Archaeopteryx

Evolution of the Pectoral Girdle

Because most images of important fossils such as Archaeopteryx (see below) are protected by copyright, I have structured parts of this section as a do-it-yourself project.

Fig. 1 - Click for larger image.

Ten specimens have been discovered since 1861. All are from the Solnhofen deposits in Germany. They have been assigned to two species and appear to be closely related to the partial fossil of Rahonavis from Madagascar.

Step 1

"Google" images of Archaeopteryx (considered the earliest ancestor of birds) and images of small, feathered dinosaurs such as Microraptor (see right), Scansoriopteryx, Buitreraptor, etc. or browse the Dinosauricon and similar websites:

http://dino.lm.com/images/
http://homepages.ucalgary.ca/~longrich/archaeopteryx.html

Step 2

Look for structures that indicate that Archaeopteryx is not just another feathered dinosaur.

Note: Archaeopteryx Specimen #10 (Wyoming Dinosaur Center, Thermopolis WY) is particularly well preserved. Its features suggest that the Archaeopteryx was more similar to the dromaeosaurs than previously thought. This specimen shows the characteristic erectile claw on the foot and typical raptor-like teeth (Gerald Mayr e.a. Science 2 Dec. 2005 310(5753):1483-1486).

Perhaps the most important characteristic of Archaeopteryx is that it’s fossils are 10-25 million years older than those of any other feathered dinosaur.

Step 3

Look for large bones in the body to which these animals might attach flight muscles for the wings.

Note: In general Archaeopteryx and the other small feathered dinosaurs have elongated, lizard-like bodies. There is little in the way of skeletal architecture at the front end of the body that compares to the hipbones or tailbones that anchored the muscles for the hind limbs. Like other theropods Archaeopteryx had a simple furcula (wishbone) and long blade-like scapulas (shoulder blades).

Not only are there no large bones for the attachment of flight muscles but there is a lack of stiffening structures that would have helped Archaeopteryx maintain its posture during flight. J.M.V. Rayner has suggested that it was capable only of continuous vortex flight, i.e. it needed a raised platform to take off but was able to fly once airborne (ref 1, 2). Many modern birds use the more specialized vortex-ring flight to take off. They reach forward and up with their wings to increase the lifting surface and generate a powerful downward thrust. Archaeopteryx seems to have lacked the joint structure to support such movement.

Recently, Nick Longrich has published a paper that describes long, aerodynamically shaped feathers on the thighs of Archaeopteryx (ref 3). They make Archaeopteryx appear much more like Microraptor and perhaps these two animals shared a similar flight style. In neither case is their any suitable attachment for muscles that would allow the animal to “flap” its hind legs.

References

1. Rayner, J.M.V. 2001. On the origin and evolution of flapping flight aerodynamics in birds. Pages 363-85 in J. Gauthier and L.F. Gall (eds.). New perspectives on the origin and early evolution of birds: Proceedings of the international symposium honoring John H. Ostrom. Peabody Museum of Natural History, Yale University, New Haven, CT.

2. Gatesy, S.M. and D.B. Baier. 2005. The origin of the avian flight stroke: a kinematic and kinetic perspective. Paleobiology 31:382-399.

3. Longrich, N. 2006. Structure and function of hind limb feathers in Archaeopteryx lithographica. Paleobiology 32: 417-431.

Modern Birds and Dinosaurs

Although modern birds still share some basic architecture with dinosaurs, the specialized modes of locomotion found in all living birds have made them very different from their ancestors in three important ways:

Sensory reception and neurological control

Birds have exceptionally large brains that they use to integrate the flood of information arriving from equally large eyes. Obligatory pedestrians, such as dinosaurs, do not need such a sophisticated optical system or such large brains. A relatively large brain is one of the few characteristics that distinguishes Archaeopteryx from other feathered dinosaurs. Even among modern birds, pedestrians such as ostriches or chickens have brains that are relatively small compared to those in flying species.

Forelimbs and the pectoral girdle

In modern birds the forelimbs are specialized as wings and the pectoral girdle is adapted for flapping flight. Those adaptations include the large keel on the breast for the attachment of flight muscles, the unique triosseal gap in the shoulder that acts as a pulley so that the supracoracoideus muscle can raise the wing when it contracts, and a specialized shoulder joint that receives the head of the humerus. The unique shape of the head of the humerus allows the bird to make exceptionally deep down strokes but it also allows the bird to rotate its wing forward to increase the area of the lifting surface. Both feathered types of dinosaur and the early types of bird lacked some or all of these adaptations making it very difficult to draw a line between ancestral birds and feathered dinosaurs.

Hindlimbs and the pelvic girdle

Modern birds have a three-part hind leg moved by muscles in the thigh. Its femur is short and thick and does not normally rest in a vertical position. They also have wide hips that form a broad abdominal vault with no skeletal support for the lower abdomen. Finally the tail skeleton is short and its musculature does not have a major role in the movement of the hind limb. Dinosaurs and early birds had narrow hips that supported a two-part hind limb. The femur was long and thin and typically rested in a near-vertical position beneath the hipbones. It was moved by muscles that lay along the long reptilian tail. Beneath the hips, the abdomen was supported by the fused tips of large pubic bones.