Article: The Cretaceous ammonite genus Leymeriella, with a systematic account of its British occurrences
Ammonites of the genus Leymeriella Jacob characterize the Northern Hemisphere and are found in neritic deposits of Lower Albian age extending from the Urals to east Greenland. Known occurrences are restricted to the Zone of Leymeriella tardefurcata. Investigation of anomalous records of Leymeriella in the overlying Zone of Douvilleiceras mammillatum discloses that the classic 'Mammillatum' bed of the Ardennes and Meuse Departments of France, which furnished d'Orbigny with much type material, is a condensed deposit in which elements of both Tardefurcata and Mammillatum age are combined. Account is taken of 18 species and 3 varieties of Leymeriella (including the subgenus Epileymerietta Breistroffer), of which 4 species and 1 variety are new, and 4 species of Proleymeriella Breistroffer, 1 new. Systematic treatment is given 7 species and 3 varieties of Leymeriella of British provenance.Mode of coiling, ontogenetic development of the septal sutures, and certain sculptural features indicate that the affinities of Leymeriella lie with the Lyelliceratidae (Acanthocerataceae) rather than with the Hoplitidae (Hoplitaceae) to which it has been generally attached. It is believed that Leymeriella provides the key to the origin of the great ammonite superfamily Acanthocerataceae. An eruptive phase of evolution at the beginning of the Albian is postulated in which the Lyelliceratidae (including Leymeriellinae), Brancoceratidae, and Mojsisovicsiidae are conceived as simultaneous and rapidly differentiating offshoots of the Desmoceratid Callizoniceras. Support for this hypothesis is given by 'Hoplites' haidaquensis Whiteaves, from the Albian of the Queen Charlotte Islands, a member of the Lyelliceratid-Brancoceratid complex for which the nominal genus Pseudoleymeriella is proposed. It is suggested that the Acanthocerataceae and the Hoplitaceae may represent synchronous radiations from the Lytocerataceae and Phyllocerataceae respectively.Leymeriellinae and Hoplitinae may have been mutually exclusive, the one shunning areas favourable to the other. The apparent extinction of Leymeriella coincided with the appearance in Europe of new, virile Hoplitid stocks in the Mammillatum Zone.