Redpathoceras, Flower, 1963

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 92-95

Redpathoceras magnum sp. nov.

urn:lsid:zoobank.org:act:5B495FBE-A8DE-4886-ABF4-FB22AC2D588F

Figs 42 , 43 A-C

Diagnosis

Mature body chamber with slightly depressed cross section, moderately curved with maximum curvature close to aperture; inflated in lateral view with maximum height of ca. 70–80 mm at mid-length of body chamber; mature body chamber margin at concave side of conch curvature nearly straight on adapical two-thirds; conch cross section compressed to circular in earlier growth stages; premature conch expands with angle of expansion 15°; mature conch with ca. five phragmocone chambers adoral of the terminal septum of truncation; diameter of terminal septum of truncation ca. 55 mm; ornamented with fine longitudinal striae and irregularly spaced transverse growth lines, which form a wide hyponomic sinus.

Etymology

Latin magnus refers to the large adult size of this species.

Type material

Holotype

PMU 26935 .

Paratypes

Fourteen paratypes ( PMU 26936–26948), Kallholn, Dalarna, Sweden, Boda Limestone, late Katian.

Type locality and horizon

Kallholn, Dalarna, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

The holotype is a fragment of a mature shell with a nearly complete body chamber combined with a phragocone consisting of five chambers ( Fig. 43 A-B). The maximum width of the body chamber is ca. 70 mm, the maximum height 68 mm at a position approximately at body chamber mid-length (width/ height ratio 1.03). The complete length of the body chamber is ca. 100 mm. The width of the bodychamber at the aperture is 70 mm and its height 63 mm (width/height ratio 1.11), at the base of the body chamber the width is 58 mm and the height 60 mm (width/height ratio 0.97). The body chamber is moderately curved, with a narrower curvature in adapertural sections.

The cross section of the aperture is slightly depressed with a more rounded margin on the convex side of conch curvature. The shell is thickened at the aperture with a thickness of ca. 2.5 mm, compared with a thickness of 1.3 mm at body chamber mid-length. The surface of the shell is poorly preserved but apparently smooth.

The phragmocone has a length of 40 mm and expands with an angle of 19°. The four adoral chambers are shallow with a depth 0.15 that of the corresponding conch height; their septa are simple and cap shaped, with a shallow convexity, whilst the sutures are directly transverse, but slightly oblique and projecting toward the aperture on the concave side of the shell.

The septum of truncation is deeply convex, its depth is such that it is almost equal to the depth of the next two chambers. The siphuncular perforation is narrow, with a diameter of ca. 3 mm (ca. 0.07 of corresponding conch height) and positioned eccentrically at a distance of 8 mm from conch margin on convex side of conch curvature (SPR 0.18).

PMU 26942 is the largest fragment assigned to this species. It is a fragment of a body chamber with a maximum width of 78 mm and a maximum height of 70 mm (width/height ratio 1.11). The length of the body chamber is 100 mm. The shell surface is ornamented with faint, irregularly spaced rounded striae or growth lines which form a broad, shallow hyponomic sinus on the convex side of the shell curvature.

In well preserved fragments of earlier growth stages with conch heights of less than 50 mm, the shell surface is ornamented with irregularly, ca. 0.5–1 mm distant longitudinal striae and irregularly spaced growth lines that form a wide u-shaped hyponomic sinus on the convex margin of the shell ( Fig. 43C ).

Remarks

Five specimens (PMU 26945–26947; NRM Mo 8795, 152300) are interpreted as truncated parts of the phragmocone (see, e.g., Fig. 43C ). All five specimens possess at their apical end a deeply curved septum of truncation. Specimens NRM Mo 8795 and 152300 possess seven and eight chambers, respectively. The conch height at the septum of truncation varies between 36 and 45 mm and the adoralmost diameters are 55 mm, resulting in angles of expansion of 14–15°( Fig. 42 ). The conch cross section in these specimens is slightly compressed to circular and they are more curved longitudinally than the mature fragments. The suture is nearly directly transverse, but forms a shallow, wide lobe on the convex side of the conch curvature. The assignment of these fragments to R. magnum sp. nov. is justified by their similar conch cross section and by the sizes of their adoral-most septa, which agree well with the size of the septum of truncation of the mature stage of the holotype.

Based on the fragments of different growth stages a conch can be reconstructed, that, if it had not been truncated at several stages, would possess an angle of expansion of ca. 15° in juvenile growth stages, a rate of expansion of more than 20° at the adoral end of the phragmocone and a decreasing height during its final growth stages. The inflation of the body chamber is a result of a nearly straight outline of the shell margin at the antisiphuncular side of the conch during the initial stages of growth of the mature body chamber.

Comparison

This is a large species of Redpathoceras , its mature conch width being more than twice that of the type species. It differs from the type species in having a conch cross section which changes during growth from compressed to depressed. R. bullatum sp. nov. differs in having a smaller adult size and a less inflated mature body chamber, with a concave outline of the conch margin on the concave side of the conch curvature. In R. magnum sp. nov. the outline of the mature body chamber at the antisiphuncular side is nearly straight in the apical two-thirds.

Stratigraphic and geographic range

Boda Limestone, late Katian, Dalarna, Sweden.

Figures

Fig. 42. Variation in angle of expansion in Redpathoceras Flower, 1963, Boda Limestone. R. magnum sp. nov. (grey dots); R. bullatum sp. nov. (black dots); R. depressum sp. nov. (black circles).

Fig. 43. Redpathoceras Flower, 1963, Kallholn, Boda Limestone. A-C. Redpathoceras magnum sp. nov. A. PMU 26935, holotype; mature truncated body chamber, lateral view. B. Same specimen, view from antispihuncular side. C. PMU 26946, premature truncated phragmocone, lateral view. D-F. Redpathoceras depressum sp. nov. D. PMU 26927, holotype; mature truncated body chamber, lateral view. E. Same specimen, view from antispihuncular side. F. PMU 26931, premature truncated phragmocone, lateral view. Scale bar = 50 mm for all figures.