Gorbyoceras Shimizu & Obata, 1935

Kröger, Björn, 2025, The Lyckholm acme of cephalopods - Review of the late Katian (Vormsi-Pirgu regional stages) Ordovician cephalopods of Estonia, European Journal of Taxonomy 978, pp. 1-169 : 38-40

Genus Gorbyoceras Shimizu & Obata, 1935

Type species

Orthoceras gorbyi Miller in Miller & Faber, 1894, from Franklin County, Indiana, probably Saluda Formation, latest Katian, USA; by original designation.

Diagnosis

Slender, circular or slightly compressed orthocones with symmetrically curved septa and straight transverse sutures; sutures parallel to annulations in each groove of the annulations; annulations with fine transverse ornament or growth lines; annulations more prominent in adult growth stages; distinct irregularly spaced longitudinal ridges that may form nodes at the ridges of the annulations; siphuncle subcentral, expanded in early growth stages, with short cyrtochoanitic septal necks, nearly tubular in later growth stages, with suborthochoanitic septal necks; mural cameral deposits developed much farther anteriorly than endosiphuncular deposits. (Adopted from Kröger 2013.)

Remarks

Gorbyoceras and Dawsonoceras share many similarities. Following the diagnosis in the Treatise ( Sweet 1964c), both are slender, annulated, orthocones with a transverse and longitudinal ornamentation; in both genera species with curved mature growth stages occur; in both genera the siphuncle is centrally positioned or nearly so, and tubular to slightly expanded; and in both genera the septal necks are very short, varying between achoanitic to suborthochoanitic to very short cyrtochoanitic, and small endosiphuncular annuli occur. The asserted difference between the two genera is the shape of the siphuncle and its ontogenetic change ( Flower 1946; Sweet 1964c). In G. gorbyi and closely related species, such as G. grossi Flower, 1946 , the siphuncle is clearly expanded ( Flower 1946: 146). Flower (1946: 144–145) emphasized the ontogenetic variability of the siphuncular segments of species of Gorbyoceras . In several of them, “in general the outline of the segments becomes gerontically simplified, approaching an orthochoanitic condition […]” ( Flower 1946: 145). Therefore, Gorbyoceras is placed within the Proteoceratidae of the Pseudorthocerataceae in the Treatise ( Sweet 1964c: K256). In contrast, Frey (1995: P63) stated that no evidence exists for an ontogenetic change of siphuncular segments from expanded barrel-shape to subcylindrical in Gorbyoceras , and thus questioned the classification of Gorbyoceras within the Proteoceratidae .

The siphuncular segments of the Dawsonoceras are described as subcylindrical, and as “abruptly constricted at septal foramina” ( Sweet 1964c: K238). Therefore, in the Treatise, Dawsonoceras as type of the Dawsonoceratida is placed within the Orthoceratacaea ( Sweet 1964c: K238). However, the shapes of the siphuncle and septal necks are not known from the type-species D. hyatti (see Foerste 1928a: 273– 276, and Foerste 1928b: 28–29). In the Treatise, the illustrated details of the siphuncle and septal necks of Dawsonoceras are taken from Flower (1946: fig. 11), which is a generalized scheme not attributable to a specific species. The concept of Dawsonoceras in Sweet (1964c) was largely adopted by Kröger & Isakar (2006) and in later publications ( Kröger 2007, 2013), placing the Late Ordovician Cycloceras fenestratum Eichwald, 1860 into Dawsonoceras .

However, Sweet (1964c) and Kröger & Isakar (2006) were not aware of, or ignored, Horny’s (1956) revision of Dawsonoceras . In this revision, a much narrower concept of Dawsonoceras was suggested, restricting the genus to annulated forms with a frilled ornamentation ( Horny 1956: 462). Furthermore, Horny (1956: pl. 3 figs 2–4) described and illustrated the details of the well-preserved siphuncle and septal necks of specimens closely related to the Silurian type of Dawsonoceras ( D. hyatti ), and type material of D. annulatum ( Sowerby, 1816) from the British Wenlock. The siphuncle of these specimens is tubular, slightly expanded into the chambers, and adnate to the septa, and the septal necks are very short achoanitic or cyrtochoanitic. Hence, the ornamentation and the internal details of Ordovician species attributed to Dawsonoceras , such as Cycloceras fenestratum Eichwald, 1860 ( Kröger & Isakar 2006: fig. 8b; Kröger 2013: fig. 13a) differ considerably from the Silurian material described by Horny (1956) and are outside of his diagnosis of Dawsonoceras .

Accepting Horny’s (1956) diagnosis has consequences for recent approaches to classifying Dawsonoceras and Gorbyoceras ( Pohle et al. 2022) , which were based on descriptions of specimens assigned to C. fenestratum and G. textumamareum ( Roemer, 1861) by Kröger & Isakar (2006) and Kröger (2013). Therein, Gorbyoceras was placed within the Dawsonoceratidae ( Pohle et al. 2022) .

Here, Horny’s (1956) diagnosis is accepted. Therefore, C. fenestratum (probably together with the similarly ornamented D. gregarium Kröger et al., 2011 ) need to be assigned to a genus other than Dawsonoceras (a task beyond the scope of this paper). Moreover, the Estonian material studied herein, supports Flowerʼs (1946) opinion that the siphuncle of Gorbyoceras changes from an expanded barrel shape towards subcylindrical (see Remarks chapter of G. textumamareum ) during ontogeny, and hence the genus is best classified within the Proteoceratidae as suggested earlier (e.g., Sweet 1964c).