Cambrian Selkirkia Priapulid Worm Fossil

Priapulid worms are among the fossils found in the Burgess shale, and ancestors are still extant today, though not diverse. They are as biologically complex as arthropods, and are thus important in the Cambrian fossil record. Taxonomically, they belong to a group named the Cephalorhyncha that are ecdysozoan animals -- encompassing the arthropods (insects, spiders, and crustaceans) and the nematodes. The grouping is based on a set of shared characters including the presence of a cuticle and the fact that they periodically shed their cuticle in a process called ecdysis. All cephalorhynchs have a spiny proboscis that is used to gather food. The Priapulid worms were probably primary predators of the early to middle Cambrian.

This Selkirkia comes from the Marjum Formation, that like other formations in Utah's House Range contains a diverse fauna exhibiting Burgess Shale-like soft tissue preservation, normally as carbon-rich films. The rare exposures with this type of preservation were oxygen-starved, delaying decay, and allowing time for two dimensional carbon films to form, usually with minimal mineral replacement. There are many Burgess Shale like deposits worldwide, though they are very rare after the middle Cambrian.