Formerly: Lepidocyclus capax, Rhynchotrema capax
- 1842 Atrypa capax Conrad, Jour. Acad. Nat. Sci. Philadelphia, 8, p. 264, pl. 14, fig. 21.
- 1893 Rhynchotrema capax Hall and Clarke, Pal. New York, 8, pt. 2, pp. 183, 185, pl. 56, figs. 14-18, 20-27; pl. 83, fig. 31.
- 1924 Rhynchotrema capax Foerste, Upper Ordovician Faunas in Ontario and Quebec, p. 124-125, pl. 10, fig. 16a-b.
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- C6 Sequence (Elkhorn)
- C5 Sequence (Lower Whitewater, Liberty, Waynesville)
- C4 Sequence (Arnheim)
Identification in Hand Sample
- Medium sized shell with suboval to subtriangular outline
- Dorsibiconvex or globose in alteral view
- Average length 16.1 mm (maximum 21.2 mm), width 18.3 mm (maximum 17.4 mm), length/width ratio 0.91, and thickness/width ratio 0.74
- 15-20 costae
- three major plications in the sulcus, rounded beak, and growth lines crossing the plications
Hiscobeccus capax from Waynesville formation of Franklin County, Indiana (OUIP 1616)
Jin & Zhan (2001):
- Description: (Southern Manitoba material) Shell medium-sized, barely reching large size, suboval to subtriangular in outline, strongly dorsibiconvex to globose in alteral view, with average length 16.1 mm (maximum 21.2 mm), width 18.3 mm (maximum 17.4 mm), length/width ratio 0.91, and thickness/width ratio 0.74; shell attaining greatest width and thickness at about two-thirds of shell length from apex. Ventral umbo narrow with incurved beak. Dorsal umbo broad, strongly arched, with beak curved into delthyrium. Deltidial plates absent. Fold and sulcus well developed, beginning immediately anterior of umbonal areas, occupying about 45% of shell width at anterior margin. Shell costae strong, simple, subrounded, typically four on fold, three on sulcus, and six to eight on each flank. Growth lamellae well developed, most conspicuous in anterior part of shell, averaging 3-4 per 1 mm.
- Ventral interior: Teeth small, wedge-like, supported by thickened shell wall. Dental plates absent. Muscle field deeply impressed.
- Dorsal interior: Hinge sockets deep. Cardinal process short, blade like (fig. 27). Septalium small, shallow. Median septum high and thick posteriorly, extending anteriorly to become low myophragm. Crura radulifer, extending antero-ventrally. Adductor scars well impressed, without bounding ridges.
- Remarks: The internal structures of Hiscobeccus capax are nearly identical to those of H. gigas. Externally, however, H. gigas has notably lower convexity, and much stronger and coarser growth lamellae. The specimens of H. capax from the Red River Formation are generally smaller than the type specimen (Conrad, 1842) and those from the Hudson Bay Lowlands (Jin et al., 1997). The cardinal process in the sectioned southern Manitoba shell appears to be weaker (also lacking an inflated crest) than that of Hiscobeccus from other regions (e.g., Jin, 1989; Jin and Lenz, 1992). The shells of Hiscobeccus capax from the uppermost Bighorn Dolomite of Wyoming (Ross, 1957) are identical to the Red River material except for their larger shell size.
- Articulate brachiopod. Note the three major plications in the sulcus, the rounded beak, and the growth lines crossing the plications. Entire Richmondian.
Jin et al. (1997):
- Remarks: In the Hudson Bay collections, both H. capax and H. gigas have relatively large shells. Hiscobeccus capax can be distinguished from H. gigas, however, by its greater convexity (thickness/width ratios of 0.8 to 1.06), finer growth lamellae (2 or 3 per 1 mm), and higher anterior tongue directed almost perpendicularly from the commissural plane and giving the shell a truncated appearance (pl. 29, fig. 1 shows a tongue projecting dorsally from the level of the commissural plane to a height of about 18 mm). Extremely globular forms may be thicker than wide (pl. 27, figs. 1-5). As is typical of the genus, four simple, rounded costae are associated with the fold and three with the sulcus. Both ventral and dorsal umbones are strongly convex and incurved, with the former tightly appressed to the latter, completely obscuring the delthyrium. The strong convexity is also shown by the specimens of H. capax illustrated by Alberstadt (1973) and Amsden (1983)
- DESCRIPTION – Shell of average size for the genus; width somewhat greater than length. Lateral profile unequally biconvex, the brachial valve having the greater depth. Anterolateral margins rounded in outline. Anterior commissure strongly uniplicate, the depth of sulcus averaging about one-half of the total thickness of the shell. Surface multicostate, about 15 to 20 costae, of which four occur on the fold and three in the sulcus. Surface crossed by closely-spaced zigzag lamellae.
