The old creationist claim that there are no transitional forms in the fossil record is starting to look a bit tired
A perennial contention of creationists opposed to evolution is that transitions or intermediates between the major groups (classes) of vertebrates (animals with backbones) do not exist. The most persistent critic of the part played by the fossil record in providing evidence for evolution is Dr Duane Gish of the Institute for Creation Research in the United States. His arguments are expressed in two books, Evolution: The Fossils Say No! and the updated version, Evolution: The Challenge of the Fossil Record. The aim of this paper is to show that the above contention is without foundation. A classic example of a transitional form (the ancient bird, Archaeopteryx lithographica) will be examined, as well as an example of evolutionary transformation, the evolution of ear bones in vertebrates.
Discovered in 1860, one year after publication of the Origin, Archaeopteryx is of late Jurassic age. About the size of a magpie, it lived some 150 million years ago. The species is represented by seven skeletons and one isolated feather. Close examination reveals a mixture of reptilian and bird features with many more of the former than the latter. (The table below lists some of the key features). In fact, two specimens in which the feathers were not immediately recognized were initially misidentified as Compsognathus, a small bipedal dinosaur. It is often stated that if it were not for its feathers, Archaeopteryx would be classified as a small dinosaur. A transitional form between major groups is defined as a fossil which possesses a mixture (or a mosaic) of features usually associated with each of the two groups, one set ancestral (“old”), the other derived (“new”). Archaeopteryx fits the bill perfectly. Its reptilian ancestry is patently obvious.
|Table 1. Characteristics of Archaeopteryx|
But not according to the creationists. In spite of the evidence outlined above and more fully discussed in advanced textbooks, they continue to proclaim that “a bird, is a bird, is a bird”. Thus Dr Morris: “The Archaeopteryx is a bird – not a reptile-bird transition.” And Dr Gish: “It was not a half-way bird, it was a bird”. In this regard it should be emphasized that a fossil does not have to be exactly intermediate in its features in order to be considered transitional. A mixture of definitive features, old and new, is sufficient. The period of transition between bony fish and the first amphibians, for example, is characterized by forms in which the mosaic patterns show varying rates of change of specific features in different genera.
Archaeopteryx hit the headlines a few years ago with the allegation that it was a fraud.
This assertion was made by the astronomer, Sir Fred Hoyle. He claimed that a forger had tampered with the fossilized skeleton of Compsognathus, adding impressions of feathers. This prompted scientific testing at the Museum of Natural History in London. Hoyle’s view, which must have been welcomed as grist to the anti-evolutionary mill, was proved groundless. The feather impressions were naturally formed. This early bird is still the de luxe example of a transitional form.
Now to a classic example of evolutionary transformation, a process whereby a structure becomes modified over time and changes in its primary function. Mammals almost certainly arose from a group of reptiles, aptly named the mammal-like reptiles, some 200 million years ago. The more advanced of these reptiles show trends towards the mammals in a number of features, such as improved locomotion by adopting an upright posture and differentiation of the teeth for the efficient exploitation of food sources. Palaeontologists normally are restricted to skeletal features for classifying a fossil. Soft tissues are seldom fossilized. The lower jaw or mandible in mammals is a single bone (the dentary which carries the teeth), in contrast to that of reptiles which comprises several bones. In addition, the middle ear of mammals contains three ear bones; reptiles have but one, the stapes.
The stapes can be traced to the fish stage of vertebrate evolution. (See fig. 1). The first fishes lacked true jaws. Hence many were filter feeders, extracting food from the stream of water entering the mouth and filling the pharynx. The filtered water then passed out through holes (gill slits) in the wall of the pharynx. The regions between the slits were supported by a basket of linked bones forming the branchial or gill arches. Jaws probably arose from a pair of these arches (another example of transformation). The upper element of the arch immediately behind the jaws eventually became transformed from an unspecialized part of a gill arch into a prop (the hyomandibular) to support the jaws at their region of articulation. It was thus ideally positioned, given its upper attachment to that region of the braincase which housed the organs of balance and hearing, to become a specialized sound transmitter, a potential realized later in the amphibians. The stapes (the transformed hyomandibular) greatly improved hearing on land.
The origin of the other two ear bones in mammals is even more intriguing. During the evolution of the mammal-like reptiles, the dentary bone in the lower jaw expanded greatly in order to provide greater surface area for the attachment of more powerful jaw muscles. At the same time the canines enlarged as efficient instruments for capturing and dismembering prey. Fig.2 shows the lower jaw of an advanced mammal-like reptile, Cynognathus For the sake of clarity the articular bone of the lower jaw is shown detached from the quadrate bone of the skull. In life these two bones form the jaw joint of reptiles. The expansion of the dentary involved two regions, the ascending coronoid process and the triangular articular process at the back (not to be confused with the articular bone).
In some mammal-like reptiles the articular process had grown back to the point where it touched the skull itself. This development created the potential for a new jaw joint formed by the dentary of the lower jaw and the squamosal bone of the skull. In fact, there are several examples of varying degrees of development of the “new” jaw joint, from rudimentary to fully functional, perfect examples of transitional stages, making the classification of such forms (reptile or mammal?) difficult. Should we be concerned? Not at all. Such “tricky” forms are to be expected in evolution. There is a continuity here which negates the creationist thesis of there being no transitional forms in the fossil record.
But the story is not yet over. The “new” mammalian jaw joint, once it became fully functional, rendered the “old” reptilian one superfluous. The bones of the “old” joint now relieved from a jaw articulation function were free to assume a new primary role. In this case it was not strictly a change of function but an enhancement of an existing minor function – sound transmission. The articular and quadrate bones were already somewhat inefficient conductors of sound to the inner ear in the early land vertebrates. The two bones underwent transformation to become ear bones and joined the stapes or stirrup in the middle ear to form a trio of efficient sound transmitters, greatly improving the conduction and amplification of sound waves from the outer to the inner ear. The quadrate became the incus (anvil) and the articular became the malleus (hammer). The improvement in hearing is linked to the importance of this faculty (along with smell) in promoting the survival of the first mammals as small nocturnal animals in a world dominated by large and aggressive dinosaurs.
What has Gish to say on the subject? He refers to the “unbridged gap between reptile and mammal” and questions how the “intermediates” managed to hear while the changes described above were going on. He seems to have overlooked the fact that the stapes was still present. In addition, as was pointed out above, the “old” jaw joint bones were already sound conductors. He also expresses concern as to how the animals continued to chew while the changes were in progress. But there was never a time when an “intermediate” was without functional jaws. The sequence of change with respect to jaw joints was: Old > Old + New > New.
Diarthrognathus epitomises the transition from reptile to mammal. In this animal, not only was the “old” reptilian joint between a reduced quadrate and articular present, but also a “new” and fully functional mammalian one. To cite a further example, Probainognathus also possessed a double articulation between skull and jaw. Furthermore, the quadrate bone, now only loosely joined to the rest of the skull, was intimately articulated with the stapes bone of the middle ear.
On the above evidence I rest my case. Transitional fossils between major groups of vertebrates do exist and lend powerful support to the reality of evolution.