Created 2023/02/08
Updated 2025/01/04

Ammonoidea – Classification and species names

lyell
Charles Lyell (1797-1875)

Classification. Zoological classification forms a tree that branches into phyla, classes, orders, families, genera, and species. For example, the class Cephalopoda groups together marine animals with tentacles. It includes the order Coleoidea (octopuses, squid, and cuttlefish), the order Nautiloidea (nautiluses), and the extinct order Ammonoidea. It is beyond the scope of this site to present the complete classification of the Ammonoidea with its 560 genera and subgenera. Our Genera page details it only for the Albian species on this website. For an overview, see Wright et al. (1996) and Lebrun (2003).

Species name. It comprises two Latin words in italics: the genus with a capital letter, and an adjective or noun complement for the species. The name is often dedicated to a person, such as Lyelliceras lyelli in honor of the English paleontologist Charles Lyell (see portrait from Wikipedia) or Otohoplites raulinianus for the French geologist Victor Raulin. Originally, "lyelli" and "raulinianus" had a capital letter, but this practice disappeared in the 1950s. The dog Latin used is sometimes hideous, as with the ammonite Bucaiella cayeuxi, dedicated to the Norman paleontologists Bucaille and Cayeux! Note that names beginning with "as" are neuter in Latin: Dipoloceras bouchardianum.

Creation. The name of a species may be followed by the author and the year of creation: Pseudosonneratia crassa Casey, 1965. The author must designate a reference specimen, preserved in a museum, the holotype. If two researchers independently create a genus or species, the name retained is the oldest: Sonneratia strigosa Saveliev, 1973, is in fact a junior synonym of Sonneratia caperata Casey, 1965. The genus name may be followed by a subgenus in brackets, such as Sonneratia (Eosonneratia) parenti. As classification evolves, a species may change genus. The original author and the year are then in brackets: Aioloceras besairiei (Collignon, 1949) was originally called Cleoniceras besairiei Collignon, 1949. To review a species, it is necessary to establish its synonymy, that is to say, the history of its successive names.

Too many species?. Some authors, like Spath and Casey, created a large number of ammonite species based on sometimes minor differences. The champion is Etayo-Serna, who created dozens of Colombian species quite similar to known European ammonites. The current trend is to classify individuals with common characteristics, found in the same layer, as belonging to the same species. Statistics confirm this, showing high intraspecific variability: ratios such as H/E (see page Measurements) follow a normal distribution. Many "species" have therefore been reduced to the status of simple variants of a reference species. For example, Amédro (1992) considers Hoplites dentatus (J. Sowerby, 1821), spathi Breistroffer, 1940, and paronai Spath, 1925, as variants of increasing thickness of H. dentatus, which has anteriority. But one can write, for example, H. dentatus var. paronai to emphasize the thick shape.

Sexual dimorphism. In Jurassic species are known small microconch shells, presumed to be male, and larger macroconch forms, presumed to be female. For example, the Oxfordian genus Creniceras is the microconch form of the genus Taramelliceras (Quereilhac et al., 2009). The identification of dimorphs in Cretaceous ammonites is still in its early stages, but it could further reduce the number of species. W. J. Kennedy, whom I had contacted regarding the existence of numerous, closely related, and poorly defined species in the Aptian genus Acanthohoplites, replied: "the present taxonomy is that of splitters who see difference rather than intraspecific variation and dimorphism". Well said!