- Pedicle vavle moderately convex in lateral profile, having the greatest convexity at the middle. Beak strongly incurved but rarely pressed against the brachial umbo. Umbo swollen, merging anteriorly with a progressively widening sulcus which occupies about one-half the width of the valve at the front. Flanks gently rounded.
- Interior of pedicle valve: delthyrium open or partially closed by rudimentary deltidial plates; delthyrial cavity large, occupied by scar of pedicle attachment. Teeth well defined; dental plates slopping toward midline, generally obscured by shell thickening. Muscle field flabellate in outline; adductor scars small incised, and elongate , tapering anteriorly and commonly emarginated along the posterior margin. Diductor muscle scars large, expanded anteriorly to enclose the adductor impressions.
- Brachial valve strongly convex in lateral profile, the greatest convexity observed at the middle. Umbo broadly swollen, slightly depressed medially, merging with a fold which becomes moderately elevated at the front. Flanks inflated.
- Interior of brachial valve: hinge plates divided thick, gently concave, slopping toward midline. Crura extended from anterior edge of hinge plate plate, becoming strongly curved near the anterior ends. A high bladelike cardinal process divides a narrow notothyrial cavity. Median septum thick adjacent to the cardinalia but becoming a sharply defined ridge where it divides the muscle field. Adductor muscle scars suboval in outline and subequal in size; the posterior pair smaller than anterior pair in specimens in which they can be distinguished.
- DISTRIBUTION – This species represents one of the most common brachiopods in the Richmond Group of Tennessee. It is found throughout the “Arnheim”, but it extends upward into the basal beds of the Fernvale at several places including localities 1, 2, 3, 5, 8, and 13. Etched limestones from the “Artheium” locality northwest of Goodlettville (loc. 3) yielded an especially large and well preserved assemblage. Additional specimens examined I the collections of the U.S. National Museum and the stratiagraphic collections of the Geological surveys include USNM 80612. USNM 48964, USGS 370W and Usgs 426N.
- A less costae, larger and possibly subspecific form of this species is found in the “Fernvale” limestone of Oklahoma (Howe, 1966) and in the Upham and Aleman Formation (Montoya Group) of west Texas (Howe, 1967). To my knowledge, the Upham form (Trentonian or early Cincinnatian) represents the earliest occurrence of this species.
- COMPARISON- Large samples of L.? capax from Richmond localities in the Ohio Valley were compared directly with specimens from the “Arnheim” and Fernvale horizons, and many were found to fall within the variation of shape demonstrated by the Tennessee are generally smaller and possess a slightly narrower fold (Text-figs. 6,7). The Tennessee variant is also commonly more costate than representative specimens from the Ohio Valley (Table 4). These differences probably reflect the two populations. The “Arnheim” form in Tennessee in fact may be ancestral to the Ohio Valley species, because the latter did not appear in the Richmond section there until after Arnheim deposition (Caster and others, 1955).
- Lepidocyclus? capax differs significantly from L.manniensis and L.cooperi in having a Rynchotrema-like open delthyrium or one partially closed by rudimentary deltidial plates and a wide delthyrial cavity which is wholly occupied by the scar of the pedicale attachment. Lepidocyclus rowleyi is possibly conspecific with L. capax; however, the former has a more elliptical outline, somewhat fewer costae, and a larger and more anterior muscle field in the pedicle valve.
- DISCUSSION- the specific name Atrypa capax was proposed by Conrad (1842, p. 264) for a “profoundly ventricose” form from “Silurian” shale exposed at Richmond, Indiana. It is not clear what Conrad intended to include in his concept of “Atrypa capax” for the description is short and in partial conflict with the single illustration. The description indicates that the species possessed about 16 ribs (costae), whereas his figure (a side view) shows 24 costae on the brachial valve assuming 10 costae on can flank and four on the fold. Examination of several hundred specimens of L.? capax in the collections of the U.S. National Museum and in my own collection from the Ohio Valley failed to reveal a single specimen having more than 21 costae; moreover, only one was thicker than Conrad’s illustrated specimen.
