Chesapecten quinarius Waller, 2018
WALLER, T. R. 2018. Systematics and biostratigraphy of Chesapecten and Carolinapecten (Mollusca: Bivalvia: Pectinidae) in the upper Miocene and Pliocene "lower Tamiami Formation" of southwestern Florida. Bulletin of the Florida Museum of Natural History, 56 (1): 1-47, figs. 1-11. [p. 17, figs. 6A-6F]
2018 Chesapecten quinarius Waller, 2018
T. R. Waller, 2018, figure 6.
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«Chlamys (Lyropecten) jeffersonius (Say, 1824). Mansfield, 1932b:59, pl. 11, fig. 1.
Chesapecten palmyrensis (Mansfield, 1936): Petuch, 2004:143, pl. 44C; Petuch and Roberts, 2007:64, fig. 3.11Eb. Zoobank Nomenclatural Act.— 293E9C29-9278-4C72-86E5-A60853E1BDF9.
Diagnosis.—Chesapecten with 4–8, most commonly 5 or 6, broad radial ribs that are rounded throughout ontogeny and covered by moderately coarse radial costae; byssal notch shallow, with poorly differentiated byssal fasciole becoming costellate in late ontogeny. Description.—Shell of moderate size, commonly with height of 60–100 mm but reaching maximum known height of 141 mm, equilateral to slightly posteriorly extended, both valves moderately convex, with left valve more convex than right. Byssal notch shallow, its depth about 14% of length of anterior hinge, with active ctenolium still present on shells 70 mm in height but absent in larger individuals; byssal fasciole not depressed, with fine radial costellae present on dorsal side of fasciole in young individuals and across entire fasciole in adult individuals; byssal sinus of left valve shallow. Disks with 5 or 6, rarely 7, broad moderately high radial ribs, rounded in early to mid-ontogeny, becoming somewhat flattened but not fading out in late ontogeny; ribs and interspaces covered by moderately coarse radial costae of fairly even strengths beginning in interspaces at shell heights of 12–16 mm; all costae with fine scales, costae increasing in number by intercalation. Total hinge length about 50–60% of shell length, with anterior hinge slightly longer than posterior hinge; anterior margin of right anterior auricle shallowly curved, meeting disk flank at about a right angle in larger shells; anterior margin of left anterior auricle outwardly convex, forming moderately shallow byssal sinus; posterior margins of posterior auricles in young growth stages shallowly sigmoidal, becoming outwardly convex in late ontogeny with overall trend about perpendicular to dorsal margin or forming slightly acute angle; all auricles densely and finely costate, 15–19 at margin of right anterior auricle, 18–20 at margins of other auricles. Interior ribs flattened, with carinate edges near margins of valves; interior rib interspaces extending dorsal to adductor scar into umbonal region in early to mid-ontogeny, becoming filled in umbonal region with thick inner foliated calcitic layer in gerontic individuals. Moderately strong dorsal hinge teeth and weak intermediate and resilial teeth present in early to mid-ontogeny, becoming obscured by ventral migration of ligament system in late ontogeny. Pallial line inset far from shell margin at about two-thirds height of shell. Umbonal inner foliated calcite layer extending ventrally beneath ventral margin of adductor scar in gerontic individuals with greatly thickened shells. Etymology.— From the Latin quinarius, meaning “of five”, with reference to the common presence of only five radial ribs. Type material and measurements.— Holotype: USNM 716590, pair of matching valves, Ht 69.0 mm, L 70.5 mm, convexity of articulated shell 28.9 mm. Type locality.— USGS 26936. Sarasota County, Florida. Warren Brothers Pit, E½ sec. 12, T36S, R18E, from base of section on east wall of pit. Evelyn Bradley, May 1978. Other material.— About 80 specimens from localities in Sarasota, Pinellas, and Calhoun counties, Florida. Comparisons.— Chesapecten quinarius resembles Ch. jeffersonius and has been confused with that species in the past. Chesapecten quinarius differs from Ch. jeffersonius in lacking sharply squared ribs throughout ontogeny and in having fewer ribs, commonly only five or six compared to eight to twelve in Ch. jeffersonius. Chesapecten quinarius differs from Ch. middlesexensis bayshorensis in having fewer, more rounded ribs, finer secondary costation, a shallower byssal notch, and a poorly differentiated byssal fasciole that becomes costellate in late ontogeny. Compared to Ch. septenarius, Ch. quinarius lacks sharply squared, flattened ribs throughout ontogeny. Sharply squared ribs are also present in the early ontogeny of Ch. palmyrensis (Mansfield, 1936), a species synonymized with Ch. septenarius by Ward and Blackwelder (1975:15). Occurrence.— Sarasota County: Sarasota pits (Warren Brothers, APAC, Quality Aggregates, Richardson Road pits), USGS 25174, USGS 25183, USGS 26934, USGS 26935, USGS 26936, SO001A, SO001C; other localities in Sarasota County: USGS 26921, USGS 24527; Pinellas County: USGS 21900; Calhoun County: USNM 371776 (specimen figured by Mansfield, 1932b:pl. 11, fig. 1). Distribution.— Units 11 and 12 in the Sarasota pits and correlative strata in the southwestern Florida Peninsula. In the present study Ch. quinarius is assigned to pectinid zone PZ4, early Pliocene (late Zanclean) in the absence of Ch. madisonius sarasotensis. It co-occurs with Ch. madisonius sarasotensis in pectinid zone PZ5, late Pliocene (Piacenzian). Remarks.— Petuch and Roberts (2007:63) listed Ch. palmyrensis (Mansfield, 1936) as an index fossil for their newly named Jupiter Waterway Member of the Murdock Station Formation (= Pecten biostrome or lower bed of Murdock Station Member of Hunter, 1968), but the specimen that they illustrated (their figure 3.11E) is not that species and was not found in the present study to be present in the Murdock Station Member. Rather it is Ch. quinarius and probably came from either Unit 12 or Unit 11 in the Sarasota pits. The specimen of Chlamys (Lyropecten) jeffersonius illustrated by Mansfield (1932b:59, pl. 11, fig. 1, USNM 371776) is a large right valve (Ht 141 mm) with only six major ribs and a minor rib on each side. This low rib count and lack of rib squareness in early ontogeny allow a tentative assignment to Ch. quinarius. Mansfield’s specimen is from the Ecphora zone of the Jackson Bluff Formation “at a dripping spring about three-fourths of a mile north of Clarksville, Calhoun County, Florida.” It is the only specimen of Ch. quinarius thus far known from outside the Florida Peninsula and is also a size record for the new species. The age of the Ecphora zone is well established as late Pliocene (Piacenzian) (Akers, 1974; Ward and Huddlestun, 1988; Jones et al., 1991; Allmon, 1993). A specimen of Chesapecten quinarius collected from low in the section at the Warren Brothers Pit in the Sarasota area (USGS 25174) has attached barnacles identified by the author as Arossia aurae Zullo, 1992b (Fig. 6G). This species has previously been reported only from its type locality in the lower Yorktown Formation of Zanclean age at the Lee Creek Mine in Aurora County, North Carolina (Zullo, 1992b:30). In Unit 11 of the Sarasota pits, however, Ch. quinarius is associated with a molluscan fauna that indicates an early late Pliocene age correlated with the Rushmere Member of the upper Yorktown Formation (Ward, 1993:163).» THOMAS RICHARD WALLER, 2018
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