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THE THERAPSID--MAMMAL TRANSITIONAL SERIES

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... The fossil transition from reptile to mammal is one of the most extensive and well-studied of all the transitions, and detailed series of fossils demonstrate how this transition was accomplished. ...

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The mammals are believed to have evolved from a class of Permian and Triassic reptiles known as therapsids. Taxonomically, mammals are distinguished by a number of features, the most obvious of which are hair (even such aquatic mammals as whales and dolphins still retain bristly hairs in their skin), and the presence of mammary glands which secrete milk, used to nourish the young. Neither of these structures is preserved in the fossil record, but fortunately, mammals can also be distinguished by a number of skeletal characteristics (particularly in the skull and teeth). In particular, mammals are distinguished from reptiles by a number of skeletal traits. Reptiles have a much larger number of individual bones in their skulls than do mammals. In reptiles, the teeth are all of the same shape, and although they vary slightly in size, they all have the same simple cone-shaped form. Mammals, however, possess a number of different types of teeth in their jaws, from the flat, multi-cusped molar teeth to the sharp cone-shaped canines. In reptiles, the lower jaw is made up of a number of different bones, and the jaw joint is formed between the quadrate bone in the skull and the angular bone in the jaw. In mammals, by contrast, the lower jaw is made up of a single bone, the dentary, which articulates with the squamosal bone in the skull to form the jaw joint. Reptiles also have a single bone in the middle ear, the stapes. In mammals, there are three bones in the middle ear, the malleus, incus and stapes (also known as the hammer, anvil and stirrup). At the top of the skull, reptiles have a small hole through which the pineal body, or "third eye", extends--this is absent in mammals. Finally, the reptilian skull is attached to the spine by a single point of contact, the occipital condyle. In mammals, the occipital condyle is double-faced.

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Paleontologists point out that the therapsids possessed many of the characteristics of both reptiles and mammals:

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"In advanced forms, the skull was intermediate in type between that of a primitive reptile and a mammal; many of the bones absent in mammals were on their way toward reduction or were already lost. A small third eye was still generally present in the top of the skull, but its opening was a tiny one." (Romer, 1967, p. 226)

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"The differentiation of the teeth progressed in the therapsids to high levels of development, with the advanced genera showing sharply contrasted incisors, canines, and cheek teeth, which in some of these reptiles were of complex form, often with accessory cusps or broad crowns. In many therapsids, the occipital condyle became double, as in the mammals." (Colbert and Morales, 1991, p. 118)

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"In many respect, the tritylodont skull was very mammalian in its features. Certainly, because of the advanced nature of the zygomatic arches, the secondary palate and the specialized teeth, these animals had feeding habits that were close to those of some mammals . . . . Yet, in spite of these advances, the tritylodonts still retained the reptilian joint between the quadrate bone of the skull and the articular bone of the lower jaw. It is true that these bones were very much reduced, so that the squamosal bone of the skull and the dentary bone of the lower jaw (the two bones involved in the mammalian jaw articulation) were on the point of touching each other." (Colbert and Morales, 1991, p. 127)

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The reptiles, as we have noted, have one bone in the middle ear and several bones in the lower jaw, and mammals have three bones in the middle ear and only one bone in the lower jaw. On the other hand, the jaw joints in the reptile are formed from different bones than they are in the mammalian skull. Thus, it is apparent that, during the evolutionary transition from reptile to mammal, the jaw joints must have shifted from one bone to another, freeing up the rest of these bones to form the auditory ossicles in the mammalian middle ear. (In fact, in most modern reptiles, the jawbones in question actually function in transmitting sound waves to the inner ear, so the transformation postulated above is not a functional change, merely an improvement in a fnction that these bones already had). As Arthur N. Strahler puts it, "A transitional form must have had two joints in operation simultaneously (as in the modern rattlesnake), and this phase was followed by a fusion of the lower joint." (Strahler 1987, p. 414) The creationists find this process to be impossible to conceive, and claim there is no fossil evidence for it:

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"The two most distinguishable osteological differences between reptiles and mammals, however, have never been bridged by a transitional series. All mammals, living or fossil, have a single bone, the dentary, on each side of the lower jaw, and all mammals, living or fossil, have three auditory ossicles or ear bones, the malleus, incus and stapes. In some fossil reptiles the number and size of the lower jaw bones are reduced compared to living reptiles. Every reptile, living or fossil, however, has at least four bones in the lower jaw and only one auditory ossicle, the stapes. . . There are no transitional fossil forms showing, for instance, three or two jawbones, or two ear bones. No one has explained yet, for that matter, how the transitional form would have managed to chew while his jaw was being unhinged and rearticulated, or how he would hear while dragging two of his jaw bones up into his ear." (Gish, 1978, p. 80)

