Lyropecten terrysmithae Powell, Millard & Garcia, 2020
POWELL II, C. L., C. D. MILLARD & C. GARCIA. 2020. A new Lyropecten (Pectinidae, Bivalvia, Mollusca) from the central California Miocene, USA. PaleoBios, 37: 1-12, figs. 1-7. [p. 2, figs. 1-5]
2010 Lyropecten terrysmithae Powell, Millard and Garcia, 2020
C. L. Powell II, C. D. Millard & C. Garcia, 2020, figures 1, 4, 5.
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«Pecten (Lyropecten) estrellanus Conrad. Arnold (1906), pl. XXI, f. 2, 2a, 2b. Not of Conrad (1857a, b). Hanna and Hertlein (1941):172, fig. 63-7. Not of Conrad (1857a, b).
Argopecten sp. Smith 1991:56, pl. 22, fig. 5, 6. Powell (2007), appendix 2. Diagnosis — Lyropecten terrysmithae n. sp. differs from similar central California Neogene Lyropecten [L. estrellanus (Conrad, 1857a) and L. catalinae (Arnold, 1906)] in being equivalve and lacking ledges, having beaks that do not project dorsal above the hinge line, a proportionately shorter hinge line, and non-rectangular auricles. The interior of the shell shows well-defined ribs, which extend much further inward from the shell margin than those of similar-sized fossil California Lyropecten.
Holotype — CAS 5900 (articulate valves).
Paratypes — Specimen numbers: CAS 78554 (right valve), CAS 78555 (right valve), CAS 78556 (right valve), CAS 78557 (right valve), CAS 78558 (right valve), CAS 78563 (left valve).
Type localities — Holotype locality: CAS 28473. Paratype localities: CAS 78553 (spec. no. CAS 78554–78556), CAS 28473 (spec. nos. CAS 78557, 78558). See Appendix 1 for locality details.
Referred specimens and occurrences — The specimens in parentheses are bulk cataloged under the following locality numbers: CAS 28473 (19 articulate specimens; CAS 448 (four double valve specimens mostly missing their auricles); CAS 67089 (=LSJU 2607) (ten single valves); CAS 78553 (=R. Arnold field no. C1011) (seven valves); CAS 68632 (=LSJU locality 1081) (one articulate specimen, three small right valves as cf.); CAS 78561 (=LSJU specimen no. 30145) (one articulate specimen), CAS 74822 (=Schenck acquisition 146) (one articulate specimen, five small right valve, four small left valves, and three small indeterminate valves); CAS 74827 (=Schenck acquisition 1904) (one articulate specimen, one valve as cf.), CAS 74824 (=Schenck acquisition 1885) (one valve as cf. encased in sediment), CAS 68631 (=LSJU 1054, =SU1054a) (one juvenile right valve, one valve as cf.); USGS M1969 (this collection contains poorly preserved pectinids two of which are identified to this species and three more are identified provisionally [cf.]), USGS M3995 (three articulate specimens, and three single valves, one with a cast of the other valve, one partial single valve). See Appendix 1 for locality details.
Etymology — Named in honor of Dr. Judith Terry Smith for her contributions to our understanding of pectin biodiversity.
Description — Specimens of Lyropecten terrysmithae in the CAS collections range in height from 42.0 to 92.5 mm, and 42.6 to 95.4 mm in length (Table 1). These specimens are medium-sized with relatively flat, unledged valves, of roughly uniform convexity (see Smith 1991, pl. 22, fig. 5), and are much flatter in profile than L. estrellanus (see Smith 1991, pl. 22, fig. 7). Lyropecten terrysmithae displays a length slightly greater than its height. Both valves exhibit between 16 to 20 radial ribs that are flat topped and visible on the interior of the valve from one-third to two-thirds the height of the shell. The umbos meet at the dorsal margin of the auricles with an umbonal angle between 113° to 115°.
