EOPHILOBRYOIDELLA SINOANISICA NEW GENUS AND SPECIES, AN EARLY PHILOBRYID BIVALVE FROM THE UPPER ANISIAN (MIDDLE TRIASSIC) OF QINGYAN, SOUTHWESTERN CHINA

Journal of Paleontology, Mar 2004 by Stiller, Frank, Jinhua, Chen

EOPHILOBRYOIDELLA SINOANISICA new species

Figures 2, 3

Philobryidae n. gen. A n. sp. A STILLER, 2001, p. 99, 268.

Diagnosis.-Shell small, equivalved, inequilateral; outline obliquely oval; valves inflated; umbones prosogyrate, prominent, beaks situated anterior but not terminal; shell moderately thick, outer surface smooth except for fine growth lines. Internal shell margin smooth, hinge margin edentulous. Hinge plate long, rather broad, slightly arched, tapering at the anterior and posterior ends; ridgelike swelling of internal shell below hinge plate, forming a supporting structure. Ligament internal, located in a narrow, long groove running from below the beak obliquely backwards and slightly curvedly across the hinge plate for some distance behind the beak, thereby dividing the hinge plate into two overlapping portions; anterior portion distinctly larger than posterior portion; both portions tapering anteriorly and posteriorly and with numerous fine crenulations directed about perpendicular to the dorsal shell margin; crenulations of the anterior portion altogether coarser than those of the posterior portion.

Description.-Shell small (typically less than 1 cm in length), equivalved, inequilateral, not gaping. Outline an obliquely rounded oval, ranging from distinctly oblique with slightly developed anterior end and somewhat elongated, produced posterior-ventral portion to, more rarely, only slightly obliquely oval, almost subcircular. Dorsal margin slightly curved; all other portions of the shell margin also curved, outline without angular or straight parts. Valves moderately to distinctly inflated, but the convexity being somewhat variable. Umbones prosogyrate, incurved, prominent, swollen, with beaks of valves close together, situated anterior but not terminal. Shell moderately thick. Outer surface smooth except for fine growth lines, no radial sculpture. Sometimes color pattern preserved (?): light zones parallel to growth lines (Fig. 3.18, 3.28) or light irregular zones (Fig. 3.1-3.5).

Internal shell margin, including dorsal portion, smooth; hinge margin edentulous, no teeth or ridges.

Hinge plate long, rather broad, slightly arched, often somewhat narrowing posteriorly, tapering at the anterior and posterior ends; starting slightly anterior to the beak and reaching to the posterior end of the dorsal shell margin. Hinge plate projecting into valve; appearing as structurally separate unit, but attached to dorsal shell margin. Ridgelike swelling of internal shell below hinge plate, forming a supporting structure; swelling distinctly longer than the hinge plate anteriorly and posteriorly. Dimension of hinge plate relative to shell dimension somewhat variable. Length of hinge plate equalling about 0.55 to 0.7, mostly 0.6 to 0.7 of shell length; length of hinge plate about four to almost seven times, mostly about five times its breadth.

Ligament internal, located in a narrow, long groove on the hinge plate; groove running from below the beak obliquely backwards and slightly curvedly across the hinge plate for some distance behind the beak (opisthodetic), initially at only a very small angle to the dorsal shell margin, some distance behind the beak curving more strongly and descending across the hinge plate, thereby dividing the hinge plate into two separate, overlapping portions. Anterior, subumbonal hinge plate portion distinctly larger, up to almost twice as broad as the smaller posterior portion; both portions tapering anteriorly and posteriorly; continually narrowing anterior end of posterior portion stretching far anteriorly towards the beak. Each hinge plate portion with numerous distinct, fine crenulations directed about perpendicular to the dorsal shell margin; crenulations on anterior portion very delicate and slightly curved anteriorly in anterior part, becoming coarser and straight posteriorly, with direction somewhat changing, always about perpendicular to the slightly curved dorsal shell margin, altogether somewhat coarser than those of the posterior portion; crenulations on posterior portion weaker than on anterior portion, striae almost straight, perpendicular to the dorsal and posteriordorsal shell margins, hence forming an angle with those of the anterior portion, striae relatively coarse posteriorly and becoming finer and shorter to almost indistinct anteriorly.