Photo by Jack Kallmeyer, used with
permission of the Dry Dredgers
|What are ichnofossils?
They are trace fossils: preserved tracks or other signs of the behaviors of animals in the substrate. Ichnofossils can provide some very intriguing insights on the ecology and behavior of an extinct animal such as a trilobite. It is very rare that the animal itself is found in direct association with the ichnofossil it created, but that kind of co-occurrence allows scientists to link ichnofossils with the species that created them. In fact, in the pre-Cambrian fossil record, there are no trilobites, but there are trace fossils which very closely resemble those made by trilobites, suggesting that before trilobites developed their hard calcite shells, their ancestors were crawling about leaving traces. While there are hundreds of named ichnofossil types, there are three main named categories of ichnofossils associated with trilobites: Rusophycus, Cruziana, and Diplichnites, described below.
The three main types of trilobite
are related to behaviors of the trilobite in its habitat:
Adapted from the Treatise of Invertebrate Paleontology, Part W. Trace Fossils (Revised)
all images this page ©1999, 2000 by S. M. Gon III. Created using Macromedia Freehand 8.0
In this trilobite ichnofossil, the animal approached from the right side, leaving Cruziana across the substrate, then settled into the silty bottom, leaving a Rusophycus.
Image courtesy of Stefano Novello, July 2008
Ichnofossils and trilobite movement
When a trilobite is resting, partially buried in the mud of the benthos, it leaves a bi-lobed impression called a Rusophycus, that is sometimes marked by parallel marks that correspond to its legs. Cruziana are tracks showing rather clearly defined edge furrows, probably created by a trilobite moving, partially buried, through the mud. The widest part of the shell leaves the parallel outer furrows, while the leg movements create the chevron-shaped marks in the center of the track. Finally, when a trilobite is walking or striding freely upon the surface, it leaves paired leg marks called Diplichnites, which can be reduced to rather widely-spaced impressions when the animal is striding at full speed across the substrate. As noted in the illustration above, the three types of traces can grade into one another, and the transitions are related to whether the animal is stationary or moving, the speed of movement, and whether the trilobite is partially buried or fully emerged from the substrate. Below are some ichnofossil specimens of the three types.
|A very detailed Rusophycus specimen from Australia show gnathobase impressions along the midline, as well as the bi-lobed, oval impression of the trilobite body.||Cruziana trail showing the central paired leg grooves and the parallel furrows on the outside edges. The curved shape suggests a searching pattern.||These Diplichnites tracks are attributed to Isotelus trilobites walking on the surface, rather than plowing (furrowing) through the surface layers.|
Ichnofossils and Feeding
Sometimes trace fossils leave evidence of how trilobites hunted or fed. Many Cruziana are thought of as feeding tracks, since to create them, the trilobite would be moving rather slowly through the upper layers of the ocean floor, as if searching for something. There have been some key specimens that show Cruziana changing direction toward a worm burrow, then stopping when it presumably intersected with the worm. Struggle marks at the end of the Cruziana track indicate that the trilobite captured and ate the worm. Rusophycus are created when a trilobite is stationary upon loose substrate. In filter-feeding trilobites such as Cryptolithus, the movement of limbs under the circular cephalic shield create distinctive trace fossils that mark this kind of feeding behavior, called Rusophycus cyptolithi. You can learn more about this in the section on Trilobite Feeding.