The Fate of Oligochiton

Chitons are one of the most distinctive and evolutionarily divergent groups of molluscs alive today. But compared to other groups of molluscs, the fossil record of chitons is rather sparse—or at least sparsely studied. It's not hard to see why. The multi-plated nature of the chiton shell means that it tends to fall apart after death, and the structure of the plates is such that critical features are easily abraded.

(Clockwise from top left) head, intermediate and tail valves of Lepidochitona lioplax, from Dell'Angelo et al. (2011).

Lepidochitona lioplax is one example of a fossil chiton. It was originally described from Oligocene rocks belonging to the Sooke Formation of southern Vancouver Island in British Columbia. Only four moderate-sized valves were initially identified: one head valve, one intermediate, and two tails (so at least two individuals were involved). The valves had a smooth outer surface without a strong distinction in appearance between the central and lateral areas. The insertion plates (lateral projections of the lower surface of the valves that in life anchor them into the surrounding girdle) were very short. The sutural laminae (anterior projections of the lower surface of the intermediate and tail valves that articulate with the valve in front) were low, wide, and divided in the middle by a broad shallow surface. Slits in the lateral insertion plates were numerous, with several in the tail valves and probably two or three on each side in the intermediate valves (Smith 1960). When first described, this species was thought distinct enough to belong in its own genus Oligochiton.

Oligochiton lioplax would then go little reported on until 2011 when Dell'Angelo et al. described an assemblage of chiton fossil from the latest Eocene or early Oligocene of the Lincoln Creek Formation in Washington State. Specimens of lioplax were relatively numerous in this collection and Dell'Angelo et al. were able to examine close to a hundred valves. Their observations would lead to something of a downgrade in the species status. Rather than deserving its own extinct genus, Dell'Angelo et al. felt that lioplax could be comfortably accommodated in the living genus Lepidochitona. Its smooth valves are unusual within Lepidochitona but not unique. The supposed multiple slits in the sides of the valves did not stand up to scrutiny. Instead, intermediate valves of L. lioplax bore only a single slit on each side, in line with other Lepidochitona species. The original inference of multiple slits was an error due to the original specimen being still partially embedded in the surrounding matrix.

Lepidochitona lioplax is one of the earliest known representatives of its genus but its exact significance is obscure. It has been suggested as a direct ancestor of the modern subgenus Spongioradsia but this, again, was based on the supposed slits in the intermediate valves that Dell'Angelo et al. refuted. To know how L. lioplax connects to the big picture of Lepidochitona evolution, we would probably need a better picture of Lepidochitona evolution overall.

REFERENCES

Dell'Angelo, B., A. Bonfitto & M. Taviani. 2011. Chitons (Polyplacophora) from Paleogene strata in western Washington State, U.S.A. Journal of Paleontology 85 (5): 936–954.

Smith, A. G. 1960. Amphineura. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I41–I76. Geological Society of America, and University of Kansas Press.