9/00 mechanical engineering: input output

input/output

an old fossil goes high-tech

by Henry Baumgartner, Contributing Editor

Computer-aided design and related tools, it turns out, are not just for designing machinery. In fact, they can be used to reverse-engineer nature's own designs—even of models that have been off the market for some time.

Five hundred million years ago, the seas were teeming with a multitude of creatures, many of which would look very strange to us today. Among the most ubiquitous of them were the trilobites, a large group of water-dwelling arthropods whose characteristic three-lobed forms are among the most familiar images of ancient life. Researchers at the University of California at Riverside are using Fastrak

3-D tracking technology from Polhemus Inc. and Superlite for CADKey software from HighRES Inc. to create 3-D models of ancient trilobite fossils and of the modern-day horseshoe crab. Polhemus, located in Colchester, Vt., specializes in 3-D motion capture, tracking, and digitizing technologies; HighRES, in La Jolla, Calif., is a supplier of reverse engineering and 3-D digitizing software.

Trilobites have been called the timekeepers of life for the Paleozoic period, approximately 540 million to 245 million years ago. They existed throughout the Paleozoic period, especially in the earlier part of it, the Cambrian and Ordovician periods, when the oceans exploded with variety and abundance of life. Trilobites, the most abundant of all skeletonized Cambrian metazoans, are found around the world. They are often remarkably well preserved, allowing scholars to infer a great deal about them and about ancient life in general.

To learn about how trilobites interacted ecologically and biologically with other organisms, the UC Riverside researchers, led by Nigel Hughes, an associate professor in the Department of Earth Sciences, study similar creatures in modern environments and compare them with animals in ancient systems.

Brenda Hunda, a team member and graduate student, is examining trilobites as well as horseshoe crabs off Long Island to find out if the interactions between modern-day horseshoe crabs and their encrusters, or epibionts, could tell her something about the biology and ecology of encrusted trilobites.

Reverse engineering of trilobites adds to knowledge of the ancient world.

The team used the Fastrak system and a stylus in conjunction with HighRES Superlite software to create 3-D images of the trilobites and the crabs. Points were recorded along the carapace in x, y, and z coordinates. The computer software then extrapolated a grid between these points to create a framework from which to surface the image. This created a solid three-dimensional reconstruction of the carapaces. The same procedure was used for each area of encrustation on the exoskeleton.

There is a general relationship between horseshoe crab surface area and the surface area coverage by encrusters. Smaller horseshoe crabs have a smaller surface area and shed their exoskeletons more frequently. As a result, encrusters rarely appear on these exoskeletons.

According to Hunda, "Larger males have more surface area coverage by epibionts. In addition, the specimens with the most encrustation suggest they have reached a terminal molt— the final molt as they stop growing. We're not sure if trilobites have a terminal molt, and so these patterns of encrustation in horseshoe crabs may give us an idea if there is a terminal molt in trilobites.

"I have also learned that the encrusters on horseshoe crabs have preferred orientations and locations on the carapace," she continued. "They generally tend to occur on areas of high relief, and areas close to the margin of the exoskeleton. Areas without epibionts can tell us that portions of the exoskeleton were not exposed to encrustation—possibly because of burrowing by the horseshoe crab.

"We can look at encrusted surfaces on trilobites and make similar connections. Although we will never know for sure, areas that are not covered by encrusters may be areas that the trilobite hid under the sediment surface—suggesting a partial burrowing habit for the trilobite."

She continued, "Finally, I learned that I may be able to tell if the trilobites were encrusted during life or after they died. This is important because we need to establish the order of events before we can say something about the biology and ecology of the animal. What we learned from the horseshoe crab implies that the animal was alive and interacted with the encrusters, and we cannot tell that for sure from the fossil record alone.

"For instance, patterns in horseshoe crab encrustation tell us that living horseshoe crabs will not have encrusters on areas of their carapace that will limit their normal everyday behavior. You certainly don't want encrusting animals on your joints or your eyes because it will inhibit your movement and sight. Encrusters on these areas must have arrived after the animal died. With this information, we can try to tell if the fossils we have were encrusted while they were alive or after they died."