Palaeodawsonocerina Kroger & Isakar 2006
Creators: name: Kröger, Björn
Publisher: name: Zenodo
Year: 2013
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Orthocerida
Dawsonoceratidae
Palaeodawsonocerina

description: Palaeodawsonocerina? nicolletoides sp. nov.
urn:lsid:zoobank.org:act: 4E1D7CA7-028A-4174-94DF-D86B525B5F94
Figs 12 D-E, 18B
Diagnosis
Slender, large cyrtocones with a slightly compressed conch cross section and a pronounced annulation; prosiphuncular conch margin wider and more flattened than antisiphuncular margin in cross section; adult conch diameter ca. 30 mm; annuli relatively wide, ca. 4 per distance equal to conch diameter, ridges of annuli sharp and narrow, grooves wider and more rounded; conch surface ornamented with fine longitudinal striae and faint, very shallow, rounded longitudinal ridges; siphuncle eccentric, lying on concave side of conch curvature, septal perforation ca. 0.14 of conch cross section.
Etymology
The name refers to Orthoceras nicolleti Clarke, 1897, which is very similar to Palaeodawsonocerina? nicolletoides sp. nov. However, this species differs from O. nicoletti in having a less centrally positioned siphuncle and a more circular conch cross section.
Type material
Holotype
PMU 26727.
Paratype
One paratype: PMU 26728 from Kallholn, Siljan District, Sweden; Boda Limestone, Boda Core Member, late Katian, Late Ordovician.
Type locality and horizon
Kallholn, Siljan District, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.
Description
Specimen PMU 26728 is a 105 mm long fragment of a nearly complete curved body chamber, with a diameter that increases from 28 to 32 mm (angle of expansion 2°; Fig. 12 D-E). The conch cross section is nearly circular at the base of the body chamber and slightly compressed at the aperture. The shell is strongly annulate, with a distance between annuli that increases adorally from 7 mm (0.24 of corresponding conch diameter) to 10 mm (0.31 of corresponding conch cross section). The second most adoral of the ridges is significantly lower in amplitude. Generally the ridges of the annuli are sharper than the valleys, which are broadly rounded. The annuli are slightly oblique, inclined adorally on the concave side of the conch curvature.
The holotype (PMU 26727), is a small piece of phragmocone 28 mm in diameter. The only preserved septum of this specimen is shallow convex (height of curvature 6 mm, 0.21 of conch diameter). The conch cross section is not perfectly circular, but rounded triangular with a pointed antisiphuncular and a broad prosiphuncular side. The septal perforation is 4 mm wide and eccentrically positioned, with a distance of 10 mm from the conch margin (SPR 0.36) on the concave side of the conch curvature. The shell is thin, 0.5 mm in the grooves of the annulation and thickens toward the ridges. The shell surface is ornamented with very fine longitudinal striae (ca. 10 per one millimeter), and very shallow, rounded, inconspicuous longitudinal ridges (Fig. 18B).
Remarks
The generic assignment of this species is uncertain because only fragments of the mature body chamber are known and internal details are a matter of speculation. However, because the conspicuous ornamentation is characteristic of other species of Palaeodawsonocerina, the species is provisionally assigned to that genus.
Comparison
The specimens described above are nearly identical to Orthoceras nicolleti from the Platteville Formation, Sandbian-early Katian of Wisconsin (Foerste 1932) with respect to the wide, pronounced annulation and the curvature of the conch. The Platteville specimen is poorly known, as no details of the conch surface are preserved and this is the only known specimen. It differs from Palaeodawsonocerina? nicolletoides sp. nov. in its larger size, central siphuncle and nearly circular conch cross section.
Teichert (1930) assigned a specimen from the Pirgu Regional Stage of Estonia to Syproceras textumaraneum (Roemer, 1861) which can be compared with Orthoceras nicolleti and Palaeodawsonocerina? nicolletoides sp. nov. with regard to its relatively large adult size, curved body chamber and distant annulations. The ornamentation of this specimen is not preserved, casting a degree of uncertainty on Teichert’s (1930) assignment to Spyroceras textumaraneum. The specimen of Teichert (1930) differs from Palaeodawsonocerina? nicolletoides sp. nov. in having a narrower and shallower annulation and a larger adult size.
Balashov (1975: pl. 1, figs 3-6) figured several annulated and longitudinally striated orthocerids under the name Gorbyoceras duncanae Flower, 1946. However, conch shape, ornamentation and internal characters indicate that these specimens are better assigned to Palaeodawsonocerina. The ornamentation, annulation and general shell shape of the specimen figured in Balashov (1975: pl. 1, figs 7) is similar to Palaeodawsonocerina? nicolletoides sp. nov., but a direct comparison based on the incomplete material is not possible.
Stratigraphic and geographic range
Boda Limestone, Siljan District, Sweden; late Katian, Ordovician.
description: Published as part of Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, pp. 1-110 in European Journal of Taxonomy 41 on pages 36-37, DOI: 10.5852/ejt.2013.41, http://zenodo.org/record/3814221