An Odd Bird

I recently read Pat Shipman's delightful book Taking Wing, in which she discusses Archaeopteryx, its beautifully preserved fossils, and controversy around its place in the history of avian development. I wish to add a few morsels to that speculative stew:

For any reader who does not already know: Archaeopteryx ("Ancient Wing") was a creature of late Jurassic time (about 150 million years ago). Known to us from a half-dozen complete or partial skeletons discovered in Bavarian limestone quarries, Archaeopteryx shows a perplexing mix of bird-like and dinosaur-like features. Anatomically, Archaeopteryx closely resembles small bipedal dinosaurs like Compsognathus; however, Archaeopteryx has wings, structured much like wings of modern birds, with feathers preserved in amazing detail by fine-grained Solnhofen limestone.

Interrelated elements of dispute about Archaeopteryx include:

• Could Archaeopteryx fly?
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• If Archaeopteryx could fly, how did it evolve the equipment and ability to do so?
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• What creatures were ancestors of Archaeopteryx?
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• What place does Archaeopteryx hold in the lineage of modern birds?

Archaeopteryx lived and died long before there were humans to observe it. All our knowledge of this creature derives from inspection of and inference from the fossil record. I will not here repeat Pat Shipman's presentation of data and arguments; however, briefly to summarize a few points:

• Compsognathus fossils also occur in Solnhofen limestone. Archaeopteryx's skeleton is so similar to Compsognathus that at least one specimen was mis-identified until its feathers were observed.
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• At least three groups of large animals have evolved flight capability: pterosaurs, bats, and birds. Archaeopteryx's wing structure resembles that of birds, and is dissimilar to those of pterosaurs and bats.
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• While Archaeopteryx has avian wing structure and modern-looking flight feathers, it lacks other features of modern birds, e.g.:
  • Beak
  • Bird-like tail
  • Keeled breastbone for attachment of flight muscles
  • Shoulder structure to permit raising wings over its back
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• Lacking modern avian shoulder structure, Archaeopteryx may not have been able to rise from the ground in still air.
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• Anatomic and aerodynamic analyses suggest that Archaeopteryx was a mediocre flier, perhaps comparable to some modern shore birds.
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• Based on the geologic strata in which its remains are found, Archaeopteryx probably lived in a coastal environment, with few if any large trees.

Ms. Shipman concludes -- and I agree -- that:

• Archaeopteryx could fly, perhaps not well
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• Archaeopteryx evolved from a small bipedal dinosaur like Compsognathus
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• Archaeopteryx probably is a forerunner (foreflier?) of modern birds

How did Archaeopteryx come to be? That question now has no clear answer. In general, opinion favors two competing hypotheses: Trees-down or Ground-up.

The Trees-down hypothesis proposes that Archaeopteryx originated as a tree-dwelling or tree-climbing dinosaur that evolved ability first to glide -- like flying squirrels -- then to fly. This hypothesis presents a logical sequence of development: inevitably, some Archaeopteryx ancestors fall from their arboreal home; those with equipment to slow their fall (e.g. elongated forelimbs and fluffy feather-like insulation) survive more often; in time, natural selection favors development of controlled flight. However: it's difficult to picture how a Compsognathus-like creature could climb a tree, and no other likely arboreal ancestor has yet been found; Archaeopteryx fossils show little evidence of adaptation for -- or wear from -- tree-climbing, and Archaeopteryx's habitat likely had few trees.

The competing Ground-up hypothesis contends that Archaeopteryx's ancestor was a small ground-dwelling bipedal dinosaur (Theropod) like Compsognathus that somehow learned to fly. The Ground-up hypothesis explains Archaeopteryx's resemblance to terrestrial dinosaurs; however, no one has yet proposed a believable mechanism by which a ground-dwelling Archaeopteryx developed flight. For aircraft and birds, takeoff is the most demanding period of flight. It is easy to see that, once Archaeopteryx is airborne, natural selection favors improving flight skills; however -- as an evolving organism -- how did Archaeopteryx make the first leap into air? Without an avian skeleton and musculature, how could Archaeopteryx rise? Perhaps it jumped from a tree, although there is little evidence that Compsognathus or Archaeopteryx could get up a tree, if they could find one to climb.

I think that the Ground-up hypothesis best fits direct physical evidence, and is mostly correct. But how did Archaeopteryx get off the ground?

As noted above, Archaeopteryx apparently lived in a coastal environment. Driven by differential temperature and heating/cooling rates -- as well as larger weather systems -- wind is a near-constant feature of most shores. Why did the Wright brothers (flight pioneers too) choose North Carolina's Outer Banks? Probably for isolation, soft sandy landings, few obstacles, and wind. Why is Nag's Head today a favorite venue for hang gliders? Same reasons. Why are some shore birds only modest fliers? Because they don't need to be strong fliers. Given strong reliable wind, a shore bird can lift without vigorous flapping. I think the same was true for Archaeopteryx.

Wind may provide the missing element in the Ground-up hypothesis. We have found other dinosaur fossils with evidence of a "hairy" coat resembling downy feathers, which may have provided insulation. I imagine that a small bipedal dinosaur covered with fluffy proto-feathers easily could have been transported involuntarily by strong wind. Natural selection would favor those best able to control or exploit wind. Forelimbs -- not used in bipedal locomotion -- evolved into wind-catchers, then wings. To glide, Archaeopteryx or its ancestors had no need to climb a tree.

How do you learn to fly? In a strong wind, stand up and hold out your arms! It helps if you are small, light, and fluffy.

Consider Archaeopteryx as a hang-gliding Compsognathus.

C. G. Cochran
July 10, 1999