Propeamussium (Parvamussium) mistensis, 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 : 33-36

PROPEAMUSSIUM (PARVAMUSSIUM) MISTENSIS N. SP.

FIGS. 21A–E . 22A–C , 23A–D

Propeamssium n. sp. R.C. Moore and Vokes (1953). p. 119.

Propeamussium sp. Hickman (1984). p. 1224.

ZooBank LSID — urn:lsid:zoobank.org:act:3BEBE80B-2C47-4694-A13C-DB5A3B9FAB0D

Diagnosis —Shell small (<5 mm),thin, outline circular, equilateral with length slightly greater than or equal to height; inequivalve: larger, right valve with smaller calcified portion and missing uncalcified maginal flexible apron (not preserved) that fits adpressed to the ventral margin of larger left valve; hingeline narrow with small auricles distinctly set off from disc, right anterior auricle with byssal notch but no ctenolium; exterior surface sculpture discrepant; right valve with numerous sharply defined commarginal riblets and no radial riblets, left valve scabrous, with prominent primary, secondary and tertiary radial riblets crossed by weaker commarginal lirae with semi-erect spines or nodules forming at intersections; interior of both valves smooth with prominent radial slats of lathic calcite imbedded in crossed-lamellar aragonite.

Description —The squamose or cancellate surface texture of the larger left valves is well-preserved in exterior molds ( Fig. 21A, B ). Although the surface of the smaller right valves is more subdued in the absence of radial riblets, the commarginal riblets are well defined in exterior molds ( Fig. 21C, D ). Radial riblets are absent in the umbonal region of the left valve, with inception occurring at approximately 1/3 of the distance from the umbones and continuing to the ventral margin of the disc. Secondary and tertiary riblets are added by intercalation. The total number of riblets is variable, with 55 on the holotype. The total number of commarginal riblets on the right valve is approximately 25–30. Sculpture of the anterior and posterior auricles of the right valve includes both radial and commarginal sculpture while those of the left valve are predominantly concentric on the posterior ear, with strongly developed radials on the anterior ear. Interior molds preserve deep impressions of 12 to 14 narrow slat-like ribs.

An incomplete interior mold of a valve from Mist ( UCMP 110764) lacks auricles (it may be either a right or a left valve), but the disc preserves deep impressions of 14 ribs that extend to the ventral margin of the valve. A small fragment of adhering shell, removed from the mold, preserves portions of five of the imbedded slat-like ribs.

Discussion —This is the first population of an unequivocal propeamussiid named and described from the Paleogene of the Cascadia Margin. It is also the earliest documentation of Eastern Pacific propeamussiids from a methane seep hardground. However, Kiel (2006) extracted a suspect propeamussiid shell with well-preserved foliated calcite microstructure from an Oligocene limestone float block in Washington State yielding δ 13 C values indicating methane seep origin, describing it in open nomenclature as Catillopecten sp.

Byssally- attached “living fossil” propeamussiids were discovered earlier on the periphery of hydrothermal vent systems at 13 o and 9 o N on the East Pacific Rise (Schein-Fatton 1985). Described as a new genus and species, Bathypecten vulcani Schein-Fatton (1988) , it is now assigned to Catillopecten Iredale (1939) . Detailed anatomical study of this peri-hydrothermal vent species ( Beninger et al. 2003, LePennec et al. 2003) reveals a unique and putatively archaic particle processing mechanism. This reinforces previous evidence for a peripheral zone of seep mollusks uniquely adapted to hypoxic and chemically hostile environments as well as opportunistic suspension feeding on an unconventional carbon source—in this instance chemosynthetic microbes in the water column.

The new Keasey propeamussiid is easily distinguished from all previously described Eastern Pacific species. Details of the discrepant sculpture on the discs and auricles of the right and left valves, delayed ontogenetic appearance of the slat-like internal ribs, the presence of a byssal notch, and squamose sculpture on the right valve support its assignment to Parvamussium rather than Propeamussium s.s.

