Another lesson from beautiful monsters: the case of 'sex reversals' in the Ammonoidea and their significance

Abstract Background Expression of sexual dimorphism is recognised in various fossil groups of molluscs such as the Ammonoidea, an extinct group of shelled cephalopods. During the Mesozoic, the best-documented sexual dimorphic examples are seen in the Jurassic superfamily Perisphinctoidea. It is usually expressed by distinct adult size and apertural modifications between the antidimorphs. Putative males (otherwise referred to as microconch) are small in size and develop lappets at the end of the shell while the females (macroconch) are larger and bear a simple peristome. Dubious cases are, however, known in that superfamily, which often relate to taxonomic biases or lack of diagnostic characters, and some others expose ontogenetic anomalies illustrated by ‘sex reversals’ in the shell morphology and ornamentation. Results The discovery of two specimens of the Callovian Aspidoceratidae Peltoceras athleta (Phillips), having both female and male features, questions the significance and causes of ‘sex reversals’ in the Ammonoidea. The two specimens have started with the macroconch ontogeny of Peltoceras athleta and show an apparent change toward maleness in the adult, as illustrated by their rounded whorl section, ribs retroversion, fading of the tubercles and lappets typical of the microconchs. Few other cases of female-to-male, as well as male-to-female ‘sex reversal’, are known in the fossil record, all belonging to the Jurassic Perisphinctoidea (families Perisphinctidae or Aspidoceratidae). Since all Jurassic Perisphinctoidea are strictly gonochoristic, these ‘sex reversals’ are pathological in nature and are herein referred to as a new forma-type pathology: namely “forma hermaphrodita”. Conclusions In the absence of any clear evidence of injury or parasitism, we hypothesize that such “forma hermaphrodita” individuals illustrate pathologic cases of intersexuality. Little is known about the ammonoid soft parts, and it is not possible to determine which internal sexual organs occur in specimens having both male and female external shell features. Abnormal feminisation and/or masculinisation also occur in modern cephalopods, the latter also grouping only gonochoric species. This phenomenon is similarly illustrated by a change in the adult body size and a mixing of both female and male structures. In that case, intersexuality is either advantageous in the population or caused sterility. The causes of intersexuality are not clearly established but environmental p