Dendraster gibbsii (Rémond,1863) - late Miocene to Pliocene fossil sand dollar
Dendraster gibbsii (Rémond, 1863) is found in the Siphonalia Member of the late Miocene to Pliocene Etchegoin Formation of the Kettleman Hills in California. Auguste Rémond initially described this fossil as Scutella gibbsii in the Proceedings of the California Academy of Science in 1863. To indicate that the original generic name has been changed, a parentheses is placed around the name of the author and the date of description. The age of this fossil is often given as Lower or Middle Pliocene, but recent work has extended the time range for the sandstone in the Etchegoin Formation.
Related to starfish, sand dollars have pentameral (five-part) radial symmetry. Normally covered in tiny spines and tube feet that are lost after the organism dies, they lie flat on the surface or buried in the seafloor. In quiet waters, they stand on edge, using their spines to drive into the sand. The calcium carbonate shell or "test" is composed of interlocking plates, easily seen in these specimens, as they have been naturally stained by petroleum. These spectacular specimens are naturally weathered out and have not been polished. They vary in preservation from delicate to boldly marked.
The Kettleman Hills oilfield is one of the largest in California. In the 1930s it was one of the largest oilfields in the world. The oil is trapped in two elongate domes. The producing horizon, over 1,500 feet thick, is the Lower Miocene Temblor Formation. The domes have been breached by erosion parallel to their axis, exposing a prolific fossil fauna ranging from Lower to Upper Pliocene in the Etchegoin and San Joaquin Formations respectively. The Etchegoin was deposited largely in bay and estuarine environments. Its sandstones were derived from the Sierran magmatic arc, the Quien Sabe volcanics, and uplifted Franciscan-Coast Range source terranes.
These fossils are superbly preserved and vary from pale and delicate to contrasty and bold. The field photo shows how we find them, though they are not always not that abundant, and the last two photos show what they look like as they come in from the field and then after cleaning. There is a huge range of preservation, not evident until they are prepared.
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