The right valve (Fig. 1) has flat-topped, flat-sided, strong radial ribs showing only intermittent looped growth lamellae as sculpture (Fig. 1). The posterior disk flank has four to six radial riblets, while the anterior disk flank shows about half that number (Fig. 1). Rib interspaces are variable and generally about half as wide as the ribs, with a wide riblet down the middle of each interspace that is channeled on both sides (Fig. 1). The anterior auricle is vertically truncate with rounded upper and lower corners, with three to five radial costae that are narrow and rounded, a relatively shallow byssal notch, and small distinct ctenolium on well-preserved valves. The posterior auricle slants towards the posterior margin making the auricle wider at the base than at the top. The valve has a number of indistinct radial costae that radiate away from a point near the umbo. The minimum hinge width when compared to the overall width of the shell is approximately 47%, with a maximum of about 55%. The top of both auricles slant slightly downward towards the umbonal region. The hinge of the right valve (Fig. 2) consists of a large resilifer, with two low, resilifer teeth adjacent, three dorsal teeth to the anterior of the broad resilifer, or and two to the posterior, expressed at different angles across the resilifer, and a large dorsal tooth on each side of the umbo, which is flat on top, with a very wide triangular base. Only a single immature left valve (Fig. 3) from locality CAS 78553 is well enough preserved for description. This hinge consists of a triangular resilifer, with faint resilifer teeth on each side, followed by two dorsal teeth on either side, and then by a very faint linear depression just below the top of the auricles. The interior of each valve exhibits impressions of the external ribs, which extend from the shell margin one-third to two-thirds the height of the shell. Radial ribs on the left valves are also flat-topped, and they may have an equal number or one less rib than on the right valve. On well-preserved specimens the ribs have moderately strong, looped lamellar growth lines. Well-preserved specimens also exhibit an interstitial channeled riblet that also show the looped lamellar growth lines. The anterior auricle is ornamented with up to five narrow, rounded ribs radiating outward, while the posterior auricle has numerous fine, radial costa with three to five subdued ribs which radiate at different angles than the anterior auricle. The bending downward of the ribs on the anterior auricle on the holotype is probably the result of a repaired injury, as indicated by disturbed growth lines. SPECIES COMPARISONS, STRATIGRAPHIC AND
GEOGRAPHIC DISTRIBUTION Currently, there are nine Lyropecten species in the Paleogene–Neogene of California: L. catalinae, L. cerrosensis, L. crassicardo, L estrellanus, L. miguelensis, L. miguelensis supervariant form of Smith (1991), L. pretiosus, L. terminus, and L. tiburonensis (Smith 1991). Although first illustrated by Arnold (1906, pl. XXI, figs. 2, 2a, 2b), L. terrysmithae was not recognized as a species separate from L. estrellanus (Fig. 5E, F) until Smith (1991). Arnold (1906) regarded L. terrysmithae as a flat form of L. estrellanus grading into L. catalinae. In a discussion of L. estrellanus, Smith (1991, p. 56) noted the difference in the following statements: “Arnold’s ‘flat form grading into catalinae’ is a flat, short-hinged scallop from the Vineyard Canyon-Indian Valley area … is a convergent Argopecten sp. related to but different from undescribed forms from eastern Baja California Sur.…” She distinguished it from L. estrellanus and L. catalinae “… in [having] hinge crura and a proportionally shorter length of the dorsal margin of the auricles that is not perfectly straight. Beaks meeting at the dorsal margin of the auricles, auricles are not rectangular, and valves are flat and unledged. Looped lamellar growth lines are distantly spaced.” This description is interesting as neither L. estrellanus nor L. catalinae have auricles with straight dorsal margins, although they are straighter than in L. terrysmithae.
