On the Rarity and Peculiarity of the Early Toarcian (Lower Jurassic) Leukadiellinae Ammonites—Systematic Review and Insights on the Interplay of Environmental Stress, Evolution and Biodiversity

The subfamily Leukadiellinae Macchioni and Venturi, 2000 includes the two rare genera Leukadiella Renz, 1913 and Renziceras Arkell, 1953. Genus Leukadiella is characterised by ornamental and structural features unusual to the family Hildoceratidae Hyatt, 1867; for this reason, it has been occasionally grouped with other “odd” representatives of this family, as, for instance, Frechiella Prinz, 1904 and Paroniceras Bonarelli, 1893. While Renziceras comprises only one species, the eleven species currently recognised within Leukadiella show a noticeably discontinuous variability field; together with rarity of findings, the interpretation of this variability represents a renowned obstacle to their classification and phylogenetic reconstruction. Following an analysis of the morphological characters commonly used for defining Leukadiella species, a revised taxonomic scheme is herein proposed. Two clusters of characters are defined, based on which two morphologically distinct groups of species are distinguished, referred to as Helenae Group and Ionica Group. The genus name Leukadiella is maintained for species of the Helenae Group, which are restricted via synonymy to Leukadiella helenae Renz, 1913 and Leukadiella jeanneti Renz, 1927. The new genus name Neoleukadiella gen. nov. is proposed for species of the Ionica Group, which, by analogous limitation, are Neoleukadiella ionica Renz and Renz, 1946 and Neoleukadiella gallitellii Pinna, 1965. The reduction in the number of species from eleven to four is consistent with their rarity and limited paleogeographic distribution. The closely related Renziceras is considered the direct progenitor of Leukadiella; in turn, the Apennine genus Cingolites Sassaroli and Venturi, 2010 is proposed as transitional between Hildaites Buckman, 1921 and Renziceras. Conversely, the progenitor of Neoleukadiella remains uncertain, although a possible relationship between Leukadiella and Neoleukadiella species is tentatively traced. Finally, some challenging assumptions are made by emphasizing the role of environmental stress in controlling developmental dynamics that may drive striking phenotypic modification, of the kind observed in the Leukadiellinae species.