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.
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
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Two Openings in Skull Behind Eye Hair, Mammary Glands,
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
tL D malleus
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. All Between Reptiles and Mammals? living mammals, including ourselves, go through a The Predictions stage in which the lower jaw has a cartilage(Meckel's Creationists are fond of testing their "creation cartilage) lying along the inside of the developing model" against the "evolution model," and I pro- dentary. The ossified rear end of the cartilageforms pose to do the same. Gish (1981) argues that the malleus, just as it forms the articularbone of the mammal-like reptiles are just reptiles showing a lower jaw in reptiles. The goniale and the upper arm chance resemblance to mammals. I claim that the of the tympanic lie alongside the cartilage in pre- mammal-like reptiles are indeed transitional forms cisely the same positions as the prearticularand an- between primitive reptiles and mammals. Therefore, gular of the mammal-like reptile. So, as mammals, I predict that the relevant fossils can be arrangedin a we go through a stage in our development when sev- sequence based on their morphology so that they
18 THE AMERICAN BIOLOGY TEACHER, VOLUME 49, NO. 1, JANUARY 1987 will show the progressive acquisition of increasingly AB temporal opening mammalian features, irrespective of what aspect of their skeletal anatomy is used. This means that a se- q quence based on stages in the lower jaw will be es- max ~~~~~q s q sue ~~~~~~~~~~~~~~~~q sentially the same as a sequence based on some other part of the skeleton such as the palate or the denti- tion. The creationist prediction would be that, be- PRIMITIVESYNAPSID cause the resemblances to mammals are accidental,it SPHENACODONTID D should be impossible to order a large series of fossils C in a closely-spaced sequence based on essentially all features of the skeleton. The likelihood of such a se- quence occurringby chance would be infinitesimally small. My second prediction is that the series of mammal-like reptiles ordered on the basis of mor- PRIMITIVETHERAPSID (BIARMOSUCHIAN) EOTITANOSUCHIAN phology will also form a series in geologic time. That E
is, increasingly mammal-like forms will appear pro- Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 gressively later in the fossil record. The creationist prediction, of course, would be that there will be no qj correlationbetween a sequence ordered on the basis of morphology and one based on time.
If both of the evolutionary predictions are borne PRIMITIVETHERIODONT out, then the conclusion that we are dealing with a Figure4. A series of skulls of non-cynodontsynapsids. The lower true evolutionary transitionbetween two higher cate- jaw of Eotitanosuchus(D) is not known and its presumed outline is reconstructed from that of other primitive synapsids. Abbrevia- gories (and, presumably between the creationists' tions: ang, angular; art, articular;dent, dentary; j, jugal; max, distinct "created kinds") is strongly supported. maxilla;q, quadrate;qj, quadratojugal;ref lam, reflectedlamina of angular;sq, squamosal;sur, surangular. Methodsof PhylogeneticAnalysis The most widely-used method for ordering or- ganisms according to what evolutionists interpret to be degree of genealogical relationship is called cla- those taxa. For example, the dentary-squamosaljaw distic analysis. A clear discussion of cladisticmethod- joint is a feature unique to mammals and so permits ology is given by Patterson (1980). Cladisticsseeks to us to group all organisms possessing it in the taxon group taxa on the basis of their shared possession of Mammalia. The alternative state of this feature, an novel features, that is, features that are unique to articulationbetween articularand quadrate,occurs in all other jawed vertebrates, including the mammal- like reptiles, so they are excluded from the taxon Mammalia. In evolutionary terms, features like the dentary-squamosaljaw joint would be termed "de-
4+ P rived"; such features delimit a more restrictedgroup su 0U f ,o +0 ~+o 0 o than does the alternative features which would be termed "primitive."It can be seen from this example that whereas derived features delimit naturalgroups, | EUTHERIODONTIA primitive features do not.
