Gastropod operculum

                              apertural view of Megastraea undosa, height 5 cm, diameter 7 cm.

This post concerns the operculum ("trap door") that closes the aperture of certain shells, like that of the turbinid shallow-marine gastropod Megastraea undosa (W. Wood, 1828) shown above. The reason that I picked this topic is because when beginning fossil collectors find a gastropod operculum they commonly do not have a clue as to what kind of animal made it.

Exterior side of the operculum of M. undosa. The width is 3 cm. 

Interior side of the operculum shown above.

Same specimen of Megastraea undosa as shown above but now with its operculum, which very effectively seals the aperture from predators.

Megastraea undosa is found today in central California and southward along the outer coast of Baja California, Mexico. The species is restricted to hard substrates in the intertidal zone and grazes on algae attached to rocks. The species has a fossil record extending back to the approximately 10 million years ago (i.e., late Miocene).

Tentaculites

Tentaculites is a genus of a problematic group of Paleozoic fossils called tentaculids. The shells of Tentaculites consist of conical tubes (5 to 20 mm long) that are  straight or slightly bent and have transverse rings (ribs) on the surface. The tubes are closed at the small pointed end and open at the opposite, larger end called the aperture. The three specimens shown above are incomplete Tentaculites sp. from the same locality in Devonian rocks of New York. The specimen on the left is 5.8 mm in length; the middle specimen is 6 mm in length; and the specimen on the right is 4 mm in length.

One of the interesting features of most shells of Tentaculites is the presence of chambers in the early stage of growth. A photomicrograph (i.e., taken through a microscope) of a cross-section of a Tentaculites shell, 7 mm in length, shows these chambers. Jasmyn Nolasco took the photomicrograph, and James Ingraffia facilitated the process.

This is a cluster of Tentaculites sp. from the Manlius Limestone of Devonian age near Ravenna, New York. The largest specimens are approximately 10 mm in length. 

Tentaculites ranged in geologic age from the Early Ordovician to the Late Devonian (see time diagram below) (a span of 128 million years).