For millions of years, the itty-bitty left forefoot of an ancient lizard has been preserved in a honey-brown amber capsule formed from tree resin. Measuring just 2 cubic centimeters in size, the foot belongs to a reptile of the Anolis genus, ancestors of modern anole lizards, and shows each minuscule detail under the microscope.

Now, a collaborative team of researchers is analyzing the Dominican Republic's “time capsule” as a way to understand what happens to an organism throughout fossilization.

Though the foot appears to be in good condition, scientists say that paleolithic looks may be deceiving. Its physical structure is largely the same as it had been between 15 and 20 million years ago, but much of the bone has decomposed and its chemical characteristics transformed since its Miocene encapsulation. The researchers say that this is odd given amber’s excellent composition as a preservative. Amber serves as a barrier between organisms that become trapped within it and the external environment, often preserving a plant or animal in a “three-dimensional, life-like posture”. Most organisms found in amber are insects that may have died as tree sap gradually rolled over them, hardening, and preserving them for eternity.

For an organism to become fossilized, it must first be embedded rapidly in the resin, which prevents other animals from eating it or microorganisms from decomposing it. Over time, the original components of the organism are gradually replaced by minerals.

“The preservation of organisms or parts of them over long geological timescales requires exceptional conditions before and after the death of the organism,” write the authors in PLOS ONE, adding that natural chemical processes that occur within the amber will sometimes deteriorate organic soft tissue of the preserved organism. 

Geoscientists from the University of Bonn working in collaboration with the German Research Foundation and the Stuttgart State Museum of Natural History examined the little lizard foot by preparing thin sections of the fossil to analyze under the microscope for further analysis using micro-computed tomography. Claws and toes on the foot were so clearly visible that researchers were able to determine that the left forefoot was broken in two places. One of the fractures was surrounded by slight swelling, an indication that the animal may have been injured by a predator before its death. The other fracture happened after the fossil was embedded in tree resin in the same places where a small crack in the amber occurred.

Very little is left of the original tissue and the bone’s elastic component has been degraded. In fact, the bone tissue has been transformed from hydroxyapatite, a mineral found in tooth enamel and bone, into a common phosphate mineral known as fluorapatite.

“This is surprising because we assumed that the surrounding amber largely protects the fossil from environmental influences,” said Jonas Barthel, a doctoral student at the Institute for Geosciences at the University of Bonn, in a statement.

It is possible that the crack may have allowed external solutions and conditions to influence the transformation. Barthel adds that mechanisms behind fossilization are currently the “subject of intense scientific investigation.”