THERIODONTIA Deciding whether a feature is primitive or derived in a particulargroup is done by making comparisons with other groups which are termed "outgroups." For example, the cladogram in Figure 1 shows the tetrapods that are grouped as Amniota because THERAPSIDA they share the derived feature of an egg containing spe- cialized membranes, including an amnion. We know SYNAPSIDA that this feature is derived because amphibians, the outgroup most similar to amniotes, as well as all Figure 3. Cladogramshowing the interrelationshipsof the prin- other organisms, lack such egg membranes. Within cipal groups of synapsids. Mammals are included as a subgroup within cynodonts, the relationships of which are shown in Fig- the amniotes, three of the taxa share the derived ure 6. characters of hair, mammary glands and a dentary-
MAMMAL-LIKE REPTILES 19 squamosal jaw joint; they thus form a natural group well off the line to mammals are included on the -the Mammalia. Among the mammals, two taxa cladograms. Figure 3 presents the relationships of form a subgroup characterized by live birth (vivi- the major groups of synapsids, with the mammals parity); these are the Theria. And, of these, one included as a subgroup of the Cynodontia at the taxon possesses a complex allantoic placenta and upper right. In Figure 6, the relationships of the cyn- gives birth to young at a fairly advanced stage of de- odonts are shown, with the most derived group velopment;' these are placental mammals, or Eu- being the Mammalia. theria. In each case, increasingly derived characters The primitive members of the Synapsida (that is, define increasingly more restricted subsets nested the mammal-like amniotes or right-hand branch of within the larger, more general set Amniota. Such a the cladogram in Figure 1) are very little different hierarchicalarrangement of taxa into nested sets is a from the most primitive members of the Sauropsida feature of all life and is a necessary consequence of (the reptile/bird group in Figure 1). All synapsids an evolutionary origin of the living world. (Creation- possess a hole in the side of the skull behind the eye ists explain the hierarchical arrangement of living socket known as the lateral temporal opening (addi- things as a reflection of the creator'splan.) This taxo- tional characteristics are given by Reisz 1980). A nomic hierarchy is discovered by working out the primitive synapsid, or pelycosaur, is illustrated in distribution of derived features. Figure 4A, with relevant bones of the skull and lower Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 Note that in this discussion of cladistic methods, jaw labeled and the temporal opening indicated. The the ordering of taxa and the determination of most important things to note about it are: the small whether features are primitive or derived were based size of the temporal opening and the moderate size entirely on comparison of morphology without re- of both the dentary and the visible post-dentary gard to relative geologic age of the organisms. bones (angular and surangular). The articular is a small bone, but the quadrate actually is a tall bone, CladisticAnalysis of Mammal-likeReptiles for most of it is covered laterally by the squamosal and quadratojugal.The teeth are all simple points. Figures 3 and 6 present the results of a cladistic Figure 4B shows a sphenacodontid synapsid. It is analysis of the mammal-like reptiles, or Synapsida, more mammal-like than the primitive synapsid in carried out by Dr. H.R. Barghusen and myself having the angular bone deepened to form a plate- (Hopson & Barghusen 1986). The analysis is based like keel with a distinct notch at its back end sepa- on our studies of the skulls of most of the relevant rating the keel from the articularregion. This special fossil specimens of mammal-like reptiles. The fol- process of the angular, the reflected lamina, is ex- lowing discussion concentrates only on those groups tremely significant because it represents the recog- relevant to the problem of the transition to mam- nizable beginning of the transformationof the repti- mals, but the groups forming evolutionary sidelines lian angular to the mammalian tympanic bone. The
PRIMITIVETHERIODONT PRIMITIVEEUTHERIODONT PRIMITIVECYNODONT (GORGONOPSIAN) (THEROCEPHALIAN) (PROCYNOSUCHUS) Figure 5. Dorsal views of the skulls of three theriodont synapsids to show the narrowing of the temporal roof in the transition from primitive theriodonts (A) to eutheriodonts (B) and the flaring of the zygomatic arches in the transition to cynodonts (C). These modifications are related to the expansion of the jaw muscles on to the skull roof and the outer surface of the lower jaw. Abbreviations: po, postorbital; sq, squamosal.
