Figure 2.

Sweetognathus P₁ elements. (a) Terminology used to describe Sweetognathus P₁ morphology (for more information on element orientation refer to [30]). The red line is an example of a cross section taken through the denticle. The oral surface of the element contains the denticles. (b) 3D rendered model of aboral surface illustrating the basal cavity indentation. (c) Scanning electron microscopy image of Sw. whitei from the Tensleep Section in Wyoming, small nodes cover the surface of the denticles; the element displays left curvature. 3D rendered models of Sw. whitei (d), Sw. asymmetricus (e), Sw. clarki (f), Sw. cf. adenticulatus (g), Sw. aff. binodosus (h), Sw. obliquidentatus (i), Sw. binodosus (j) and Sw. anceps (k). (Online version in colour.)