Artist impression of Enantiornithes. Credit: Raúl Martín.

A nearly complete skeleton of a baby bird that died shortly after birth shows scientists the nuances of millions of years of evolution during the Mesozoic era, according to a new paper in Nature Communications.

“It is amazing to realize how many of the features we see among living birds had already been developed more than 100 million years ago,” said Luis Chappe, one of the authors from the Los Angeles Museum of Natural History, in a statement.

The fossil, less than 5 centimeters long, dates from approximately 127 million years ago.

Its value lies mostly in that it died shortly after birth, showing the crucial postnatal biological development of the avian species during the age of the dinosaurs.

The skeleton was nearly complete – only the feet, most of the hands, and the tail’s tip were missing, they write.

The skull was partially crushed, and the bones were mostly disarticulated, they write.

The cranium evidence as it stands places the specimen in between the ancient Archaeopteryx, and the later Cerebavis, which is close to that seen in most birds that currently inhabit the planet, they write. That places the bird within the group of Enantiornithes.

The technique used to assess the tiny features of the fossil was propagation phase-contrast synchrotron microtomography, otherwise known as synchrotron radiation. The extreme level of detail extended down to the submicron level, they write.

“New technologies are offering paleontologists unprecedented capacities to investigate provocative fossils,” said Fabien Knoll, of the University of Manchester, one of the authors. “Here we made the most of state-of-the-art facilities worldwide including three different synchrotrons in France, the UK and the United States.”

The baby bird would not have been able to fly, since its sternum was still mostly made of cartilage. That would potentially mean the young chick was altricial – meaning it was reliant on parents for care, feeding and protection.

But the researchers added that the Enantiornithes may have had a more diverse set of strategies for early life – potentially meaning it showed other precocial traits, meaning newborns can be independent, like modern chickens.

“Comparisons between this new specimen and other known early juvenile Enantiornithes support a clade-wide asynchronous pattern of osteogenesis in the sternum and the vertebral column, and strongly indicate that the hatchlings off these phylogenetically basal birds varied greatly in size and tempo of skeletal maturation,” they write.

Evolution back to the Mesozoic Era – extending from approximately 250 million years ago until a mass extinction event about 65 million years ago – has been a focus of scientists from a diverse range of disciplines. Some biologists have even re-created dinosaur features in birds, from legs to reptile snouts, using processes of reverse-genetic engineering.