Many people have become familiar with the concept of amber over the past few decades thanks in part to one of the biggest dinosaur themed movies ever, Jurassic Park. This movie brought not only dinosaurs, but also the idea of animals trapped in tree resin, into the spotlight. Fossilization by tree resin is not the most common way to be preserved, but when it does happen, the preservation of the animals inside is near perfection. Oftentimes they can retain a detail not otherwise found in other forms of fossilization. Through such exceptional preservation a lot can be learned from these animals, including linking them to modern day relatives. In a recent article published in Science Advances, collected specimens of tropical lizards have been brought together to construct and refine the evolutionary lines that gave us many species of lizards, chameleons and geckos.
Amber is the hardened and fossilized form of what used to be tree resin, not sap. For amber to form, a specific environment and special circumstances must occur. The resin must be resistant to several factors, or be in an environment that excludes them altogether. Such factors include decay by microorganisms such as bacteria or fungi, weathering, and severe temperature changes. Sometimes the sweet resin, before hardening, attracts insects and other animals, which then become trapped in the sticky substance. On rare occasions larger animals can be caught, such as birds, lizards or even small mammals. Things trapped in resin are also not limited to animals. Other plants, pollen, feathers, dirt and more can become trapped as well, and make for equally interesting studies. Once trapped in the resin, whatever parts of these animals or objects that is covered becomes preserved over millions of years. What this means is that all tissues that are inside the resin do not degrade. The animal or object retains all details and quality it had when it became ensnared in the resin. Finds like lizards where colors, scales and body shapes are preserved give incredible insight into the evolutionary history of that animal.
The specimens that were looked at in this particular study were excavated in Myanmar (Burma) at the Albian-Cenomanian boundary by Juan Daza and his team. Here the team collected 12 specimens radiometrically aged at 99 million years, putting them in the Mid-Cretaceous. This Burmese amber, also called burmite, is some of the oldest amber collected with such high quality specimens. The lizards are mostly a collection of disarticulated body parts or bones, and not complete organisms. Many of the 12 findings were only tails, feet or skin. It has been determined that these lizards are examples of organisms that survived the transformation of a landscape from a coniferous one (trees with cone-like seeds and needle-like leaves) into one of broad leaves (such as those found in modern tropical rainforests). Because they survived this change the species found are incredibly diverse, suited for a wide variety of microenvironments within a tropical climate.
All lizards belong to the order of Squamata, including chameleons and geckos. Squamates are scaled reptiles that include not only lizards, but also snakes. The lizards found were also included in this order, and some provided basal forms of reptiles we see today. One specimen appeared to be the ‘missing link’ between ancient lizards and modern chameleons. Its toes are not fused like that of modern chameleons who exhibit an almost mitten-like foot form, but instead still separately articulated. The anatomical features that do link it to modern chameleons are the bones in the face, most importantly the lingual process and the orbitals. The lingual process is the way in which the animal uses its tongue. For humans we use the hyoid bone, and in chameleons the process is similar. A formation of small bones along with muscles propels the tongue forward when they are feeding in a motion that is referred to as ‘ballistic’. This fossil lizard has similar bones in its throat that would mimic the same process. The lizard also has large eye sockets that suggest eyes that may have functioned separately from one another, much like that of a modern chameleon. It is important to note that no stem forms of the chameleon have previously been reported, and this is the first of its kind. Although no chameleons currently live in Myanmar, fossil evidence suggests that these lizards once roamed the tropical regions of this land.
The second discovery that marked this excavation as unique was a stem form of early geckos. Also Squamates, these reptiles are well known for their sticky pads that enable them to climb nearly any surface, including glass. Their pads are not actually sticky but are instead specialized scales called lamellae that use Van der Waal’s forces to cling onto surfaces. The fossil gecko was found to have toe pads very similar to extant species of geckos, showing that these specialized pads had already evolved more than 100 million years ago. Other fossil records show that these pads have independently evolved in many species of geckos separate from one another, and have also been lost in evolution. Not all modern geckos retained this feature, such as the leopard gecko. The presence of these sticky pads in this fossil lizard has called into question the time at which these pads developed. What this fossil suggests is that the evolution of these sticky pads far outdates any previously found fossils, which has created controversy among scientists regarding what a new phylogeny would look like.
All of the specimens found in this excavation were used to help develop their respective phylogenies, leading to a problem common among scientists. It is not often that physical evidence can be provided for ancestors of species. When we look at dinosaurs 99% of the time we only have the fossilized bones to look at, not skin, scales, or flesh. These burmite preserved animals give us physical characteristics of lizard ancestors, which oftentimes helps to create accurate phylogenetic trees. On the other hand, scientists also have molecular evidence in living reptiles that disputes this evidence. Both are legitimate ways by which to create a phylogeny, but more often than not the phylogenies will look very different. It is difficult to settle on a ‘correct’ phylogeny with so much evidence coming both ways. Hopefully with more discoveries and the advancement of technology these problems can be resolved, but for now it is uncertain which family tree is more accurate, not only with these lizards but with many phylogenetic trees.