- Through the courtesy of H.G. Richards, I examined one of Conrad’s supposedly original specimens (ANSP 6029). According to a label written by T.G. White, “This specimen is apparently Conrad’s type of Atyrpa capax as figured by Conrad ….This specimen and others labeled capax and inaequivalvis were in the tray bearing Conrad’s erased label of cotype increbesceus.” Measurments in mm from this specimen are as follows: length, 18.5; width, 20.2; thickness, 19.0; height of sulcus, 10.7; and fold width, 13.2 The surface ornamentation shows only 14 costae on the brachial valve. Because this specimen is representative of those found at Richmond, it could serve as the lectotype for the species.
- Wang’s (1949,p.12) original diagnosis of Lepidocyclus and the definition given in the Treatise (Williams, and others, 1965, p.H555), emphasized conjuct deltidal plates as a diagnostic character of the genus. Howe (1965) reassessed Lepidocyclus and recognized the variability of this character by noting the Rhynochotrema-like open delthyrium (or a delthyrium partially closed by narrow deltidial plates) and the large dethyrial cavity wholly occupied by the pedicle cellist in specimens of L.? capax. Because these features are so unlike the conjunct deltidial plates and tubular pedicle tunnel observed in other species of the genus (for example, contrast figs. 10 and 12, illustrated in Plate 156 with Pl. 155 figs. 20 and 22), Howe (1966) questioned the generic assignment of L.? capax. This interpretation has been reaffirmed by a study of more than 100 well-preserved pedicle valves from the “Arnheim” and Fernvale, which failed to reveal a single specimen displaying conjunct deltidial plates (see PL. 156, fig. 10). Regardless of the manner in which these differences are interpreted, it is clear that L.? capax and similar species (for example, L.? rowleyi) occupy an intermediate position between Lepidocyclus and Rhynchotrema, and I feel that the genetic assignment of such species is arbitrary. In previous papers, Howe (1966,1967) considered this problem and tentatively retained L.? capax in the genus Lepidocyclus, mainly because of its large size and obscured dental plates. However, there is little reason to doubt that L.? capax was derived directly from Rhynchotrema and its Rhynchotrema-like features and phylogenic position from equally valid reasons for reassigning the species to Rhynchotrema. Williams (1965, p.H214) aptly called attention to the problem of classification by stating, “Taxonomic procedure is a practical method of segregating organisms into identifiable groups, each with a clearly defined status within a classificatory scheme, whereas the theory of evolution postulates continuity between all such groups within the framework of time.” It is precisely at the generic level that this incompatibility often reaches its most serious level. One can apply various methods in paleontology to the problems of species differentiation, but at the generic level the procedures are more empirical and less likely to reflect true phylogenetic relationships.
- Typical Form: The type of Rhynchotrema capax illustrated by Conrad was a very obese gerontic form, such as occurs in the Whitewater at Richmond, Indiana. This type was considerably more gibbous than any other specimen figured since, though similar specimens are common in the Whitewater in Indiana and Ohio. Conrad’s figure represents a lateral view. Distance from the beak to the anterior margin of the shell 23 mm; maximum thickness from pedicle (ventral) to brachial (drosal) valve 26 mm, convexity of pedicle valve 12 mm, that of brachial (dorsal) valve 14 mm. The most conspicuous feature of this lateral view is of the flatness of its outline along anterior part of shell, due to the strong, almost geniculate curvature of the shell toward its anterior margin, especially in pedicle (ventral) valve.
- Although it is customary to identify as R. capax the various specimens of Rhynchotrema in the Richmond of Ontario and Quebec, none has the characteristic obese aspect of the Whitewater type, but they resemble more closely the unnamed forms found in the Waynesville in Indiana and Ohio. Occasionally these Waynesville forms have been identified as R. perlamellosum, which also is different.
- R. increbescens (Hall) from the Trenton (pl. X, fig. 15), is a much smaller species, less gerontically incurved.
- A form resembling R. capax found south of the west end of lac Calvaire, 12 miles southwest of the city of Quebec, was listed as R. increbescens solely on account of its occurrence in strata apparently as low as the lower Eden of Cincinnatian areas. However, its resemblance to R. capax was startling to one accustomed to regard the latter species as a safe index fossil for the Richmond. Unfortunately the Lac Calvaire specimen was found in a soft shale hat crumbled before it was photographed or preserved.