Archaeopteryx
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[edit] Definition
Archaeopteryx is perhaps the most famous example of an intermediate form in the fossil record, having feathers, and forelimbs adapted to function as wings, yet possessing many reptilian features not found in modern birds.
[edit] Anatomy
A more detailed picture comparing Archaeopteryx to a pigeon may be found here: the similarities and differences are readily apparent.
We turn now to the question of its reptilian characteristics. Scientists universally agree that Archaeopteryx, and by extension modern birds, are descended from archosaurs, the group which includes dinosaurs, crocodilians and pterosaurs, besides a bunch of basal archosaurs which are neither of the above, and which are sometimes referred to a thecodonts (the term is becoming obsolete). Scientists are also in universal agreement that birds are not descended from pterosaurs, an easy call to make when one observes the very different specialisations for flight exhibited by the two groups.
The reader should be aware that some scientists are still fighting a valiant rearguard action against the placement of birds among the theropod dinosaurs[1], asserting that they must be descended from some other kind of small bipedal archosaur; as this is a larger subject than the single genus that is the subject of this essay, we shall deal with the topic in a separate article. But by a broad consensus of paleontologists, Archaeopteryx is an avialan maniraptoran coelurosaurian saurischian dinosaur, and its closest known relatives are the dromaeosaurs.[2]
The University of California Museum of Paleontology provides a list here of twenty characteristics which place Archaeopteryx and other early birds amongst the coelurosaurian dinosaurs, and among the Maniraptora in particular.
Here, paleontologist Gregory Paul lists some characteristics which are shared only by early birds, including Archaeopteryx and the dromaeosaur dinosaurs, thought to be the sister group of the birds.
We shall look over the various parts of Archaeopteryx, and point out the more significant of its anatomical features.
[edit] Forearm and hand
One point of particular interest is the unfused semi-lunate carpal (the semicircular bone in the wrist shown on the diagram) which is a feature found only in maniraptoran dinosaurs and in early birds such as Archaeopteryx.
[edit] The furcula (wishbone)
The furcula played an important historical role in the classification of birds. The furcula is a fusion of two clavicles (collar bones). Clavicles are basal to archosaurs, but were thought for a long time to be absent in theropod dinosaurs. This, it was agreed, meant that birds could not be descended from theropods, because this would involve the lineage losing its collarbones and then getting them back again, a most improbable event in the light of the theory of evolution.
It is now known that not only do theropods have clavicles, but in many of them the clavicles have fused to form veritable furculae[3][4] . This, then, is a common anatomical feature to theropods, to Archaeopteryx, and to modern birds.
[edit] The sternum
One specimen of Archaeopteryx, the The Solnhofen-Aktien-Verein specimen, has been found with a sternum, which is small and flat, not the massive keeled affair so prominent in the bony anatomy of modern birds.
[edit] The pelvis
[edit] Legs and feet
The legs and feet of Archaeopteryx show a number of birdlike traits, including fusion of the proximal tarsals to one another and to the tibia and fibula, and the distal tarsals fused to the metatarsals.
It also has a reversed hallux (big toe) such as is found in most modern birds. Acceptance of this fact has been gradual, and there is still some debate about exactly how much it is reversed. For this reason, there are still websites which claim that the hallux of Archaeopteryx was not reversed. They are wrong.
One fascinating aspect of Archaeopteryx's legs is that they may well have borne feathers suitable for flight[5]. It may, in effect, have had four wings, a trait which it shares with the gliding dinosaur Microraptor gui[6]
[edit] The skull
In other ways its skull has undergone some birdlike adaptations: the braincase is larger than you would expect in a dinosaur, especially in the areas connected with vision.[7]
Note how in Archaeopteryx the attachment of the cervical vertebrae to the skull is at the back of the head as in dinosaurs, not to the base of the skull as is found in modern birds.
Note the sclerotic rings of bone around the eyes. These are common to reptiles and birds; they are often omitted in illustrations, and we have also seen some illustrations which omit the ring in some species and not others, exaggerating the morphological differences.
[edit] Vertebrae
In modern birds, the bones of the pelvis and the lumbar, sacral, and a few caudal vertebrae are fused into a single, solid structure called the synsacrum; a good look at it may be obtained here. In Archaeopteryx, as in dinosaurs, the vertebrae are free and mobile.
Archaeopteryx also retains a long bony tail, to which, as we know from the fossils, feathers were attached. However, we can already see a reduction in the tail; in particular, the ventral spines projecting outwards and downwards from the caudal vertebrae are much reduced; the tail of Archaeopteryx must have been lighter and less muscular than that of flightless dinosaurs.
In modern birds, the caudal vertebrae are much reduced in number, so that they extend back no further than the pelvis does, and end in a unique fused structure known as a pygostyle. A detailed photograph of the skeletal tail of a modern bird can be found here
[edit] Ribs
First of all, we should deal with the interesting question of uncinate structures. An uncinate structure is a strut projecting backwards and upwards from a rib and overlapping the rib behind it, giving the rib cage structural strength.
