Ontogenetic heterochronies: a tool to study both variability and phyletic relationships? Example: Nigericeras, Ammonitina of the African Upper Cretaceous

• Published in: Comptes rendus. Palevol Vol. 2; no. 6-7; pp. 535 - 547
• Main Authors: Courville, P, Cronier, C
• Format: Journal Article
• Language: French
• Published: 01.01.2003
• Subjects: Nigericeras gadeni; Pseudocalycoceras; Africa, Sahara Desert
• ISSN: 1631-0683
• DOI: 10.1016/j.crpv.2003.09.003

The Nigericeras gadeni ontogenesis (Saharian Upper Cretaceous) is characterized by three ornamental stages: multi-tuberculate inner-whorls, ombilical bi-tuberculate medium-whorls, then finally smooth adult stage. Ontogenetic Heterochonies account for their adult variability, which extends between both paedomorphic (thick/ornamented) morphotype, and peramorphic (thin/smooth) morphotype. That species may take root in the older Pseudocalycoceras. A peramorphocline characterizes the evolution of these ammonites and their younger relatives, leading to different Nigerian and Nigerien, highly compressed taxa. Such morphologies seem to be related to shallow-water palaeo-environments. The general transgressive context leads to the progressive scarcity of their supposed ecological niches.Original Abstract: Laontogenese de Nigericeras gadeni (Cretace superieur saharien) est caracterisee par trois stades ornementaux: multitubercule dans les tours internes, bitubercule puis lisse en fin de croissance. Les Heterochronies du Developpement permettent de decrire sa variabilite adulte, entre un type orneuepais pedomorphique et un type lisse-mince peramorphique. Cette espece saenracine au sein du genre Pseudocalycoceras. Une derive peramorphique marque laevolution de ces ammonites et de leurs descendants, aboutissant a des taxons morphologiquement tres comprimes, differents au Niger et au Nigeria. Les taxons paraissent en adequation avec des milieux tres peu profonds. Le contexte transgressif general induit une rarefaction progressive de leurs niches ecologiques.