Two nautiloids are preserved in the above image of a slab from the Upper Ordovician of northern Kentucky. (I wish I knew which specific locality. This is why paleontologists are such fanatics about labeling specimens.) The top internal mold (meaning it is sediment that infilled a shell now dissolved away) has been covered in a previous blog entry. This week I want to concentrate on the nautiloid at the bottom.
These nautiloids belong to the Family Orthoceratidae McCoy, 1844, which existed from the Early Ordovician (490 million years ago) through the Triassic (230 million years ago). They had conical, aragonitic shells with walls inside separating chambers (camerae) and a central tube (the siphuncle) connecting them. They were swimming (nektic) predators that could control their buoyancy through a mix of gases and liquids in the camerae mediated by the siphuncle.
What is most interesting here is the preservation of these nautiloids. The aragonitic shells were dissolved away at about the same time the internal sediment was cemented, forming the internal molds. These molds were exposed on the seafloor, attracting encrusting organisms. This means the dissolution and cementation took place quickly and in the marine environment, not after burial. This rapid dissolving of aragonite and cementation by calcite is typical of Calcite Sea geochemistry, something we don’t see in today’s Aragonite Seas.
Above is a close view of the cemented siphuncle of the lower nautiloid, heavily encrusted by a trepostome bryozoan.
Even more cool, the outside of the lower nautiloid was encrusted by several trepostome bryozoan colonies. When the shell dissolved it left the undersides of these bryozoans exposed, as seen above. These undersides often contain the remains of shelly organisms the bryozoans encrusted (see the Independent Study project by Kit Price ’13) and even soft-bodied animals (epibiont bioimmuration; see Wilson et al., 1994).
A neat package here resulting from biological, sedimentological and geochemical factors.
References:
Palmer, T.J., Hudson, J.D. and Wilson, M.A. 1988. Palaeoecological evidence for early aragonite dissolution in ancient calcite seas. Nature 335 (6193): 809–810.
Sweet, W.C. 1964. Nautiloidea — Orthocerida, in Treatise on Invertebrate Paleontology. Part K. Mollusca 3, Geological Society of America, and University of Kansas Press, New York, New York and Lawrence, Kansas.
Wilson, M.A., Palmer, T.J. and Taylor, P.D. 1994. Earliest preservation of soft-bodied fossils by epibiont bioimmuration: Upper Ordovician of Kentucky. Lethaia 27: 269-270.
