Belongs within: Orthocerida.
Straight to the wastebasket
Published 5 March 2025
During the period from the Middle Ordovician to the Early Devonian, the cephalopod fossil record was dominated by orthocones. This is the term used for straight, conical to cylindrical shells, the greater number of which were carried by members of the order Orthocerida. Despite their abundance, however, orthoconic cephalopods have been poorly served taxonomically. Their simple form provides very few characters for comparison, with every indication that the same combinations of characters have arisen on multiple occasions. Michelinoceras was doomed from the start.
Because of their aforementioned uniformity, early authors tended to just dump every new generic-looking orthoconic taxon into the genus Orthoceras. As a result, Orthoceras became an ill-defined, unwieldy mass. In 1932, A. F. Foerste recognised that the majority of ‘Orthoceras’ species were not close relatives of that genus’ type species, and established the new genus Michelinoceras for such segregates. As a result, instead of just dumping taxa into Orthoceras, authors began just dumping taxa into Michelinoceras. As explained by Pohle & Klug (2024), “Michelinoceras became a waste basket taxon for inconspicuous orthoceratoids with smooth shells, low apical angle, central siphuncles and wide cameral spacing that occur from the Ordovician up to the Triassic”, a truly stupendous length of time. Sweet (1964) considered the genus’ post-Devonian representation, at least, was not well established, but did not go so far as to exclude the offending taxa.
Sweet (1964) also recognised a number of subfamilies in the Orthoceratidae, the largest of which was the Michelinoceratinae. However, this subfamily was as vaguely defined as its type genus, comprising smooth or indistinctly ornamented orthoceratids that generally had well-developed cameral deposits. Two of the other three subfamilies were similarly shakily based, distinguished primarily by the presence of longitudinal (Kionoceratinae) or annular (Leurocycloceratinae) ornamentation. Ironically, the only subfamily that had a claim to be based on more than just surface ornamentation was the Orthoceratinae, characterised by the presence of longitudinal furrows in the body chamber.
As such, there is little to determine whether the Michelinoceratinae represented an evolutionarily coherent group. Pohle & Klug (2024) considered that the Triassic representatives of ‘Michelinoceras’ were not only distinct from that genus, they did not even belong to the same order. Instead, the Triassic taxa belonged to the order Pseudorthocerida, with true Orthocerida not extending past the end of the Palaeozoic (except, perhaps, in their ammonoid and coleoid descendants). Kröger & Isakar (2006) argued for the complete abandonment of subfamily divisions within the Orthoceratidae. It is possible that, with further study, a more robust concept of Michelinoceras will narrow it down to the direct relatives of its Silurian type species, but for now it remains an overflowing wastebasket.
Systematics of Michelinoceratinae
Michelinoceratinae [Michelinoceratidae, Michelinocerotidae]S64
|--Arkonoceras Flower 1945S64
| `--*A. arkonense (Whiteaves 1898) [=Orthoceras arkonense]S64
|--Balticoceras Teichert 1940S64
| `--*B. discors (Eichwald 1857) [=Trematoceras discors]S64
|--Eotripteroceras Flower 1943S64
| `--*E. minutum Flower 1943S64
|--Hesperoceras Miller & Youngquist 1947S64
| `--*H. laudoni Miller & Youngquist 1947S64
|--Pleurorthoceras Flower 1962 (see below for synonymy)S64
| |--*P. clarkesvillense (Foerste 1924) [=Orthoceras clarkesvillense]S64
| |--P. osmundsbergense Kröger 2013PK22
| `--‘Orthoceras’ selkirkense Whiteaves 1892 [=*Foersteoceras selkirkense, *Shimizuoceras selkirkense]S64
|--Trematoceras Eichwald 1851S64
| |--*T. elegans (Münster 1841) [=Orthoceratites elegans]S64
| `--T. politumS64
|--Bitaunioceras Shimizu & Obata 1936S64
| |--*B. bitauniense (Haniel 1915) [=Orthoceras bitauniense]S64
| |--B. coahuilenseS64
| `--B. texanum Miller & Youngquist 1947P68
|--Michelinoceras Foerste 1932S64
| |--*M. michelini (Barrande 1866) [=Orthoceras michelini]S64
| |--M. anguliferumT64
| | |--M. a. anguliferumT64
| | `--M. a. alpenenseT64
| |--M. dunbariT64
| |--M. dutroi Gordon 1957P68
| |--M. floridaense Hook & Flower 1977K93
| |--M. melleni Hook & Flower 1977K93
| |--M. niagarenseH87
| |--M. primum Flower 1962K93
| `--M. unicameraM17
`--Acanthomichelinoceras commutatumKE19
Pleurorthoceras Flower 1962 [incl. Foersteoceras Shimizu & Obata 1936 non Ruedemann 1925, Shimizuoceras Zhuravleva 1962]S64
*Type species of generic name indicated
References
[H87] Holland, C. H. 1987. The nautiloid cephalopods: a strange success story. Journal of the Geological Society 144 (1): 1–15.
[K93] King, A. H. 1993. Mollusca: Cephalopoda (Nautiloidea). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 169–188. Chapman & Hall: London.
[KE19] King, A. H., & D. H. Evans. 2019. High-level classification of the nautiloid cephalopods: a proposal for the revision of the Treatise Part K. Swiss Journal of Palaeontology 138: 65–85.
Kröger, B., & M. Isakar. 2006. Revision of annulated orthoceridan cephalopods of the Baltoscandic Ordovician. Fossil Record 9 (1): 137–163.
[M17] Mutvei, H. 2017. The new order Mixosiphonata (Cephalopoda: Nautiloidea) and related taxa; estimations of habitat depth based on shell structure. GFF 139 (3): 219–232.
Pohle, A., & C. Klug. 2024. Orthoceratoid and coleoid cephalopods from the Middle Triassic of Switzerland with an updated taxonomic framework for Triassic Orthoceratoidea. Swiss Journal of Palaeontology 143: 14.
[PK22] Pohle, A., B. Kröger, R. C. M. Warnock, A. H. King, D. H. Evans, M. Aubrechtová, M. Cichowolski, X. Fang & C. Klug. 2022. Early cephalopod evolution clarified through Bayesian phylogenetic inference. BMC Biology 20: 88.
[S64] Sweet, W. C. 1964. Nautiloidea—Orthocerida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K216–K261. The Geological Society of America and the University of Kansas Press.
[T64] Teichert, C. 1964. Morphology of hard parts. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K13–K53. The Geological Society of America and the University of Kansas Press.