Friday, August 29, 2014

An exceptional trilobite-trace fossil

Trilobites (see the link ) were Paleozoic arthropods that crawled along the shallow-sea floor. When a trilobite stopped to rest, it made a shallow burrow in the mud or silt. These burrows were commonly filled with sediment and later fossilized as "resting" trace fossils called by the Latin name, Rusophycus. Remember from one of my earlier blogs, a trace fossil shows behavior of a fossil organism.

The above picture is the bottom of a Cambrian Rusophycus from the Inyo Mountains, central California. The burrow is 10 cm in length. Scratch marks made by the trilobites legs are visible on the bottom of the burrow.

These two slabs (both about 11 cm, widest dimension) of slightly metamorphosed Cambrian siltstones from eastern California contain a cluster of Rusophycus. The burrows were probably aligned parallel to an ancient current that brought food to the trilobite. Rusophycus commonly has a bilateral symmetry that was caused by its two rows of legs that moved back and forth in its burrow. This leg action was necessary because its gills were attached to its legs, thus it had to move its legs (even when stationary in its burrows), in order to obtain oxygen from the shallow-sea water.

Ahh, finally we get to a truly exceptional specimen of Rusophycus. It is undoubtedly the best specimen I have ever seen. It is preserved three-dimensionally (8.5 cm long, 3.5 inches) and is of Cambrian age from the Salt Spring Hills, eastern California. The above picture is the bottom view, which shows the scratches made by the legs of the trilobite. The specimen is a plaster replica of the actual specimen, which is now in a museum collection. I painted the plaster replica so as to make it look more like the actual specimen.

This is a side view of the same specimen shown above. The bottom of the burrow is at the top of the picture. I painted the antennae red, so you can readily see them. The fact that the antennae are visible indicates that the remains of the trilobite that made this resting trace are within the burrow, thereby proving the trilobite made this burrow. In situ (in place) specimens of trilobites in their burrows are very rare.


  1. Trilobites are by far the coolest arthropods. What I found most interesting about the trilobite trace fossils is the indication of its crawling and resting phase. This was difficult for me at first but after a while it became a lot easier to determine whether the trilobite was resting or crawling. This is an interesting and extremely helpful tactic. Unfortunately, I did not find a trilobite on our most recent field trip but I am hoping to go on many trips in the future to collect these amazing fossils.
    Thanks for sharing!

  2. While in Cochabamba Bolivia i hiked up Pico Tunari. Around the top in all the truck slides there were tons and tons of fossils, many Rusophycus. Near the top of the climb (somewhere between 4900 meters and the top there was this giant rock facing the trail with easily 100 Rusophycus fossils just sitting together. The guide said they were trilobite footprints. Definitely Rusophycus even if they weren't from trilobites