Wednesday, 20 July 2016

Cretaceous Birds From The Amber Time Machine

The phrase amber time machine, first used in an eponymous documentary presented by David Attenborough, perfectly captures the palaeontological importance of preserved tree resin. The views into the past offered by amber are perhaps the most detailed in existence and certainly some of the most extraordinary. From intricate predatory battles to intimate mating embraces, the amber time machine has presented a rich variety of snapshots of the past. Discarded vertebrate fragments, such as shed bird feathers or reptile skins, have been found in amber. Larger fragments and organisms on the other hand are rare as most vertebrate species are typically too large to be trapped in blobs of resin or are at least large enough to wriggle free.

MicroCT scan data of the two specimens, showing
the remarkable preservation of the skeletal anatomy
Small lizards have been recorded on a number of occasions, but recently two bird wings were discovered in Burmese amber from the Angbamo site in the Katchin Province of the country. One pair probably came from a dismembered individual, discarded by a predator or scavenger trying to avoid consuming the larger wing weathers. Claw marks in the amber and evidence of decay projects in the other specimen, however, suggest that the individual was engulfed while still living.

The bird was most likely a hatchling - a theory backed by the osteology of the specimens. The digit proportions and arrangement themselves indicate that these hatchlings belonged to a now extinct group of birds known as the enantiornithes. At 98 million years old, these specimens were not primitive enantiornithes, but their juvenile anatomy makes precise placement within the group's phylogeny difficult. The association of the three dimensional preserved feathers with the skeletal remains - something never seen before in the fossil record - offered the researchers other angles of analysis.

The remarkable association of the feathers with skeletal remains in three dimensions.
The red arrows indicate the position of the claws. The scale bar is 2.5 millimetres
The profiles of the rachis (the central supporting quill of the feather), the degree of interlocking of the barbules to confer rigidity, and the asymmetry of the overall feathers suggests that they were used for powered flight.

In turn the angles between the rachis and the barbs of the feathers are consistent with advanced flying birds, making it likely that these hatchlings were more advanced enantiornithes.

Intriguingly, the developmental characteristics of the feathers were closer to those of adults than those of juveniles, suggesting that feather development in enantiornithes skipped the downy juvenile stage seen in modern birds. Most incredibly, the original colour patterns are preserved in fantastic detail without the need to resort to the SEM analysis of melanosomes, used in previous studies of feather colouration in fossilised birds and dinosaurs.

While not paradigm-changing, these bird wings preserved in amber are truly remarkable due to the wealth of information they contain. Rarely do fossils give us such a complete view of vertebrate anatomy and morphology, and in virtually all cases, such fossils are preserved as flat films, like the spectacular theropods and avians from the Liaoning fossil beds. 3D preservation of such detail, however, is known only from amber and even amongst amber, these specimens are unique. This particular view through the amber time machine is nothing short of astonishing.