Trematoceras elegans Münster, 1841

Pohle, Alexander & Klug, Christian, 2024, Orthoceratoid and coleoid cephalopods from the Middle Triassic of Switzerland with an updated taxonomic framework for Triassic Orthoceratoidea, Swiss Journal of Palaeontology (14) 143 (1), pp. 1-32 : 22-26

Trematoceras elegans Münster, 1841 .

Figures 2A–H , 9A–F , 10D

1841 Orthocera elegans Münster ; p. 125; pl. 14, fig. 2a–c 1843 Orthocera Freieslebense Klipstein ; p. 143; pl. 9, fig. 4a–b

1843 Orthocera politum Klipstein ; p. 144; pl. 9, fig. 6 1847 Orthoceras dubium Hauer ; p. 260–261; pl. 7, fig. 3–8

1849 Orth. elegans Münster –Quenstedt; p. 478–479; pl. 31, fig. 3–5

1851 Trematoceras elegans Eichwald ; p. 124; pl. 1, fig. 3 1859 Orthoceratites dubius Hauer –Stoppani; p. 112; pl. 24, fig. 1–4

1867 Orthoceras cf. dubium (Hauer) –Beyrich; p.138, pl. 3, fig. 3

1869 Orthoceras Campanile Mojsisovics ; p. 590

1869 Orthoceras elegans Münster –Laube; p. 59; pl. 36, fig. 9 1869 Orthoceras politum Klipstein –Laube; p. 60; pl. 36, fig. 8

1873 Orthoceras dubium Hauer –Mojsisovics; p. 3–4; pl. 1, fig. 4, 5

1877 Orthoceras elegans Münster –Barrande; Supplément, p. 65–66; pl. 483, fig. 4–15

1882 Orthoceras campanile Mojsisovics –Mojsisovics; p. 291; pl. 93, fig. 1–4, 11

1882 Orthoceras elegans Münster –Mojsisovics; p. 291; pl. 92, fig. 10–12

1882 Orthoceras politum Klipstein –Mojsisovics; p. 293; pl. 92, fig. 13–14; pl. 93, fig. 7–8

1899 Orthoceras campanile Mojsisovics –Tommasi; p. 16; pl. 2, fig. 1, 1a

1899 Orthoceras politum Klipstein –Tommasi; p. 16; pl. 2, fig. 2, 2a

1907 Orthoceras campanile Mojsisovics –Reis; p. 113– 114; pl. 1, fig. 1

1952 Orthoceras elegans Münster –Leonardi & Polo; p. 8; pl. 1, fig. 1, 4, 9, 11, 12; pl. 2, fig. 51–52

1964 Trematoceras cf. politum (Klipstein) –Sweet; p. K229; fig. 156, 4a

1964 Trematoceras cf. elegans (Münster) –Sweet; p. K229; fig. 156, 4b–c

1973 Michelinoceras campanile (Mojsisovics) –Rieber; p. 71–72; pl. 17, fig. 11, 26–27

1991 Trematoceras elegans (Münster) –Bizzarini & Gnoli; p. 112; pl. 1, fig. 1–4; pl. 2, fig. 1–2

Lectotype: Despite the long research history and its status as type species of Trematoceras , a lectotype was never designated for T. elegans . Münster (1841) mentioned that the species was not rare in the Cassian Formation, but it is not clear how many specimens he had at his disposal, though he illustrated three specimens that can be considered as syntypes. Only the specimen on his pl. 14, fig 2c (SNSB-BSPG AS VII 1014) has been sectioned to show the structure of the septal necks ( Fig. 9A, C ). This specimen is thus best suited as lectotype and is here designated as such. Nevertheless, the specimen is strongly corroded and does not show cameral deposits. These are clearly visible in specimen SNSB-BSPG AS VII 1015, figured on pl. 14, fig 2b in Münster (1841), which we designate as paralectotype ( Fig. 9B, D–F ). We do not designate the other specimens in Münster’s collection as paralectotypes, as their internal characters are unknown and accordingly, there is a small chance that these specimens represent separate species. Nevertheless, we think that it is likely that there is only a single species in the Cassian Formation.

