The Mammal-like Reptiles James A. Hopson is a professor of anatomy and evolutionarybi- ology at the Univ. of Chicago, 1025 East 57th St., Chicago, IL A study of transitionalfossils 60637. He received his B.S. in Geology from Yale Univ. and his Ph.D. in Paleozoology from the Univ. of Chicago. From 1953 to 1957, he curated and did researchon fossil vertebratesin the Pea- body Museum at Yale. Since 1957, he has been teachingvertebrate paleontology and evolution at the Univ. of Chicago. His research interests have centered mainly on mammal-likereptiles and the earliest mammals, but he also has published on brain size and be- havior in dinosaurs. He is past president of the Society of Verte- James A. Hopson brate Paleontology and was editor of the journalPaleobiology. More than 500 species of mammal-like reptiles these should be rare isolated examples. Therefore, have been named on the basis of fossils found in the evolutionary prediction, to be supported satisfac- Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 rocks of all continents, including Antarctica. These torily, should have a fairly extensive sequence of in- fossil-bearing rocks date from about 320 to about 170 termediate forms. On the other hand, paleontolo- million years ago, indicating a total span of existence gists do not consider the fossil record to be as com- for the mammal-like reptiles of about 150 million plete as Gish claims it to be (Raup 1983), and so they years. My purpose in this articleis to summarize cur- expect gaps to exist between taxa, especially for rent knowledge of the reptile to mammal transition, groups with a relatively poor fossil record. However, considered by paleontologists to be the best-docu- it can be predicted that for many groups with a good mented example in the fossil record of an evolu- fossil record, numerous specimens exist which tionary sequence connecting two major structural bridge' the gap, both morphologically and tempor- grades. This treatment of the mammal-like reptiles ally, between currently-recognizedhigher categories, also serves as a case study of how paleontologists de- and, therefore, between what creationists would ac- termine that a collection of fossil specimens repre- knowledge as distinct "created kinds." A more de- sents an evolutionary series. tailed discussion of these predictions is given later. One often reads statements by "scientific creation- ists" that transitional fossils, forms intermediate be- tween higher taxonomic categories, do not exist. The creationist explanation for this is, of course, that "all What is a Mammal-like Reptile? basic animal and plant types (the created kinds) were Figure 1 is a diagram called a cladogram which brought into existence by acts of a supernaturalCre- shows current ideas as to how the living groups of ator using special processes which are not operative amniote tetrapods-reptiles, birds and mammals- today" (Gish 1979, p. 11). As a test of the conflicting are related to one another. Note that living reptiles theories, or "models," of the origins of distinct plus birds form one major subgroup, often called the groups of organisms, Gish (1973, 1979)makes the fol- Sauropsida, and the mammals form another. If fossil lowing predictions about the fossil record: forms were placed on this cladogram, the mammal- Creation Model Evolution Model like reptiles would form a series of branches between Sudden appearance of Transitional series link- the point at which the sauropsids branch off to the each created kind with ing all categories.No sys- left and the point of divergenceof the modern groups ordinal characteristics tematic gaps. of mammals. They would be placed here because complete. Sharp bound- they share many skeletal features with mammals, yet retain numerous primitive features shared with rep- aries separating major taxonomic groups. No tiles. transitional forms be- We must decide where to place the boundary be- tween higher categories. tween mammals and premammals, and, with the fossils taken into account, this is in some ways an At the outset, we must qualify these predictions to arbitrarydecision. In cases where soft-part anatomy make them realistic with respect to the actual nature cannot be used, mammals traditionallyare defined as of the fossil record. Gish (1973) acknowledges that having a lower jaw consisting of a single bone, the "apparent transitional forms," which appearto be in- dentary, which forms a joint at its back end with a termediates because of resemblances due either to bone of the skull called the squamosal (Figure2A). A chance or similar adaptation, might be found, but fossil in which the back of the dentary has a knob 16 THE AMERICAN BIOLOGY TEACHER, VOLUME 49, NO. 1, JANUARY 1987 H - SAUROPSIDAPSI - MAMMALIA- l C3~~~~~~~~~~~~~~~ C3~~~C 0 ~~fr C3~~c 