Amaurolithus

Thu, 11/10/2007 - 15:20 — Jeremy Young

Genus Amaurolithus Gartner and Bukry, 1975

Description: Ceratoliths non-birefringent or only weakly birefringent. The c-axis is perpendicular to the plane of horseshoe, but spines on the surface may result it in being tilted from the horizontal and so showing some birefringence, but birefringence is lower than in Ceratolithus and extinction is rarely exactly in line with axis of the ceratolth.

In the SEM Amaurolithus ceratoliths (sometimes termed amauroliths) usually come to rest with the more ornamented surface upward, in this orientation the left hand arm is distinctly curved whilst the right hand arm is relatively straight, but more heavily ornamented.

Remarks: Raffi et al. (1998) documented likely evolutionary relationships between Ceratolithus and the fossil genera Amaurolithus and Triquetrorhabdulus. They presented evidence that A. delicatus, A. amplificus and C. acutus all independently evolved from Triquetrorhabdulus. Since this indicated that Amaurolithus was polypyhletic they proposed dividing the genus in two; Amaurolithus and Nicklithus.

Taxa included: