Bulletin of the Southern California Academy of Sciences, Vol. 72, No. 2, August 1973, pp. 111-112
A NEW SPECIES OF STRAPAROLLUS (ARCHAEOGASTROPODA) FROM THE MISSISSIPPIAN OF NEVADA
During a recent biostratigraphic study (Mount, 1972a, 1972b) of the larger invertebrates in the Upper Paleozoic section of the central Pancake Range, Nye County, Nevada, I placed particular emphasis on the distribution of Mollusca in the Mississippian and Pennsylvanian rocks. Excluding the Cephalopoda, mollusks of this age are poorly known in the Great Basin area of North America. In his pioneering study of the Eureka district of Nevada, Walcott (1884) described several new bivalves and gastropods, however in later years so little attention has been given to this group that at the present time it is virtually useless for age determination and correlation. Recently, the stratigraphic positions of Walcott's species have become better known as a result of the biostratigraphic work of Gordon (1971). It is the purpose of this paper to describe a new species of the gastropod Straparollus. Numerous fossil lists include reference to this genus, but it is generally represented by material too poorly preserved or too scanty to allow species assignment. References to the Department of Geological Sciences, University of California, Riverside, are hereafter abbreviated as UCR.
Family Euomphalidae de Koninck, 1881
Genus Straparollus de Montfort, 1810
Subgenus Euomphalus Sowerby, 1812
Straparollus (Euomphalus) pancakensis,
Straparollus (Euomphalus) n. sp. A: Mount, 1972b,
p. 75, pl. 2, figs. 18, 19, 20. Diagnosis: Shell relatively small; whorls 5; spire slightly elevated; single high, broad, rounded spiral rib medially situated between the upper suture and outer shoulder.
Figure 1. Straparollus (Euomphalus) pancakensis, new species. Holotype UCR 7101/1. A. Top view. B. Basal view. C. Apertural view. x 2.
Description: Shell relatively small for the genus, subdiscoidal; spire slightly elevated, the first two whorls flattened; moderately phaneromphalus, umbilicus deep with nearly vertical sides, umbilical angle near 80 degrees. Whorls 5, with moderately rounded angulations on outer-upper and -lower shoulders, outer sides of whorl moderately convex, umbilical profile of whorl weakly convex to flattened. Upper sutures shallow and slightly impressed, passing around the previous whorl just below the upper shoulder. Protoconch unknown. Spiral sculpture consists of a single high, broad, rounded rib between upper suture and angulation on the outer shoulder, width of rib about one third that of whorl, slopes of rib slightly concave. Growth lines orthocline, extending outward to rib on upper whorl, there bending into a sharply defined but shallow sinus; beyond rib growth lines curve slightly backward around whorl to lower suture forming a slightly projecting upper lip over the aperture. Holotype: UCR 7101/1; height 7.8 mm, diameter 20.8 mm, height of aperture 6.0 mm. Paratypes: UCR 7101/2; height 7.3 mm, diameter 20.5 mm, height of aperture 6.7 mm (shell incomplete). UCR 7101/3; height 5.2 mm, diameter 18.8 mm, height of aperture 4.1 mm (shell incomplete). Type locality: UCR locality 7101; 6530 feet south and 675 feet west of long. 115° 50' W., lat. 38° 55' N.; at elevation of 7,240 feet on crest of ridge extending west from first hill north of hill 7865, Brown Summit 7½ minute quadrangle (1968 edition), Nye County, Nevada (same as Department of Geology, University of California, Los Angeles, locality 6058-60). Stratigraphic Position: Ely Group, Moleen Formation, unit 60 of Mount (1972a, 1972b); the fossils occur in platy, thin-bedded, dark gray, silty limestone 20 feet above the base of the formation and can be collected best from the abundant weathered platy slabs that cover the slopes. Age: In addition to the gastropod described here, the following fossils have been identified:
SCYPHOZOA: Paraconularia sp.