0~ 4~~~b~ 0 o+C Feathers Al antoic F u I - DividdlacentaI L _Fully-Divided Heart Live Birth Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 Two Openings in Skull Behind Eye Hair, Mammary Glands, Squamosal - Dentary Jaw Joint, Uric Acid Excretion, 3 Ear Ossicles Two Systemic Aortae Amniote Egg Figure 1. Cladogramshowing the interrelationshipsof the living amniote tetrapods. A few of the features which characterizesome of the subgroups are indicated. (condyle) for articulatingwith a depression (glenoid As is well known from comparative anatomy, the cavity) on the squamosal is defined to be a mammal. quadrate and articularcan be homologized with the This seems a rather simple feature to use as a major incus (anvil) and malleus (hammer)of the middle ear taxonomic distinction. However, other important of mammals (Figure 2C). These bones lie in an air- changes in the skull are correlated with it in the filled cavity between the inner ear, which houses the known fossils, so we can be pretty secure that when sensory apparatus for hearing, and the eardrum, we encounter such an articulation we are dealing which picks up sound vibrations from the air. The with a true mammal. malleus possesses a hook-like process which extends Figure 2B depicts the skull of a moderately ad- down and forward to attach to the eardrum. Vibra- vanced, mammal-like reptile. Although strikingly tion of the drum causes the malleus and, with it, the mammalian in some features, it lacks an articulation incus to rock, which in turn causes the stapes, a between dentary and squamosal. The dentary is pro- small stirrup-shaped bone attached to the incus, to portionately larger than in typical reptiles but it does move in and out of an opening in the inner ear cap- not extend far enough back to contact the squamosal. sule. This movement sets up vibrationsof the fluid of Instead, the back part of the lower jaw is made up of the inner ear, thereby stimulatingthe sensory cells of several additional bones and the jaw joint is formed the organ of Corti. In modern reptiles, the stapes is between the rearmost of these bones, the articular, the only bone extending between the eardrum and and a bone of the skull, the quadrate.The quadrateis the inner ear capsule. connected to the squamosal. An articular-quadrate The mammalian middle ear mechanism contains jaw joint is characteristicof all nonmammalianjawed two bones in addition to the three mentioned above; vertebrates, but the dentary-squamosaljaw joint is a these are the tympanic bone which supports the specialization unique to mammals. eardum and has the form of a horseshoe in primitive In the transition to mammals, what became of the mammals, and the goniale, which is fused to the an- quadrate and articular. and the other bones of the terior part of the malleus but is a separate bone in lower jaw that are present in mammal-likereptiles? fetal mammals. MAMMAL-LIKE REPTILES 17 A MAMMAL-OPOSSUM squamosal C adult ear ossicles incus stapes tL D malleus tympanic B MAMMAL-LIKEREPTILE squamosal D pouch young dentary Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 incus stapes 4; t } td ra~~~~qudrte malleus tympanic dentary articular angular Meckel's cartilage Figure 2. A, the skull of the opossum Didelphisshowing the dentary-squamosaljaw joint characteristicof mammals. B, the skull of a mammal-likereptile, the cynodont Thrinaxodon,showing the primitive articular-quadratejaw joint. C, the middle ear bones of the adult opossum, greatly enlarged. D, internalview of the lower jaw of a pouch young opossum, showing how the elements of the middle ear are attachedto the dentaryin early development. The same patterningis used in B, C, and D to indicate homologous bones. The darkerstipple in D indicates the goniale (prearticular)which forms the anteriorprocess of the adult malleus. The tympanic bone can be homologized with the eral of our ear bones are actually part of our lower angular bone of the lower jaw of reptiles (Figure2B). jaw. Later in development the middle part of In most mammal-likereptiles the angular possesses a Meckel's cartilage atrophies, so that the set of ear free-standing process, the reflected lamina, which is bones loses its skeletal connection with the dentary notched behind. Possibly this notch held the ear- and is then connected only to the skull. To my drum as the tympanic does in mammals (Allin 1975; knowledge, this reflection of our reptilian past has Crompton & Parker 1978). The goniale is generally never been mentioned by creationists. identified with the reptilian prearticularbone. Sev- eral additional reptilian jaw bones have been lost in Are the Mammal-like Reptiles modern mammals. an Evolutionary Transition Figure 2D shows the lower jaw of the pouch young of an opposum which is still essentially a fetus.