Chesapecten middlesexensis hunterae 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. 11, figs. 3A-3K]
2018 Chesapecten middlesexensis hunterae Waller, 2018
T. R. Waller, 2018, figure 3.
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«Zoobank Nomenclatural Act.—E87793D5-F4B0-4ACF-80D4-415F6153A6CA.
Diagnosis.— Chesapecten middlesexensis with 9–12 radial ribs, most commonly 10–11, rarely as many as 14, with coarse radial scabrous costae on ribs and interspaces; byssal notch remaining moderately deep throughout ontogeny; byssal fasciole slightly depressed and either non-costellate or with a few faint costellae appearing in late ontogeny; active ctenolium commonly absent in individuals greater than shell height of 100 mm; hinge dentition weak or absent in mature individuals. Description.— Shell large, reaching heights of about 145 mm, equilateral or slightly posteriorly extended, with length exceeding height in mid- to late ontogeny; both valves moderately convex, with left valve more convex than right and with narrow anterior and posterior disk gapes; byssal notch moderately deep in late ontogeny, with an active ctenolium commonly absent on right valves greater than 100 mm height; broad byssal fasciole slightly sunken, becoming faintly costellate in late ontogeny; byssal sinus of left valve shallow. Disks with 9–12 interlocking radial ribs, most commonly 10–11, rarely as many as 14, high with rounded crests and steep sides in early ontogeny, becoming lower and rounded or slightly flattened in later ontogeny; ribs, interspaces, and disk flanks covered by coarse scabrous radial costae beginning at shell height of 15–25 mm, costae increasing by intercalation, numbering 3–5 in interspaces in central sector at shell height of 50 mm and 4–7 at shell height of 100 mm. Disk flanks costate, narrow, and steep, curved inward on posterior side. Total hinge length about 60% of shell length, with anterior hinge slightly longer than posterior hinge; anterior margin of right anterior auricle shallowly curved, then curving more sharply on ventral side into deep byssal notch with apex acute in early ontogeny but forming a right angle between edge of fasciole and disk flank in later ontogeny; anterior margin of left anterior auricle shallowly outwardly convex to nearly straight, with byssal sinus decreasing ontogenetically to very shallow in largest shells; margins of posterior auricles nearly straight, forming slightly acute angle with dorsal margin in early ontogeny, becoming slightly obtuse to dorsal margin in late ontogeny; all auricles densely and finely costate, with about 10–15 radial costae at margin of right anterior auricle and 15–20 at margins of other auricles. Hinge teeth weakly developed or absent 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 posteroventral margin of adductor scar. Etymology.— Named for the late Muriel E. Hunter, who was the first to recognize the stratigraphic significance of marine strata in the Port Charlotte area. Type material and measurements.— Holotype: USNM 716578, right valve, Ht 74.5 mm, L 77.0 mm, convexity 10.4 mm (Fig. 3A–E). A non-matching left valve, USNM 716579, of nearly the same size as the holotype is shown in Figure 3F–G to show relative convexity of the valves. Type locality.— USGS 26913. Cocoplum Waterway at Collingswood Boulevard, North Port, Sarasota County, Florida. Other material.— About 150 specimens (single valves plus pairs of matching valves) from 27 localities in the Port Charlotte area. Comparisons.— Within the Chesapecten middlesexensis species complex in the Mid- Atlantic Coastal Plain, Ch. middlesexensis hunterae most closely resembles Ch. middlesexensis ceccae Ward, 1992b, but differs from that subspecies in having a lower rib count (10–13, compared to 15–16). The deep byssal notch and differentiated non-costellate byssal fasciole of Ch. middlesexensis hunterae separate it from Ch. jeffersonius and Ch. madisonius sensu lato. Compared to Ch. madisonius sarasotensis n. ssp. of Unit 11 in the Sarasota pits, Ch. middlesexensis hunterae has a much deeper byssal notch, ribs that are less squared in profile in late ontogeny, and a hinge length that is more than half the length of the shell (64% in holotype of Ch. middlesexensis hunterae, but only 47% in holotype of Ch. madisonius sarasotensis). Occurrence.