BRACHIOPODA: Composita cf. C. subquadrata Hall, Leiorhynchus carboniferum Girty, Ovatia sp. indet.
GASTROPODA: Bellerophon (Bellerophon) sp.
BIVALVIA: Aviculopecten cf. A. affinis Walcott, Aviculopinna cf. A. consimilis (Walcott), Schizodus cf. S. semistriatus Girty.
SCAPHOPODA: Plagioglypta sp. indet.
TRILOBITA: Paladin sp.
BLASTOIDEA: Pentremites sp. indet.
CONODONTA: Cavusgnathus gigantus Gunnell, C. lautus Gunnell, C. spathus Dunn, Gnathodus aff. G. girtyi simplex Dunn, G. muricatus (Dunn), Spathognathodus cf. S. minutus (Ellison), Streptognathodus unicornis Rexroad and Burton.
This fauna is judged to be Late Chester (Late Mississippian) in age and is assigned to the upper part of the Rhipidomella nevadensis Assemblage Zone of Sadlick (1954). This assignment is based primarily on the presence of the conodont Gnathodus aff. G. girtyi simplex which occurs in this zone in southern Nevada (Webster, 1969) and of the characteristic Mississippian brachiopods Composita cf. C. subquadrata and Leiorhynchus carboniferum. Also, about 175 feet higher in the same section Rhipidomella nevadensis (Meek) is abundant in beds that also contain Mississippian species of the brachiopod genera Anthracospirifer, Cleiothyridina, Flexaria, Inflatia, and the bryozoan Archimedes. Discussion: Straparollus (Euomphalus) pancakensis is easily distinguished from all other species of the subgenus Euomphalus by its more strongly pronounced revolving rib which is more medially located between the upper suture and outer shoulder. The only North American species that approaches it is S. (E.) umbilicatus (Meek and Worthen, 1860. 462) from the Middle Pennsylvanian Desmoines Series of Illinois (Knight, 1934: 151), the latter form having a much higher spire, larger number of whorls, narrower spiral rib, and more angulated whorl profile. Straparollus (Euomphalus) planodorsatus (Meek and Worthen, 1860: 462) from the Mississippi Valley is the only species of this subgenus recorded from the Chester Series and it is readily distinguished from the new form described here by the lack of a spiral rib, more rounded whorl profile, higher spire, and somewhat larger size.
The new name proposed here refers to the Pancake Range, the type locality.
Gordon, M., Jr. 1971. Biostratigraphy and age of the Carboniferous formations, in Mississippian stratigraphy of the Diamond Peak area, Eureka County, Nevada. U. S. Geol. Survey Prof. Pap. 661:34-55.
Knight, J. B. 1934. The gastropods of the St. Louis, Missouri, Pennsylvanian outlier; 7, The Euomphalidae and the Platyceratidae. J. Paleo., 8:139-166.
Meek, F. B., and A. H. Worthen. 1860. Descriptions of new Carboniferous fossils from Illinois and other western states. Proc. Acad. Nat. Sci. Philadelphia, 12:447-472.
Mount, J. D. 1972a. Late Paleozoic biostratigraphy of the Pancake Range, Nye County, Nevada. M.S. thesis, Dept. Geology, Univ. California, Los Angeles, viii + 79 pp.
_____. 1972b. Late Paleozoic biostratigraphy of the Pancake Range, Nye County, Nevada. So. California Paleo. Soc. Spec. Publ., No. 1:180.
Sadlick, W. 1954. Mississippian-Pennsylvanian boundary in northeastern Utah. J. Paleo., 28: 506-507. (Abstr.)
Walcott, C. D. 1884. Paleontology of the Eureka district. U. S. Geol. Survey Mon., 8:1-298.
Webster, G. D. 1969. Chester through Derry conodonts and stratigraphy of northern Clark and southern Lincoln Counties, Nevada. Univ. California Pubt. Geol. Sci., 79:1-121.
JACK D. MOUNT, Dept. Geological Sciences, University of California, Riverside 92502.