20 THE AMERICAN BIOLOGY TEACHER, VOLUME 49, NO. 1, JANUARY 1987 A e C, . 10
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Figure 6. Cladogram showing the interrelationships of the prin- .L - Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 cipal groups of cynodonts, with mammals as the most derived subgroup.
Figure 7. A series of skulls of progressively more derived cyno- articularhas a downturned process, absent in more donts (A-D) and the early mammal Morganucodon(E). The den- primitive synapsids, which represents the beginning tary-squamosaljaw joint of Morganucodonis indicated by broken of the downturned process of the mammalian lines. Abbreviations:po, postorbital;pref, prefrontal. malleus, though here the process appears to have been for attachment of a muscle rather than an ear- drum. Other advanced features are reduction of the quadratojugalbone and its replacement in the cheek there is a split into three major groups of advanced by the forwardly-expanded squamosal; though therapsids (Figure3). Two of these groups, the Dino- scarcely recognizable here, the meeting of the jugal cephalia and Anomodontia, became specialized away and squamosal bones below the temporal opening is from the direction expected in the ancestry of the forerunner of the mammalian cheekbone. Fi- mammals and so are not discussed further in this ar- nally, certain teeth toward the front of the upper and ticle. Only the third group, the Theriodontia (Figure lower jaw are greatly enlarged to form canines; note 4E), shows a number of derived mammal-like fea- also the increased height of the maxillarybone to ac- tures. The skull of theriodonts is much lower and commodate the enlarged canine roots. flatter than in other therapsids and so has a more The remaining mammal-like reptiles are known as mammalian form. The temporal opening is greatly therapsids. The primitive therapsids (Figure4C), col- elongated and the rear end of the dentary bone is lectively called biarmosuchians, still resemble the raised above the level of the post-dentary bones to sphenacodontids, but are advanced in a mammalian form a coronoid process. The post-dentary bones are direction in having a larger temporal opening, a somewhat reduced in height, but the most significant much enlarged upper canine, and a much higher feature of theriodonts is the great reduction in height maxilla. The reflected lamina of the angular has be- of the quadrate and quadratojugaland the loosening come deeply notched above to form a distinct pro- of these bones so that they are movable on the rest of cess. the skull. This, of course, is what one would expect A somewhat more derived therapsid is repre- of the bone that, at a later stage, is to become the sented by Eotitanosuchus(Figure 4D), in which the mammalian incus. temporalopening is furtherenlarged, especially in its Figure 5A shows the skull of a primitive theriodont upper and rear portion which flares up, back and in dorsal view. Note that the skull roof between the outward. In primitive reptiles, the region of the skull temporal openings is rather wide. In Figure 5B is the behind the eye and internal to the cheek serves as an skull of a primitive eutheriodont, the group that in- important attachment area for jaw-closing muscles. cludes the Therocephaliaand Cynodontia. Here, the In Eotitanosuchus,expansion of the temporal opening skull roof is very narrow, the temporal opening indicates that the jaw musculature is increasing in having expanded toward the midline so that only a mass and is beginning to expand outward through remnant of the old roof remains above the narrow the opening (Barghusen 1973). braincase. The jaw muscles are now no longer en- At the next level, characterizedby still further ex- closed beneath the broad roof and they have ex- pansion of the temporal opening (correlatedwith still panded up over the braincase to attach to a promi- greater development of the jaw-closing muscles), nent midline crest. Note, also, that the postorbital
MAMMAL-LIKE REPTILES 21 A 000 c 00