Diagnosis: Trematoceras with expansion rate (ER) of about 5°, relative siphuncular size (RSS) of approximately 0.1 and relative cameral length (RCL) between 0.5 and 1.5. Septal necks very short, constricting siphuncle slightly at septal foramen. Lamellar cameral deposits present, arranged radially in petal-shaped sectors. Embryonic shell relatively large, up to 2 mm in diameter compared to an adult size of at least 25 mm, possibly up to 50 mm (poor data). Endosiphuncular deposits unknown.

Material: 16 specimens from the Besano Formation are comfortably assigned to Trematoceras , e.g., due to visibly central siphuncle. Two specimens were sectioned, one of them revealing septal necks, confirming the species assignment. The rest of the material was assumed to represent the same species due to the general low morphological variability. Further 19 specimens were only poorly preserved (e.g., as external moulds) but agree in cross section and expansion rate and thus, an assignment to Trematoceras elegans is assumed. In addition to the three specimens ( PIMUZ 3770 , 3781 , 3782 ) already illustrated by Rieber (1973), we include PIMUZ 30979 , 38 , 255 , 38 , 348 , 38 , 625 , 38,651– 38,654, 38,669, 38,693, 38,966, 38,993, 39,021, 39,024– 39030, 39,056– 39065, 39,486, 39,587. Furthermore, PIMUZ 29943 is noted as loaned to the Museo dei fossili del Monte San Giorgio, Meride ( CH) and was not investigated. In total, this adds up to 36 specimens from the upper Lower and middle Besano Formation. Stratigraphic distribution of the material: bed 41 ( PIMUZ 29943 , 39 , 056 , 39,586 ), bed 45 ( PIMUZ 3781 , 39,486 ), bed 47 ( PIMUZ 3782 , 30,979 ), bed 49 (39,587), bed 58 (39,057, 39,058), bed 61 ( PIMUZ 3770 , 39,024 – 39030 , 39,065 ), bed 69 ( PIMUZ 38966 , 38,993 ), bed 72 ( PIMUZ 38255, 38,348, 38,625), bed 74 ( PIMUZ 39021), bed 83, ( PIMUZ 38693), bed 88 ( PIMUZ 39059), bed 94 ( PIMUZ 39060), bed 98 ( PIMUZ 38651– 38,654), bed 100 ( PIMUZ 38669), bed 104 ( PIMUZ 39061, 39,062), bed 148 ( PIMUZ 39063), bed 162 ( PIMUZ 39064).

Description: The specimens from Monte San Giorgio are either preserved as internal moulds of phragmocones or body chambers, or as external moulds, leaving essentially a hollow tube in the matrix. The conch is virtually straight with circular cross section. The apertural diameter of the fragments is on average 14.0 mm. The largest specimens, PIMUZ 38348 and 38625 (both from bed 72), have an apertural diameter of 25 mm, while the smallest specimen, PIMUZ 38693 from bed 83, reaches from 2 to 4 mm. External moulds are usually preserved over a considerable length up to 183 mm (PIMUZ 3782, bed 47; see Rieber, 1973), while internal moulds are usually broken into shorter fragments. The expansion rate remains constant within te same specimen, on average 4.9°.

Remarks: Rieber (1973) considered only the species “ Michelinoceras ” dubium (Hauer, 1847) and “ M.” campanile as possible candidates for the material from the Besano Formation (both differing only in the length of the body chamber and septal spacing) and assigned all material to the latter species. Because he did not section any specimens, his taxonomic identifications are uncertain, as Schastlivceva (1981, 1986, 1988) showed that internal characters set apart the genera. The specimen sectioned here (PIMUZ 39586, Fig. 2D, E ) confirms an assignment to Trematoceras . Due to the poor data on most species of Trematoceras , it is challenging to differentiate between the about 50 species of Triassic orthoceratoids that have been established to date. Internal characters are missing entirely for many Triassic orthoceratoids (listed under “ Orthoceras ” in Table 1 ), and intraspecific variation has typically not been investigated. As the Trematoceras morphotype seems to represent the majority of the known trematoceratids, it is likely that many of them belong to this genus ( Schastlivceva, 1988). However, at the same time it is also likely that many species represent junior synonyms. Externally, they are all very similar and it is doubtful whether slight variation in the spacing or coarseness of the growth lines justify species separation. It is probable that cameral and endosiphuncular deposits provide additional insights, but knowledge on them is very patchy. There may also be some variation in the length and shape of the septal necks, but they have similarly rarely been investigated. It is furthermore problematic that many species are only available as drawings in old monographs and the original type material has never been photographed and published (as exemplary shown for the type species). These species are thus mostly known from short descriptions that are insufficient from a modern taxonomic perspective. Indeed, it is quite possible that some type specimens will prove to be difficult to find or are lost.