Modern birds have uncinate processes on the ribs; to be strictly accurate, almost all modern birds have this feature, a point we shall return to below. Archaeopteryx lacks these structures, and we have sometimes seen this cited as a saurian feature of Archaeopteryx. However, we should note that uncinate processes do appear in the very groups of dinosaurs which are morphologically closest to the birds[8]. It seems as though it is Archaeopteryx that is the odd one out!
Within the conventional phylogeny of dinosaurs, there is only one simple explanation for this: Archaeopteryx, as we have said, is not the ancestral bird, merely a primitive bird. Bearing this in mind, we may hypothesise that the uncinates are primitive to all birds, inherited from maniraptoran ancestors, and that they were lost in the group of primitive birds that included Archaeopteryx.
This is quite possible, as demonstrated by the Anhimidae, a modern family of South American birds related to ducks, which also have no uncinates and which do perfectly well without them[9]: it seems, then, that natural selection can dispense with the uncinates under certain circumstances. It is plain from this also that these uncinate structures are not necessary either for flight (most Anhimidae are flighted birds) nor for the function of the flow-through respiration of birds, as some have speculated.
Other issues with the ribs of Archaopteryx are more clear-cut: like dinosaurs, and unlike modern birds, the ribs of Archaeopteryx do not articulate with the sternum.
Also like dinosaurs, and unlike modern birds, Archaeopteryx has gastralia (which we mention in this section because gastralia are sometimes called "abdominal ribs"). Today, gastralia are found only in reptiles.
[edit] Lungs and pneumatized bones
It is now known [10][11] that Archaeopteryx had the pneumatized bones suitable for the flow-through respiration [12] of modern birds, as is common in saurischian dinosaurs, both theropod and sauropod. [13][14].
The common claim that uncinate processes are necessary for this method of breathing is wrong, as we have seen above in the section on the ribs: the modern Anhimidae manage to breathe without them.[15]
There is, then, no reason to doubt that Archaeopteryx breathed like modern birds; indeed, no reason to doubt that the trait is basal in saurischian dinosaurs.
[edit] Creationists versus Archaeopteryx
Archaeopteryx has attracted more than the usual amount of creationist venom, partly because its such a good example of an intermediate form, and partly because its one of the few they've ever heard of.
The most common creationist tactic is to pretend that Archaeopteryx is anatomically a modern bird. As we have seen from our discussion of the anatomy, above, this is ridiculous. One variant on this is to point out that Archaeopteryx is classed as a bird[16]. It is indeed; it is also classed as a dinosaur, as are all living birds.
- [Modern] birds are a kind of dinosaur. Specifically, they are ornithurine ornithothoracine pygostylian avialan eumaniraptoran maniraptoran maniraptoriform tyrannoraptoran coelurosaurian avetheropod tetanuran neotheropod theropod saurischian dinosaurs. [17]
The classification of Archaeopteryx as a bird has, then, nothing to do with whether it is an intermediate form between theropods and modern birds.
The second creationist tactic is to borrow Fred Hoyle's idea that the Archaeopteryx fossils were fraudulently manufactured by adding feathers to the fossils of a dinosaur (specifically, Compsognathus): for a debunking of this conspiracy theory, see our main article, Archaeopteryx a Hoax?
You will notice that these two errors are mutually contradictory: Archaeopteryx cannot both have the skeleton of a modern bird and of Compsognathus, a dinosaur. Occasionally one has the pleasure of seeing the same creationist use both arguments consecutively, as in the following from Dr Jolly F. Griggs' endearingly ridiculous tract Evolution 101:
- Fred Hoyle and Chandra Wick-Ramasinghe have made a strong case for fossil forgery in their book Archaeopteryx, The Primordial Bird. This reviewer of the book was convinced. Taking the fossil at face value, the fossil was a complete bird, not half reptile and half bird.
How, you ask yourself, can a man convinced by Hoyle that Archaeopteryx is really Compsognathus also deny that it has reptilian features? That, my friends, is the magic of creationism.
This is often what happens when creationists are faced with a really good example of an intermediate form: they are determined to shoe-horn it into one taxon or another, but of course the facts (with which they are, in any case, largely unfamiliar) don't allow them to decide between taxa.
The third creationist tactic is to borrow from the arguments of those scientists who think that the dinosaurs are descended not from theropod dinosaurs, but from some more basal archosaur. These opinions they quote out of context in order to conceal the fact that every scientist agrees that Archaeopteryx is an intermediate between archosaurs and modern dinosaurs, and that for the last hundred years the only question has been what kind of archosaur.
Meanwhile, Creationists still can't decide whether to pretend that Archaeopteryx is identical to an extinct reptile or a modern bird. Perhaps they should toss a coin.
Besides these tactics, creationists have their usual recourse to making stuff up, failing to understand the theory of evolution, redefining the phrase "intermediate forms", et cetera.