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Figure 8. Ventralviews of the skulls of A, a therocephalianeutheriodont, B-D, three cynodonts, and E, an early and, F, a modern mammal, to show the progressive development of the bony secondary palate and the posterior displacement of the internal opening of the nasal passage. Abbreviations:max, maxilla;pal, palatine;q, quadrate. bone, which forms a bar behind the eye, does not temporal opening but which moved down through extend back to contact the squamosal as it does in all the new gap between lower jaw and cheek bone. other synapsids. These are all steps in a mammalian This muscle, the masseter, can be felt on your cheek direction. when you clench your teeth. It is a mammalianinno- Figure 5C shows the skull of a primitive cynodont, vation that first appears in the earliest cynodonts based on the genus Procynosuchus,which differs from (Barghusen 1968). that of the primitive eutheriodont, and from the Early cynodonts show a number of additional therocephalians, in having the cheek bones (zygo- mammalian innovations. The posterior part of the matic arches) greatly flared laterally. Whereas the dentary has expanded back over the post-dentary lower jaw in other therapsids lies very close to the bones, and the lower part of the dentary extends cheek bone, in cynodonts there is a large gap be- down below the angular bone as a distinct free tween the jaw and the cheek bone. The meaning of "angle." The reflected lamina of the angular is this feature becomes apparent when we examine the greatly reduced in size compared with other the- skull in side view (Figure 7A); the small depression rapsids. Finally, the cheek teeth are no longer simple high on the coronoid process of the dentary indicates points; they have become complicated in shape by the presence of a new muscle in cynodonts which the development of additional points (cusps). In ven- originated as a slip of the main mass that filled the tral view (Figure 8), we see in the primitive eutherio-
22 THE AMERICAN BIOLOGY TEACHER, VOLUME 49, NO. 1, JANUARY 1987 A MORGANUCODON dentary condyle sage comes to lie between the back teeth (Figure8C). Beyond the Thrinaxodonlevel (Figure6), there was \ quadrate an enormous diversity of cynodonts which were all stapes more derived than Thrinaxodonin the following fea- tures (Figures7C, 8D): the dentary is very greatlyen- K ~~~~~articular larged and comes very close to the jaw articulation, angular though it does not quite reach it; the post-dentary B bones are much reduced in height; the reflected lamina is still further reduced; and the secondary palate is closed anteriorly, though its rear end still lies between the rear teeth. At this advanced cynodont stage, there was a great variety of adaptive types which diverged away from the line to mammals. Only the Probainognathidae Thrinaxodon Probainognathus Morganucodon (Figures 7C, 8D), small meat- or insect-eaters with Figure 9. A, internal view of the lower jaw of the early mammal teeth reminiscent of those of Thrinaxodon(Figure 9B), Morganucodon showing the presence of the new condyle on the resemble the expected structural intermediate be- Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 dentary for articulation with the squamosal bone of the skull. The reptilian jaw bones are still part of the lower jaw but they are very tween Thrinaxodonand the most primitive true reduced in size. The old reptilian jaw joint between articular and mammals. quadrate lies adjacent to the new mammalian jaw joint. The pat- The most mammal-likegroup of cynodonts, the Ic- terning on the reptilian jaw bones is the same as in Fig. 2. B, inner views of cheek teeth of the cynodonts Thrinaxodonand Probainog- tidosauria, consists of small forms that were contem- nathus and of the molar of the early mammal Morganucodon to poraries of the earliest mammals. They share with show the similarity of morphology. mammals the possession of a very large dentary ex- tending back to lie close to the squamosal and a post- dentary rod of very reduced size (Figure7D). The re- flected lamina is now a slender hooklike structure and the angular bone greatly resembles the tympanic dont (Figure 8A) that plates growing in from the bone of mammals. The prefrontal and postorbital sides of the palate are beginning to separate the in- bones are lost from the border of the eye socket, as is ternal opening of the nasal passage from the mouth the postorbitalbar. The bony secondary palate is ex- in the cavity; early cynodont (Figure 8B) these plates tremely long, extending back beyond the end of the almost meet on the All midline. of these novel fea- tooth row. Ictidosaurs, though extremely mammal- tures of cynodonts appear to be correlated with one like, are nevertheless still reptile-like in possessing another and the suggest beginning of the ability to numerous replacements of the teeth. Uncertaintyre- process food in the mouth before swallowing it, an mains as to whether the rear of the dentary actually important feature of mammals. contacted the squamosal. Although the dentary ex- In the cynodont ear region, the stapes has a mam- tends sufficiently far back, both it and the squamosal