Deltoceras beluga, 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 : 63-64

Deltoceras beluga sp. nov.

urn:lsid:zoobank.org:act:197BA0EF-574D-4EEF-ADC3-DF120829D7C5

Figs 41A , 42–43 , 44A

Diagnosis

Discoidal, slightly involute conch with moderately expanding whorls with relative whorl width (rW) of ca 0.7–08 and with slightly flattened venter. Ornamented with fine growth lines, which form broad lateral lobe and deep hyponomic sinus.

Etymology

From the Beluga whale Delphinapterus leucas (Pallas, 1776) a common guest offshore of Profilstranda in the Hinlopen Strait, Svalbard.

Type material

Holotype Specimen FMNH-P30321 .

Paratypes Two additional specimens, FMNH-P30322 and FMNH-P30326 , from type locality and horizon .

Type locality and horizon

From Profilstranda section, Ny Friesland, Svalbard, from bed PO 07, 4 m above base of Olenidsletta Member, V 1a trilobite zone, Blackhillsian, Floian.

Description

The holotype is a complete specimen of ca 2.5 volutions, with a conch diameter of 42 mm and an apertural conch width and height of 12 mm and 15 mm, respectively (WWI = 0.8) ( Fig. 42 ). The relative whorl width (rW) varies between 0.7 and 0.8 in the holotype. The conch grows with a WER of 2.78–1.96 between the first and last whorl and thus decreases with increasing diameter ( Fig. 43 ). The whorls are rounded at the flanks with greatest width at ca the first third of the conch height from dorsum, ventral side slightly flattened and dorsum with shallow zone of impression ( Fig. 44A ). Zone of impression ca 1 mm deep at aperture.

The conch surface is ornamented with irregularly spaced fine growth lines which form a broad lobe across the flanks and a deep, rounded, V-shaped hyponomic sinus. Hyponomic sinus at aperture of holotype ca 10 mm deep. The completely preserved body chamber has a length of 250° at the dorsum and 220° at the distal end of the hyponomic sinus.

The septa form a broad lateral sinus and have a ventral distance of ca 4 mm at a conch diameter of 28 mm. The umbilical window is ca 3 mm in diameter in the holotype.

Comparison

Besides the type species only one other species of Deltoceras was known. This second species, D. vangeni Ruedemann, 1906 from the Chazy Group, Valcour Island, New York, USA, has a nearly circular cross section and probably is an estonioceratid. The new species differs from the type species of Deltoceras in having a conch cross section with a slightly flattened venter and a shallow impression zone at the dorsum. Because the early volutions of the type species are not known and the three specimens known from the Olenidletta Member are relatively small compared with the type specimen, the possibility exists that the Olenidsletta Member specimens are conspecific with D. planum and represent earlier growth stages. The assignment of Deltoceras to the Estonioceratidae (see Evans 2011) or alternatively into the Tarphyceratidae (herein) is questionable and depends on a better knowledge of the type species. Here we place the genus provisionally in the Tarphyceratidae because of an assumed shallow dorsal impression during earlier growth stages in D. planum .

Figures

Fig. 41. Coiled cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. Deltoceras beluga sp. nov., FMNH-P30322, from bed PO 07. B. Trocholitoceras walcotti Hyatt, 1894, FMNH-P30328, from bed PO 131. C-D. Cycloplectoceras hinlopense sp. nov., FMNH-P30327, holotype, from bed PO 131. Scale bar = 10 mm for all figures.

Fig. 42. Deltoceras beluga sp. nov., FMNH-P30321, holotype, from bed PO 07, from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. View from right side. B. Ventral view; note the deep u-shaped hyponomic sinus. C. View from left side. Scale bar = 10 mm.

Fig. 43. Diagrams of whorl expansion rate (WER) and whorl width index (WWI) of Deltoceras beluga sp. nov., from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. Solid lines connect measurements from individual specimens. Data of type specimens of D. planum Hyatt, 1894 from Ulrich et al. (1942). See Supp. file 2 for details of measurements

Fig. 44. Cross sections of coiled cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen with details of siphuncular position and impression zone of conch. A. Deltoceras beluga sp. nov., FMNH-P30322, from bed PO 07. B. Litoceras profilbekkenense sp. nov, FMNH-P30329, holotype, from bed PO 131, C. Trocholitoceras juvenicostatum Ulrich et al., 1942, FMNH-P30324, from bed PO 131. D. Trocholitoceras walcotti Hyatt, 1894, FMNH-P30328, from bed PO 131. Scale bars = 10 mm.