Labyrinthoceras and Frogdenites (Sphaeroceratidae, Ammonitina) from Western Tethys: The origin of the Sphaeroceratidae

Abstract

A biostratigraphical and systematic revision of the genera Labyrinthoceras and Frogdenites is presented, based mostly on new specimens from the Lower Bajocian of the Betic Cordillera (Southern Spain). Types and other supporting specimens are described, including some from southwestern England with precise biostratigraphy. Frogdenites is shown to span the Lower Bajocian (Ovale Zone to lowermost part of the Propinquans Zone), whereas Labyrinthoceras ranges from the Laeviuscula Zone (Trigonalis Subzone) to the Propinquans Zone. Both genera show well-marked dimorphism; the microconchs may have reduced lateral lappets. Labyrinthoceras meniscum (Waagen) [M] & [m] = Manselites mansellii (J. Buckman) is described along with specimens that are placed in the new species Labyrinthoceras dietzei nov. sp. [M]. Frogdenites is grouped into three morphospecies: Frogdenites fernandezlopezi nov. sp. [M] & [m?], an evolute morph with poorly developed tubercles; F. spiniger Buckman [M] & [m], which is more involute with strongly forward curving projecting ribs; and F. extensus (Buckman) [M] & [m] with a more eccentric coiling. Docidoceras tolleyi nov. sp., which possesses a morphology intermediate between Docidoceras and Frogdenites, is described. Phylogenetic analysis shows that the Sphaeroceratidae originated in the Earliest Bajocian (Ovale Zone) and that Frogdenites is the oldest genus of this lineage. Docidoceras rather than Emileia may be the ancestor of the Sphaeroceratidae. The evolutionary path from Frogdenites to Labyrinthoceras, with “Chondroceras obornensis” Parsons, 1980 (unpubl. manuscript), as an intermediate step, involves size increase and disappearance of tubercles.