Size reduction and ornamental oscillation within a Barremian lineage of giant heteromorphic ammonites (Early Cretaceous, northwestern Tethyan margin)

Abstract

Field sampling data in the Vocontian Basin (southeastern France) and Mallorca (Spain), at the transition between the lower and upper Barremian (upper Moutoniceras moutonianum Zone and lower Toxancyloceras vandenheckei Zone), provides new information about the early representatives of the ammonite family Ancyloceratidae. Several successive species of the genera Moutoniceras and Toxancyloceras are identified. A review of the history of the acquisition of the Astier collection by the Natural History Museum of London, which contains the holotype of the type-species T. vandenheckei, clarifies some misinterpretations which this species previously suffered. T. vandenheckei and M. eigenheeri are revised, and a new species is described: T. canuti sp. nov. With respect to an evolutionary perspective under biostratigraphic control, their phylogeny is considered (M. eigenheeri -> T. canuti sp. nov. -> T. vandenheckei). The hypothesis of the origin of the genus Toxancyloceras within the Moutoniceras is strengthened; this link is consistent both stratigraphically and morphologically. Moutoniceras appears to be the oldest known representative of the Ancyloceratidae, which is rooted in the early Barremian. The ontogenetic and evolutionary patterns of the phyletic lineage Moutoniceras/Toxancyloceras are twofold: the first concerns the ornamental changes (itself determined by three imbricated patterns) and the second involves the adult size. Both patterns determine two evolutionary phases through time: (1) the giant Moutoniceras and the progressive disappearance of the tubercles through heterochrony (paedomorphosis), and (2) the drastic size reduction and the reappearance of the tubercles from the “small” Moutoniceras to the Toxancyloceras (through heterochrony, with peramorphosis and a combination of pseudo-dwarfism, acceleration and graduaptation). The oscillation in disappearance and reappearance of the tubercles demonstrates a possible case of evolutionary reversibility where heterochrony helped by the progenesis impact, favours character repeatability in the evolutionary patterns. The results for the genera Moutoniceras and Toxancyloceras have significant biostratigraphic implications for the Tethyan Barremian. The status of T. vandenheckei as a zonal and subzonal index species (basal upper Barremian) is reinforced, and three new ammonite horizons are defined: the Moutoniceras eigenheeri, Toxancyloceras canuti and T. vandenheckei horizons. The stratigraphic distribution of all their index species is very restricted, indicating a well established evolutionary context.