Girtypecten (Sinopecten) newelli, Fang & Sun & Baliński, 2006

Fang, Zong-Jie, Sun, Yuanlin & Baliński, Andrzej, 2006, A new aviculopectinid bivalve from the Early Carboniferous of Guizhou, China, Acta Palaeontologica Polonica 51 (3), pp. 599-604 : 601-603

Girtypecten (Sinopecten) newelli sp. nov.

Fig. 2.

Derivation of the name: The new species is dedicated to the late Professor Norman D. Newell of the American Museum of Natural History in recognition of his exemplary studies of Paleozoic pectinoid bivalves. He contributed to the knowledge of the Bivalvia and paleontology for nearly 70 years.

Types: Holotype, PKUM02−0085 ( Fig. 2A), is a nearly complete left valve, showing both exterior ornamentation and asymmetrical resilifer. There are five paratypes, PKUM02−0086 – PKUM02−0090 ( Fig. 2B–F), with additional details of spines and reticulated ornamentation. No right valve has been found, supposedly due to the normally thinner nature of its shell and being easily damaged during the acid treatment. Similar preservation bias was observed in the type species by Newell and Boyd (1995) who suggested, in consequence, that the genus probably had paradiscordant valves.

Type locality and horizon: Top part of the Tournaisian Muhua Formation at Muhua, Changshun, Guizhou, China .

Diagnosis.— Girtypecten with auricles subequal in length; disc covered only by five simple, coarse, wide spaced primary costae, which are intersected by slightly closer spaced commarginal ridges to form reticulated ornamentation; strong costae and ridges maintained throughout growth; the interspaces smooth, broad, lacking costellae and/or lines; pyramidal costal spines distally pointed, with a dorsal groove along the outerside; resilifer anteriorly elongate. From G. spinosus Chen, 1962 new species differs in being smaller and in having subequal auricles and up to five radial costae. It is differentiated from G. carboniferous Zhang, 1987 by fewer costae and well defined spines, and from G. chitralensis Reed, 1925 by evidently fewer costae.

Description.—Shell small, probably less than 10 mm in length, nearly acline or slightly prosocline, weakly inflated. Auricles elongate, nearly equal in length, each with acute end above sinus, well differentiated from disc by umbonal fold, and without radial ornament. Beak of the left valve subcentral and orthogyrate, projects slightly beyond the hinge; the hinge margin straight, subequal to shell length. Disc subcircular, surface covered by five simple, coarse, rounded, wide spaced primary costae, set at subequal distances apart and of equal relief; costae crossed at regular intervals by 12 or more, strong, slightly closer commarginal ridges, which continue across auricles and fade away near dorsal margin; strong commarginal ridges are maintained throughout growth in all six specimens. Crossing of costae and concentric ridges produces a reticulated ornamentation; no new ranks of costae and/or lines added with growth, so interspaces between costae are smooth. Costal spines are distally pointed, rising from ridge intersections, circular in cross section, with a dorsal groove along the outerside. Ligament area with a large resilifer, having a breadth of nearly 4 mm, with greater part (3/4) of resilifer anterior to beak. Right valve and shell structure unknown.

Measurements.—The holotype restored height is 7 mm; restored length 9 mm.

Discussion.—The new species is similar to Girtypecten spinosus (see Chen 1962: 194, pl. 1: 2a, b) in general form and ornamentation. Both of them have only a few widely spaced primary costae and commarginal ridges and should be included in the new subgenus. Our specimens differ in being very small, possessing subequal auricles, and surface marked with only five radial costae. G. spinosus has seven costae and evidently a larger posterior auricle. G. carboniferous Zhang, 1987 ( Zhang 1987: 277, pl. 1: 7, 8, 10, 15, 16) from the Late Carboniferous Maping Formation of Guangxi, China resembles the present species in lacking secondary radial costellae and belongs to the new subgenus, but the former has 10 costae without spines at the intersection of the radial and concentric ornamentation, and smaller anterior auricle. G. chitralensis Reed, 1925 ( Reed 1925: 56, pl. 10: 4) from the Early Permian of India and Pakistan is also classified in the new subgenus, but it is easily distinguished by possessing more numerous radial costae (16–17).

Occurrence.—Conodont Lower Siphonodella crenulata Zone , near the top of the Muhua Formation of the Lower Carboniferous (Tournaisian) at Muhua, Changshun County, Guizhou Province, China.