When lizards prove too fast, slippery or downright squirrely to catch using other methods, herpetologists will sometimes turn to glue traps to snare their quarry. This technique is particularly useful when trapping elusive scansorial species (e.g. Bauer & Sadlier 1992, Ribeiro-Júnio et al. 2006), but is also proving useful for those of us who study fossil species. Sticky tree resin provides a naturally occurring glue-trap, and inclusions in fossilized resin, or amber, show that this material has been snaring and preserving small animals for many millions of years. Amber fossils are particularly important for three main reasons. Firstly, they preferentially preserve small, delicate specimens that are rare in the fossil record by dint of being too fragile to survive the fossilization process. Secondly, because they tend to be formed in tropical forested environments where decomposition happens rapidly and deposition is limited, they often contain snapshots of ecosystems that are rare in the fossil record. Finally, they preserve specimens in startling detail, often retaining soft anatomy at a resolution that no other fossilization process is capable of.
The term “Amber fossils” inevitably brings to mind images of frozen mosquitos, stuffed to the eyeballs with DNA-rich dinosaur blood and, while it is true that an enormous amount of arthropod diversity has been recovered from amber fossils, lizards have also been found in fossilized amber, from three continents and across six horizons that span 100 million years. Last year this blog discussed a series of anoles in amber that were featured in an impressive study by Sherratt et al. (2015) which documented the presence of ecomorphs 20 million years ago in the Hispaniola and Mexico. These fossils allow us to see directly into the past, and in the case of anoles, it was possible to document the early morphological diversification into some of the five basic ecomorphs (although, not all of them fitted in such categories).
Earlier this month, a paper in Science Advances revealed the oldest amber lizard assemblage known to-date. This work is the result of teamwork between Ed Stanley, Phillip Wagner, Aaron M. Bauer, David Grimaldi and myself (Juan Diego Daza). We document the remains from 12 lizards that lived 99 million years ago in a tropical forest in an area of what is today Myanmar. Most modern lizards are pretty easy to identify to at least to family level–skinks have cycloid scales, geckos have toepads (not all), lacertoideans tend to have square scales, etc.–so naturally, you start looking for these traits in order to identify these animals. When I learned from my mentor, Dr. Richard Thomas, to use plastic bags in the field to collect lizards, these bags can be labeled and this makes sorting easy after a day of work. I remember the day that when we all met at the American Museum of Natural History with David Grimaldi; he started to bring plastic containers with small amber inclusions. It was like taking a Herpetology exam, and we kept asking each other, “what do you think it is?” You could only hear our wild guesses–gecko, skink, anguimorph, teiid, agamid–but none of us was entirely sure. This is because when these lizards were trapped, they looked very different to their modern relatives.
To determine the identity of these animals, we decided to look deeper, using a combination of light microscopy and high-resolution Computed Tomography (CT) to extract as much data from the specimens as possible. For some specimens, there were no bones left, so we had to generate models of the void space inside the fossil. The morphological data gathered allowed us to include the most complete specimens into a series of phylogenetic analyses and helped us determine that two of these lizards were closely related to Gekkota and Chamaleonidae. Although many of the press releases keep referring by approximation that we described the oldest chameleon, this fossil fits more as a transitional form toward the evolution of chameleons, in the same way that Archaeopteryx is not part of the modern bird clade. The same situation occurs in one of the geckos found in this amber rocks. Despite having a distinctly modern appearance, the specimen was recovered outside of the seven families recognized within crown Gekkota.
So, what’s special about these fossils? After all, these are not the first Cretaceous lizards ever described. First of all, the diversity of the specimens helps inform us about the poorly known small-vertebrate diversity of the ancient paleotropics. These fossils also provide exquisite detail preserved in both soft and hard tissue. The scales (sometimes pigmentation), claws and even the tongue of one specimen can be seen through the sherry-orange colored fossils. The skeleton and even the surface of the specimens can be visualized using CT Scans, which is an impressive imaging technique that provides high-resolution images with no distortion of these animals. Check this paper out and let your mind travel into ancient times to see how lizards were 99 Million years ago.
Bauer, A. M., and Sadlier R. A. 1992. The use of mouse glue traps to capture lizards. Herpetological Review 23:112–113.
Daza, J. D., Stanley, E. I., Wagner P., Bauer A. M., and Grimaldi D. A. 2016. Mid-Cretaceous amber fossils illuminate the past diversity of tropical lizards. Science Advances 2016:2.
Ribeiro-Júnior M. A., Gardner T. A., and Ávila-Pires T. C. S. 2006. The effectiveness of glue traps to sample lizards in a tropical rainforest. South American Journal of Herpetology 1:131–137.
Sherratt, E,, Castañeda M. D.R., Garwood R. J., Mahler D. L., Sanger T. J., Herrel A., de Queiroz K., and Losos J. B. 2015. Amber fossils demonstrate deep-time stability of Caribbean lizard communities. Proceedings of the National Academy of Sciences of the United States of America, 112(32): 9961-9966.