Lyropecten catalinae occurs in rocks of late middle Miocene age (“Margaritan” CPMS; Fig. 6) on Santa Catalina Island in the southern California Bight and in the eastern Ventura Basin, southern California (Smith 1991). Lyropecten catalinae appears most similar to L. terrysmithae but can be distinguished from L. terrysmithae in having comarginal rounded ledges on its valves and wider interspaces. Addicott in Durham (1968), reported L. catalinae from the Santa Margarita Sandstone in the Bradley (locality USGS M1936) and Tierra Redondo Mountain 7.5’ (locality USGS M1940) quadrangles, Monterey County. In a review of “Margaritan” CPMS mollusks in the Salinas Valley, Powell (2007) examined Cenozoic specimens from USGS M1936 and assigned them to Smith’s (1991) indeterminate Argopecten sp. here assigned to L. terrysmithae. Cenozoic specimens reported from USGS M1940 could not be located (Powell, 2007). Elimination of the L. terrysmithae specimens from L. catalinae, restricts the geographic occurrence of L. catalinae to southern California and Baja California, Mexico. Therefore, the geographic range of L. terrysmithae is now limited to the southern Salinas Valley, Monterey County in central California. Smith (1991), citing an oral communication and field maps of T.W. Dibblee, Jr., reported the stratigraphic occurrence of L. terrysmithae as restricted to a hard sandy facies of the Monterey Formation in the Cholame Hills, Monterey and San Luis Obispo Counties. Other occurrences reported here are in the Santa Margarita Sandstone. In several collections L. terrysmithae is found in association with L. estrellanus, which is restricted in age to the “Margaritan” CPMS. Unfortunately, several collections including L. terrysmithae cannot be attributed with confidence to a specific California provincial molluscan stage because index taxa are lacking, so the stratigraphic range of this species includes, but may not be restricted to the “Margaritan” CPMS. Occurring from the Salinas Valley south to Baja California Sur, Mexico in late Miocene to late Pliocene rocks (“Etchegoin” to “San Joaquin” CPMS; Fig. 6) (Smith 1991), L. crassicardo is easily distinguished from L. terrysmithae by its larger size, beaks that projects slightly beyond the dorsal margin of the auricles, and rounded radial ribs that commonly have radial striae. Lyropecten estrellanus in part co-occurs with L. terrysmithae in central California in middle late Miocene (upper “Margaritan” CPMS) age rocks. This "Margaritan" index fossil is distinguished by commonly having rounded comarginal ledges on more inflated valves, beaks that project beyond the dorsal margin of the auricles, and a wider shell at a similar size. Both L. miguelensis and L. miguelensis supervariant form of Smith (1991) are distinguished from our new species by having valves that are strongly inflated and with the right-valve beak extending beyond the left-valve beak and both extending beyond the dorsal margin of the auricles. Also the right valve has slightly fewer ribs and both valves have radial striae when well preserved. Lyropecten miguelensis is only reported from the early Miocene (upper “Vaqueros” CPMS) of San Miguel Island, Santa Barbara County in the southern California Bight (Smith 1991), however, we believe these rocks are likely middle Miocene in age (“Temblor” CPMS) as rocks referred to the “Vaqueros Formation" are on nearby Santa Cruz Island, Santa Barbara County were recently determined to be middle Miocene in age (Powell and Geiger 2019, Powell unpublished data). The supervariant form of L. miguelensis is reported in rocks of early to middle Miocene age (upper “Vaqueros” to lower “Temblor” CPMS age) from the northern Channel Islands (San Miguel, Santa Cruz, and Santa Rosa islands), Cuyama Valley, and Santa Madre Range in Santa Barbara County, the La Panza Range, San Luis Obispo County, the Santa Lucia Mountains, Monterey County, and the Temblor Range, San Luis Obispo and Kern counties. Occurring in older rocks L. pretiosus is reported by Smith (1991) from late Oligocene(?) to early Miocene age rocks (upper lower to middle “Vaqueros” CPMS) from the northern Channel Islands, Santa Barbara County, south to Baja California Sur, Mexico, and possibly (poorly identifiable specimens) as far north as the La Panza Range, San Luis Obispo County, central California (Smith 1991). L. pretiosus tends to form slightly angular ledges in the left valve and has subdued rounded radial ribs, beaks that extend slightly above