''Antarcticoceras'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of crioconic

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

s in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Shasticrioceratidae. It lived during the

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

Period. ''Antarcticioceras''

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s can be found in the Cretaceous rocks of

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

and

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

.M.R.A. Thomson, « Ammonite faunas of the Lower Cretaceous of south-eastern Alexander Island », ''British Antarctic Survey Scientific Reports'', 80 (1974), p.1-44.

Taxonomy and evolution

The type species of ''Antarcticoceras,'' ''A. antarcticum,'' was described from 6 (possibly 7) specimens that were recovered from Alexander Island: the largest antarctic island. When these specimens were first recovered, it was thought they represented an unknown taxon within the Ancyloderid subfamily Helicancyanae. Thmpson, M. 1971. Ammonite Faunas of south eastern Alexander Island and their stratigraphical significance. In: Aide, R. ed. Antarctic Geology and Geophysics This original

hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...

was made because the first two specimens recovered were smaller in overall size and displayed apparent aspinoceratid coiling, which is typical within several helicancyan taxa. Once more specimens were recovered, it became clear that the overall form of the shell was more similar to the Crioceratidae than the Ancyloceratidae. The exact placement of ''Antarcticoceras'' remained uncertain because the specimens displayed high dorso-lateral tubercule, single-sized ribbing, and fairly simple suture, none of which were Crioceratid features. A later study found major similarities in the ventral morphology and ornamentation '' Shasticrioceras'' and ''Antarcticoceras'' which suggest a shared evolutionary origin. Bert, D. Bersac, S. Canut, L. 2020.Implications of the ‘hemihoplitid-like’ ammonites iterative morphology in the context of the late Tethyan Barremian (Early Cretaceous). Cretaceous Research. Volume 106. A new family, Shasticrioceratidae, was then erected to accommodate both genera. Following the placement of the genus into the Shasticrioceratidae, it appears most likely that ''Antarcticoceras'' evolved sometime during or before the Barremian from ''Shasticrioceras'' or a ''Shasticrioceras''-like common ancestor. ''A. domeykanum'' appeared first, followed by ''A. perezi'' and ''A. antarcticum,'' however the nomen nudum status of ''A. perezi'' casts uncertainty on the late-Berremian to early Albian evolutionary history of the genus, until the appearance of ''A. antarcticum.''

Species

Three species of ''Antarcticoceras'' are known in the literature: ''A. antarcticum,'' the type, was recovered from Albian- aged rocks of Alexander Island. Following the recognition of ''Antarcticoceras'' as a genus another species, ''A. domeykanum'' (from the earlier Barremian) was placed into the genus as well. Bayle, M. & Coquand, H. 1851. Mémoire sur les fossilessecondaires recuillis dans le Chili ar M. Ignace Domeyko émoires de la Société Géologique de France 2,4(1): 1–47. Kennedy, W. Klinger, H. 2009. The heteromorph ammoniteNdumuiceras variabilegen. et sp. nov.from the Albian Mzinene Formation,KwaZulu-Natal, South Africa. African Natural History, Volume 5, A third species ''A. perezi'' is generally regarded as a

nomen nudum In taxonomy, a ''nomen nudum'' ('naked name'; plural ''nomina nuda'') is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published with an adequate descr ...

(the description is not considered complete enough to justify a valid taxonomic name). F.A. Mourgues Paléontologie stratigraphique (ammonites) et évolution tectono-sédimentaire du Bassin d’arrière arc de Chañarcillo (Berriasien – Albien, Nord du Chili) Unpublished PhD (2007), p. 295

Distribution and habitat

''Anarcticoceras'' fossils are known from marine deposits randing from 113 to 129 million years in age. ''A. domeykanum'' is found in the Chañarcillo Basin of northern Chile, While ''A. antarcticum'' is known only from the rocks of Antarctica's Alexander Island.

References

{{Taxonbar, from=Q2852622 Ancyloceratoidea Ammonitida genera