Cambrian Soft Body Arthropod Naraoia with Digestive System

"Often called soft-bodied trilobites"

Naraoia sp.

Family Naraoiidae, Soft-bodied Arthropod); having resemblance to Naraoia compacta from the Burgess Shale

Geologic Time: Middle Cambrian Utah Cambrian Explosion Fossil

Size: inches (25.4mm=1 inch): Specimen 20mm in matrix 75mm X 80mm

Fossil Site: Marjum Formation, Millard County, Utah

If this Cambrian animal seems familiar, it is for good reason. It looks like Naraoia compacta (described by Walcott in 1912) from the Burgess Shale. This would make it a member of a clade of animals known as Nectaspida, an order of soft-bodied Arthropods, closely related to Trilobita. This specimen shows the two complex branching digestive glands and the gut diverticula, which is even more rarely preserved. The right portion of the head shield is turned in a bit, but otherwise is a unusaully well preserved specimen.

The prevailing view is that the Nectaspida are a sister group to the clade which includes the calcified trilobites. The revised Treatise adopts the tactic of treating the naraoiids as a family within the class Trilobita, order "uncertain," thereby acknowledging a systematic position within the class, but allowing the formal definition of Trilobita to be based upon the characters of calcified forms (Fortey 1997, p. 294).

For your further study:

  • Fortey, R.A. 1997: Classification. In Whittington, H.B. et al. 1997: Treatise on Invertebrate Paleontology Part O Arthropoda 1 Trilobita, Revised, Volume 1: Introduction, Order Agnostida, Order Redlichiida. The Geological Society of America and The University of Kansas.
  • Gould, Stephen Jay 1989: Wonderful Life. Penguin. 347 pp.
  • Walcott, C.D. 1912: Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata. Smithsonian Miscellaneous Coll. Publ. 2051, v. 57: 145-228.
  • Chen, J.-Y.; Zhou, G.-Q. 1997: Biology of the Chengjiang Fauna. In Chen, J.-Y.; Chen, Y.-N.; Van Iten, H. (eds.) 1997: The Cambrian Explosion and the Fossil Record. Bulletin of the National Museum of Natural Science, 10. Taichung, pp. 11-105
  • For a more extensive discussion and references, visit Sam Gon's web site.

Gould wrote: "Even if complexity is only a drift away from a constraining left wall, we might view trends in this direction as more predictable and characteristic of life's pathway as a whole if increments of complexity accrued in a persistent and gradually accumulating manner through time. But nothing about life's history is more peculiar with to this common (and false) respect expectation than the actual pattern of extended stability and rapid episodic movement, as revealed by the fossil record."

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