Ruthenoceras Korde, 1949

Dzik, Jerzy, 2020, Variability of conch morphology in a cephalopod species from the Cambrian to Ordovician transition strata of Siberia, Acta Palaeontologica Polonica 65 (1), pp. 149-165 : 162-163

Genus Ruthenoceras Korde, 1949

Type species: Ruthenoceras elongatum Korde, 1949 , Boguchany on the Angara River , Siberia , Ust-Kut Formation, probably latest Cambrian .

Emended diagnosis.—Moderately elongated compressed conch strongly endogastrically bent near the apex and near the base of the living chamber but with gently curved phragmocone showing a change from septa arranged transversely to their strongly oblique orientation.

Remarks.— Korde (1949), while studying thin petrographic sections of the “algal limestone” collected from an exposure located 4 km from the village of Boguchany on the Angara River (that time considered to be late Cambrian age), noticed two oblique sections of nautiloid conchs, each a few millimetres long. She misinterpreted the sediment-filled siphuncle as the living chamber and considered her two new genera and species, R. elongatum and Angaroceras globosum, to represent ascoceratids (“Mixochoanites”). Balashov (1962: 124) pointed out that the “algal limestone” in the area represents the Ust-kut Formation of early Ordovician age. Although the Korde’s (1949) locality is about twenty kilometres away from the place where the here described material was discovered, the strata in the area are virtually horizontal and little doubt remains that her material, as well as the Balashov’s (1962) Clarkoceras angarense come from the same rock unit. This introduces a serious nomenclatorial problem. Korde’s (1949) thin sections do not show diagnostic characters of the species, but there is nothing that could contradict its conspecificity with my material that comes from the same formation in the same region. At first glance the siphuncle appears to be wider than in specimens from the Pashino block No. 1 sample but the Korde’s (1949) specimens were apparently cut obliquely close to the transverse plane near the conch venter, like the centrally located specimens on the polished slab from the Pashino block No. 1 ( Fig. 1D ). The most parsimonious solution would thus be to choose one of Korde’s (1949) names and interpret the genus and species based on the new material as well as on data of Balashov (1962).

The material from the loose block No. 1 fits well the morphology of the only nautiloid species identified by Balashov (1962) from the Ust-Kut Formation on Angara. A few other nautiloids from the same Formation on Chunya, Lena, and Podkamennaya Tunguska rivers were reported by him, but whether they are strictly coeval with the Angara material remains to be proven by conodont studies. Their conchs are straight, except for Levisoceras from Chunya.

Balashov (1962) attributed his new species from the Ust-kut Formation to Clarkoceras Ruedemann, 1905 , but the type species of this genus, C. newtonwinchelli ( Clarke, 1897) is breviconic and quite different from the Siberian species. Possibly, the genus Ectenolites Ulrich and Foerste, 1936 , with the type species E. subgracilis from the earliest Tremadocian Gasconade Dolostone in Missouri, is a senior synonym of Ruthenoceras . It shows a similar morphology of the phragmocone but the living chamber is much more elongated in some of its species ( Kröger and Landing 2007) although it is not preserved completely in the holotype of E. subgracilis ( Ulrich et al. 1943: pl. 58:1, 2). The taxonomic value of this character has to be determined. It may be noteworthy that its fossils come from a stromatolite-rich rock unit ( Overstreet et al. 2003). Among other North American type species of ellesmeroceratid genera, that of Albertoceras Ulrich and Foerste, 1936 , i.e., A. walcotti , is similar to the Siberian nautiloids except for its narrow siphuncle that is somewhat departed from the conch wall and for a tendency to exogastric curvature ( Ulrich et al. 1944: pl. 23: 14). It is known from a minute, probably juvenile specimen from the Mons Formation of Alberta, together with equally small specimens classified in A. gracillimum . Despite its small size, its living chamber is somewhat constricted but there is no sign of maturity in septal crowding. Some similarity to the Siberian species is noticeable in Eremoceras pergracile ( Ulrich, Foerste, and Miller, 1943) from the Ellenburger limestone in Texas (but not the type species of Eremoceras Hyatt, 1884 , which is breviconic) and E. obliquum ( Ulrich, Foerste, and Miller, 1943) from the Oneota dolomite in Wisconsin.

Among the latest Cambrian ellesmeroceratids from China, the type species of Acaroceras Chen, Qi, and Chen, 1979 , A. endogastrum Chen, Qi, and Chen, 1979 , based on a single specimen from the upper part of the Fengshan Formation in Inner Mongolia, fits probably the general slightly endogastric conch shape and inclined septa of the Siberian species, although it is known only from fragmentary longitudinal sections of the phragmocone ( Chen et al. 1979; Chen and Teichert 1983). The Siberian species differs from the Chinese one in a wider siphuncle and more densely distributed septa. Altogether eleven species of this genus and 122 of other genera were named by Chinese authors based on a rather limited material from the Wanwankou Member of the Fengshan Formation.

The only other Chinese Cambrian ellesmeroceratid with similarly inclined septa and a wide siphuncle, Qiushugouceras inclinatum Chen and Teichert, 1983 , differs from the Siberian species in an orthoconic conch appearance (but only two longitudinally sectioned, probably juvenile, specimens are available) and in longer, gently curved (loxochoanitic) septal necks ( Chen and Teichert 1983).

Stratigraphic and geographic range.—Latest Furongian or earliest Tremadocian near Kodinsk, Siberia.

Figures

Fig. 1. Loose block No. 1 of a stromatolitic limestone with abundant nautiloid conchs of the Ust-kut Formation (latest Furongian or earliest Tremadocian) found on the left bank of the Angara River at the former village Pashino. A. The block partially exploited for fossils. B. Stromatolite columns with empty cavities and a laminar cover above.C. A piece of the rock with exposed nautiloids. D. Polished rock surface (note similarity of the specimen in the middle to the holotype of Ruthenoceras elongatum Korde, 1949). E. Naturally abraded upper surface of the stromatolite columns.