Bathyisognomon smithwickensis, 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 : 21-24

BATHYISOGNOMON SMITHWICKENSIS N. SP.

FIGS. 12A–F , 13A–C

Isognomon clarki ( Effinger, 1938) Squires (1989) , in part, p. 279.

Zoobank LSID — urn:lsid:zoobank.org:act:3E81A342-9A89-4CBD-8A0B-B84164CA2CB0

Diagnosis —Same as diagnosis of the genus, which is monotypic.

Description —The hinge line is slightly arcuate and there is no distinct posterior wing or auricle. The beaks are produced, forming a narrowly acute angle (40–50°). There is a sharp anterior byssal indentation beneath the beaks followed by a nearly straight anterior shell margin. The posterior and ventral margins are evenly rounded posterior to the hinge line. The ratio of height to length=0.7.

Discussion —The description of this species is necessarily broad and subject to qualification based on additional specimens and better preservation of the outer calcitic shell and ornamentation that are not present in the type material. The thickness of the interior aragonitic shell is unknown due to exfoliation of individual layers, which is common in fossil isognomonids with disintegration of the organic layers separating successive thin nacreous increments. This phenomenon is illustrated here ( Fig. 14A–C ) in a set of stacked layers of isognomonid nacre ( UCMP B-5162) from Miocene strata of the Calvert Cliffs, Chesapeake Bay, Maryland.

Squires (1989) conservatively assigned all isognomonid specimens from the Eocene of California, Oregon, and Washington to Isognomon clarki , considering it a single highly variable species with preservation inadequate for morphological separation. Specimens described as Isognomon n. sp.? by Givens (1974) from older Eocene strata of the Juncal Formation in California (Loc. UCR 4752) have a similar mytiliform shape but are proportionally wider and less long than the new Keasey species and come from a calcareous sandy conglomerate lens in a shallower depositional facies. Word-of-mouth references to Keasey specimens in private collections from the Smithwick Quarry could not be confirmed, and access to the abandoned site is now prohibited.

Etymology —The name refers to the now abandoned Smithwick Concrete Products Haydite Quarry, commercial source for manufacture of lite-rock, a mid-20 th century strong, lightweight aggregate material used in dam and bridge construction. Large quantities of volcanic ash in the massive tuffaceous siltstone beds at the Smithwick and Empire Quarries produced a composite of exceptional quality.

Material examined —The description is based on the two double-valved type specimens ( Figs. 12A–F ) and a tentatively-assigned large, double-valved partial specimen ( Fig. 13A–C ) from the upper middle member of the Keasey Formation.

Holotype — UCMP 110744 , length 10.2 mm, height 8.6 mm. Loc. UCMP IP7984.

Paratype — UCMP 110745 , length 7.1 mm, height 5.0 mm. Loc. UCMP IP7984 .

Tentatively-assigned —Loc. LACMIP 5806, length (incomplete) 6.5 mm, height (incomplete) 7.5 mm.

Type locality— UCMP IP7984. Upper middle member.

PECTINIDA J.E. GRAY, 1854

PECTINOIDEA RAFINESQUE, 1815

The ancient order Pectinoidea originated in the Carboniferous. Two of the major clades recognized by Waller (1978, 1991, 2006) are represented in the Keasey Formation. Both appeared in the Middle Triassic. The pectinids are here recognized as having a ctenolium ( Fig. 16C ), a feature that is absent in propeamussiids. However, the ctenolium is variable in its morphological expression and may have arisen multiple times as a series very fine denticles along the ventral margin of byssal notch, serving functionally to separate fine byssal threads and increase attachment stability. Pectinoidean family-group relationships remain unresolved, and molecular data further suggest that propeamussiids are not monophyletic (Smedley et al. 2019). While supraspecific allocation of the Keasey taxa is necessarily provisional, intensive collecting has yielded well-preserved specimens that are morphologically distinct and assignable to family-group shell morphotypes and ecotypes.

Stratigraphic range —Carboniferous–Holocene.

PECTINIDAE RAFINESQUE, 1815

Pectinids are the best known of the pteriomorph bivalves. Commonly known as scallops, the family includes many living and fossil species in cosmopolitan genera. Shell microstructure and mineralogy and the prismatic or foliated calcite of exterior microsculpture are often exceptionally well preserved in fossil pectinids, and the abundance and short stratigraphic ranges of many species have made them especially important in Cenozoic biostratigraphic zonation and correlation. The fossil record of Northeastern Pacific pectinids is predominantly a shallow water record. Less well known are the small-shelled deep-water (> 200 m) pectinids and propeamussiids (mud pectens and glass scallops). They are the most abundant bivalves in the Keasey fauna. However, they are seldom collected and poorly represented in museum collections because the thin translucent shells are fragile and often crushed. They often occur in dense but highly localized aggregations ( Fig. 15A–D ). Shells in some of these assemblages are peculiarly associated with large numbers of small echinoid spines ( Fig. 15C ) or ambercolored fragments of fish scales ( Fig. 15D ). Preservation is usually inadequate for generic and species recognition, and even when delicate ornamentation is well preserved it is difficult to photograph. Low angle illumination required for photography often produces optical illusions in which negative features appear positive. Scanning Electron Microscopy contributes significantly to characterizing microsculpture of the new species described below.

Stratigraphic range —Lower Triassic–Holocene.

Figures

Figure 12. Bathyisognomon smithwickensis n. sp. A–C. Right valve exterior, left valve exterior, and exterior view of ligament pits, double-valved holotype, UCMP 110744. D–F. Right valve exterior, left valve exterior, and exterior view of ligament pits, double valved paratype, UCMP 110745. Scale bars=5 cm.

Figure 13. Bathyisognomon smithwickensis? Right valve exterior, left valve exterior, and exterior view of ligament pits on large, incomplete specimen from Loc. LACM IP5806. Scale bar=5 cm.

Figure 14. Unidentified isognomonid shell with successive nested nacreous layers separated by diagenetic loss of organic shell layers, unidentified species, Miocene of Maryland, Loc. UCMP B-5162. Scale bar=1 cm.

Figure 15. Dense concentrations of mud pectens in matrix. A. Delectopecten keaseyorum n. sp. in silty mudstone, middle member. B. Delectopecten keaseyorum in tuffaceous siltstone. C. Delectopecten kieli n. sp. in silty mudstone with echinoid spines. D. Delectopecten kieli in silty mudstone with fish scales and bone. Scale bars= 1cm.

Figure 16. Delectopecten vancouverensis (Whiteaves, 1893), North Pacific live-collected, articulated representative of the type spe- cies of Delectopecten. SBMNH 130085. A. Left valve. B. Right valve. C. Enlargement of byssal notch with ctenolium (yellow circle). Scale bars=1 mm.