File:Ophiocrossota sp. (fossil brittle stars) in fossiliferous sandy limestone (Santa Margarita Formation, Miocene; Lopez Lake area, San Luis Obispo County, California, USA) 4.jpg

English: Ophiocrossota sp. - fossil brittle stars in fossiliferous sandy limestone from the Miocene of California, USA. (cut/polished surface; slab is ~16.3 centimeters across at its widest)

These specimens may be Ophiocrossota baconi or Ophiocrossota oweni.

"Brittle stars" are technically called ophiuroids. They are starfish-like organisms with a central disc and slender, snake-like arms. Brittle stars first appear in the Ordovician and continue into the Holocene. Unlike starfish, brittle stars do not have coelomic extensions in their arms. Each arm is skeletal and composed of pieces of calcite called vertebrae (vertebral plates). Brittle star arms move in a serpentine manner and tend to break off easily. The arms may be complexly branched. In terms of locomotion, ophiuroids are more mobile than starfish. The feeding style for most brittle stars is filter feeding or scavenging, but some are predatory.

The fossiliferous limestone seen here is remarkable - it is composed almost entirely of brittle star bodies (click on the photo once or twice to zoom in). The limestone matrix has a significant component of quartz sand.

Classification: Animalia, Echinodermata, Asterozoa, Ophiuroidea, Ophiurida, Ophiuridae

Stratigraphy: Santa Margarita Formation (possibly the lower Santa Margarita Formation), Upper Miocene

Locality: private ranch in the Lopez Lake area, above the town of San Luis Obispo, southern California, USA

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See info. at: en.wikipedia.org/wiki/Brittle_star and

en.wikipedia.org/wiki/Ophiuridae