Trematoceratidae Zakharov, 1996

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 : 21-22

Family Trematoceratidae Zakharov, 1996

Diagnosis: Relatively small orthocones without any indication of conch curvature with the possible exception of the apicalmost parts. Expansion rate constant, around 5° with relatively little variation. Siphuncle central or very slightly subcentral, with straight or slightly expanded siphuncular segments and short ortho- or suborthochoanitic septal necks that lack the inner prismatic layer and have a reduced outer prismatic layer. Cameral deposits present, petal- or star-shaped. Endosiphuncular deposits parietal, possibly restricted to the septal necks and early ontogenetic stages. Embryonic shell where known comparatively large, up to 2 mm in diameter with blunt conical apex and cicatrix (modified after Zakharov, 1996).

Included genera: Trematoceras Eichwald, 1851 ; Paratrematoceras Schastlivceva, 1981 ; Pseudotemperoceras Schastlivceva, 1986 . Doubtful: Phatthalungoceras Tongtherm & Nabhitabhata, 2018 ; Zhuravlevia Doguzhaeva, 1994 .

Remarks: Several characters, such as the embryonic shell or the shape of the cameral deposits are known only from a limited number of taxa. However, due to the high consistency in other characters that are usually considered to be more variable between species (e.g., apical angle and shell sculpture), we assume that the variability in these lesser-known characters was likely low as well. Of course, new evidence for distinctly different morphologies may overturn our hypothesis but until then, we propose that the best approach is to classify all Mesozoic orthoceratoids within a single family. This includes the Early Cretaceous Zhuravlevia Doguzhaeva, 1994 , although we include it here only tentatively due to the 90 million years gap between it and Triassic trematoceratids.

Like the Triassic orthoceratoids, there have been very few studies on Permian taxa. It is thus conceivable that the Trematoceratidae can be extended into the Permian. Nevertheless, the few available studies indicate that orthoceratoid diversity (or at least disparity) was comparatively higher during the Permian (see, e.g., Sweet, 1964; Niko et al., 2000; Yang et al., 2021; Niko & Ehiro, 2023). Thus, a possible extension of the Trematoceratidae to include Permian taxa must await a better phylogenetic understanding of the latter.

Occurrence: Global; Early–Late Triassic (Induan? Olenekian–Norian, Rhaetian?), Early Cretaceous? (Aptian?).