By Jack D. Mount
During a field trip of the Paleontological Society of Southern Arizona (PSSA) in March 1986 to
sections of marine Permian limestone in central Cochise County, Arizona, specimens of the pectinid mollusk Aviculopecten were
collected from a locality south of the town of Tombstone. Pectinids, commonly referred to as scallops, are members of the bivalve
superfamily Pectinacea and are known from the Ordovician to the Recent. The genus Aviculopecten is reported from the
Mississippian through Permian (Cox and others, 1969). The specimens of Aviculopecten described in this note constitute a new
species and the first record of the genus from the Permian of southern Arizona.
The fossil locality is in the Tombstone Hills in limestones of the Colina Formation as mapped by Gilluly and others (1956). The Colina Formation was named and described by Gilluly and others (1954) with the type section on Colina Ridge in the Tombstone Hills. The Colina is dominantly a dark-gray, thick-bedded limestone ranging in thickness from 190 m. to 284 m. (Keith and Wilt, 1978). These rocks were originally included in the Naco Formation of Ransome (1904). Ransome regarded the Naco as Pennsylvanian in age but noted that the fossils of the upper part were similar, to the Hueco Formation of Texas now known to be Permian in Age. Gilluly and others (1954) raised the Naco to a group and divided it into six formations in ascending order: Horquilla Limestone, Earp Formation, Colina Limestone, Epitaph Dolomite, Scherrer Formation, and Concha Limestone.
No other fossils were collected with the Aviculopecten. However, the Colina Limestone is moderately fossiliferous; and the fauna is characterized by a large variety of gastropods including Omphalotrochus, Straparollus, Orthonema, Naticopsis, Euphemites, Meekospira, and Anomphalus. Other less common elements of the fauna include brachiopods, scaphopods and echinoid spines (Gilluly and others, 1954 and 1956).
Genus AVICULOPECTEN McCoy
Aviculopecten new species
Figures 1 and 2
Valves medium in size for the genus, orbicular in shape. Length of the hinge about 0.7 times the
length of the valve. Auricles without ornamentation. Valves with concentric ornamentation of numerous fine slightly raised lines
crossing both costae and interspaces.
Left valve with 18 to 19 low flat primary radial costae, increasing in number by intercalation, spaces between costae narrow. Byssal sinus shallow and broadly curved.
Right valve with 17 to 18 low flat primary radial costae, costae slightly narrower than on left valve, increasing in number by bifurcation and becoming finer and more numerous than on left valve.
Figure 1. Aviculopecten new species,
specimen 86-3/1, left valve, x2.3.
Specimen no. 86-3/1, left valve, 18 mm. high, 17 mm. long. No. 86-3/2, right valve, 17 mm high, 16 mm. long. No. 86-3/7, incomplete right valve, 33 mm. long; if complete, estimated height would be approximately 35 mm.
Two figured specimens: numbers 86-3/1 and 86-3/2. Five other specimens: 86-3/3 to 86-3/7. The specimens are currently in the possession of the author.
PSSA locality 2; Mount's field locality 86-3. In the eastern Tombstone Hills, Cochise County, Arizona; in middle of roadcut on west side of U. S. Highway 80, 2.1 km. south of intersection with Cowan Road; 670 m. west and 240 m. south of the northeast corner of Section 31, Township 20 South, Range 23 East, Tombstone 7 1/2 Minute Quadrangle, 1978 edition.
The locality is in dark gray thinly bedded limestone of the Colina Limestone. The exact stratigraphic position of the site is probably not determinable. Based on the position of the locality as plotted on the geologic map of Gilluly and others (1956), it appears to be very low in the formation.
Early Permian; probably Wolfcampian. Gilluly and others (1954 and 1956), based on the assemblages of invertebrates collected by them, suggested that most of the Colina Limestone is Wolfcampian in age and that the upper part of the formation may be Leonardian. Sabins and Ross (1963) and Ross and Tyrrell (1965) made collections of fusulinids from Pennsylvanian and Permian sections in Cochise County. They determined that the upper beds of the underlying Earp Formation are Wolfcampian age. They found no fusulinids in the Colina Limestone or the overlying Epitaph Dolomite but suggested that the Colina is probably Wolfcampian and Leonardian in age and the Epitaph Dolomite is possibly Leonardian. Dirks (1966) and Lyons (1989) collected Leonardian fusulinids from the upper part of the Colina Limestone.
Aviculopecten new species bears some resemblance to A. mazonensis Worthen from the Pennsylvanian of Illinois (Newell, 1937). However, the later species has ornamented auricles, more numerous radial costae, and less rounded valves.
A. kaibabensis Newell (1937) from the Permian Kaibab Limestone near Flagstaff, Arizona, is a much larger species with a longer hinge line and much more numerous and finer radial costae.
The Colina species is more circular in outline and has smoother, more evenly sized radial costae than A. vanvleeti Beede from the Permian Whitehorse Sandstone of Oklahoma and Texas (Newell, 1937).
Cox, L. R., and others, 1969. Bivalvia. Treatise on Invertebrate Paleontology. Boulder, CO: Geological Society of America, part N, vol. 1, 489 p.
Dirks, Thomas N., 1966. The Upper Paleozoic Stratigraphy of the Quimby Ranch Area, Southern Guadalupe Canyon Quadrangle, Cochise County, Arizona. Tucson: University of Arizona, MSc thesis, 79 p.
Gilluly, James, and others, 1954. Late Paleozoic stratigraphy of central Cochise County, Arizona. U. S. Geological Survey Professional Paper, no. 266, 49 p.
______. 1956. General geology of central Cochise County, Arizona. U. S. Geological Survey Professional Paper, no. 281, 169 p.
Keith, Stanley B., & Wilt, Jan C., 1978. Second day. road log from Douglas to Tucson via Bisbee, Tombstone, Charleston, Fort Huachuca and Sonoita. Land of Cochise. Southeastern Arizona. Albuquerque: New Mexico Geological Society, 1978. pp. 31-76.
Lyons, Timothy W., 1989. Stratigraphy and Depositional Environment of the Colina Limestone (Lower Permian), Southeastern Arizona. Tucson: University of Arizona, MSc thesis, 260 p.
Newell, Norman D., 1937. Late Paleozoic pelecypods: Pectinacea. State Geological Survey of Kansas, Reports, vol. 10, part 1, pp. 1-123.
Ransome, Frederick L., 1904. The geology and ore deposits of the Bisbee Quadrangle, Arizona. U. S. Geological Survey Professional Paper, no. 21, 168 p.
Ross, Charles A., & Tyrrell, Willis W., Jr., 1965. Pennsylvanian and Permian fusulinids from the Whetstone Mountains, southeast Arizona. Journal of Paleontology, vol. 39, pp. 615-635.
Sabins, Floyd F., & Ross, Charles A., 1963. Late Pennsylvanian-Early Permian fusulinids from southeast Arizona. Journal of Paleontology, vol. 37, pp. 323-365.
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