Delectopecten keaseyorum, 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 : 28-30

DELECTOPECTEN KEASEYORUM N. SP.

FIGS. 15A, B ; FIG 19A–E

Delectopecten n. sp. Vokes in Warren, Norbisrath and Grivetti (1945) (checklist).

Delectopecten n. sp. R.C. Moore and Vokes (1953), pp. 115, 119.

Delectopecten sp. Hickman (1984), p. 1221; fig. 5D.

ZooBank LSID — urn:lsid:zoobank.org:act:8B64E474-0E4C-4152-B5FE-8EB1F2F2AC5A

Diagnosis —Shell small (average length= 1 cm), equivalved, with strongly compressed valves of equal length and width; shell material extremely thin, typically preserved with distinctive hyaline or glassy appearance; valves compressed, with slightly raised, sharply pointed umbones; posterior auricles not clearly delimited from disc, flat with fine camptonectes surface sculpture; anterior auricles large, strongly set off from disc; posterior auricles flat, not clearly delimited; right anterior auricle with deep byssal notch and sculpture of 10 to 12 fine radial riblets crossed by fine growth lines; left anterior auricle sharply delimited from disc, flat, with fine camptonectes surface sculpture; disk surfaces of both valves covered with extremely fine camptonectes sculpture, weak commarginal undulations sometimes developed beneath umbones on early portion of disc.

Discussion —Assignment to Pectinidae is based on the presence of the same pattern of camptonectes microsculpture on right and left valves ( Fig. 19A–D ) and the presence of a ctenolium along the ventral margin of the byssal notch ( Fig. 19E ).

This is undoubtedly the most abundant bivalve species in the Keasey Formation. It was collected from many localities and in all three members of the formation during geologic mapping and noted in the checklist of Warren et al. (1945) as Delectopecten sp. Harold Vokes had intended to describe it, and his material and prospective types were examined in the present study. However, better material collected subsequently by the author is the basis of the new name. Lack of representation of this species in collections is attributable to the relative rarity of isolated concretions and crushing of the delicate shells. Less frequent and abundant species traditionally attract collectors with a search image for the rare, single, entire, and well preserved Keasey gastropod and bivalve taxa. Collectors skilled in the preparation of delicate fossils may be rewarded by working with this species.

A preservational peculiarity of this species, especially in the massive tuffaceous siltstone of the upper part of middle member, its occurrence in small, dense concentrations or concretionary masses, isolated from other mollusks at any given locality ( Fig. 15 A, B ). Within these concentrations shells have no predominant orientation and are stacked on top of one another. Although valves are entire, they have been fragmented by compression or distorted by pressure from adjoining individuals (e.g., Fig. 15A, B ; Fig. 19 ). Articulated specimens are present in these concentrations indicating lack of significant preburial transport or exposure.

Etymology —Named for members of the Keasey Family of Vernonia, Oregon for their longstanding interest in the fossil fauna and encouragement of its collection and study, including access to outcrops on their property.

Comparisons — Delectopecten keaseyorum is clearly distinguished from Delectopecten kieli by its larger and thinner shell, hyaline surface, and finer, submicroscopic camptonectes sculpture ( Fig. 19A–E ). The sculpture is visible with a light microscope only at higher magnification and with strongly oblique illumination. As noted above, meaningful comparison with the ill-defined D. peckhami is confounded by Gabb’s generalized original description, persistent lack of a clearly designated type, and inadequate illustration. The broad stratigraphic range (Eocene–Miocene) of specimens assigned to D. peckhami is consistent with the suggestion that it is a complex of deep-water species in need of detailed study and revision. In the experience of this author, the most similar material to D. keaseyorum is from the Paleogene Kreyenhagen Formation of the Coalinga district in California, removed from D. peckhami and described as D. lillisi Hertlein (1934) . E.J. Moore (1963, 1984b) provides a detailed history of the treatment of these small mud pectens and helpful illustration of relatively well- preserved Eocene to Holocene specimens.

Material examined —More than 500 entire valves (some articulated) and recognizable and shell fragments, including material from numeous USGS localities originally collected by Vokes, Warren, Norbisrath, and Grivetti.

Holotype — UCMP 110760 , incomplete left valve in matrix, exposed length 8 mm, exposed height 9.5 mm, Loc. UCMP IP2543 .

Figured Paratypes — UCMP 110761, incomplete right valve in matrix, partially overlapping holotype, UCMP Loc. IP2543; UCMP 110762, incomplete right valve partially overlapping paratype 110761, Loc. UCMP IP2543; UCMP 110763, partial right valve with byssal notch and ctenolium, Loc. UCMP IP16620.

Type locality — UCMP IP2543 (Empire Lite-Rock Quarry) (formerly USGS 25036) (Coll. C. Hickman and Undergraduate Invertebrate Paleontology Class field trip; 1967, C. and J. Hickman, 1970).

Other localities — USGS 15263, 15265, 15306, 15307, 15308, 15309, 15584 (lower member). UCMP IP16620; USGS 15267, 15268, 15274, 15276, 15279, 15280, 15281, 15282, 15283, 15217, 15313, 15314, 15316, 15318, 15509, 15525, 15582, 15602 (middle member). USGS 15315, 15318 (upper member).

Figures

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 19. Delectopecten keaseyorum n. sp. Holotype and two paratypes in matrix. A. Partial left valve exterior over nested right valve fragments, Holotype UCMP 110760. B. Right valve of paratype exterior partially overlapping holotype, UCMP 110761. C. Right valve exterior partially overlapping adjacent paratype, UCMP 110762. Scale bar for A–C=5 mm. D. Detail of fine campto- nectes sculpture on holotype. E. Right valve exterior of paratype in matrix preserving ctenolium denticles in byssal notch (yellow arrow), UCMP 110763. Scale bar=1 mm.