MORPHOLOGICALLY DISPARATE PALEOZOIC CHITONS AND THE ORIGIN OF THE CROWN GROUP CHITONIDA

Newly recovered chiton shell plates (valves) from the Permian of the Guadalupe Mountains of Texas show fine details of tegmental (outer shell layer) sculpture, configurations of the fine internal sensory aesthete canals, and lateral and anterior projections of the articulamentum (underlying shell layer) including cuts and grooves therein. These new fossils add to the already diverse chiton fauna from in and around the Permian Capitan Reef, which overall show high variability in aspect ratio, tegmental coverage and sculpture, and shape and extent of incisions in the lateral projections of the articulamentum (insertion plates). In contrast, known Mesozoic chiton fossils are sparse and morphologically uniform, and similar in valve form to those of Recent chitons. The chiton tree was significantly pruned during the end Permian mass extinction, during which abundance, diversity, and morphological disparity of chitons all plummeted. Paleozoic chiton fossil lineages appear to have frequently converged in both tegmentum reduction and in the various incisions observed in the lateral projections of the articulamentum. Chitonida (Mollusca: Polyplacophora), including most living chitons, has diagnostic slits coinciding with slit rays, which are rows of pores on the ventral valve surface produced by aesthete innervations in the insertion plates. Some Late Paleozoic chitons have been considered to be members of the Chitonida based on the presence of slits in the articulamentum, consistent with molecular clock analyses. However the slits in the Late Paleozoic chitons differ from those in Chitonida in their: (1) greater abundance; (2) shallower incisions; and (3) lack of an association with slit rays. In addition, pectinations and grooves occur in the anterior sutural laminae extensions of the articulamentum layer of valves from various Late Paleozoic chitons, and it is likely that these are convergent adaptations for anchoring the valves in place, given their scattered distribution in different chiton lineages. The oldest chiton fossils with slits as in Chitonida are possibly from the Jurassic, but best verified from the Cretaceous, in either case significantly younger than the Late Paleozoic fossils that have been claimed as corresponding to the origin of Chitonida.