Is it a Bird?

Introduction

Archaeopteryx (discovered in 1861) is perhaps one of the most famous fossils and for many years was considered to be a classic intermediate between theropod dinosaurs and birds and evidence that birds evolved from dinosaurs.

In the 1970s John H. Ostrom of Yale University, compared the skeletons of Archaeopteryx and the theropod Deinonychus and because of the many similarities he concluded that birds are the direct descendants of theropods.

These two-legged reptile-like creatures are found in the Jurassic and Cretaceous rocks (which according to evolutionists are 175 to 65 million years old).  Two well know examples are Velociraptor and Deinonychus, which were running predators with a large forward-facing killing claw on their feet.

The discovery of small theropod fossils in China with “dino fuzz” (believed by some to be proto-feathers) during the 1990s and more recently other similar creatures with feathers is now presented as confirmation that birds evolved from theropod dinosaurs.

“Dino-bird” fossils

Two of the first Chinese “dino-birds”, Sinosauropteryx and Protarchaeopteryx were reported in 1996 and 1997, respectively.

The turkey-sized Sinosauropteryx, had fringed, filamentous structures along its backbone and on its body surface which are thought to be precursors to feathers.
Sinornithosaurus (reported in 2001) had filamentous integumental appendages which are composed of multiple filaments.

Caudipteryx appears to have had feathers on its tail and arms and was similar to secondarily flightless birds.

The order in which these fossils appear in the fossil record is however not consistent with an evolutionary progression, since Archaeopteryx, which had asymmetrical flight feathers and was clearly able to fly is, in evolutionary terms, older than all of them.

This temporal paradox existed for many years until the discovery of creatures like Anchiornis and Aurornis, which are claimed to have had less well-developed feathers than Archaeopteryx and therefore resolve the ‘temporal paradox’ since they are considered to be “earlier” than Archaeopteryx.

Creatures with four wings

Some feathered theropods appear to have had four wings.

The first of these four-winged, long-tailed flying raptors (Microraptor) was reported in 2003.

Changyuraptor was another four-winged creature from the Early Cretaceous of China.  It had tail feathers which are nearly 30 cm long, roughly 30% the length of the skeleton.

These are examples of small four-winged, long tailed theropods also known as “micro-raptorine birds”.

What is a bird?

The way in which we interpret these fossils depends on the definition of a bird.

All modern birds have the following features:

  •       Asymmetrical flight feathers
  •       A pygostyle
  •       A sophisticated respiratory system connected to airsacs
  •       Light weight, air-filled bones
  •       A brain with enlarged cerebellum and visual cortex (the avian brain)

Archaeopteryx also had airsacs, air-filled bones, an avian brain and asymmetrical flight feathers, but it had more in common small theropods than modern birds.  These features include jaws with sharp teeth, three fingers with claws, a long bony tail, a hyper-extensible second toe ("killing claw") and various other skeletal characteristics not found in living birds.

All living birds have a pygostyle which is a blade like bone at the end of the tail and there are some feathered fossils with a rod-like pygostyle, such as Confuciusornis and Sapeornis.

Confuciusornis is a Cretaceous flying bird with a horny beak.  The size and shape of the humerus and the keel of the sternum shows that it would have been able to fly.

Sapeornis also had a beak and a pygostyle.  However, these creatures were different from living birds as among other things they all had unfused wing (hand) bones and claws on their wings.

Living birds have a skeleton designed for powered flight with a rigid airframe (rib cage, shoulder blade, furcula and coracoid).  In addition, the coracoid has an acrocoracoid process which guides the supracoracoideus tendon through a pully like system and enables birds to lift their wings high over the back and produce a powerful lift-generating downstroke.  Creatures such as Archaeopteryx and the microraptors did not have this pulley system and would not have been powerful flyers.

Feathers

It is clear that some of the fossil theropods had pennaceous feathers with a stalk or quill similar to most living birds.  However, there has been some disagreement about whether or not some of the integumentary structures seen on some fossils were proto-feathers.

Examination of specimen of Sinosauropteryx suggested that the integumental structures proposed as proto-feathers are the remains of structural fibres that toughened the skin.  The conclusion that these are not proto-feathers is based on the observation that the fibres show a striking similarity to collagen found under the skin in many creatures which are not birds.

On the other hand, the discovery of quill knobs on a Velociraptor forearm is good evidence that Velociraptor had feathers.  Caudipteryx also appears to have had feathers.  But neither of these creatures could fly.

Discussion

It is clear that some theropod dinosaurs had feathers and some could fly.

We would suggest that the fossil record shows the existence of different kinds of feathered creatures.

Flying feathered theropods

The flying feathered theropods had the following features:

  •        Long bony tails
  •        Teeth
  •        Wings with claws
  •        No acrocoracoid process and no supracoracoideus pulley

Archaeopteryx is a good example of such a creature.

Four winged gliders

Some feathered theropods had four wings and long feather covered tails.  Microraptor and Changyuraptor are good examples.  Although these creatures did not have the well-developed acrocoracoid process and the supracoracoideus pulley, the additional aerofoils on the legs and long tails would have enabled them to glide effectively.

Non-flying feathered theropods

Other fossil theropods with feathers were clearly non-flying because, among other things, they were too big to fly.  These would probably be secondarily flightless creatures which grew much larger and were similar to modern running birds.  Examples include Caudipteryx and possibly Velociraptor.

Birds

Birds (fossil and modern) are those creatures with the following features:

  • Pygostyle
  • Acrocoracoid process and supracoracoideus pulley
  • Keel like sternum (not always)
  • Fused “fingers” (not always)
  • Beak with no teeth (not always)

Conclusion

The above classification is consistent with the theory that the feathered theropods and fossil birds are representatives of the original created kinds and their direct descendants.

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