Trematoceras Eichwald, 1851

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

Genus Trematoceras Eichwald, 1851

Type species: Orthocera elegans Münster, 1841 .

Included species: T. elegans ( Münster, 1841) ; T. boreale Schastlivceva, 1986 ; T. clarum Schastlivceva, 1986 ; T. hikichii Niko et al., 2016 ; T. insperatum Schastlivceva, 1988 ; T. lytosiphon (Gemmellaro, 1904) comb. nov.; T. mangishlakense Schastlivceva, 1981 ; T. solidum Schastlivceva, 1988 ; T. subcampanile (Kiparisova, 1954) ; T. vulgare Schastlivceva, 1981 ; T. watanabei Niko & Ehiro, 2020 .

Emended diagnosis: Trematoceratid with moderate to wide septal spacing and relatively wide, tubular, central siphuncles and orthochoanitic to suborthochoanitic septal necks with gradual septal neck transition.

Remarks: Differs from Michelinoceras by its shorter septal necks, a much larger protoconch with an initial chamber that is shorter than the subsequent chambers and a conical apex with a cicatrix. Additionally, the pronounced lamellar cameral deposits are unique to Trematoceras , producing star- or petal-shaped impressions. However, until the taphonomy, growth and formation of cameral deposits are better understood, the absence of this character should not be treated as diagnostic. Paratrematoceras differs mainly in having a narrower siphuncle with more sharply bent septal necks, while Pseudotemperoceras possesses expanded siphuncular segments and a tendency towards narrower cameral spacing.

Note that the above list of species includes only those with known internal structures. Many of the about 50 described species of Triassic orthoceratoids are only known from external characters and cannot be assigned to any genus with confidence and are considered nomina dubia. We also made only a minimal attempt to synonymise some of the previously described species, although we consider them more prevalent. “ Orthoceras ” lytosiphon Gemmellaro, 1904 from the Norian of Sicily is here reassigned to Trematoceras , as its internal characters appear to be virtually identical with the type species. We do not synonymise it with the latter species because we follow previously established or at least suspected synonymies. To further clarify the taxonomic status of the various Trematoceras species and other Triassic orthoceratoids, the original type specimens should be reinvestigated together with new material and analysed with quantitative morphometric methods.

Occurrence: Worldwide; Early–Late Triassic (Olenekian–Norian, Rhaetian?).