Lawrenceoceras larus, Kröger & Pohle, 2021

Kröger, Björn & Pohle, Alexander, 2021, Early-Middle Ordovician cephalopods from Ny Friesland, Spitsbergen - a pelagic fauna with Laurentian affinities, European Journal of Taxonomy 783 (1), pp. 1-102 : 12-14

Lawrenceoceras larus sp. nov

urn:lsid:zoobank.org:act:B0503E12-D63A-4656-BAE7-8C22CFB40D5E

Figs 5A–C , 6B , 7A

Diagnosis

Exogastrically curved conch with angle of expansion of conch width of ca 3° and slightly depressed conch cross section (rW = 1.16); conch surface smooth with fine irregularly spaced growth lines which

form shallow hyponomic sinus at convex side of conch curvature; siphuncle small, 0.1 of conch height and near conch margin.

Etymology

From the gull genus Larus Linnaeus, 1758, a common companion during fieldwork at Profilstranda.

Type material

Holotype Specimen FMNH-P30351 .

Paratypes Five specimens ( FMNH-P30337 , P30338 , P30339 , P30342 , P30355 ) from bed PO 131, 128 m above base of Olenidsletta Member, V2 b trilobite zone, Blackhillsian , Floian .

Type locality and horizon

Profilstranda section, adjacent to Hinlopenstretet, Spitsbergen, bed Po123.3, 120.3 m, above base of Olenidsletta Member, V 2a trilobite zone, Blackhillsian, Floian.

Description

The holotype is a fragment of a curved phragmocone with a length of 65 mm, its width expands from 13.5 to 16.5 mm over a length of ca 48 mm (angle of expansion ca 4°). The conch surface is almost smooth, ornamented only with faint irregularly spaced growth lines, which form a very shallow sinus on the convex side of the conch curvature and a broad and shallow, but distinctive hyponomic sinus at the concave side of the conch curvature.

The conch cross section is depressed (rW = 1.16–1.23) with the dorsum slightly flattened compared with the ventral side. Ten measurements of the relative conch width including the additional material result in a mean value of rW = 1.3 (1 st –3 rd quantile =1.24–1.33, n =10).

The septa are narrowly spaced, ca 10 chambers occur per distance similar to corresponding conch height and the sutures form a shallow lobe at the lateral flanks (rCL = 0.1). The septal curvature is relatively strong, ca 2–3 mm at the adapical end of the specimen.

The septal perforation is positioned almost at the margin of the conch on the convex side of the conch curvature and has a diameter of 1.5 mm at a position where the conch height is 13 mm.

In specimen FMNH-P30355 details of the siphuncle and septal necks are preserved. There, the connecting ring is thickened with concave segments and the septal necks are short loxochoanitic. No endosiphuncular and/or cameral deposits are known.

Comparison

The new species differs from other species of Lawrenceoceras in having a relatively low angle of conch width expansion; it also differs from the type species in having a more depressed conch cross section (rW in L. collinsi is ca 1.06). Lawrenceoceras confertissimum (Whitfield, 1886) from the Fort Cassin Formation, New York, USA has a larger rate of expansion of the conch width (12.5° compared with 4° in L. larus sp. nov.). Lawrenceoceras australe ( Ulrich et al., 1944) from the Odenville Limestone, Alabama, USA differs in having a more depressed conch (rW = 1.34) with a distinctive triangular conch cross section.

Figures

Fig. 5. Cyrtoconic cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A–C. Lawrenceoceras larus sp. nov., FMNH-P30351, holotype, from bed PO 131. A. Lateral view with siphuncle on left side. B. Ventral view, prosiphuncular side. C. Adoral view of septum and septal perforation.D–E. Lawrenceoceras ebenus sp. nov., FMNH-P30335, holotype, from bed PO 123.3. D. Lateral view with siphuncle on left side. E. Ventral view, prosiphuncular side. F–G. Olenidslettoceras farmi gen. et sp. nov., FMNH-P30334, holotype, from bed PO 123.3. F. Lateral view with siphuncle on left side. G. Ventral view, prosiphuncular side. H–N. Vallhalloceras floweri King & Evans, 1990. H–I. Specimen FMNH-P30337, from bed PO 131. H. Lateral view, siphuncle on right side I. Ventral view, prosiphuncular side. J–L. Specimen FMNH-P30343 from bed PO 123.3, J. Lateral view with siphuncle on left side. K. Ventral view, prosiphuncular side. L. Dorsal view, antisiphuncular side.M–N. Specimen FMNH-P30338 from bed PO 131. M. Lateral view with siphuncle on left side. N. Ventral view, prosiphuncular side. O–Q. Cyclostomiceras profilstrandense sp. nov., specimen FMNH-P30336, from bed PO 123.3. O. Lateral view with siphuncle on left side. P. Ventral view, prosiphuncular side. Q. Dorsal view, antisiphuncular side. Scale bar = 10 mm for all figures.

Fig. 6. Median sections of cyrtoconic cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. Lawrenceoceras ebenus sp. nov., FMNH-P30335, holotype, from bed PO 123.3. B. Lawrenceoceras larus sp. nov., FMNH-P30355, paratype, from bed PO 131. C. Vallhalloceras floweri Evans & King, 1990, FMNH-P30340, from bed PO 131. D. Olenidslettoceras farmi gen. et sp. nov., FMNH-P30334, holotype, from bed PO 123.3. Scale bar = 5 mm for all figures.

Fig. 7. Details of siphuncle and septal necks, median sections of cyrtoconic ellesmeroceratids from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. Note the loxochoanitic septal necks and the concave siphuncular segments. A. Lawrenceoceras larus sp. nov., FMNH-P30355, paratype, from bed PO 131. B. Lawrenceoceras ebenus sp. nov., FMNH-P30335, holotype, from bed PO 123.3. Scale bar = 1 mm for all figures.