Weyla (Lywea) yukonensis Aberhan, 1998
ABERHAN, M. 1998. Early Jurassic Bivalvia of western Canada. Part I. Subclasses Palaeotaxodonta, Pteriomorphia and Isofilibranchia. Beringeria, 21: 57-150, text-figs. 1-14, pls. 1-19. [p. 130, pI. 16, fig. 1; pI. 17, figs. 5, 6; pI. 18, figs. 2, 4, 5; pI. 19, figs 1, 3, 5, 7; text-figs. 14, 15]
1998 Weyla (Lywea) yukonensis Aberhan, 1998
M. Aberhan, 1998, plates, 16-19.
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«v 1998 Weyla sp. A - Aberhan: fig. 4H.
Derivation of name. After southern Yukon, where the holotype was found. In an unpublished sketch on the "phylogeny of the Lower Jurassic pecten Weyla", SIMON W. MULLER referred to this species as W. yukonensis and his suggestion is followed here.
Holotype. GSC 112354 from GSC locality 10248, articulated specimen, Figured on PI. 17, Figs. 5a-d.
Additional material. 1 left valve and 2 articulated specimens from GSC locality 10244; 1 left valve (GSC 112574) from GSC locality 10247; 1 right valve and 1 articulated specimen (GSC 112582) from GSC locality 10248; 1 articulated specimen (GSC 112575) from GSC locality 19419; 2 right valves (including GSC 112581) and 2 articulated specimens (including GSC 112584) from GSC locality 28500; several valves (including GSC 112568) From GSC locality 42091; 7 left valves and 8 right valves (including GSC 112552, 112573) from GSC locality 84187; 1 left valve (GSC 112583) from GSC-locality 84193; 4 left valves and 1 right valve from GSC locality C-81308; 2 left valves and 2 right valves from GSC locality C-81316; several left and right valves from GSC locality C-186954. In shell preservation and as internal and external moulds.
Stratigraphic range. Hettangian to Sinemurian.
Diagnosis. Medium- to large-sized, pectini- form, biconvex shell with the right valve more strongly convex than the left valve. External ornamentation of right valve consisting of broad, prominent radial ribs with flat to slightly rounded crests and moderately steeply dipping flanks; ribs separated by triangular interspaces. External ornamentation of left valve consists of narrow, radial ribs, acutely triangular in cross-section, and separated by interspaces, which are wider than the ribs and have slightly concave bottoms.
Description. Shell of medium to large size, pectiniform, biconvex, thick-shelled; inequivalve, with right valve strongly convex and larger than left valve, which is only moderately convex; length greater than height in both valves. Right valve umbo very prominent, mesogyrous, and projecting distinctly beyond hinge-line; left umbo extends only slightly beyond hinge-line. Auricles large, but incompletely preserved; not delimited from disc of shell by a deep sulcus. Surface of both right auricles convex. Free margin of posterior right auricle meets dorsal margin at an approximately right angle and disc at an acute angle. Surface of left valve posterior auricle feebly convex; its free margin is straight, meeting disc at an acute angle.
Shell surface ornamented with 13 to 15 straight radial ribs, which bifurcate and are different on each valve. Ribs of right valve strong, broad, and in cross- section with flat to slightly rounded crests; flanks of ribs dipping at a moderately steep angle, which becomes smaller towards ventral margin of shell. Interspaces between ribs triangular in cross-section, becoming somewhat more rounded towards the ventral margin. Ribs of left valve narrow and acutely triangular in cross-section. Interspaces always wider than ribs and with slightly concave bottoms. All auricles carry several fine radial riblets. Ribs also present on shell interior, but style of ribbing very different from external ornamentation. In both valves each external rib corresponds to a rib on the shell interior. Internal ribs of left valve consisting of very fine, central ridge with flat to very gently outward dipping surfaces on both sides of ridge. Each of these surfaces is separated from interspaces by a narrow, rounded ridge, which forms the most prominent part of each rib (Text-fig. 15B). Interspaces feebly concave and commonly with centrally placed, delicate, radial riblet. Internal mould of left valve often preserved in a way that fill of interspaces and fill between prominent ridges of each rib appear as positive relief. At the ventral margin these fills are about equal in size, and from internal moulds alone it cannot be determined which one correlates with external ribs and interspaces respectively (Text-fig. 15D). Internal ribs of right valve, also correlating with external ribs, consist of broadly rounded main ridges (Text-fig. 15A). Towards the ventral margin one accompanying ridge branches off on each side of main ridge. Interspaces feebly concave with a centrally placed, low, radial riblet. Towards the ventral margin the main ridges become continuously weaker, and finally the pattern of ribs and interspaces is reversed and an interspace is developed corresponding to the position of main ridges of earlier growth stages (Text- fig. 15C). Resilifer of left valve wide and deep, triangular in outline, and centrally placed just ventral to umbo on a surface that dips slightly towards interior of right valve. Ventral parts of inner surface of both left auricles with several auricular crura that are sub-parallel to dorsal margin. Other internal characters unknown. Remarks. The only other known species of Weyla with a biconvex shell is W. (Lywea) unca (Philippi 1899). The latter can be readily distinguished from W. yukonensis by its ornamentation, which consists of V- shaped ribs and interspaces on both valves. Similarly, W. meeki can be separated from W. yukonensis by the presence of triangular ribs and interspaces on both valves. In addition, the left valve of W. meeki is flat.
The strongly convex right valves of W. alata and W. bodenbenderi differ from right valves of W. yukonensis by the development of slightly concave to flat interspaces between ribs rather than triangular interspaces as is typical of W. yukonensis. Left valves of W. alata and W. bodenbenderi are flat and in this respect differ from the convex left valves of W. yukonenesis. Furthermore, W. yukonensis lacks the flanking riblets typical of left valves of W. alata, and, in contrast to left valves of W. bodenbenderi, its ribs are triangular in cross-section rather than flat-topped.» MARTIN ABERHAN, 1998
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