Rare Wiwaxia Enigmatic Soft Body Animal Fossil


Wiwaxia (Walcott, 1911)

Superphylum Lophotrochozoa, Phylum incertae sedis, Family Wiwaxiidae

Soft-bodied animal, first decribed from the Burgess Shale biota

Geological Time: Middle Cambrian

Size: 21 mmm

Fossil Site: Marjum Formation, House Range, Millard County, Utah


WiwaxiaWiwaxia was first described in 1899 from a single spine found across the valley from the Burgess Shale, where complete specimens were subsequently discovered. The slug-appearing animal had two rows of spines along the back, ostensibly affording some protection from predators. The remainder of the upper (dorsal) body was covered with small, flat, overlapping hard plates, termed sclerites. Each of these little scales was attached with a root-like base and it is assumed Wiwaxia grew by molting the Wiwaxiaplates. Because of the sclerites, some researchers have attempted to place Wiwaxia with the annelids, and indeed, it may be ancestrial or closely related to the segmented worms. The polychaete annelid worms are spiny with chaetae that are mineralized.

Since there are none on the bottom (ventral) surface, the animal partly resembles the slug, a member of the mollusk family. However, mollusks do not have any sclerite armor so the animal's affinity to present day species is unsettled. It did have an anterior jaw with two rows of teeth on the ventral surface, suggesting it was a bottom feeder. Fossil sizes range from 6 to 50 mm. The animal is rare with but 140 specimens known from the Burgess Shale.

It is likely that trilobite collectors in Utah have discarded many Wiwaxia and other soft-bodied animals over the years. The material is harder to see than trilobites and many local collectors were simply unfamiliar with soft-bodied animals and didn't recognize what they were. Nonetheless, these animals are as rare as they are hard to see in the Marjum Formation.

Also See: Utah Cambrian Explosion Fossils

References

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Wiwaxia


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