Ellesmeroceras sp.

Cichowolski, Marcela, Vaccari, N. Emilio, Pohle, Alexander, Morón Alfonso, Daniel A., Vaucher, Romain & Waisfeld, Beatriz G., 2023, Early Tremadocian cephalopods from Santa Rosita Formation in NW Argentina: the oldest record for South America, Acta Palaeontologica Polonica 68 (4), pp. 583-601 : 594-596

Ellesmeroceras sp.

Fig. 8A .

Material.— CEGH-UNC 27496 b from the Alfarcito Member , Santa Rosita Formation , early Tremadocian , Lower Ordovician , Quebrada de Arenal, Trancas section, Jujuy, Argentina .

Description.—The specimen consists of a small, slightly cyrtoconic, endogastric fragment of phragmocone of ca. 20 mm long and lacking the apical portion. It is 4 mm in diameter at the adoral end and 2.5 mm in diameter adapically ( Fig. 8A ). It is estimated that the complete conch could easily reach 30 mm in length. The chambers are short, ranging from 0.4 to 0.6 mm, with an RCL of 0.12 or eight chambers within a distance equivalent to the maximum diameter. The sutures are sinuous ( Fig. 8A 1 , A 2 ). The siphuncle has a diameter 35% that of the conch diameter at the same point of measurement, is marginal and ventral. The expansion rate is low, with an apical angle of 5.7°. The shell wall is not preserved. In the adoral part of the phragmocone, seems to be an inflexion followed by a slight increase in the expansion rate (see arrow in Fig. 8A 1 ). This could be a pathological trait or a diagnostic character, but as we only have the single specimen, it is impossible to evaluate its significance.

Remarks.—Despite the similarities to E. humahuacaensis , the notably larger size of the conch and the sinuosity of the suture line lead us to regard this specimen as a separate taxon. Until other material exhibiting additional characters is available, we prefer to leave this specimen in open nomenclature. Nevertheless, the slightly cyrtoconic, endogastric conch and the very short chambers suggest that it may be assigned to Ellesmeroceras .

Order undetermined

Family Bassleroceratidae Ulrich, Foerste, Miller, and Furnish, 1944

Remarks.—The family Bassleroceratidae has been assigned both to the Ellesmerocerida , the Tarphycerida, and the later assignment of these specimens. Compare with this material, Lawrenceoceras Ulrich, Foerste, Miller, and Furnish, 1944 , has a smaller siphuncle relative to the conch diameter, and it is not completely marginal ( Kröger and Pohle 2021). In Anguloceras the septa slope steeply apicad from the venter to the dorsum ( Unklesbay and Young 1956). In Avaoceras the siphuncle is submarginal ( Ulrich et al. 1944). Hitherto, the oldest reported bassleroceratids were late Tremadocian in age ( Evans 2011). The questionable Bassleroceras from the Balnakeil Formation ( Evans 2011) and the two species of Bassleroceras from the Rochdale Formation ( Kröger and Landing 2008) are both slightly younger than the Argentine specimens described herein. The formers are from the Paltodus deltifer –Macerodus dianae Conodont Zone (which is lower Tr 2). If the presence of the group in the Alfarcito Member of the Cordillera Oriental, Jujuy, is confirmed, it would be one of the oldest records. It is interesting to note that similar forms, but yet undescribed, appear to occur in the earliest Tremadocian of Australia (AP unpublished data).

The age of the specimens further highlights the problematic state of ellesmerocerid taxonomy, as it is also difficult to differentiate between bassleroceratids and certain late Cambrian ellesmerocerids such as the Huaiheceratidae and the Xiaoshanoceratidae , which apparently also had an exogastric conch. The Xiaoshanoceratidae supposedly have a depressed cross-section, while the only character distinguishing the Huaiheceratidae from the Bassleroceratidae is the thinner connecting ring of the former. In our opinion, these characteristics are questionable, because the xiaoshanoceratids are only known from randomly oriented cross-sections ( Chen and Teichert 1983a) and a depressed cross-section may also be produced by an oblique cut, meanwhile the connecting rings are frequently missed and/or altered during diagenesis.

