Scelidotoma aldersoni

Powell, Charles L. II & Geiger, Daniel L., 2019, Two new Miocene limpets (Fissurellidae) from southern California, with notes on other fossil occurrences of the family in northwestern North America, PaleoBios 36, pp. 1-7 : 4-5

SCELIDOTOMA ALDERSONI PoWELL AND GEIGER, N. SP. FIGS. 1, 3, 4

Diagnosis — Scelidotoma aldersoni n. sp. differs from all other northeastern Pacific fossil and modern fissurel- lids by a combination of its large size, oblong-oval shape with convergent margins towards the posterior end, and lacking an oval or circular opening or slit that is common to most Fissurellidae .

Holotype — LACMIP 14846.

Paratype — SBMNH 634129.

Type locality —Locality LACMIP 31503, approximately 1200 feet north and 1800 feet east of the southeast corner of section 35, T 1N., R 17W. (Malibu Beach 7.5’ Quadrangle), from a Tellina Linnaeus (1758) bed about 5–6 m above 50 foot sandstone bed in amphitheater along Old Topanga Road. Collected by John Alderson, ca. 2001. Coordinates are latitude 34.12°, longitude -118.64°. This locality was collected from the Cold Creek Member of the Topanga Canyon Formation, lower to middle Miocene (“Temblor” California provincial molluscan stage, =the uppermost Burdigalian/Langhian Stage of the International Commission on Stratigraphy).

The paratype comes from “lower Kinton Point” formation (informal; =”Vaqueros” Formation) in the first sandstone north of Posa Canyon on the southwest side of Santa Cruz Island, Santa Barbara County, at an approximate latitude of 34.0°, longitude -119.8°. The “Kinton Point formation” is an undescribed manuscript name and does not have any lithostratigraphic significance. Collected by Ron Hoeppel and Bob Bereskin sometime in the 1960s. Associated with the specimen are the bivalves Dosinia merriami Clark (1915) , Macoma Leach (1819) sp. , and Yoldia Möller (1842) sp. (identified by Ron Hoeppel; information provided by Greg Wahlert of UCSB).

Occurrences —Known from a single specimen from the type locality ( LACMIP 31503) in the early and middle Miocene Topanga Canyon Formation in the Santa Monica Mountains and provisionally (cf.) from an internal mold from the middle Miocene “Vaqueros” Formation exposed in Canada Posa on the southwest coast of Santa Cruz Island in the northern southern California Bight (locality UCMP 1402, =locality UCSB 1682).

Etymology —This new species is named for John Alderson who collected the Topanga Canyon Formation specimens and is an expert on the Topanga Canyon Formation in the Santa Monica Mountains.

Description

The shells are shaped like an oblong oval with the sides slightly inclined with a rounded, but blunt posterior end that on the holotype may be slightly impressed. The holotype is 54.4 mm wide, 39.6 mm wide and 23.2 mm tall ( Figs. 1, 4) and the paratype is 48.2 mm long, 35.5 mm wide and 19.2 mm tall ( Fig. 3). The apex is approximately ⅓ of the shell length from the anterior end. On the holotype, the surface of the shell shows very low, faint, rounded top growth lines, although it appears nearly smooth to the naked eye. A profile through the apex and anterior and posterior margins show a slight concavity between the anterior end and the apex, and a relatively straight profile between the apex and the posterior end. There is a wide, shallow, rounded selenizone running from about a centimeter below the apex to the dorsal margin. Towards the apex from this point the preservation differs and the depression cannot be observed. The shell is of moderate thickness and light to medium gray in color, although the coloration appears to be an artifact of preservation and not related to the original coloration. The aperture is broken off on the holotype and the paratype is an interior mold.

Discussion

Scelidotoma aldersoni is the first fossil member of the genus described from western North America. Inclusion in the genus is indicated by the limpet-shape of the shell, the presence of axial sculpture, the anterior notch, and the interior channel. The only known modern (Holocene) member of Scelidotoma is the northeastern Pacific S. bella , from which S. aldersoni differs by a more eccentric apex (~55% vs. 70% from anterior margin). It is similar in appearance to the genus Scutus from the western Pacific differing in many features of the shell and living animal, but is distinguished above.

The only previous fossil records of Scelidotoma in western North America is MacNeil (1957) who reported Emarginula (Subemarginula) aff. E. yatesii Dall (1901) (= Scelidotoma aff. S. bella ) from the Nuwok Member of the Sagavanirktok Formation, exposed in the Marsh Anticline at Carter Creek, Arctic National Wildlife Refuge on the North Slope of Alaska. Age determination of the Nuwok Member are wide ranging with ages of Oligocene ( McNeil and Miller 1990), Miocene (in part Dall 1920, MacNeil 1957), and Pliocene (in part Dall 1920, Fouch et al. 1990, Marincovich and Powell 1991) having been determined for exposures at Carter Creek, where Dall’s (1901) specimen was collected.

The genus Scelidotoma is also reported here as a fossil from several widely separated locations and ages in the western United States. A single juvenile Scelidotoma ( Fig. 6) from the middle Eocene Crescent Formation in the Black Hills, near Olympia in western Washington State, can be positively identified to genus based on the interior axial channel from the anterior notch and the axial cords on the exterior of the shell and is illustrated here ( Fig. 6). In addition, collections from the San Diego Formation (LACMIP and SDSNH collections) contain specimens of S. bella , extending its biostratigraphic range from the Holocene back to the Pliocene and the biostratigraphic range of the genus Scelidotoma from the Holocene to the Eocene in the western western North America.