Although Paleogene propeamussiids have a long history of recognition in California, most of the material is preserved in mudstone facies as poorly-preserved interior casts recognized primarily by sharply-incised impressions of the radial ribs. Propeamussium interradiatus ( Gabb, 1869) has been used for much of this material, and difficulties with the typification and handdrawn illustrations of Gabb’s species have been discussed by many authors (e.g., Stewart 1930, Grant and Gale 1931, Addicott 1971). Five California Paleogene species of Parvamussium are recognized and illustrated by E.J. Moore (1984b). Propeamussiids in Alaska (Scholl et al. 1970), in rocks originally considered Paleozoic, were identified, and described by Addicott (1971) along with Eocene Foraminifera, extending the distribution to high latitude in the Northeastern Pacific.

Unfortunately, the exterior and interior molds of the new Keasey species are difficult to illuminate and photograph, appearing in a common optical illusion that reverses positive and negative features ( Fig. 21 ). Latex casts of these specimens, coated with ammonium chloride ( Fig. 21 ), provide a true indication of negative and positive but fail to capture the natural appearance of specimens as they appear in the field and in museum collections. The diagnosis and description of P. mistensis is based microscopic study and drawings combined with data extracted from three different image sets described below.

In spite of the difficulties of specimen illustration, this is the best-preserved fossil propeamussiid species from the Northeastern Pacific, prompting the following note on the image sets.

Image sets —Three kinds of images were assembled for the holotype and each of eight paratypes from the Mist locality. Figure 21A–E illustrates the holotype and four of the paratypes, each represented by a color photograph of the actual specimen paired with a black and white photograph of a latex cast coated with ammonium chloride. Figure 22A–C illustrates three paratypes represented by photographs of uncoated latex casts. In some instances, photographs of actual specimens under different lighting reveal details useful for interpretation. In addition, Fig. 23A–D provides rendered drawings as a summary guide for interpreting future collections of fragments and imperfect specimens.

Etymology —named for the classic crinoid locality, cold seep, and river bluff outcrops at Mist, Oregon.

Material examined —Although the primary types (collected by Harold Vokes) are all from the carbonate hardground of the main crinoid lagerstätte at Mist, specimens of the propeamussiid occur in float from a higher siliciclastic horizon with abundant spines and tests of two echinoid species. To avoid confusion, a new UCMP locality number is not assigned (see below).

Holotype — USNM 561849 , (incomplete) length 8 mm, height 9 mm.

Figured paratypes — USNM 561850, length 5 mm, height 5 mm; USNM 561852, length 6 mm, height 6 mm; USNM 561853, length of hingline 3.3 mm; USNM 561854, length 7 mm, height 6 mm; USNM 561856, length 8.1 mm, height 8.3 mm. Measurements are approximate, greatest dimensions are all within 5–8 mm.

Unfigured paratypes — USNM 561851 , USNM 561855 , USNM 561857 .

Additional Unfigured paratype — UCMP 110764 . This specimen is from a float block with echinoid spines from above the main crinoid layer and is the only specimen preserving shell material .

Type locality — USGS 15318= UCMP A5018. Numbers refer to the generalized Mist locality, including a prominent bench adjacent to the river bluff that was removed by commercial quarrying of the crinoids. For an excellent description of the stratigraphy at Mist see Burns et al. (2005).

Figures

Figure 21. Propeamussium (Parvamussium) mistensis n. sp., paired color images of molds in matrix and corresponding ammonium chloride-coated casts. A. Left valve exterior, holotype USNM 561849, length 8 mm. B. Left valve exterior, paratype USNM 561856, length 8.1 mm. C. Right valve exterior, paratype USNM 561850, length 5 mm. D. Right valve exterior on top of left valve, paratype USNM 561854, length 7 mm. E. Left valve interior, Paratype USNM 561852, length 6 mm.

Figure 22. Propeamussium (Parvamussium) mistensis n. sp., uncoated latex casts of three paratypes. A. Left valve exterior, USNM 561853. B. Left valve exterior, USNM 561856. Right valve exterior, USNM 561850. Scale bars=3 mm.

Figure 23. Propeamussium (Parvamussium) mistensis n. sp., reconstruction of major features of shell morphology based on all material examined. A. Left valve exterior. B. Left valve interior. C. Right valve exterior. D. Right valve interior. Dashed lines in C, D indicate extent of the larger left valve and uncalcified flexible apron of right valve.