Rank Genus | Subfamily Hammatoceratinae | |
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Similar Kossmaticeras, Lytoceras cornucopia, Ludwigia, Liparoceras, Hildoceras bifrons | ||
Hammatoceras is a genus of ammonites belonging to the family Hammatoceratidae which lived during the Late Toarcian stage of the Early/Lower Jurassic between about 184 and 175 million year ago.
Contents
Description
The shell of Hammatoceras varies from evolute to sub-involute, according to species and growth stage. All are compressed, i.e. discoidal, in form, with strong ribbing and a keeled ventor. Ribs arise from tubercles on the umbilical shoulder, the inner, dorso-lateral margin of the whorls, and bifurcate on the flanks, stopping at the ventral keel. Juvenile forms tend to be more evolute with more of the inner whorls showing, whereas mature forms tend to be more involute with outer whorls more strongly embracing.
Taxonomy
Hammatoceras is included in the subfamily Hammatoceratinae, which in the outdated 1957 Treatise on Invertebrate Paleontology was part of the Hammatoceratidae, but of the Phymatoceratidae in D.T. Donovan et al. 1981, a case of what to base the family name on. Both accordingly contain the earlier Phytmatoceratinae and derived Hammatoceratinae. The superfamily Hammatoceratoidea in which the Hammatocaratidae and related families are currently placed is abandoned in favor of the older, equivalent, Hildoeratoidea. (M. K. Howarth. 2013)