Nautiloids - Fossil Record

Fossil Record

Nautiloids are often found as fossils in early Palaeozoic rocks (less so in more recent strata). The shells of fossil nautiloids may be either straight (i.e., orthoconic as in Orthoceras and Rayonnoceras), curved (as in Cyrtoceras) coiled (as in Cenoceras), or rarely a helical coil (as in Lorieroceras). Some species' shells—especially in the late Paleozoic and early Mesozoic—are ornamented with spines and ribs, but most have a smooth shell.

The shells are formed of aragonite, although the cameral deposits may consist of primary calcite.

The rocks of the Ordovician period in the Baltic coast and parts of the United States contain a variety of nautiloid fossils, and specimens such as Discitoceras and Rayonnoceras may be found in the limestones of the Carboniferous period in Ireland. The marine rocks of the Jurassic period in Britain often yield specimens of Cenoceras, and nautiloids such as Eutrephoceras are also found in the Pierre Shale formation of the Cretaceous period in the north-central United States.

Specimens of the Ordovician nautiloid Endoceras have been recorded measuring up to 3.5 meters (13 ft) in length, and Cameroceras is (somewhat doubtfully) estimated to have reached 11 meters (36 ft). These large nautiloids must have been formidable predators of other marine animals at the time they lived.

In some localities, such as Scandinavia and Morocco, the fossils of orthoconic nautiloids accumulated in such large numbers that they form Orthoceras limestones. Although the term Orthoceras now only refers to a Baltic coast Ordovician genus, in prior times it was employed as a general name given to all straight-shelled nautiloids that lived from the Ordovician to the Triassic periods (but were most common in the early Paleozoic era.