Archive for January 23rd, 2011

Wooster’s Fossil of the Week: A most unlikely clam — rudists from the Upper Cretaceous of the Oman Mountains

January 23rd, 2011

This week’s fossil was collected on a memorable trip in 2000 to the United Arab Emirates and Oman with my friend Paul Taylor, an invertebrate paleontologist at the Natural History Museum in London. We were there to study hard substrate faunas (sclerobionts) in an Upper Cretaceous (Maastrichtian) unit known as the Qahlah (pronounced “coke-lah”) Formation. We traveled along the border between these two countries in an old Toyota Landrover plotting out the distribution and characteristics of the Qahlah and its fossils. If you want a pdf of the resulting paper (and I’m sure you do), just click here: Wilson & Taylor (2001).

One of the most interesting fossil types common in the Late Jurassic through the Cretaceous is the rudist clam. The image above is one of our Qahlah specimens known as Vaccinites vesiculosus. There are two conical rudists growing together here, with the one on the left still retaining most of its upper valve.

Rudist clams are an example of just how far evolution can go with a basic body plan. They are heterodont clams sharing a common ancestry with the typical modern Mercenaria we so love to eat (and dissect). Starting in the Jurassic, the left valve began to elongate into a cone and the right valve became a cap-like cover. They attached to each other and formed reef-like masses throughout the warm, shallow tropical seas of the Cretaceous. They were so successful that they appear to have competitively excluded most of the coral reefs. Corals had the last gurgly laugh, though, because the end-Cretaceous extinction completely wiped out the rudists, allowing the later rise of modern coral reefs.

A typical heterodont clam is in the upper left of this diagram; the rest are rudist clams. In the lower right is a drawing of the type of rudist photographed above. Diagram from Schumann & Steuber (1997; Kleine Senckenbergreihe 24: 117-122).

When I see our rudist clam specimen I’m reminded not only of its complex evolutionary heritage, but also of our own desert odyssey with grim musket-bearing Omani tribesmen, endless sand dunes stretching west, and delicious banquets of lamb and dates.