| Archaeopteryx
lithographica, from the Jurassic of Germany, is the world's
earliest and most primitive bird. Note that the photo above is the real
thing; most of the photos you see are of plaster casts, and not the
original fossil. The
original is difficult to get near even as a scientist, as it is
currently housed in a massive safe in the Berlin's Humboldt Museum. There are eight specimens of Archaeopteryx currently known, named after the cities that house them: they are the London, Berlin, Maxberg, Eichstätt, Haarlem, Munich, Solnhofen, and Thermopolis specimens. All come from the lithographic limestones of Solnhofen, in Bavaria, which were laid down in shallow lagoons roughly 150 million years ago. There are
probably two
or more species of Archaeopteryx: Archaeopteryx
lithographica (Berlin, London, and Solnhofen specimens) is
characterized by short thumbs, peglike teeth and a short snout. A.
recurva (Eichstätt, Munich, and Thermopolis specimens) is
characterized by longer fingers, pointed, procumbent teeth, and a
longer, more slender snout. |
| The
first specimen of Archaeopteryx
was discovered in 1861. This was nice timing for Charles Darwin,
who had just published The Origin of Species a couple years earlier
(1859), and pissed a lot of people off by claiming that all life on
earth had descended from one, or a few, kinds of organisms (evolution).
However, it was hard to figure out how birds fit into the tree of life:
there were no animals which seemed to be clearly intermediate between
birds and other animals, and the avians had a bizarre mixture of
features- they had warm blood like mammals, laid eggs like reptiles,
and bore feathers, unlike anything. Birds didn't seem to fit
comfortably with the idea of common descent. Archaeopteryx was the perfect intermediate fossil. It had feathers and a wishbone, so it was clearly a bird. On the other hand, it had the classic "reptile" features- a long bony tail , clawed fingers, and teeth. It was exactly what you would predict to see if Darwin and the other evolutionists were correct, and all organisms had arisen via transformation from populations of other organisms. Intermediate fossils really do exist. In recent years, Archaeopteryx has been important to understanding two main questions. First, what exactly did birds evolve from? Second, how did birds evolve flight? |
| While it has long been
recognized that birds evolved
from reptiles, it has been unclear which group was most closely related
to birds. Although the idea of a close relationship between birds and
dinosaurs was proposed as far back as the late 19th century, it never
found much support and the features shared by dinosaurs and birds (for
instance, bipedalism, a three toed foot, hollow bones) were simply
assumed to result from convergent evolution instead of common ancestry. Finally, in the 1970s, John Ostrom- spurred by his discovery of the dromaeosaur Deinonychus- published a series of papers describing the extensive similarities that existed between Archaeopteryx and the theropod dinosaurs. The 1980s and 1990s saw this pile of evidence become a landslide, culminating with the discovery of dinosaurs from China bearing feathers and protofeathers. Although there are some vocal dissenters, most paleontologists now accept that birds are simply a group of dinosaurs which have become highly adapted for flight. |
| The other major question about avian
origins concerns the origin of avian flight. Two hypotheses have
dominated the discussions. One proposes that flight and flapping of the
forelimbs evolved in the context of running, this has been dubbed the
cursorial or "ground up" hypothesis. The other is the arboreal or "trees down" hypothesis, in which birds descend from arboreal parachuters and gliders, similar to modern tree squirrels and flying squirrels. An interesting piece of evidence here concerns the hind limbs. When originally discovered, the hind limbs of the Berlin specimen bore large feathers, leading some scientists to propose that Archaeopteryx used the hind limbs in flight. This kind of adaptation is seen in modern gliding lizards such as Draco and Ptychozoon, but wouldn't be expected if primitive birds were relying on their legs to become airborne; large hindlimb feathers would hinder a cursor. |
|
Recently my
paper on this was published; I found
that the feather structure and arrangement indicated that they were
used as lift-generating winglets, and calculated that these structures
could have significantly decreased both the stall
speed and turning radius of the bird. (Longrich, N. 2006. Structure and
function of hindlimb
feathers in Archaeopteryx lithographica. Paleobiology
32(3):417-431).The
idea of a
multi-winged Archaeopteryx
has been around for
more than a century, but it hasn't recieved much attention, for several
reasons. First, the feather impressions on the main slab(see left),
which so impressed early workers, were prepared away. Second, having
the Berlin specimen behind the Iron Curtain for decades probably made
it difficult to study. Third, people tend to see what they want or
expect to see. Everybody "knows" that birds don't have four wings so it
became possible to overlook them even when they are in plain sight. I
first studied the Berlin specimen back in 2001 and, if I even noticed
them at all, it never would have occurred to me to examine the hind
limb feathers: I "knew" that the hindlimbs weren't relevant to the
aerodynamics of early birds. It wasn't until the 2003 discovery of the
four-winged dromaeosaurs from China (Microraptor
gui and its relatives) that I was prepared to see what had been
right in front of my nose just a few years before.
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| References Here are a few of the more important and/or interesting things out there that have been written on Archaeopteryx over the past century; it's by no means a complete list.
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