Archive for July 8th, 2016

Wooster’s Fossil of the Week: An ammonite from the Middle Jurassic of southern England

July 8th, 2016

Leptosphinctes microconch Jurassic Dorset 585We’re featuring just a workaday fossil this week because of other summer activities. This is the ammonite Leptosphinctes Buckman 1929 from the Inferior Oolite (Middle Jurassic) at Coombe Quarry, Mapperton, Dorset, southern England. Cassidy Jester (’17) and I collected it last month during our 2016 England research expedition. Our friend Bob Chandler generously identified it. It popped out of a rock we were pounding into submission, providing a direct application of ammonite biostratigraphy to our work. As with many ammonites, the group is well known but the names are still a bit dodgy.

This specimen is a microconch, meaning it is the smaller version of a species pair, the larger being the macroconch. It is presumed that this is sexual dimorphism and that the microconch is the male because it didn’t need to carry resources for egg-laying. This is one reason why the taxonomy of these ammonites is in perpetual revision.

References:

Buckman, S.S. 1909–1930. Yorkshire Type Ammonites & Type Ammonites. Wesley & Son, Wheldon & Wesley, London, 790 pl.

Chandler, R B., Whicher, J., Dodge, M. and Dietze, V. 2014. Revision of the stratigraphy of the Inferior Oolite at Frogden Quarry, Oborne, Dorset, UK. Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen 274: 133-148.

Galácz, A. 2012. Early perisphinctid ammonites from the early/late Bajocian boundary interval (Middle Jurassic) from Lókút, Hungary. Geobios 45: 285-295.

Pavia, G. and Zunino, M. 2012. Ammonite assemblages and biostratigraphy at the Lower to Upper Bajocian boundary in the Digne area (SE France). Implications for the definition of the Late Bajocian GSSP. Revue de Paléobiologie, Vol. spéc, 11: 205-227.