Aalenian Tmetoceras (Ammonoidea) from Iberia: taphonomy and palaeobiogeography

Abstract

From different areas of the Iberian Peninsula more than 600 specimens of Aalenian Tmetoceras have been found. This taxonomic group represents less than 20% of the whole ammonoids recorded in Opalinum, Murchisonae, Bradfordensis and Concavum biozones. Tmetoceras representatives, as well as Phylloceratina and Lytoceratina, were more frequent in shelfal basins than in epicontinental platforms. Taphonomic data suggest a eudemic character of the representatives of T. scissum in shelfal basins or oceanic areas. Exceptional immigrants and drifted shells of this species arrived in shallow environments of neighbouring platforms. In contrast, representatives of T. regleyi inhabited preferentially shallow environments of epicontinental platforms. T. scissum was a pandemic species, inhabiting oceanic or shelfal environments in the early Aalenian. However, some species of Tmetoceras, such as T. regleyi and T. flexicostatum, were geographically restricted in very distant areas. T. regleyi has been found only in European areas of the West Tethyan Subrealm. A pattern of adaptive radiation may have taken place in the Western Tethys during the Opalinum-Murchisonae biochrons, giving rise to T. regleyi from T. scissum. Specialized forms of Tmetoceras (k-strategists such as the individuals of the species T. regleyi) are widespread in the epicontinental platforms around the Western Tethys during the Murchisonae and Bradfordensis biochrons. Epicontinental, specialized forms of T. regleyi suffered extinction in the latest Bradfordensis Biochron. Shelfal or oceanic, generalist forms of T. scissum disappeared in the Western Tethys or the Mediterranean Province in the latest Bradfordensis Biochron, but they survived in the East-Pacific Subrealm.