The discovery of what is considered the first bird, Archeopteryx, took place in the already distant year of 1861. This magnificent specimen, which had both bird and reptile characteristics, was from the beginning considered revolutionary as direct evidence of the evolutionary process. One of Darwin’s colleagues, Thomas Henry Huxley published a study of Archeopteryx interpreting its primitive features as reptilian. We can even consider that he already suggested the dinosaurian origin of birds by pointing out the similarities between Archeopteryx, Megalosaurus, Compsognathus and Iguanodon. He even proposed a name for the lineage that included dinosaurs (as they were then conceived) and birds, a group he named Ornithoscelida .
Over the decades, fossil bones of bird-like dinosaurs turned up. And little by little, the line that separated dinosaur from bird began to blur. In 1969 John Ostrom published the theropod dinosaur Deinonychus . Deinonychus was a theropod dinosaur whose proportions were far from the image of slow and heavy dinosaurs, it was slender and graceful. It had long hind legs, perfect for running and jumping, topped by sharp claws, especially the claw of finger II, the “terrible claw” of raptors. Its arms were long and closely resembled the arms of birds, only topped, again, with sharp claws. In that sense, his arm was very much like Archeopteryx’s. The tail, very long and held high in a horizontal position, would help as a counterweight during running and jumping. The pubis, the hip bone that in birds appears pointing backwards but in the rest of saurischian dinosaurs still points forward, here it already acquires a position similar to that of the avian. We are facing a graceful and fast predator that had nothing to do with that image of silly and slow dinosaurs doomed to extinction. This finding played an important role in the so-called Dinosaur Renaissance, when we stopped seeing dinosaurs as obsolete beings and began to see them as active animals perfectly adapted to their environment that they were. And these little theropods began to be seen as birds…
The Liaoning sites in China became internationally famous in the 1990s for the discovery of dinosaurs. These deposits belong to the so-called “exceptional conservation” or Konservat-Lagerstätten in German, in which, due to their peculiar formation, invertebrates or the soft tissues of vertebrates can be found. The Solnhofen deposit in Germany or Las Hoyas in Cuenca, Spain, belong to this same type of deposit. In the case of Liaoning, the burial and preservation of this fauna is related to volcanism phenomena, which on the one hand caused abundant volcanic ash to be deposited, and on the other hand, produced mass mortalities due to its toxic gases. That must have been a catastrophe for the fauna of Liaoning, but it was lucky for paleontologists 130 million years later. Above all, when perfectly preserved pterosaurs and dinosaurs with their stomach contents or skin began to be found in its outcrops. The biggest surprise was the finding of non-avian dinosaurs with feathers , both primitive and more developed, on their skin. These dinosaurs belonged to several theropod lineages, allowing us for the first time to hypothesize about the evolution of feathers.
This is how, little by little, we learned that many of the characteristics that we thought were exclusive to birds had actually been inherited from their ancestors . Even its hollow bones, which allowed the great sauropod dinosaurs to become giants.
But surprisingly, in recent years we have learned that birds were not the only dinosaurs that could take flight. Two very closely related lineages, at the very least, were able to glide through the treetops.
One of them is a subfamily of the dromaeosaurs or raptors, the microraptorines . The first of them that was found, Microraptor, was already surprising for having imprints of remiges feathers on both arms and legs, in addition to the tail feathers. This group of small raptors would be able to use their two pairs of wings to glide through the trees of ancient Cretaceous forests.
And the second of these groups is even stranger: the scansoriopterygids . Other theropods closely related to raptors, which were not only feathered, but developed membranous wings . Thanks to the elongation of one of their fingers, and the development of a new elongated bone that started from their wrists, these theropods supported a membranous wing, similar to that of pterosaurs or bats . In life, they must have been the closest thing to a dragon that has ever existed. And they also must have glided or fluttered through the trees.
Undoubtedly, dinosaurs, and especially those closely related to birds, show us once again that evolution does not have a predetermined direction or meaning. And also that when we believed that the fossil record could not surprise us, it leaves us stunned. And again.
References:
Ostrom, J. H. 1976. Archaeopteryx and the origin of birds. Biological Journal of the Linnean Society, 8 (2): 91-182.
Ostrom, J.H. 1969. A new theropod dinosaur from the Lower Cretaceous of Montana. Postilla, 128: 1-17.
Xu, X. et al. 2003. Four-winged dinosaurs from China. Nature, 421(6921): 335-340
Xu, X. et al. 2015. A bizarre Jurassic maniraptoran theropod with preserved evidence of membranous wings. Nature, 521 (7550): 70-73.