the dorsal margin of the auricles and has few radial ribs (14–15; Smith 1991) and fine striae on well preserved specimens all of which serve to distinguish it from our new species. The “Jacalitos” CPMS (late Miocene) index fossil L. terminus occurs from the southern Salinas Valley to the Jacalitos Hills and canyons south of Coalinga in the western San Joaquin Valley. It is distinguished in having fewer ribs (right valve, 14–15; left valve, 13–14), beaks that project slightly, but equally above the dorsal margin of the auricles, juveniles with one riblet in interspaces and four or more costae in adults, while the left valve interspaces may contain four to five or more costae. It is similar to L. terrysmithae in having valves with rectangular ribs. Lastly, L. tiburonensis, occurs in rocks associated with the proto-Gulfo de California from the Salton Trough in south-central California south to Isla Tiburon, Sonora, Mexico, and on islands in the Gulfo de California; all these occurrences are late Miocene in age (Matti et al. 1985, McDougall et al. 1994, 1999, Rymer et al. 1994, 1995, Bennett et al. 2015), not late middle to late(?) Miocene as reported by Smith (1991). The late Miocene age is equivalent, at least in part to the “Jacalitos” CPMS in coastal California. It is easily distinguished from L. terrysmithae by its beaks, which project equally slightly above the dorsal margin of the auricles, radial ribs with costae and valves having significantly fewer ribs (right valve 11–12, left valve 9–10), which are rounded rectangular in shape with wider interspaces.
CONCLUDING REMARKS
Smith (1991) formerly referred specimens of Lyropecten terrysmithae to Argopecten sp. based on it having 1) hinge crura, 2) a short hinge line that is not perfectly straight, 3) beaks that meet at the hinge line, 4) auricles that are not rectangular, 5) valves that are flat and unledged, and 6) looped lamellar growth lines. However, all these features can be observed in various California Lyropecten species to some extent with the exception of beaks meeting at the hinge line. Waller (1986, 1991) identified the hinge of the Lyropecten/Nodipecten clade as having a three-element hinge dentition with a resilial, intermediate and dorsal teeth, a condition not found in Argopecten. Smith (1991) apparently was not able to observe the hinge of this new species given Waller’s (1986, 1991) early work on the subject since she lists the hinge of Lyropecten as having three pairs of hinge teeth in the right valve and two in the left valve, and the hinge of Argopecten as “...teeth weak to moderately strong,” without giving the number of teeth (Smith 1991, table 2). If she had seen the hinge she would not have misassigned the genus.»
CHARLES L. POWELL II, CHERYL D. MILLARD & CHRISTINE GARCIA, 2020
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«Arnold's "flat form grading into catalinae" (pI. 22, figs. 5, 6) is a flat, short-hinged scallop from the Vineyard Canyon-Indian Valley area of the Stockdale Mountain 7½-minute quadrangle (localities USGS M3995, USGS 12922 = 16833, CAS 28473). It is a convergent Argopecten sp related to but different from undescribed forms from eastern Baja California Sur (CAS localities 56493, 56499); in the Cholame Hills it is restricted to a hard sandy facies of the Monterey Formation (T.W. Dibblee, Jr., oral commun. 1973, and field map of the San Miguel 15-minute quadrangle).
This form is an Argopecten that has been identified as both L. estrellanus and L. catalinae. It differs from both of them in hinge crura and a proportionally shorter hinge line that is not perfectly straight. Beaks meet at the hinge line, auricles are not rectangular, and valves are flat and unledged. Looped lamellar growth lines are distantly spaced. Despite all of these Argopecten features, the species is easily confused with L. estrellanus; it also has 17-18 ribs and a single midriblet in each interspace.» SMITH, J. T. 1991. Cenozoic Giant Pectinids from California and the Tertiary Caribbean Province: Lyropecten, "Macrochlamis", Vertipecten, and Nodipecten species. United States Geological Survey Professional Paper, 1391: v + 1-155, figs. 1-18, pls. 1-38. [p. 56]
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Argopecten sp.; J. T. Smith, 1991, Cenozoic Giant Pectinids from California and the Tertiary Caribbean Province: Lyropecten, "Macrochlamis", Vertipecten, and Nodipecten species, plate 22, figures 5, 6.
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