit is epibyssate and acline, closely attached with valves horizontal to the hard substrate. Pulvinitids are the most easily distinguished from the other duplivincular families by the byssal foramen in the functionally ventral valve. Brooding has been documented in living Foramelina, (Tëmkin 2006) although planktotrophic development is inferred Isognomonids and other pterioids.

Pulvinitids appeared in the Late Jurassic and have a highly discontinuous fossil record and biogeographic distribution. However, two species described by Zinsmeister (1978) from the Paleogene of southern California along with a new species from the Cretaceous of the Antarctic Peninsula demonstrate survival across the K–Pg boundary. A Paleocene species subsequently described from the Atlantic Coastal Plain of North America (Ward and Waller 1988) further demonstrates survival, although there is a significant gap between the shallow-water Paleocene species and the living deep-water Australian species.

Isognomonidae and Inoceramidae Giebel (1852) are less easily distinguished. Cox (1954) saw no reason to recognize them as separate, although he subsequently did so in the Treatise on Invertebrate Paleontology ( Cox 1969a, b). Other authors have noted differences that include shell microstructure and attachment of the ligament to the interior, aragonitic nacreous layer in isognomonids and to the exterior prismatic calcitic layer in inoceramids. However, ultrastructural evidence suggests that architectural reorganization of hingeplate mineralogy from aragonitic to calcitic occurred within inoceramids between the Jurassic and Cretaceous (Knight and Morris 2009). Additional distinguishing features of the inoceramid ligament area include more numerous ligament pits and wider interspaces between pits. Crampton (1988) provides a detailed and well-illustrated comparison of diagnostic and differential characters of the two families, including a table with literature references and page numbers. Both families include a variety of shell shapes and both epibyssate and endobyssate species.

Isognomonidae and Retroceramidae Koschelkina (1971): Triassic and Jurassic retroceramids have been separated more recently from placement in Isognonomidae. As in isognomonids the ligament area is attached to an inner aragonitic hingeplate, but Crampton (1988, p. 972) notes that some Late Triassic and Early Jurassic species are difficult to assign to either family, an observation consistent with difficulties reconstructing evolutionary divergences in the Early Mesozoic.

Isognonomidae and Bakevillidae King (1850): Cox (1954) resuscitated Bakevillidae as a family-group name for another group of multivincular taxa from the Permian of England with distinctive obliquely-elongate shells with a prominent posterior wing. Irregular spacing of the ligament pits is a distinctive apomorphic feature of bakevillids (Waller 1998). Isognomonids may have arisen from bakevillids in the Triassic, but this is another Early Mesozoic evolutionary scenario that lacks strong support.

Stratigraphic range —Permian–Holocene.

Figures

Figure 10. Living and fossil limopsids. A, B. Exterior and interior of left valve, Limopsis panamensis Dall (1902), SBMNH 474078, scale bar=1 cm. C, E. Limopsis squiresi n. sp., Keasey Formation holotype, UCMP 110728. C. Left valve shell interior and resilifer preserved dorsally in matrix, with remnant periostracum and exterior shell of right valve preserved ventrally. E. Fragment of right valve exterior outlined in yellow on C, removed and inverted. Scale bar=5 mm. (D). Limopsis marysvillensis (Dickerson, 1913), Capay formation holotype, UCMP 11766.