Spondylus batequensis Squires & Demetrion, 1990
SQUIRES, R. L. & R. DEMETRION. 1990. New Eocene marine bivalves from Baja California Sur, Mexico. Journal of Paleontology, 64(3): 382-391, figs. 1-4. [p. 385, figs. 2.6-2.12]
1990 Spondylus batequensis Squires & Demetrion, 1990
R. L. Squires & R. Demetrion, 1990, figure 2.
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«Diagnosis.— Spondylus with left valve having about 12 radial ribs that may be spinose with interspaces containing usually three ribs with or without single intervening radial riblets; right valve (attached) with 10-12 usually fairly evenly spaced, strong, flattish radial ribs bearing upturned elongate spines with interspaces containing usually three spinose radial riblets.
Description.— Medium sized; shell up to 32 mm high (incomplete), subcircular; beaks central; shell thin, moderately inflated; left valve less convex than right valve. Left valve with about 12 primary radial ribs that may have short spines more common posteriorly; bottoms of short spines with longitudinal groove; interspaces between ribs with 1-3 smooth to spinose secondary radial ribs alternating with smooth to spinose tertiary radial riblets; secondary ribs may approach primary rib strength, especially in beak region; anterior auricle sloping (preservation?), with three ribs; small posterior auricle with two faint ribs; both auricles separated from rest of valve by somewhat swollen radial rib; hinge area poorly preserved with triangular ligamental pit elongate. Right (attached) valve with 10-12 usually fairly evenly wide-spaced, strong radial ribs with imbricated spines; spines flattish to spikelike with longitudinal groove on bottom; spines along anterior margin commonly strongest and commonly upturned; spiny nature of right valve produces "thorny" appearance; interspaces between strong radial ribs with 1-7 (commonly three) spinose imbricated riblets; anterodorsal part of right valve may have lamellose overgrowths (which represent the attachment area); small posterior auricle with two riblets, anterior auricle obscured by matrix; interior of valve showing numerous primary radial ribs separated by secondary ribs, all ribs extend from beak to ventral margin with ribs stronger, more flat-topped, and more uniform in size ventrally; hinge area not seen. Remarks.— Spondylus batequensis n. sp. is most closely allied to Spondylus dumosus (Morton, 1834, p. 59, PI. 16, fig. 8, text-fig. p. 60; Dockery, 1982, p. 49, 50, PI. 14, figs. 1-9, text-figs. 33, 34.1) known from the lower Oligocene Red Bluff Formation, Mississippi and southwestern Alabama (Glawe, 1967; Dockery, 1982). Spondylus batequensis n. sp. differs from S. dumosus in the following features: 10-12 rather than 9-10 radial ribs, weaker radial ribs on the left valve, right valve interior with radial ribs throughout, and spinose secondary and tertiary riblets over entire shell exterior rather than only on beak area. These differences become harder to discern on juvenile specimens (less than 30 mm shell height) and juvenile portions of adult specimens of S. dumosus. The evidence suggests that S. batequensis is the ancestral species and some of its morphological traits are retained in the juvenile portion of S. dumosus. Their close relationship is further strengthened by the presence of a longitudinal groove along the bottoms of the flattish radial ribs on both valves. The new species shows more affinity to Gulf Coast lower Oligocene species than to Cretaceous or Eocene species from the west coast of North America. Spondylus striatus Packard (1922, p. 422, PI. 29) and S. rugosus Packard (1922, p. 422, PI. 27, fig. 3, PI. 30, fig. 3, PI. 31, fig. 3) were present in southern California during the Late Cretaceous. These species lack spines. Spondylus cf. S. striatus in Sundberg and Riney (1984, figs. 2-13) from the Upper Cretaceous Lusardi Formation and Point Loma Formation in the San Diego area, southern California, resembles the new species somewhat but is much larger (up to 80 mm height), much less spiny, and the interribs are not spinose. Spondylus batequensis n. sp. is only the second reported Spondylus from the "Capay Stage" of the west coast of North America. Spondylus carlosensis Anderson (1905, p. 194, PI. 13, fig. 1) has been previously reported (Baldwin, 1964, p. 11) from rocks in northwestern Oregon equivalent to the middle lower Eocene "Capay Stage." This species, however, has been reported (Yokes, 1939; Squires, 1984, 1989; Moore, 1987) most commonly from strata in California equivalent to the mostly middle Eocene "Domengine Stage" and "Transition Stage." Spondylus carlosensis is only very obscurely spinose. Spondylus cliffensis Hanna (1927, p. 278, PI. 32, figs. 2, 7) is the only other Spondylus known from the Eocene of the west coast of North America. This species, known only from "Domengine Stage" strata in southern California (Hanna, 1927), lacks spines. Similarly, Spondylus perrini Wiedey (1928, p. 138, Pl. 17, figs. 6, 7) is the only Oligocene species of Spondylus from the west coast of North America, and it lacks spines. Etymology.— The specific name is for the Bateque Formation. Material.— Twenty-one right valves (with nine showing the interior) and 21 left valves. Four of the 42 valves are articulated. Occurrence.— Middle lower Eocene "Capay Stage" (Ypresian Stage). Bateque Formation, Baja California Sur, Mexico, localities CSUN 1220b and 1220c. Repository.— Holotype, IGM 5061 (=plasto-holotype), LACMIP 8064; paratypes, IGM 5062-5066 (=plasto-paratypes), LACMIP 8065-8069; locality CSUN 1220b.» RICHARD LANE SQUIRES & ROBERT DEMETRION, 1990
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