Ancient Arachnids Articulated

Rebecca C. Jernigan, rebecca.jernigan@laurin.com

Some people are fascinated by them. Many are terrified of them. Yet few can deny that spiders play an important role in the environment as well as in popular culture. But how much do we truly know about where these creatures came from?

Investigators at Imperial College London are trying to expand our knowledge of some of the early ancestors of modern arachnids by enabling others to see, in three dimensions, the details of the creatures’ structure. They hope that revealing the physical traits of these ancient animals will enable scientists to learn more about the way they moved and hunted.

The Cryptomartus hindi is shown from the left, center and right. The scientists believe that its angled front legs indicate that it was an ambush predator, grabbing its prey before killing it. Images courtesy of the Natural History Museum and Imperial College London.

Using a CT scanner, the researchers took 3000 x-rays each of two fossils – a specimen of Cryptomartus hindi and one of Eophrynus prestvicii, both of which date from the Carboniferous period 312 million years ago. The researchers used the resulting slice images to create a threshold image, marking every pixel below a certain gray level as part of the fossil. They manually removed the noise – and identified the anatomical parts – in each slice, allowing them to create renderings of the legs separately from the body. The slices were compiled into 3-D models, all using custom-designed software.

The Eophrynus prestvicii, pictured from left and right, had long legs for chasing down prey on the forest floor. The new images reveal that it also had spikes on its back, perhaps as a defensive mechanism.

The fossils had not previously been imaged with computed tomography because of the iron-rich siderite nodules that contain them. To overcome the material’s resistance to x-rays, the team used high-energy rays with long exposure times.

As for the future of this technique, said Russell Garwood, lead author of the paper, “we’re applying it to many different arthropods from the same time period, including insects, millipedes, other arachnids, scorpions and many of the other creepy crawlies that lived on these ancient forest floors. We’re hoping eventually to get a good overview of this entire community of animals.”