Archaeopteryx

Archaeopteryx:

archios – ancient

pteryx - wing

Archaeopteryx fossil with feather impressions

One of the most remarkable examples of a transitional fossil is Archaeopteryx, an animal of the late Jurassic period that exhibited characteristics of both reptiles and birds.  Its remains are bone and feather imprints found in slate deposits that are located in Germany.  When it lived, more than 150 million years ago, the area was a tropical coastal lagoon.  The Earth is always in transition.

Asymmetrical flight feathers indicate flight ability

Except for its prominent flight feathers on its arms Archaeopteryx strongly resembled a two-legged (bipedal) reptile of its time.  Its size was similar to that of a crow or robin.  It had a strong running ability, with large curved claws that could be used for perching and climbing.  Its snout was blunt and its teeth were needle-like points, much like those of modern-day insectivores.  The animal had a long, bony tail that was lined with feathers.  The skeleton of Archaeopteryx closely matched that of other small reptiles and tiny carnivorous dinosaurs of the period, except for the claw-bearing three long digits of its hands.  These were covered in asymmetrical flight feathers. 

Archaeopteryx lacks keel for large flight muscles

Flightless birds have symmetrical feathers.  The asymmetrical flight feathers of modern flying birds provide them with fine control over their wing configuration as well as providing the bird with forward thrust from the feathers on the outer portion of the wing.  In addition to having true flight feathers the wings of Archaeopteryx were about the same size as those of comparably sized birds of today.  There were also indicators this transitional animal from reptile to bird couldn’t have been a strong flier.  The most obvious reason is that it lacked a keel-like sternum, or breast bone, that serves as an anchor for the flight muscles of modern-day birds.  These powerful muscles can represent up to a third of a bird’s weight and they require a large attachment area which is lacking in Archaeopteryx.  Instead, this animal’s skeleton remains similar to other bipedal reptiles of its period, lacking a bird’s many fused bones and other refinements focused on flight. 

Small bipedal dinosaurs have skeletons similar to Archaeopteryx

Birds have reduced their pelvic bones into a single, fused structure.  Their tail is also reduced and the remaining bones fused into a structure called the pygostyle.  Where Archaeopteryx has three long boney fingers, tipped with claws, birds have a single rigid structure, although the three metatarsals – or finger bones – are evident in the embryonic stage of a bird.  Archaeopteryx also has a furcula, or fused clavicles – the ‘wishbone’ – providing attachment for the flight-related pectoralis muscles.  There are other anatomical indicators that give strength to the conclusion that Archaeopteryx is a precursor to the arrival of modern-day like birds that appear in the fossil record some twenty million years later in the early Cretaceous period.  A number of these birds retained small Archaeopteryx-like teeth in their bills. 

Cretaceous era birds retain teeth like Archaeopteryx

Up until a few decades ago it was widely believed that ancient reptiles were sluggish, cold-blooded animals.  There is evidence available today that make a compelling case for warm-blooded dinosaurs and for some of their closely related reptilian relatives, as well.  If this is true then many of these animals would require some form of insulation to retain their body heat.  Feathers appear to have evolved from reptilian scales and their ability to trap air would enable an animal to better regulate its body temperature.  With time, feathers would differentiate in purpose from warmth-protecting down to the elongated vanes of wing feathers.

Demands of flight require larger brains

Archaeopteryx would need a high metabolic rate for flight and that, in turn, would require the elevated temperatures that environment alone might not provide.  Whether or not the animal was warm blooded it is likely its body was completely covered with insulating feathers.  Flight, the powered movement through atmospheric three-dimensional space, makes demands on an animal’s brain that isn’t required for earthbound four legged animals.  The brain cavity of Archaeopteryx is significantly larger than those of other reptiles similar in size and contemporary to it.  The brain-size of modern birds has continued to grow in size as their flight abilities have become stronger and increasingly more controlled. 

Dinosaur similar to Archaeopteryx with feather insulation

Charles Darwin’s seminal work, published in the book On the Origins of Species in 1859, was quickly linked in its conclusions to the first finding of an Archaeopteryx fossil in 1861.  The fossil’s value was soon appreciated by some as an example illustrating the transformations life forms undergo to meet the challenges organisms continually confront with the changing geologic times.

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