malian it shape, though is still much larger than in a lack well-developed articulatingsurfaces. Taking the mammal. direct with By making contact the inner entire suite of reptilian and mammalian features surface of it the quadrate is taking on the relations of combined in the ictidosaurs, it is probablybest at this the mammalian stapes to the incus. stage in our knowledge to consider them to be "al- The fossil record of cynodonts is especially good most-but-not-quite" mammals. They are important and contains a large number of stages between the in pointing up the lack of a sharp transitionbetween primitive procynosuchids and the first true true mammals and their nonmammalian predeces- mammals. In the galesaurids and thrinaxodontids sors. (Figures 6, 7B), the dentary is expanded further back- wards, the coronoid process is much higher, and the masseteric fossa extends to the lower border of the The EarliestTrue Mammals jaw. These features indicate the development of an essentially mammalian pattern of jaw musculature in Figures 7E and 8E represent the skull of Morganu- the cheek and temporal region (Barghusen 1968). The codon, one of the most primitive known mammals cheek teeth are more complex than in procynosu- and undoubtedly the best known mammal from the chids and, at least in Thrinaxodon,resemble those of age of dinosaurs. Morganucodonand its close relatives early mammals (Figure 9B). The secondary palate of are now known from several nearly complete skulls galesaurids is still incomplete, as in procynosuchids, and skeletons and from literally tens, perhaps but in Thrinaxodonit is completed by the meeting of hundreds, of thousands of fragments of the skull, the plates of maxillary and palatine bones on the dentition and postcranial skeleton. They have been midline so that the inner opening of the nasal pas- extensively monographed (Kermack, Mussett &
MAMMAL-LIKE REPTILES 23 Rigney 1973, 1981;Jenkins & Parrington1976) and so quence based on the palate (Figure 8), though the are extremely well understood. former would be the more detailed because a greater Morganucodonis unquestionably a mammal be- number of distinct stages are known for the jaw. The cause the rear end of its dentary bears an articulating series presented here show no great discontinuities knob which fits against an oval depression on the anywhere-all morphological steps are relatively squamosal to form an undoubted dentary-squamosal small (as, for example, in the transition of angular jaw joint. In more than one specimen (still unde- bone to tympanic ring). Yet, if we add them all up, scribed), these bones are found in contact. The cheek we see a total amount of change that is profound. teeth of Morganucodonhave a complex pattern very Therefore, I conclude that the set of relationships set much like that of Thrinaxodonand other cynodonts out in the cladograms is strongly corroboratedin its (Figure 9B). These teeth are more advanced, how- essentials, and that the mammal-like reptiles are ever, in that they come together in a precise manner demonstrated to be the morphologically-transitional and they have divided roots. The dentition of Mor- forms that evolutionists claim them to be. ganucodonis also mammalian in being divided into a If we examine the order of evolutionary origination single series of milk teeth and a single replacement in the cladograms, we see that, from left to right, the series of permanent teeth; more primitive cynodonts groups are increasingly more derived and so must be had numerous replacements of the entire dentition. increasingly recent in time of origin. If the geologic Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 Although it possesses numerous characteristically- ranges (times of first and last appearance)of the rele- mammalian features, Morganucodonretains many vant groups in the cladograms are arranged in the reptilian features lost in later mammals. Notable same left to right sequence on a geologic time scale among these are the full complement of reptilian (Figure 10), the most primitive group indeed appears bones in the lower jaw, albeit greatly reduced in size, earliest, and progressively more derived groups ap- and a functional articular-quadratejaw joint lying pear at increasingly later times. In fact, given the medial to and on the same transverse line as the new known imperfections of the fossil record, the correla- dentary-squamosal joint (Figure 9A). (Gish (1981) tion between degree of advancement toward cites Kermack et al. (1973) as stating that the acces- mammals and time of appearance is surprisingly sory jaw bones and