Ammonites are prehistoric molluscs encased in spiral shells that lived throughout the Mesozoic Era. They were a very successful group and widely recognized. Ammonites come from the wider Ammonoid Super-Order, and the Ammonoids as a whole go back to the Devonian Period, as far as 415 Million Years, in three great orders; first there were the Goniatites, that thrived in the Devonian-Permian, and then went extinct - replaced by the Ceratities that did well in the Permian-Triassic (doing extrememly well in the Triassic, so common they could in terms of Invertebrates be considered the Age of the Ceratites) However, the Ceratites then went extinct to be replaced by the true Ammonite Ammonoids, which are the most iconic and (to many) the only memorable or known part of the three Ammonoid Orders (Goniatites and Ceratites only seem well known to the experts, Ammonites taking all the general publics attentions) Ammonite Ammonoids thrived throughout the entire Mesozoic, although towards the end of the Late Cretaceous, in the last 15 Million Years, the number of extant Ammonite genera fall from 30 or so to little more than 6 globally. Even so, it is likely that if the K-Pg Extinction did not unfold 65.5 MYA, they would have recovered and endured even to this day. The heyday of the Ammonites was in the Jurassic Period.
Ammonites display some features of sexual dimorphism, and based on size and morphology we can begin to differentiate the presumed males and females of species of Ammonites, as well as other forms of Ammonoids. Females seem to be the largest in most species, with sexual dimorphism a clear recurring aspect of these marine animals.