Chandler R., Callomon J. (2009) The Inferior Oolite at Coombe Quarry, near Mapperton, Dorset, and a new Middle Jurassic ammonite faunal horizon, Aa-3b, Leioceras comptocostosum n.biosp. in the Scissum Zone of the Lower Aalenian // Proceedings of the Dorset Natural History and Archaeological Society. V.130. P.99-132.
pdf: http://rogov.zwz.ru/Chandler,%20Callomon,%202009.pdf (Thanks to Robert Chandler for this pdf)
The biostratigraphy of Coombe Quarry at Mapperton, Dorset is described. Its special significance lies in the presence of an ammonite faunal horizon Aa-3b (hitherto not recognized elsewhere in the English Inferior Oolite) lying in the upper part of the Scissum Zone. Its exceptionally abundant and well-preserved ammonite assemblage characterizes a new species, Leioceras comptocostosum sp.nov., giving an unusually clear insight into the structure of a natural, genetic, true ammonite species - a palaeobiospecies - as expressed in the intraspecific variability of its fossils. It typifies the evolution of the genus Leioceras, regarded as a phylogenetic segment of the evolutionary lineage of the family Graphoceratidae, as an isochronous slice - a transient - through the trunk of that lineage. The formal taxonomic relationships of the new (bio)species to existing nominal Linnean (morpho)species, with which some of its variants have in the past been associated in European continental descriptions, are briefly reviewed. Leading among these are Leioceras comptum (Reinecke, 1818) and Leioceras costosum (Quenstedt, 1886), both of which have been used as zonal indices and both of which are in need of serious revision.