Question

Explain (in essay format) how birds are the last surviving dinosaurs. In particular, describe the evolutionary trends of the dinosaur lineage that gave rise to birds that were exaptations for the transition to flight.

Answer 1

By the start of the Jurassic Period Dinosaurs had become the global superstars of the animal kingdom (201 million years ago) and they held the top carnivore and top herbivore spots in food chains.

The beginning of birds

The evolution of Birds began in the Jurassic period from a group of meat eating dinosaurs called theropods dinosurs named Paraves. Tyrannosaurus rex belonged to the same group, although birds evolved from small theropods, not huge ones like T. rex. Modern phylogenies place birds in the dinosaur clade Theropoda. Birds are categorized as a biological class, Aves. The ancient birds looked quite a lot like small, feathered dinosaurs. The small theropod dinosaur Archaeopteryx lithographica was considered to have been the earliest bird. Their mouths still contained sharp teeth. But over time, these birds lost their teeth and evolved beaks.The oldest bird fossils are about 150 million years old.

The end of dinosaurs' rule: the Cretaceous extinction

Huge asteroid strike and massive volcanic eruptions caused disastrous changes to the environment and he reign of the dinosaurs came to an abrupt end. Most dinosaurs went extinct and only birds remained.These birds evolved in many ways, which enabled them to survive in lots of different habitats. Today there are at least 11,000 bird species.The birds survived from huge asteroid strike and massive volcanic eruptions because of their small size, ability to eat lots of different foods and ability to fly.

Evolutionary trends

The ancient birds looked quite a lot like small, feathered dinosaurs. The evolutionary trend among birds has been the reduction of anatomical features to save weight. The bony tail was the first anatomical feature disappeared and  reduced to a pygostyle and the tail function taken over by feathers. Confuciusornis is an example of their trend. While keeping the clawed fingers, perhaps for climbing, it had a pygostyle tail, though longer than in modern birds. The loss of a long tail was followed by a rapid evolution of their legs which evolved to become highly versatile and adaptable tools that opened up new ecological niches. Enantiornithes evolved into ecological niches similar to those of modern birds and flourished throughout the Mesozoic. Though their wings resembled those of many modern bird groups, they retained the clawed wings and a snout with teeth rather than a beak in most forms.

The modern toothless birds evolved from the toothed ancestors in the Cretaceous. Meanwhile, the earlier primitive birds, particularly the Enantiornithes, continued to thrive and diversify alongside the pterosaurs through this geologic period. All but a few groups of the toothless Neornithes were also cut short.

The Cretaceous saw the rise of more modern birds with a more rigid ribcage with a carina and shoulders able to allow for a powerful upstroke, essential to sustained powered flight. Another evolution was the appearance of an alula, used to achieve better control of landing or flight at low speeds. They also had a more derived pygostyle, with a ploughshare-shaped end. An early example is Yanornis. Many were coastal birds, strikingly resembling modern shorebirds, like Ichthyornis or ducks like Gansus. Like the Hesperornithiformes (a group of flightless divers resembling grebes and loons) many of them evolved as swimming hunters,.Most of these birds retained typical reptilian-like teeth and sharp claws on the manus The surviving lineages of birds were the comparatively primitive Paleognathae (ostrich and its allies), the aquatic duck lineage, the terrestrial fowl and the highly volant Neoaves.