An international team of researchers worked at a site in northern Myanmar when he discovered the skull of a strange and tiny animal trapped in a piece of amber. The same size as a hummingbird, the smallest bird in the world, corresponds to a type of dinosaur hitherto unknown, baptized by its discoverers as Oculudentavis khaungraae .
This is the smallest species of the entire Mesozoic Era and its finding, published Wednesday in Nature , provides new clues about the evolution of the ancestors of the first small birds.
Resin-enclosed fossils provide unique opportunities for researchers to study the anatomy of animals that lived in the remote past, as they retain tissues that do not last in other sediments. "Burmese amber creates particularly large pieces, which are capable of preserving vertebrate animals, much larger than insects, which appear much more frequently in resin," says Jingmai O'Connor of the Beijing Institute of Paleontology, first author of the work.
In the case of the skull described in the article, he was trapped about 99 million years ago. Its name, Oculudentavis , means 'bird with teeth and eyes', and reflects the most notable features of the animal: a serrated jaw and eyes similar to those of modern lizards. Scientists have been able to deduce their characteristics from a single bone thanks to the analysis of fluorescence by synchrotron radiation; that is, studying the radiation generated by charged particles within a magnetic field. In this way, they have been able to reconstruct the shape and size of the eye, which suggest daytime customs.
On the other hand, each of his jaws would have had about thirty teeth. Despite the small size of the Oculudentavis (its skull measures 7.1 mm), this suggests that it was a predator and that it probably fed on small arthropods or invertebrates. That makes an important difference with modern birds of a similar size, which lack teeth and feed on flower nectar.
"We only have the head, so the interpretation of the fossil's evolutionary position is still very provisional," says Luis Chiappe, a researcher at the Los Angeles Natural History Museum and co-author of the work. "For example, feathers were found in the same piece of amber, but we still can't tell for sure if they belonged or not."
The first birds
It is believed that the birds first appeared about 150 million years ago , during the end of the Jurassic period, one of the temporal divisions of the Mesozoic era. They evolved from small predatory dinosaurs, similar to the velociraptor, and thrived throughout the following period - the Cretaceous -, driven, in part, by the decline of the dinosaurs. When an asteroid hit the Yucatan Peninsula, 66 million years ago, they would have diversified enough for some species to survive the mass extinction that followed the cataclysm.
The authors suggest two possible relationships between the Oculudentavis and those first birds: the first is that it could have belonged to the enantiornites, the most common group of Cretaceous birds (between 145 million and 66 million years ago). The second is that it is more closely related to dinosaurs, being halfway in the evolutionary tree between birds and the Archeopteryx , the iconic winged Jurassic dinosaur, which would be the ancestor of all birds and lived 150 million years ago .
Difficult classification
In any case, the phylogenetic analysis, which helps to understand how a new species fits with others, places it as very primitive. This analysis suggests that it would be only slightly more evolved than Archeopteryx, and that, like him, it would have had a long bone tail. Although its discoverers believe that it was more bird than saury. "The skull is too weird for an analysis to really say where it fits," says O'Connor, "in reality, I don't think it was so primitive and I suspect it was probably an enantiornithine bird."
In any case, the discovery represents a new piece to understand the evolution of birds, showing that the miniaturization of their bodies occurred very early in their evolution. Dinosaurs had a body size much larger than that of birds; the smallest saurus weighed more than two hundred grams, while the smallest live bird, the hummingbird zunzuncito ( Mellisuga helenae ), only two. Deciphering the evolutionary path that led from one family to another has an important scientific value, since body size affects life span, food needs, sensory abilities and many other basic aspects of biology.
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