— Port Charlotte area, west of Murdock: Cocoplum Waterway: USGS 26913, USGS 26937, SO010A, B, SO011, SO058, SO010C; Auburn Waterway: USGS 26907; Flamingo Waterway: USGS 26924, USGS 26914, CH017, CH112; Bayshore Waterway: USGS 26932, USGS 26917, CH030A. Port Charlotte area, east of Murdock: Pellam Waterway: USGS 22911, USGS 22912; Crestview Waterway: CH119, CH118; Lion Heart Waterway: USGS 23118, USGS 26933; Sunset Waterway: USGS 23117, USGS 26912, USGS 26930. Distribution.— Known only from the Port Charlotte area in the southwestern Florida Peninsula, where it occurs in the Bayshore Clay Member of Hunter (1968), now regarded as within the upper Peace River Formation with an estimated age of late Miocene (Tortonian). In the present study, Ch. middlesexensis hunterae is assigned to zone PZ2 (see Biostratigraphy section). Remarks.— Chesapecten middlesexensis hunterae displays two intergradational morphologies that may have some stratigraphic significance. Forms that occur mainly along the Cocoplum Waterway, including the holotype (Fig. 3A–E), have somewhat thinner shells, lower convexity, deeper byssal notches, and slightly higher rib counts than forms that occur along the Sunset and Lion Heart Waterways east of Murdock (Fig. 3H–K). The former are commonly associated on the Cocoplum Waterway (USGS 26913, USGS 26937) with a barnacle-hash coquina containing abundant fragments of a broad-ribbed Ecphora in the Ecphora gardnerae group (E. cf. gardnerae whiteoakensis Ward and Gilinsky, 1988) (Fig. 4D−F) as well as Mansfieldostrea compressirostra brucei (Ward, 1992b) (Fig. 4G, H). Both of these taxa are associated with the lower upper Miocene (Tortonian) Claremont Manor Member of the Eastover Formation in the Mid-Atlantic Coastal Plain (Ward and Gilinsky, 1988; Ward, 1992b). Additional associated calcitic bivalve species include Carolinapecten murdockensis druidwilsoni n. sp. and n. ssp., described below, Euvola smithi (Olsson, 1914) (Fig. 4A), and a very worn but abundant undescribed species of Argopecten Monterosato, 1889 (Fig. 4B−C). The barnacle fragments that comprise the barnacle-hash coquina are all from a single species, Fistulobalanus klemmi Zullo, 1984 (Fig. 4I−J). Although the type material of this species from the Lee Creek Mine in North Carolina has been determined to be from the lower Pliocene Sunken Meadow Member of the Yorktown Formation (Ward, 2008:355), the genus has a long Miocene history in the western Atlantic region (Zullo, 1984), and its Florida occurrence may be late Miocene in age. The form of Chesapecten middlesexensis hunterae present along the Sunset and Lion Heart waterways occurs in a more clay-rich quartz sandy lithology with abundant rounded black phosphate grains and rare brown grains and is associated with Ca. murdockensis murdockensis n. sp. and n. ssp. Miocene indicators include non-auriculate members of the Mansfieldostrea compressirostra group, Euvola smithi, and the barnacles Chesaconcavus myosulcatus Zullo, 1992b, and Arossia sp. aff. newmani Zullo, 1992b. However, certain taxa previously assumed to have stratigraphic ranges beginning in the late Pliocene are also present at Sunset Waterway localities. These include the oysters Myrakeena sculpturata (Conrad, 1840), Undulostrea locklini (Gardner, 1945), and Cubitostrea coxi (Gardner, 1945), and the gastropod Dicathais handgenae Portell and Vokes, 1992. It is not clear whether these taxa may have originated in the Miocene or that Pliocene beds may be represented in the spoil-bank collections.» THOMAS RICHARD WALLER, 2018
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