T. elegans has been differentiated from other species by its apical angle, the length of its body chamber, cameral spacing, or details in the ornamentation. However, as elaborated above, the variation of most of these characters is so poorly known that the utility of these characters in differentiating between species is doubtful. Already Quenstedt (1849) was of the opinion that “ Orthocera ” freieslebenense Klipstein, 1843 was identical with T. elegans , as according to him, it was based on taphonomic features. This opinion appears to have been accepted by others, as the species has not been studied since then. According to Quenstedt (1849), the species O. ellipticus Klipstein, 1843 (preoccupied by O. ellipticus Münster, 1840 , thus replaced by O. subellipticus d’Orbigny, 1850 ), characterised by an elliptical cross section, represents deformed body chambers of T. elegans , although other authors treated O. subellipticus as valid. Later, Mojsisovics (1882) indicated that T. elegans and T. campanile Mojsisovics, 1869 differ only in the smaller size of the latter and that intermediate specimens would invalidate the latter species. This was corroborated by Leonardi and Polo (1952), who reported intermediately sized trematoceratids from the Cassian Formation, arguing that the two species cannot be reliably separated. The trematoceratids from the Besano Formation appear to be closer in size to other Anisian specimens. However, according to Quenstedt (1849), specimens with a “7/ 4 inch diameter” (= ca. 4–5 cm) occur as well in the Cassian Formation. Similar sizes have been reported from slightly younger orthoceratoids from Austria by Hauer (1847). Generally, the size data is so poor that it does not allow for a species separation.

We cannot find any convincing argument for splitting the alpine species of Trematoceras into more than one species. While we do not rule out the possibility of several sympatric species, the current species do not reflect natural units, because they were defined based on features that are either known to be variable within the same locality or even within a single specimen, or else are unknown in a lot of specimens. Conversely, characters that might carry some diagnostic value such as those of the siphuncle are poorly known. Therefore, in our opinion, the best approach is to provisionally accept only the type species, T. elegans , as valid until more data becomes available. Species not included in the synonymy list are therefore regarded as nomina dubia (compare species listed as “ Orthoceras ” in Table 3). Norian and Rhaetian orthoceratoids from the alps are poorly known for the most part and currently, we cannot confirm their assignment to T. elegans . Trematoceras cf. triadicum described by Jeletzky and Zapfe (1967) appears to have slightly longer septal necks and a wider siphuncle, but the limited data make it impossible to judge whether this is within the range of intraspecific variation. Of the alpine orthoceratoids where internal characters are known, only “ Orthoceras ” salinarium Hauer, 1846 appears to differ in a narrower siphuncle, which is here assigned to Paratrematoceras (see below).

Occurrence: Alps, Europe; Middle–Late Triassic (Anisian–Carnian).

Figures

Fig. 2 Orthoceratoids from the Besano Formation (Middle Triassic) of the Monte San Giorgio,Ticino, Switzerland. A–H: Trematoceras cf. elegans (MÜnster, 1841). A PIMUZ 39056,partially fixed with silicon, bed 41. B PIMUZ 39487,bed 87. C-E PIMUZ 39586, sectioned specimen, bed 45. F–G PIMUZ 39029, bed 61. H PIMUZ 39064,bed 162

Fig. 9 Lectotype and paralectotype of Trematoceras elegans (MÜnster, 1841) from the Carnian of the Cassian Formation. A, C SNSB-BSPG AS VII 1014, lectotype. B, D–F SNSB-BSPG AS VII 1015,paralectotype. A External (lateral?) view. B Apertural view. C Longitudinal section. D Apical view. E Lateral view, venter right. F Ventral view

Fig. 10 Reconstructions of non-ammonoid cephalopods from the Anisian of Monte San Giorgio. A Breviconoteuthis breviconus (Reis, 1907); B Mojsisovicsteuthis sp.; C Ticinoteuthis chuchichaeschtli gen. et sp. nov.; D Trematoceras elegans (MÜnster, 1841); F Enoploceras rieberi Pieroni, 2022