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"Mammals also have three bones in their ears, while reptiles have only one. Where did the two 'extras' come from? Evolutionary theory attempts to explain it as follows: Reptiles have at least four bones in the lower jaw, whereas mammals have only one; so, when reptiles became mammals, there was supposedly a reshuffling of bones; some from the reptile's lower jaw moved to the mammal's middle ear to make the three bones there and, in the process, left only one for the mammal's lower jaw. However, the problem with this line of reasoning is that there is no fossil evidence whatsoever to support it. It is merely wishful conjecture." (Watchtower and Bible Tract Society, 1985, p. 81)

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Not only is this explanation not "merely wishful conjecture", but it can be clearly seen in a remarkable series of fossils from the Triassic therapsids. The earliest therapsids show the typical reptilian type of jaw joint, with the articular bone in the jaw firmly attached to the quadrate bone in the skull. In later fossils from the same group, however, the quadrate-articular bones have become smaller, and the dentary and squamosal bones have become larger and moved closer together. This trend reaches its apex in a group of therapsids known as cynodonts, of which the genus Probainognathus is a representative. Probainognathus possessed characteristics of both reptile and mammal, and this transitional aspect was shown most clearly by the fact that it had TWO jaw joints--one reptilian, one mammalian:

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"Probainognathus, a small cynodont reptile from the Triassic sediments of Argentina, shows characters in the skull and jaws far advanced toward the mammalian condition. Thus it had teeth differentiated into incisors, a canine and postcanines, a double occipital condyle and a well-developed secondary palate, all features typical of the mammals, but most significantly the articulation between the skull and the lower jaw was on the very threshhold between the reptilian and mammalian condition. The two bones forming the articulation between skull and mandible in the reptiles, the quadrate and articular respectively, were still present but were very small, and loosely joined to the bones that constituted the mammalian joint . . . Therefore in Probainognathus there was a double articulation between skull and jaw, and of particular interest, the quadrate bone, so small and so loosely joined to the squamosal, was intimately articulated with the stapes bone of the middle ear. It quite obviously was well on its way towards being the incus bone of the three-bone complex that characterizes the mammalian middle ear." (Colbert and Morales, 1991, pp. 228-229)

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In a slightly later group, known as the ictidosaurians, the mammalian part of the double jaw joint seen in Probainognathus was strengthened, while the old reptilian part was beginning to become reduced in size. In describing a member of this group known as Diarthrognathus, paleontologists Colbert and Morales point out: "The most interesting and fascinating point in the morphology of the ictidosaurians (at least, as seen in Diarthrognathus) was the double jaw articulation. In this animal, not only was the ancient reptilian joint between a reduced quadrate and articular still present, but also the new mammalian joint between the squamosal and dentary bones had come into functional being. Thus, Diarthrognathus was truly at the dividing line between reptile and mammal in so far as this important diagnostic feature is concerned." (Colbert and Morales, 1991, p. 128)

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The therapsid-mammal transition was completed with the appearence of the Morganucodonts in the late Triassic. This group is described by paleontologist T.S. Kemp:

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"The axes of the two jaw hinges, dentary-squamosal and articular-quadrate, coincide along a lateral-medial line, and therefore the double jaw articulation of the most advanced cynodonts is still present . . . The secondary dentary-squamosal jaw hinge had enlarged (in the Morganucodonts) and took a greater proportion if not all of the stresses at the jaw articulation. The articular-quadrate hinge was free to function solely in sound conduction." (Strahler, 1987, p. 419)

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Thus, the fossil record demonstrates, during the transition from therapsid reptile to mammal, various bones in the skull slowly migrated together to form a second functional jaw joint, and the now-superfluous original jaw bones were reduced in size until they formed the three bones in the mammalian middle ear. The reptilian quadrate bone became the mammalian incus, while the articular bone became the malleus. The entire process had taken nearly the whole length of the Triassic period to complete, a time span of approximately 40 million years. Since the determining characteristic of a mammal in the fossil record is the structure of the jaw bone and joint, all of the therapsids up to the Morganucodonts are classified as reptiles, and all those after that are considered to be mammals. As Romer puts it, "We arbitrarily group the therapsids as reptiles (we have to draw a line somewhere) but were they alive, a typical therapsid probably would seem to us an odd cross between a lizard and a dog, a transitional type between the two great groups of backboned animals." (Romer, 1967, p. 227)

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In the unbroken evolutionary sequence from reptile to mammal, the cynodonts are intermediate between earlier and more primitive theriodont therapsids and the earliest mammals. These highly successful animals first appear in the late Permian, radiate quickly into a number of different forms (including both terrestrial and semi-aquatic) before the end of the period, reach their maximum diversity in the Early Triassic, and become increasingly mammal-like as the Triassic progresses, giving rise to true mammals in the later Triassic. By the start of the Jurassic, only the insectivore-like tritheledonts and rodent-like tritylodonts remained; the latter continuing alongside true mammals throughout the Jurassic and even into the early Cretaceous.

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In cladistic nomenclature, the term "cynodont" is also used to include mammals, which evolved from cynodonts and hence are, phylogenetically speaking, derived cynodonts.

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