reptilian jaw joint of Morganu- high. codonwere not reduced in size from that of much ear- The fossil record of the mammal-like reptiles lier cynodonts; however, Allin (1975) pointed out strongly supports the predictions expected of a true that these bones were indeed smaller in Morganu- evolutionary series between two distinct higher taxo- codon,and Kermacket al. (1981) have since agreed.) nomic categories. The prediction of the creationist The reflected lamina of the angular of Morganu- codonis a slender hooklike structurewhich may have I 1 1? I I I oIg I I I I Il already supported an eardrum (Allin 1975). The artic- PE N NS Y L- JUR- ular has a downturned process, not complete in any VA NAN PE RMIA N TRI ASSIC A S S IC specimen (it is restored in Figure 9A on the basis of an ictidosaur articular).The tiny quadrate closely re- I I______Primitive Synapsida sembles the incus of primitive living mammals (cf. Sphenacodontidae
Figures 2C, 9A), including the long process which Biarmosuchia contacts the very mammal-likestapes. Eotitanosuchia Q Here, then, we have a fossil that in its jaw and ear precisely straddles the boundary between two higher Gorgonopsia categories of traditional classifications-the Reptilia Therocephalia and Mammalia. Procynosuchidae
Evolution or Creation? Thrinaxodontidae Examining the Predictions Cynognathidae IJ In support of the evolutionists' claim that the "so- Tritylodontoidea called mammal-like reptiles" (Gish 1979, p. 85) are Probainognathidae 0 actually transitionalforms between primitive reptiles Ictidosauria and mammals, I predicted that it should be possible to order the fossils in a morphologicalseries showing Mammalia progressive acquisition of increasingly mammalian features, no matterwhat set of features is used. It can Figure 10. The geologic ranges of the main groups of synapsids, which are arranged from left to right in the same order in which be easily seen that a sequence based on the lower jaw they appear on the cladograms of Figs. 3 and 6. The intervals in (Figures4, 7) would indeed be compatiblewith a se- the time scale at left are ten million years.
24 THE AMERICAN BIOLOGY TEACHER, VOLUME 49, NO. 1, JANUARY 1987 model of "no transitionalforms between higher cate- argument that these bones were already functioning gories" is totally unsupported. in hearing while still part of the jaw. So the only step that had to be taken once the new mammalianjaw Reexamination of Creationist joint was established was to free the rod of post-den- Objections to Mammal-like tary elements from contact with the dentary. This Reptiles as Transitional Fossils happens during the ontogeny of every living mammal by atrophy of the middle part of Meckel's Having reviewed the evolutionary history of cartilage;it undoubtedly occurredin a similarfashion mammal-like reptiles, let us now examine some of in the phylogeny of mammals. In any case, Gish is the creationist arguments against interpreting these grasping at straws because the lack of a functional fossils as transitional forms between primitive rep- explanation in no way discredits a morphological tiles and mammals. My purpose is to expose the fla- transformation based on such strong observational grant misrepresentation of the scientific literature evidence. that is the foundation of their strategy for discredi- Thus far in this article, I have taken at face value ting the evolutionary interpretation of fossils (see Gish's apparent acceptance of the fossil record as also discussions by Kitcher 1982 and McGowan representing several hundred million years of time. 1983). For example, Gish (1979, p. 85-86) cites He consistently uses the terminology of the geologic Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 Crompton and Parker (1978) as providing evidence time scale throughout his writings (e.g., Gish 1973, against the idea that the mammal-likereptiles were 1979), which permits him to develop arguments really mammal-like.He selects from their description about gaps in the fossil record as evidence for the of Thrinaxodon(see Figures7B, 8C, 9B) only those fea- sudden appearance, without predecessors, of all tures in which the animal is primitive (reptile-like), higher categories of organisms. This emphasis on completely ignoring its mammalian characteristics. unbridged gaps in the fossil record is his main line of On the basis of such selection of evidence it is easy argument in support of special creation over evolu- for him to conclude that its similarity to mammals tion. "may thus have been grossly overstated." But, of course, this is not what Gish, or any other Elsewhere, Gish (1973) argues that fossils have "scientific creationist" whose work I have read, never been found showing an intermediatestage be- really thinks the fossil record represents. The views tween the reptilian many-boned lower jaw and of "creation science" are summarized by Gish as single-boned middle ear and the mammalian single- follows: "It is believed that most of the important boned jaw and three-boned ear. He writes: "There geological formationsof the earth can be explained as are no transitionalforms showing, for instance, three having been formed as the result of the worldwide or two jaw bones or two ear bones" (Gish 1973, p. Noachian Flood described in Genesis. The fossil 136). In this he is correct, of course; intermediates record, rather than being a record of transformation, such as he describes never did exist. But his argu- is a record of mass destruction, death, and burial by ment is a "red herring," intended, it would seem, to water and its contained sediments" (Gish 1979, p. mislead the uninformed. As we have seen, the four 61-62). If, as Gish believes, the fossil record was laid reptilian jaw bones were incorporatedinto the mam- down by a catastrophicworldwide flood, then a pre- malian middle ear mechanism as a unit. diction of the creation model should be that there Gish further objects to this proposed evolutionary ought to be no ordering of fossils accordingto relative transition because: "No one has explained yet, . . . degree of advancement toward present-day life. In- how the transitional form would have managed to stead, all sedimentary rocks should contain a thor- chew while its jaw was being unhinged and rearticu- ough mixture of living and extinct species, with no lated or how it would hear while dragging its jaw perceptible ordering of species from less modern bones up into its ear" (Gish 1973, p. 136). His tactic below to more modern above. here is to discredit the idea of an evolutionary trans- But creationists do recognize that the fossil record formationby emphasizing the adaptive improbability shows order; they acknowledge that above the es- or impossibility of the intermediate stages. But to do sentially unfossiliferous Precambrian ("preflood") this he grossly misrepresents how the transformation rocks, the first forms to appear are invertebrates, actually occurred. As I have shown above, the lower with fishes appearing later, land-living amphibians jaw never was "unhinged"; it went from having one and reptiles still later, and birds and mammals later hinge (reptilian) to two hinges lying side-by-side still. Their explanation for this gross ordering of life (reptilian plus mammalian) back to a single hinge involves a sorting action by the flood on the basis of (this time, mammalian). Likewise, the post-dentary differences in hydrodynamic characteristicsof ma- bones of the lower jaw were already connected to the rine organisms, which affect their sinking rates, and stapes via the quadrateeven in the earliest mammal- the varying ability of land-living species to flee the like reptiles, and Allin (1975) has made a persuasive rising flood waters (Morris& Whitcomb 1961;Morris
MAMMAL-LIKE REPTILES 25 1974;Wysong 1976). However, as Miller (1982, p. 87) Raup, D.M. (1983). Geology and creationism.Field Museum so trenchantly points out: "The more one considers of NaturalHistory Bulletin, 54(3), 16-25. the fossil record, the more laughable the notion of Reisz, R.R. (1980). The Pelycosauria:A review of phyloge- netic relationships. In A.L. Panchen. (Ed.), TheTerrestrial flood geology becomes." The sorting action of the Environmentand the Origin of Land Vertebrates,London: flood cannot conceivably explain the hundreds of or- Systematics Association Special Symposium. dered morphological sequences documented within Wysong, R.L. (1976). Thecreation-evolution controversy. Mid- taxonomic groups at all hierarchical levels from land, Michigan:Inquiry Press. phylum down to genus. Such series are known for highly-organized, active forms such as dinosaurs, horses, whales and mammal-likereptiles (Figure 10); but they also exist in such lowly, passive forms as foraminiferal protozoans, the species of which are preserved in a characteristicsequence in the layered sediments on the ocean floor everywhere in the world. In this context, of what significance are the cre- ationists' gaps when hundreds of well-documented Downloaded from http://online.ucpress.edu/abt/article-pdf/49/1/16/42529/4448410.pdf by guest on 28 September 2021 fossil sequences, even if they include gaps within them, speak so eloquently for evolution and against special creation.
