Trapped in amber – from small vertebrates to dinosaurs

The potential of amber, the fossilized tree resin, to preserve remains of small animals has been known since the 1st century when the Roman author, naturalist, and philosopher Pliny the Elder described a piece of amber with a lizard enclosed in it. However, as recent discoveries show, amber can preserve a more diverse range of animals than we previously assumed, including the dinosaurs!

Amber can be found in many places around the world but only few of those places have yielded amber that originated during the Mesozoic. One of them is in Lebanon; another in Myanmar.

And it is the latter that produced one of the most spectacular fossils described recently. That amber, called burmite, was formed almost 100 million years ago, in the middle phase of the Cretaceous. In March 2016, a team of researchers led by Juan Daza described several pieces of amber with squamates trapped inside.

New technologies, enabling the scientists to study the slightest anatomical details of the tiny animals, revealed many important and unexpected traits. One of the specimens is a hind leg that looks almost exactly like the legs of modern “flying dragons”. Another is a leg of an extinct gecko – with clearly visible toe pads! That made the animal a good climber.

The smallest of those “amber lizards” had only a centimeter in length. Still, it is probably the most interesting of all of them. It had a short, blunt snout, short legs, big eyes, and a short, curly tail. All these traits suggest that it is the earliest known chameleon, predating the second oldest member of the chameleon group by more than 70 million years! Appartently, chameleons – and probably many other groups of squamates – evolved much earlier than it was previously thought.

Now, while it is perfectly imaginable that chameleons and other small squamates could have been trapped in tree resin, there were animals that we would not normally expect to be discovered in amber. Among those, finds of dinosaurs would rank as the least expected. However, as new discoveries show, there are still things that can surprise us.

Back in June 2016, a team of researchers led by Lida Xing from the China University of Geosciences described two mummified wings, each having less than two centimeters in length. The small size and development of the preserved bone fragments suggested that the animal was probably a hatchling. The wings most likely belonged to an enantiornithine, member of a clade of small, toothed birds that were very common in the Cretaceous. These fossils supported the hypothesis that enantiornithine hatchlings were able to move almost immediately after hatching. Their feathers were similar to those present in modern birds, suggesting that these small Cretaceous birds were able to fly actively.

A new study, published on December 8 and again by a team led by Lida Xing, was even more surprising. It reported on a dinosaur tail from the mid-Cretaceous (about 99 million years old) Burmese amber, the same amber that preserved the wings mentioned above. The tail belonged to a coelurosaur, member of a clade of theropod dinosaurs that also includes birds.

Even though the dinosaur was probably still very young, it already had an “adult” plumage. This type of primitive feathers has as yet been unknown in non-bird dinosaurs. The contour feathers of modern birds are composed of main shaft, called rachis, to which numerous branches, called the barbs, are fused. Barbs themselves are branching into barbules. The anatomy of this “amber dinosaur” suggests that barbules were present on barbs before they fused to form the rachis. That’s interesting especially from the evo-devo perspective as such morphology was not predicted by the studies of the feather embryonic development.

As the new studies show, fossils trapped in amber, preserved in three dimensions and with splendid details, can easily become an extremely important source of anatomical data, thus shedding new light on many burning issues regarding the evolution of life. Let’s hope that more such discoveries are to come!

Featured image © Chung-tat Cheung.
Photo of the dinosaur tail trapped in amber © R. C. McKellar, Royal Saskatchewan Museum.

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