Genus Aioloceras Whitehouse, 1926

Suborder Ammonitina – Superfamily Desmocerataceae – Family Cleoniceratidae

Description. According to Riccardi and Medina (2002), this discoid genus exhibits involute whorls, a shallow umbilicus with a straight wall slightly inclined outwards and a narrowly rounded edge, and a compressed, ogival or lanceolate cross-section, with flanks converging towards a narrowly arched venter lacking ventrolateral shoulders. The ribs are blunt, proverse, and falciform, comprising a straight primary rib dividing into two concave secondary ribs. Additional simple ribs may be present. In juveniles, the ribs arise without tubercles on the umbilical margin and cross the narrow venter, becoming lower and forming a rounded proverse sinus. Later, on the inner whorls, they arise very gradually at some distance from the umbilicus. Finally, on the body chamber, the ribs become fewer, wider, irregularly spaced, and disappear on venter. They may even efface completely. A few shallow constrictions are frequently present.

History of genus. Genus created by Whitehouse for Cleoniceras argentinum Bonarelli, 1921, a species from Patagonia. It differs from Cleoniceras by its sharp ribs and the absence of umbilical tubercles. Due to a lack of complete specimens, the genus was long poorly understood. Since 1970, complete specimens collected near Lakes Cardiel and San Martin in southern Argentina have allowed Riccardi and Medina (2002) to refine the genus, with two local species: A. argentinum (Bonarelli, 1921) and A. rolleri (Leanza, 1970). Whitehouse's sharp ribs are no longer mentioned, as they are only present on the first whorls of A. argentinum. The genus is placed in the family Cleoniceratidae, subfamily Cleoniceratinae, alongside the genus Cleoniceras.

Malagasy species. In 1949, Collignon placed his Malagasy species besairiei in Aioloceras, at the time a subgenus of Cleoniceras. Realizing that it lacked the sharp ribs described by Whitehouse, he moved it in 1963 to a new genus, Paracleoniceras. Riccardi and Medina (2002) emphasize the strong resemblance of Paracleoniceras to the Argentinian Aioloceras (photos on the left) and consider it a junior synonym of Aioloceras. We follow their position here. They also assert that all the Paracleoniceras in Collignon's Atlas (1963) are probably variants of Aioloceras besairiei.

Remarks. This genus is known from South Africa, Antarctica, Australia, Madagascar, and Patagonia, which is not surprising since these regions were very close to each other during the Albian. Riccardi and Medina (2002) then Kennedy and Klinger (2012) have highlighted dimorphs that differ only in size. Adult microconchs range from 52 to 73 mm, while macroconchs vary from 80 to 240 mm.