References Allin, E.F. (1975). Evolution of the mammalianmiddle ear. Journalof Morphology,47, 403-437. Barghusen, H.R. (1968). The lower jaw of cynodonts (Rep- tilia, Therapsida) and the evolutionary origin of mammal-like adductor jaw musculature. Postilla, 116, 1-49. . (1973). The adductor jaw musculature of Dime- trodon(Reptilia, Pelycosauria).Journal of Paleontology,47, 823-834. This publication Crompton, A.W. & Parker, P. (1978). Evolution of the mammalian masticatory apparatus. AmericanScientist, is available in 66, 192-201. Gish, D.T. (1973). Creation, evolution, and the historical evidence. AmericanBiology Teacher, 35(3), 132-140. microform. . UniversityMicrofilms (1979). Evolution?The fossils say no! (3rd ed.). San Internationalreproduces this Diego: Creation-LifePublishers. in . (1981). The mammal-likereptiles. Impact,102, 1-8. publication microform: Hopson, J.A. & Barghusen, H.R. (1986). Analysis of ther- flS microfiche and 16mmor In P.D. 35mmfilm. For information apsid relationships. MacLean, J.J. Roth, E.C. about this publicationor any Roth & N. Hotton. III (Eds.). The Ecologyand Biologyof of the more than 13,000 titles Mammal-likeReptiles. Washington, D.C.: Smithsonian In- we mail stitution Press. offer, completeand the couponto: University Jenkins, F.A., JR. & Parrington, F.R. (1976). The post-cra- MicrofilmsInternational, 300 N. Zeeb Road,Ann Arbor, nial MI 48106. Call us toll-free for an immediateresponse: skeletons of the Triassic mammals Eozostrodon,Me- 800-521-3044.Or call collect in Michigan,Alaska and gazostrodonand Erythrotherium.Royal Societyof London Hawaii: 313-761-4700. PhilosophicalTransitions (B), 273, 387-431. Kermack, K.A., Mussett, F. & Rigney, H.W. (1973). The 0 Please send informationabout these titles: lower jaw of Morganucodon.Linnean Society Zoological Journal,53, 87-175. Name . (1981). The skull of Morganucodon.Linnean Society ZoologicalJournal, 71, 1-158. Company/Institution Miller, K.R. (1982). Special creation and the fossil record: Address The central fallacy. AmericanBiology Teacher, 44(2), 85-89. City Morris, H.M. (1974). Scientificcreationism. San Diego: Cre- State Zip ation-LifePublishers. Phone( & Whitcomb,J.C. (1961). TheGenesis flood. Philadel- phia: Presbyterianand Reformed Co. Un i Patterson, C. (1980). Cladistics. Biologist,27, 234-240. Re- printed in J. Maynard Smith (Ed.), 1982, EvolutionNow. MicRilms San Francisco:W.H. Freeman and Co. International
26 THE AMERICAN BIOLOGY TEACHER, VOLUME 49, NO. 1, JANUARY 1987