23 November 2021

Rudist fossil Durania cornupastoris - Thumamah

Whilst looking for fossils near Thummama, near Riyadh, my daughter and I found a number of Rudists fossils of Durania cornupastoris (Des Moulins). They were mainly ventral radial band (Vb) and were found in the Aruma Formation. This formation is distributed in NW-SE direction around Riyadh and consists of three members, from bottom to top, the Khanasir Limestone Member, the Hajajah Limestone Member, and the Lina Shale Member. The rudist biostrome, approximately 2 m thick, is placed in the uppermost part of the Khanasir Limestone Member and consists mainly of Durania specimens. Upper Cretaceous rudists were widely distributed throughout the northern and southern margins of the Mediterranean Tethys. They are described from the Cenomanian–Turonian sequences of Algeria, Tunisia, Egypt, Jordan, and Lebanon and the Campanian–Maastrichtian of Tunisia, Somalia, Oman, UAE, Saudi Arabia, SE Turkey, Iraq, and Iran on the African–Arabian plate. The habitat where they occurred was an inner sublittoral, warm marine paleoenvironment. This paleoenvironment was characterized by firm, stable substrate surfaces subjected to periodically intense wave and current action, within the photic zone, abundant food supply but with minimal terrigenous input. Rudists were a group, now extinct, of box, tube or ring-shaped marine heterodont bivalves that arose during the Late Jurassic and became so diverse during the Cretaceous that they were major reef-building organisms until their complete extinction at the close of the Cretaceous. They lived in shallow marine environments with fossils found in limestone rocks. They had two asymmetric valves with one valve attached to the sea floor. Today, their fossils are found throughout the tropics in the Mediterranean, the Middle East, the Caribbean, and Southeast Asia. Rudists dominated the world of reefs throughout the Cretaceous until the Cretaceous-Tertiary mass extinction event that caused their demise. Rudists grew by accretion (or the increase in size by addition) and were suspension feeders. Generally, large-bodied rudists lived longer and reproduced relatively frequently with small spawns. Rudists lived in shallow ocean waters on the sea floor. These organisms were epifaunal, which means they were usually attached to the sea floor sediment. The clustering and building up of Rudist habitats caused the creation of "Rudist Reefs" which were the dominant reef frameworks in the Cretaceous oceans. Basic external features of rudists include the umbo and thick, asymmetric right and left valves. The umbo is the rounded protrusion found just above the hinge, and the hinge is the pivoting point where the two valves meet. 

Characteristics of rudists include the following:

Unequal valves

One of the valves is usually attached to the sea floor sediment

Hinge structures: free (unattached to sea floor sediment) valve has two teeth and one socket; attached valve has two sockets and one tooth

Two adductor muscles