Article: The Leptoceratoidinae: small heteromorph ammonites from the Barremian
The Barremian Leptoceratoidinae, a subfamily of small-sized heteromorph ammonites, are revised. Despite considerable differences in the mode of uncoiling, Leptoceratoidinae are considered to represent a monophyletic unit, defined by their small size, simplified suture-lines and ubiquitous Barremian age. Three evolutionary lines are recognized exhibiting parallel trends of size increase: (1) Karsteniceras with criocone uncoiling and a planispiral to trochospiral initial coil; (2) Hamulinites with an ancylocone type of uncoiling and a small planispiral initial coil; and (3) a line compromising Eoheteroceras gen. nov. with ancylocone uncoiling and a large trochospiral initial coil, as well as Manoloviceras gen. nov. with only one slightly curved arm and a large trochospiral initial coil. It is inferred that these evolutionary lines originated in Veveysiceras gen. nov. with hamitid-like uncoiling. Veleziceras, with a straight to gently curved shell, is tentatively included in the Leptoceratoidinae, but cannot yet be assigned to one of the three lines defined above. The following new species are described: Karsteniceras ibericum, K. beyrichoide, K. hoheneggeri, K. trinidadense and E. silesiacum, the type species of Eoheteroceras gen. nov. The origin of leptoceratoids remains obscure; they are not closely related to the Berriasian to Valanginian true leptoceratids. Presumably, Eoheteroceras gave rise to the large-sized Heteroceras. Due to the simplified suture-line formula ELUI and the type of uncoiling, the Leptoceratoidinae are included in the Ancyloceratidae. Their evolutionary centre was the southern (Tethyan) margin of the European Plate. From this area, Leptoceratoidinae migrated into the central North Atlantic and even the western Pacific. While Leptoceratoidinae presumably had a vagrant epibenthic mode of life, their dispersal was most probably achieved during their nearshore juvenile stage by the North Equatorial Current.