Chesapecten madisonius sarasotensis 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. 21, figs. 8A-8D]
2018 Chesapecten madisonius sarasotensis Waller, 2018
T. R. Waller, 2018, figure 8.
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«Chesapecten jeffersonius (Say, 1824): Petuch, 2004:143, pl. 44I; Petuch and Roberts, 2007:64, fig. 3.11D; Portell et al., 2012:9, pl. 8B.
Chesapecten jeffersonius (Say) unnamed subspecies, Petuch and Roberts, 2007:65, fig. 3.12E. Zoobank Nomenclatural Act.— A7D14827-6294-48E7-BBF6-293BF10DE768.
Diagnosis.— Chesapecten with 7–12 ribs, most commonly 10 or 11, with steep, slightly undercut sides and rounded crests in early ontogeny, becoming lower with somewhat flattened crests and sloping sides in later ontogeny; secondary costae on disk coarse, scabrous, and of more than one order; byssal notch shallow and rounded, with byssal fasciole lacking costellae or with costellae limited to dorsal part of fasciole. Description.— Thick, moderate to large shell known to reach 155 mm height, equilateral to slightly posteriorly extended, both valves moderately convex to well inflated, with left valve consistently more convex than right and with narrow disk gapes between valves. Byssal notch shallow, its depth about 3–12% of length of anterior hinge, averaging 8%, with active ctenolium becoming obsolete or absent in shells greater than about 80 mm height; byssal fasciole slightly arched adjacent to disk, with weak costellae on dorsal part; byssal sinus of left valve shallow, its innermost point anterior to a line perpendicular to hinge and passing through anterior end of dorsal margin. Disks with 7–12, commonly 10 or 11, moderately high ribs, wider than interspaces on right valve, narrower on left valve, crests of ribs rounded with steep sides in early ontogeny, becoming somewhat lower and flattened with sloping sides in late ontogeny; ribs and interspaces covered by rather coarse scabrous costae, with scales small, closely spaced, and erect; costae beginning in interspaces at shell heights from 22–29 mm. Total hinge length about half of shell length, with anterior hinge slightly longer than posterior hinge; all auricles and disk flanks densely and finely costate, about 15 costae on right anterior auricle to at least 20 on other auricles. Ribs on valve interiors flattened, with carinate edges near margins of valves, the troughs between interior ribs not extending dorsal to adductor scar. Weak dorsal and resilial hinge teeth present in young individuals, becoming obscured by ventrad migration of ligament system in larger individuals. Pallial line inset far from valve margin at about two-thirds height of shell. Thick umbonal inner foliated calcite layer extending ventrally to below ventral margin of adductor scar. Etymology.— Name refers to the Sarasota pits, Sarasota County, Florida. Type material and measurements.— Holotype: USNM 716592, a pair of matching valves, Ht 108 mm, L 114 mm, convexity of articulated shell, 52 mm (Fig. 8A–D). Type locality.— USGS 24968,Warren Brothers Shell Pit, Sarasota County, Florida, 6.5 km (4.05 miles) east of Highway 301 on 17th Street, then left (north) at T-intersection and 0.8 km (0.5 mile) to pit. Collector: Mrs. Evelyn Bradley, 1970 and 1971. Other material.— About 90 specimens from 17 localities in Sarasota and Pinellas counties, Florida. Comparisons.-- Chesapecten madisonius sarasotensis most closely resembles specimens of Ch. madisonius madisonius that have fewer than 12 ribs (exemplified by a specimen from the Yorktown Formation at the Lee Creek Mine, Beaufort County, North Carolina, illustrated by Gibson, 1987:pl. 25, fig 1). As observed by Gibson (1987:73-74) these variants occur low in the upper Yorktown Formation and appear to represent an earlier stage of evolution compared to populations with higher rib numbers occurring higher in the formation. Compared to the Lee Creek specimens with lower rib counts, Ch. madisonius sarasotensis has a more convex right valve, coarser disk costae, and a byssal fasciole that is more differentiated from the remainder of the right anterior auricle in elevation and in lacking radial costellae across the entire fasciole in most specimens. The specimen of “Chesapecten madisonius subspecies ? carolinensis” illustrated by Campbell and Campbell (1995:pl. 1, fig. 2) from the lower Goose Creek Limestone of South Carolina is similar to Gibson’s (1987:pl. 25, fig. 1). It differs from Ch. madisonius sarasotensis in having a right valve of lower convexity and a less differentiated byssal fasciole. The stratigraphically highest Chesapecten in the Florida Peninsula occurs rarely in the Ochopee Limestone (Pliocene, Piacenzian) in Monroe and Collier counties. Based on the few specimens available, the Ochopee form differs from Ch. madisonius sarasotensis in having a less differentiated byssal fasciole, a higher mean rib number (commonly 12 or 13), narrower and more rounded ribs, and on average a narrower umbonal angle. Except for its thicker shell, the Ochopee form appears to be within the range of variation of more advanced Ch. madisonius madisonius and is tentatively identified as that subspecies. Chesapecten madisonius sarasotensis differs from Ch. jeffersonius in having coarser secondary costae and scales, ribs that remain prominent even at the ventral margin of large individuals, and internal grooves that do not extend into the umbonal region dorsal to the adductor scar. Compared to Ch. septenarius, Ch. madisonius sarasotensis commonly has more ribs (10 or 11 compared to 6–8), coarser secondary costae, less flattened rib crests with the edges of the crests not as sharp and the sides of the ribs less steep. Chesapecten madisonius sarasotensis differs from Ch. quinarius in having more ribs (commonly 10 or 11 compared to 5 or 6), a slight flattening of rib crests rather than rounded ribs, and lacking internal grooves dorsal to the adductor scar. Occurrence.— Sarasota pits (Warren Brothers, APAC, Macasphalt, Quality Aggregates, Richardson Road pits): USGS 24968, 25174, USGS 25183, USGS 26927, USGS 26934, USGS 26935, SO001A-C, SO021, SO026, SO017; other localities in Sarasota County: USGS 26919, USGS 22584, SO060, SO061; Pinellas County: USGS 21900, USGS 26926. Distribution.— Unit 11 in the Sarasota pits and correlative strata in the southwestern Florida Peninsula in pectinid zone PZ5 (see Biostratigraphy section). Reports that Chesapecten occurs in the Buckingham Limestone (Waldrop and Wilson, 1990) could not be verified. Remarks.— The idea that the Chesapecten in Unit 11 of the Sarasota pits may be a hybrid of Ch. madisonius and Ch. septenarius (Ward, 2008:416) is not supported by the present study, because Ch. septenarius and Ch. madisonius madisonius are both apparently absent from this unit. The so-called Ch. septenarius is the taxon described herein as a new species, Ch. quinarius, and the so-called Ch. madisonius is the new subspecies Ch. madisonius sarasotensis. In Units 11 and 12 of the Sarasota pits, Chesapecten madisonius sarasotensis is associated with a molluscan fauna that indicates a Pliocene age correlated with the Rushmere Member of the upper Yorktown Formation (Ward, 1992a:163). In Unit 11 it is associated with the pectinids Chesapecten quinarius, Carolinapecten eboreus watsonensis, and Argopecten comparilis (Tuomey and Holmes, 1855). Other common calcitic associates include auriculate oysters of the Mansfieldostrea compressirostra group and the gastropod Ecphora quadricostata.» THOMAS RICHARD WALLER, 2018
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