Genus HADRODONTINA Staesche, 1964

Type species. Hadrodontina anceps Staesche, 1964.

Type stratum and locality. Campiller member, Skyth, South Tirol, Italy.

Remarks. Tere is still an ongoing debate about the phylogenetic relationships of Pachycladina, Parapachycladina and Hadrodontina. Sweet (1988) in his prioniodinid phylogeny considered Pachycladina and Hadrodontina as sister taxa. Some species of Pachycladina were assigned to a new genus Parapachycladina by Shunxin et al. (1997), but this view is not widely accepted. Orchard (2007) observed that Hadrodontina anceps, Ellisonia aff. triassica and Pachycladina peculiaris appear to constitute a natural group, although they are currently assigned to different genera. Based on their cladistics analysis, Donoghue et al. (2008) concluded that Pachycladina is either a sister taxon to Ellisonia or stays unresolved in a polytomy with Ellisonia, Hadrodontina and Furnishius. Based on his multi-element apparatus reconstructions, Koike (2016) included Hadrodontina and Pachycladina within the subfamily Hadrodontinae (Koike, 2016), supporting the original view of Sweet (1988). We follow here this suprageneric classification and include Hadrodontina and Pachycladina within the subfamily Hadrodontinae. More recently, Sun et al. (2020) published 3 natural assemblages of Hadrodontina aequabilis and confirmed the suprageneric classification of Koike (2016).

In P 1 elements, what most distinguishes Pachycladina from Hadrodontina is the basal configuration: the inverted basal ‘attachment’ surface of Pachycladina occupies the entire lower side plus one lateral side of the carina, whereas in Hadrodontina, a basal cavity with a deep basal groove is usually formed on the lower side and the attachment surface rarely extends over the mid-part of the keel, if at all. Te denticles of Pachycladina are also less numerous but much larger in relative size than those of Hadrodontina. Based on multi-element reconstructions however, Koike (2016) showed that Pachycladina peculiaris, as well as Ellisonia aff. triassica (Koike et al., 2004), should be synonymized with Hadrodontina aequabilis.