Previously, I reviewed what we currently know about anole fossils - these fossils are preserved in amber, a fossilised tree sap/resin from Mexico and the Dominican Republic (like the one pictured right). Today, I want to share how I have been using high resolution x-ray computed tomography, a.k.a CT scanning to look at these fossils and so peer into the past.
Background to CT scanning Amber
CT scanning involves x-raying an object from many angles, and then compiling these x-rays to reconstruct 3D models of the object (more detailed description here). CT scanning works when the object being scanned is made of different materials that each absorb x-rays differently. Think of a medical x-ray; skin absorbs far fewer x-rays than bone, so the two show up as different shades of grey on the developed x-ray.
The inclusions in amber are usually subfossils, where organic material still remains (e.g., bone). This means there will be different materials with different x-ray absorption. Amber absorbs more x-rays than air (similar density to a plastic drinks bottle), but fewer than bone. I digitally remove the amber and make a 3D model of the fossil inside.
CT scanning is great technique for studying amber specimens because the x-rays do not damage the amber (no evidence of clouding), and it can be used to see inside even the most opaque of pieces. With this method, you can create see in great detail inclusion in the amber without any destruction to the piece.
Exploring Dominican amber using CT
To demonstrate how CT scanning is great for amber, I shall show a famous Dominican amber anole fossil, housed at the American Museum of Natural History. It was described by Kevin de Queiroz and co-authors in 1998, and was also featured in the Losos ‘Lizards in an Evolutionary tree’ book. The fossil is most likely a hatchling anole (27mm SVL). It died in a well laid out pose, lying on its stomach with its legs out stretched.
I chose to examine this specimen using micro CT to see what information we could glean from such a rare piece. To the left is a 3D model of the skeleton, shown from the back, side and belly (left to right).
I discovered that the inclusion in this amber fossil is remarkably well preserved – the whole skeleton is almost completely preserved, except for 3 breaks that cut thorugh the lizard that take out parts of its forelimbs.
Amazingly, the lizard is not squashed – it retains its 3D form as in life.
CT scanning has also shown me that there’s more to these amber fossils than skeleton. When I scanned some other amber specimens kindly leant to me from private collectors of Dominican amber, I found some very exciting things. Within the amber, there are often air pockets. Usually these are just air bubbles, perhaps due to gas escaping from rotting material encased in the amber. But sometimes, the air turns out to have shape!
When the air is contained within the body cavity, the skin leaves an impression on the amber, so we can see soft tissue in remarkable detail. To the right is an anole forelimb, where in addition to bone, we see the impression of the soft tissue as an air-filled void in the amber. Also, I occasionally see interesting invertebrate inclusions, such as the ant shown far right, which are also preserved as an air-filled void in the amber.
In the last few years, CT scanning amber has become increasingly popular (e.g. see here for a video of a spider in Baltic amber, and here for the surprising story of a mite that was hitching a ride on the spider!). Scientists are finding that there is a wealth of information encased in these little golden gems. Creatures that may otherwise not fossilize well can be caught in amber and preserved in remarkable detail. The future of anole fossil research certainly lies here – Watch this space!
If you have a lizard in amber, we’d love to hear from you! Please contact the author of this post.