Figures

Fig. 8. Early Tremadocian (Ordovician) cephalopods from Quebrada de Arenal, Trancas section, Cordillera Oriental, Jujuy,Argentina.A. Ellesmeroceratid Ellesmeroceras sp. (CEGH-UNC 27496b) in oblique-lateral (A1) and oblique-ventral (A2) view of the uncoated specimen, and oblique-lateral view of the coated specimen (A3). Arrow points to the siphuncle. Note the sinuosity of the suture lines. B. Bassleroceratid Bassleroceras? sp. (CEGH-UNC 27497) in lateral view (B1, B2) showing exogastric curvature, low expansion rate, and external siphuncle, in apical view of the apicalmost septum preserved (B3, B4), showing the slightly compressed shape and the siphuncle position.Photographs (B1, B3) and schematic drawings (B2, B4) Scale bars:A, 5 mm, B1, B2, 2 mm; B3, B4, 1 mm. discarded order Bassleroceratida (Evans 2011: 22, and dis- 3.2 mm correspond to part of the living chamber and 4.1 cussion and references therein). We followed here the pro- mm to part of the phragmocone. It has an oral dorsoventral posal by Pohle et al. (2022), in which the family is located diameter of ca. 3 mm, an apical dorsoventral diameter of c within the subclass but not included in any particular or- 2.6 mm, and an apical lateral diameter of ca. 2.1 mm. The der, although Kröger and Pohle (2021) included the family cross-section is slightly compressed (0.8), and the siphuncle within Ellesmerocerida. See Evans (2011) for further discus- is rather large, ventral and marginal (Fig. 8B), with a diamesion about the family Bassleroceratidae. ter 34 % of that the conch diameter. The chambers are very short, ca. 0.3 mm long (Fig. 8B1). There are 10 chambers in Genus Bassleroceras Ulrich and Foerste, 1935 a length equivalent to the conch diameter, i.e., an RCL of 0.1. Type species: Orthoceras perseus Billings, 1865, from the St. Armand At 2.3°, the expansion rate is very low. CEGH-UNC 27480a Limestone, near Phillipsburg, Missisquoi County, Quebec, Canada. is a 7.4 mm long fragment of a slightly exogastric conch, of Tremadocian, Lower Ordovician. which 4 mm corresponds to part of the living chamber and Bassleroceras? sp. 3.4 mm to the phragmocone. Adapically, the dorsoventral diameter is ca. 3 mm, and the siphuncle is 1 mm wide, or Fig. 8B. 33% of that the conch diameter. The chambers are 0.3 mm Material.—CEGH-UNC 27497 and 27480a, from the Alfa- long, indicating an RCL of 0.08. The SCI is 0.16. rcito Member, Santa Rosita Formation, lower Tremadocian, Remarks.—The slightly exogastric curvature of these spec- Lower Ordovician, Quebrada de Arenal, Trancas section, imens, along with the very low expansion rate of the conch, Jujuy, Argentina. suggests that they can be assigned to the Basslerocertidae. Description.—CEGH-UNC 27497 is a 7.3 mm long frag- We would need more material in order to investigate interment of a slightly exogastrically curved conch, of which nal structures and determine more precisely the taxonomic

Fig.1. Cambrian and earlyTremadocian cephalopod recordsfrom different localitieswithin thepaleotropical belt (compiled from literature).Abbreviations: Fm., formation; Mb., Member.

Fig. 2. A. Map of SouthAmerica showing the main Paleozoic geological provinces of NW Argentina.B. Map of the study area (Jujuy Province,Argentina). The satellite image from Google Earth. QA, Quebrada de Arenal (-23.474677, -65.337646); QY, Quebrada de Yacoraite (-23.3321972, -65.457527). C. General view of the outcrop where the specimens were collected along the Quebrada de Arenal.This interval is characterized by sandstone interbedded with a general interval of siltstone. D. Regional biostratigraphy and lithostratigraphy correlated with the map intervals used. Modified and updated from Balseiro and Waisfeld (2013) and Vaucher et al. (2020). R. f. anglica, Rhabdinopora flabelliformis anglica. E. Stratigraphic section of the Quebrada de Arenal with the main facies assemblages (FA) that defined the section. Modified from Vaucher et al. (2020). The position where the samples were collected is indicated and belongs to Kainella merdionalis Trilobite Biozon. Til., Tilcara Member; R., Ruspaca Member; K. t., Kainella teiichii.