Kayentachelys aprix has previously been hypothesized to be the oldest stem
cryptodiran turtle because of the presence of a flat, vertical plate on the
processus pterygoideus externus, and the presence of a processus trochlearis
oticum.
Just these two characters?
However, the presence of these characters cannot be confirmed in the
available specimens. Other putative stem-cryptodiran characters, such as the
prefrontal-vomer contact and the presence of an epipterygoid, are herein
corroborated as being symplesiomorphies, because they generally appear to be
present in basal turtles.
Not surprising...
Agnolin, F.L. 2007. Brontornis burmeisteri Moreno & Mercerat, un Anseriformes (Aves) gigante del Mioceno Medio de Patagonia, Argentina. Revista del Museo Argentino de Ciencias Naturales, n.s. 9(1):15-25.
What? An anseriform?!?
Thus, this test provides no support for a possible role of ionizing radiation in the K-T extinction event.