VERY RARE ARTICULATED CHITON FOSSILS

Chitons (also called amphineurans or polyplacophorans) are mollusks that belong to class Polyplacophora. The geologic range of this class is from the Cambrian Period to modern day.

Chitons are sluggish crawlers on the sea bottom and live mostly on hard substrates in very nearshore environments. Chitons can, however, also live on sunken wood or on hard substrates in deep-water (bathyal) offshore environments, where cold seeps have formed rocky edifices. Fossils of chitons specimens are usually uncommon to rare and almost always consist of disarticulated valves (i.e., broken apart).

The chiton shell consists of eight dorsally located, aragonite plates (valves) that overlap posteriorly. The anteriormost valve (head plate) has indentations on its front edge. The valves are surrounded by a band of tough, organic material (girdle) that is formed by the mantle as a soft part and is quite flexible. In a few chitons, the dorsal shell may be concealed entirely by organic material. The underside of a chiton is mostly a broad flat muscular foot. If disturbed, the chiton can roll up like a "pill bug," bending at each joint between the dorsal plates.

Sketches of the dorsal, ventral, and side views showing the morphology of a chiton (shell and soft parts).

Unless burial is quick, preservation of fully intact (i.e., with articulated valves all present) chiton shells in the sedimentary/fossil record is extremely rare. To date, there have been only six reports in the published literature of articulated fossil chitons. Squires and Goedert (1995) reported on one of the two oldest known of these occurrences, namely, in late middle Eocene to early late Oligocene cold-seep limestones, Olympic Peninsula, Washington. They identified the chitons to be the extant species Leptochiton alveolus (Lovén, 1846), which is one of two known, modern-day cosmopolitan species of chiton. This species is present in the Pacific, Atlantic, Indian oceans, and in the Mediterranean Sea. Its favored habitat is in bathyal to abyssal depths (see Squires and Goedert, 1995, p. 51).

Three views (dorsal, ventral, and side) of a modern-day chiton shell (without its encircling girdle) with its valves glued-back together. The “head” area is at the top of the first two images and to the right in the third image.

Leptochiton alveolus (Lovén, 1846) from the upper part of the Lincoln Creek Formation, early late Oligocene age, Washington; specimen is 1.6 cm length. Image is from Squires and Goedert (1995, fig. 3). This remarkably rare specimen was found by Jim Goedert. 

Leptochiton alveolus (Lovén, 1846 ), modern day, Peru; specimen is 2.2 cm length. Image is from Squires and Goedert (1995, fig. 6). 

The only other presently known Eocene occurrence of an articulated chiton (Leptochiton sp.) was reported by Cabrera and Olivero (2011) from Anarctica.  

References Cited:

Cabrera, M.I.L and E.B. Olivero 2011. An Eocene articulated Polyplacophora (Mollusca) from the La Meseta Formation, Antartica and the stratigraphy of the fossil-bearing strata. Journal of Paleontology 85(5):970–976.

Squires, R.L. and J.L. Goedert. 1995. An extant species of Leptochiton (Mollusca: Polypolacophora) in Eocene and Oligocene cold-seep limestones, Olympic Peninsula, Washington. The Veliger 38(1):47–53. Pdf is available for free via Biodiversity Heritage Library (www.biodiversitylibrary.org).