Batequeus mezquitalensis 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. 383, figs. 2.1-2.5]
1990 Batequeus mezquitalensis Squires & Demetrion, 1990
R. L. Squires & R. Demetrion, 1990, figure 2.
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«Diagnosis. — Same as for genus.
Description. — Medium sized; shell height up to 41 mm, slightly shorter than long; subcircular disk and moderately convex thin valves, equally low-convex valves; tendency for oblique growth in ornamentation in posterior region of each valve; anterior auricles shorter than posterior ones; hinge line about threefifths length of disk; hinge plate raised and lens shaped; resilifer triangular and extending to hinge line, bordered on each side by a low ridge adjacent to a flattened area. Left valve with numerous scaly radial ribs, interspaces about same width with one or two intercalaries in later growth stage (beyond about 20 mm valve height), radial ribs tend to be more closely spaced on anterior and posterior regions of valve; anterior auricle with about six scaly riblets; posterior auricle with about eight scaly riblets; dorsal margin of hinge line smooth and bordered ventrally by low ridge, fairly deep groove separating this low ridge from cardinal crus (rarely two or more) on both sides of resilifer; interior of valve with numerous wide and flat, weak radial ribs that can be fluted at valve margin. Right valve with about 30 low, flat-topped radial ribs that may become grooved and may have a single rib in flat-bottomed interspaces in later growth stage (beyond about 20 mm valve height); radial ribs wider than interspaces; commarginal growth lines extremely fine; primary ribs give way posteriorly to cluster of 8-12 scaly, secondary radial ribs; anterior auricle with short byssal notch and about six riblets; posterior auricle with about eight scaly riblets; exterior surface of each auricle bent backwards into hinge-line area (exept in resilifer area), causing interior dorsal margin of each auricle to be swollen and accentuated by vertical, scalelike growth lines; hinge line with two cardinal crura radiating from each side of apex of resilifer, uppermost one extending nearly entire length of hinge line, lowermost one much shorter and less prominent, fairly deep groove separating uppermost cardinal crus from swollen dorsal margin; interior of valve with about 23 wide, flat, weak radial ribs with fairly wide interspaces. Remarks. —The left valve of Batequeus mezquitalensis n. sp. is very similar in ornamentation to some specimens of "Chlamys" decemnaria (Conrad, 1834, p. 151; 1840, p. 49, PI. 24, fig. 2) from the upper Pliocene Yorktown Formation, Virginia and North Carolina. As shown in Gibson (1987, p. 65-69, PI. 15, figs. 2, 3, 5-7, PI. 16, figs. 3-5, Pis. 19, 20), there is a great deal of variation in "C." decemnaria. Some specimens (Gibson, 1987, PI. 20, fig. 3) are remarkably similar to B. mezquitalensis, except that in the new species the anterior auricle is smaller than the posterior. Batequeus mezquitalensis is the earliest documentable Cenozoic pectinid from the west coast of North America. Chlamys proavus (Arnold, 1906, p. 52-53, PI. 2, figs. 6-8; Moore, 1984, p. B19, PI. 3, fig. 15) may be from the Paleocene Locatelli Formation, San Mateo County, middle California, but its stratigraphic position is uncertain (Moore, 1984). Cyclopecten? martinezensis (Gabb, 1869, p. 198, PI. 33, fig. 96; Moore, 1984, p. B8-B9, PI. 1, figs. 9, 10) from the Martinez Formation, Contra Costa County, middle California (Moore, 1984) is earlier in age than Batequeus mezquitalensis, but C.? martinezensis is a propeamussiid rather than a pectinid. Etymology. — The specific name is for Arroyo Mezquital, along which the type locality of the new species is located. Material.— About 100 specimens with nearly equal number of left and right valves, all of which show the interior. A few specimens are articulated. Occurrence. — Middle Eocene CP14a Zone of Okada and Bukry (1980), which correlates within the middle Eocene part of the "Tejon Stage" (Lutetian Stage). Bateque Formation, Baja California Sur, Mexico, locality CSUN 1293. Repository.— Holotype, IGM 5058 (=plasto-holotype), LACMIP 8061; paratypes, IGM 5059, 5060 (=plasto-paratypes), LACMIP 8062, 8063; locality CSUN 1293.» RICHARD L. SQUIRES & ROBERT DEMETRION, 1990
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«Node S2 (Fig. 1.2) represents the start of extreme differential sculpture of the two valves, that of the left valve being densely costate, with all of the costae scaly, that of the right valve having only the coarser primary plicae and intercalated secondaries. Batequeus (Node S3) retains plesiomorphic biconvexity (Squires and Demetrion 1990, fig. 2(3)) while evolving medial bifurcation of its central right primary plicae. Thus far Batequeus is known only from its type species, B. mezquitalensis Squires and Demetrion, 1990, from the Bateque Formation of Baja California Sur, Mexico. Its age was originally given as middle Eocene based on planktonic foraminifera indicating zones P8 or P9 of Berggren et al. (1985). However, those zones now indicate a late Early Eocene age (late Ypresian) according to Berggren et al. (1995: 140). A second species of Batequeus, B. ducenticostatus Campbell, 1995, from the "Cross Formation" of probable Bartonian age of South Carolina, is not a Batequeus but rather is a member of the Chlamys group. Its radial sculpture is similar on the two valves, and its radial costae are more densely packed, more densely scaly, and with a more complex introduction pattern throughout ontogeny than in any member of the Batequeus-Serripecten clade.»
WALLER, T. R. 2006. New Phylogenies of the Pectinidae (Mollusca: Bivalvia): reconciling Morphological and Molecular Approaches. In: S.E. Shumway & G.J. Parsons (Ed.), 2006: Scallops: Biology, Ecology and Aquaculture, 1-44, figs. 1.1-1.4. [p. 16]
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T. R. Waller, 2006, New Phylogenies of the Pectinidae, figure 1.2.
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