A bug in the x-ray system proves informative

Lynn Savage, Features Editor, lynn.savage@photonics.com

From its ancestral beginnings, it seems, the cockroach has been built to survive.

Fossils of Archimylacris eggintoni, a time-lost relative of the modern pest, are common enough and found all around the world. Finding intact specimens, however, is not easy, and so there are a number of mysteries surrounding the insect.

Using a partially intact specimen stored at the Natural History Museum in London and an x-ray computed tomography (CT) system, researchers at Imperial College London have created a 3-D virtual fossil that has revealed hidden features of the creature.

The modern relative of A. eggintoni.

When alive, A. eggintoni and its relations grew to about 2 to 9 cm long and 4 cm wide and roamed the floors of the primeval forests looking for decayed matter to feed on and a nice place to lay their eggs. The insects, which eventually evolved into termites and mantises as well as modern cockroaches, flourished during the Carboniferous geologic period from 359 million to 299 million years ago. The fossils they left behind typically show bodies and wings, but very few limbs or other small parts survive in the fossil record.

Using x-ray microtomography and digital reconstruction, Russell Garwood and his colleague Mark D. Sutton examined a protoroach that was a fossilized void inside siderite (ferrous carbonate). They used an x-ray CT system made by Metris NV (now owned by Nikon Metrology NV) and installed at the Natural History Museum to acquire two sets of 3000+ x-ray images: one of the entire fossil at a resolution of 22.5 µm and one of the anterior portion – to focus on the limbs – at a resolution of 17.7 µm. They report their findings in the April 14, 2010, online edition of Biology Letters.

X-ray computed tomography helped create a three-dimensional “virtual fossil” of Archimylacris eggintoni, the ancestor of mantises, termites and cockroaches. Courtesy of Imperial College London and the Natural History Museum.

With software developed by Sutton, they created virtual models of the fossil that showed features of A. eggintoni that have never been seen before. They found that the insect had claws at the base of its legs, which likely facilitated tree climbing, and adhesive limb structures called euplantulae that helped it walk across slick surfaces such as leaves. These specializations likely allowed the creature to lay eggs well away from predators.

The researchers also noted that the insect’s legs – fairly long and pitched at a low angle to the body – were built for running quickly across uneven ground, and that its mandibles were perfectly suited for its diet of decomposing plants and insects. In fact, old A. eggintoni wasn’t that different from its present-day cousins.