Cynognathus, a mammal-like reptile from the Triassic period. Smithsonian.
. 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.
The most apparent characteristic in the mammalian skull is the presence of teeth that are differentiated in size and shape. In contrast to reptiles, which have uniform cone-shaped teeth, mammal teeth have several different styles of tooth, each adapted for a different task: canines for puncturing, incisors for slicing, molars for grinding. We can see this process beginning already in the mammal-like reptile Dimetrodon (whose Latin name means, literally, “two-measured teeth”).
Another notable difference is the structure of the ear. 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 function that these bones already had).
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 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.
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. The genus Diarthrognathus (“two-joint-jaw”) illustrates this most clearly.
Thus, the fossil record of the therapsids 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.
The therapsid-mammal transition was completed with the appearance of the Cynodonts in the late Triassic, of which Cynognathus is a good example. This was a dog-sized predator that lived in the early and middle Triassic period, about about 240 million years ago. Its fossils have been found in South America, southern Africa, China and Antarctica. The skeleton lacks the reptilian gastralia or abdominal ribs, indicating that it probably had a diaphragm and breathed like a modern mammal. The teeth are differentiated like a modern mammal’s, and there are openings in the bones of the snout for nerves and blood vessels to pass through to the sensory whiskers, indicating that Cynognathus was covered with fur.
Cross sections of late-Triassic therapsid bones also reveal a series of small holes called Haversian canals, which are typical of fast-growing, warm-blooded animals (and which are absent in cold-blooded reptiles), indicating that the therapsids developed a progressively more mammalian warm-blooded metabolism as time went on. And as the skull and jaws were becoming progressively more and more mammalian, the rest of the body structure was following suit.
Thus, several of the mammalian conditions were, indeed, probably present in the Cynodonts, including a diaphragm and warm-bloodedness. We do not know whether the Cynodonts laid eggs, like the modern-day platypus and echidna, or bore live young like most of today’s mammals. But in nearly every feature, the therapsids demonstrated a reptile-like condition at the beginning of the Triassic, grow progressively more and more mammal-like, and finally ended up as primitive mammals in the late Triassic.