Delectopecten kieli, 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 : 25-28

DELECTOPECTEN KIELI N. SP.

FIGS. 17A–E , 18A–L

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.? Delectopecten sp. Kiel (2006). p. 133; figs. 3.4, 3.5.

ZooBank LSID — urn:lsid:zoobank.org:act:165BAA17-9286-4E98-B4FF-616439C176CF

Diagnosis —Shell small, subcircular in outline, equivalve, length equal to or slightly greater than height; anterior auricle large, with deep byssal notch and imperceptible to weakly-developed ctenolium in right valve; posterior auricle small, weakly-developed, merging indistinctly with posterior shell margin; very fine camp- tonectes sculpture well-developed, similar on both right and left valves but variable between specimens; early portion of disc smooth, with finely beaded and slightly antimarginal striae becoming increasingly well developed; commarginal striae imperceptible and irregularly developed, most prominent as periodic growth pauses interrupting antimarginal striae; shell interior with prominent fold defining right anterior ear byssal notch; small triangular resilifer distinct in many specimens; anterior and posterior adductor scars not preserved.

Discussion —The anterior auricle is shorter and more rounded than in the Delectopecten vancouverensis and the shell shape (height≈length) differs from that of the type species (height>length). The most important feature of the shell exterior is the distinctive reticulate pattern of camptonectes microsculpture ( Figs. 17E , 18A–E, J–L ). The most important features of the shell interior are the well-preserved resilifer and the prominent fold bordering the anterior ear and byssal notch of the right valve ( Fig. 18G–J ).

Occurrences range from individual internal and external molds to concentrations of highly altered disarticulated valves and concentrations of well-preserved and occasionally articulated valves. Some localized concentrations are monospecific ( Fig. 15A, B ) while others include echinoid spines ( Fig. 15C ) and fish scales ( Fig. 15D ). The tooth of a demersal angelshark in one of these concentrations suggests that it may be a regurgitate of undigested material.

It is remarkable that this species had not been collected prior to its discovery in such great abundance at the type locality and a closely-adjacent small outcrop in the lower member of the Keasey. The type lot includes> 100 specimens recovered from disaggregated matrix. Additional shells remain in hand samples. Discovery of the fresh outcrops on a new logging road was fortuitous. A diverse fauna of rarer gastropods is associated with the pectinid concentrations. This is the type locality for Conus weltoni Hickman (1980) , Svelltella? keaseyensis Hickman (1980), and Turrinosyrinx nodifera Hickman (1976) . These three species also are restricted to the lower member. It is also the type locality of a minute (<3 mm) marginellid gastropod Granula profudorum Hickman (1980) , also known from two localities in the middle member.

Comparisons —This species is easily distinguished from other pectinids and propeamussiids in the Keasey Formation by its distinctive camptonectes sculpture, strongly developed anterior ear and byssal notch, weakly developed posterior ear, and shell proportions. It is similar to two Paleogene species from California, notably many of the specimens that have been assigned various authors to Pecten peckhami Gabb (1869) . The lack of a clearly designated type is reviewed by Stewart (1930), and the assignment of California Eocene and Oligocene forms assigned to Gabb’s species are reviewed by E.J. Moore (1963, 1984) as belonging to D. peckhami of Arnold (1906). The difficulty of comparison resides primarily in poor preservation, especially lack of the diagnostic microsculpture.

The closest comparison to the Keasey species is Delectopecten sp. Kiel (2006) from the Lincoln Creek Formation in Washington. It is known from a single small (4 mm) incomplete left valve. However, the camptonectes microsculpture on the posterior portion of the shell and auricle in Kiel’s micrograph (his Figure 15.5 ) is indistinguishable from the unique microsculpture of D. kieli . It is also notable that the Lincoln Creek specimen is from a cold seep carbonate, consistent with seep associations of the Keasey pectinids and propeamussiids. As noted above, the living type species of Delectopecten is an oxygen minimum zone species, occurring in association with chemosymbiotic bivalves and mollusks that tolerate severe hypoxic conditions.

Etymology —Named for Steffen Kiel for his insightful contributions to molluscan paleontology of hydrocarbon seeps in Cenozoic forearc strata of the Cascadia Margin.

Material examined —more than 200 entire valves (some articulated) and shell fragments from the main outcrop at type locality and a closely adjacent small outcrop on a logging road in the lower member.

Direct measurements from delicate specimens are not possible, and values for size derived from scale bars in photographs and SEM images are approximate.

Holotype — UCMP 110746 , length 3.4 mm, height 3.4 mm. Loc. UCMP IP7983 (formerly USGS 25026 ).

Figured Paratypes — UCMP 110747, length 4.5 mm, height 4.5 mm. UCMP 110748, length 5 mm, height 5 mm. UCMP 110749, length 4.3 mm, height 4.4 mm. UCMP 110750,length 3 mm, height 3 mm. UCMP 110751,length 3 mm, height 3 mm. UCMP 110752, length 3 mm, height 3 mm. UCMP 110753, length 3 mm, height 3 mm. UCMP 110754, length 2 mm, height 2 mm. UCMP 11055, length 4 mm, height 4 mm. UCMP 110756, length 3 mm, height 3 mm. UCMP 110757, length 3 m, height 3 mm. UCMP 110758,length 3 mm, height 3 mm. UCMP 110759,length 2 mm, height 2 mm.

All from Loc. UCMP IP7983 (formerly USGS 25026).

Type locality — UCMP IP7983 and closely adjacent UCMP IP2536 (formerly USGS 25026 and 52025), lower 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 17. Delectopecten kieli n. sp. A. Right valve exterior, Holotype, UCMP 110746. B. Right valve exterior, Paratype, UCMP 110747. C. Left valve exterior, Paratype, UCMP 110748. D. Right valve exterior, Paratype UCMP 110749. Scale bars=1 mm. E. Detail of camptonectes sculpture on B, with reticulations and splitting of fine radial ribs (white circles). Scale bar=0.5 mm.

Figure 18. Delectopecten kieli n. sp. Low voltage scanning Electron Micrographs. A. Right valve exterior, Paratype, UCMP 110750. B. Right valve exterior, Paratype, UCMP 110751. C. Right valve exterior UMMP 110752. D. Right valve exterior, Paratype UCMP 110753. E. Left valve exterior, Paratype, UCMP 110754. F. Left valve exterior, Paratype UCMP 110755. G. Right valve interior, Paratype UCMP 110756. H. Right valve interior, Holotype, UCMP 110746. I. Right valve interior, Paratype UCMP 110757. J. Articulated, left valve exterior rotated within right valve interior, Paratype UCMP 110758. K. Detail of sculpture on J. L. Articulated, left valve exterior displaced on top of right valve interior, Paratype UCMP 110759. Range of shell lengths: 2.2–3.2 mm.