QXR37 The mammal-like reptiles were a group of animals that lived in the Permian and Triassic periods, and they provide some of the nicest examples of "transitional fossils" in the fossil record. Although soft body parts like fur and mammary glands are not preserved, there are a great many skeletal features which clearly differentiate reptiles from mammals, and we can see almost all of these gradually developing in the mammal-like reptiles. For example, reptiles walk with their legs sprawled out sideways, while mammals carry their legs straight under their bodies, and the fossil record shows a sequence of species with increasingly mammalian gaits. Also, reptile's teeth all look basically the same, while mammalian teeth are differentiated into bicuspids, incisors, and canines; again, in the mammal-like reptiles we see the teeth becoming gradually more specialized into the mammalian form. And while a reptile's jaw is made up of several bones (dentary, splenial, angular, surangular, prearticular) the mammal's jaw is made of only one, the dentary, with several of the other bones having shrunken enormously and become part of the mechanism of the inner ear. Again, the fossil record shows a sequence of mammal-like reptiles in which the dentary is gradually crowding out the other bones, finally leading to a mammal-like jaw. These are just a few of the most important reptile/mammal differences--there are many others, and in virtually every case you can see a gradual transition from a reptile-like structure to a mammal-like structure. Here are some sites I found on the reptile/mammal transition, many with nice pictures: A site about "early mammal-like reptiles": http://www.museums.org.za/sam/resource/palaeo/cluver/early.htm And the sequel, "later mammal-like reptiles": http://www.museums.org.za/sam/resource/palaeo/cluver/later.htm "Evolution: From Reptiles to Mammals": http://genesispanthesis.tripod.com/fossils/rept_mam.html A site addressing some creationist critiques of the reptile/mammal evidence: http://www.geocities.com/osarsif/ce06.htm A site listing the various kinds of therapsids (an 'advanced' group mammal-like reptile, or alternately a group of'primitive' mammals): http://home.houston.rr.com/vnotes/unit10.5.html A site about "transitional fossils" which has a section on the reptile/mammal transition, including some niceimages—definitely worth checking out: http://asa.calvin.edu/ASA/resources/Miller.html And here's a brief summary of the skeletal features distinguishing reptiles from mammals, and the evidence of a gradual transition, from this site: http://www.cmnh.org/fun/dinosaur-archive/1994Jul/0032.html What specific details determine that a fossil is from a reptilian mammal? I think you mean what makes a mammal-like reptile? However here is a sample of *some* of the characters and *some* of the forms which make up the therapsis -> mammal transition . 1 Reptiles: Single occipital condyle Mammals: Double occipital condyle 2 Reptiles: Undifferentiated dentition Mammals: Differentiated dentition 3 Reptiles: No secondary palate Mammals: secondary palate 4 Reptiles: No diaphram Mammals: Diaphram 5 Reptiles: Uncrowned, uncuspate teeth Mammals: Crowned, cuspate teeth 6 Reptiles: Teeth with single root Mammals: Teeth with multiple roots 7 Reptiles: Lower jaw of several bones Mammals: lower jaw of dentary bone only 8 Reptiles: Jaw joint quadrate-articular Mammals: Jaw joint dentary-squamosal 9 Reptiles: Lumbar region with ribs Mammals: Lumbar region free 10 Reptiles: Separate clavical ribs Mammals: Fused clavical ribs 11 Reptiles: Flat scapular Mammals: Strong spine on scapular 12 Reptiles: Pelvic elements separate Mammals: Pelvic elements fused 13 Reptiles: Limbs out from body Mammals: Limbs under body 14 Reptiles: Cold blooded Mammals: Warm blooded 15 Reptiles: Scales Mammals: Fur/hair 16 Reptiles: Joined external nares Mammals: Separate external nares The reptiles evolved into four major groups; the anapsids, which produced the turtles, the diapsids which produced the dinosaurs, and an offshoot group, the eurapsids, which produced the icthyosaurs. The final group, the synapsids, took a radically different path than the other groups and produced the therapsids, which concentrated on osteo- and pysiological changes which eventually produced the mammals. The group called the cynodontia (dog tooth) produced a lineage of forms intermediate between reptiles and mammals. [The following is not a direct lineage, but representatives of successful, related groups which exhibit a gradual aquisition of mammalian characters during the Permian-Triassic. The group looks like this: A B C D E F G | | | | \/ | …… Cynodont ----------------------------------------> Mammal A Procynosuchus_, Latest Permian-Triassic, South Africa Has an expanded temporal region; large zygomatic arch; enlarged dentary, but the lower jaw is still made up of several bones (albeit reduced); the begining of a secondary palate; double occipital condyle (first major mammalian character). B Thrinaxodon_, Early Triassic, South Africa, Antarctica Elaborate cheek teeth; large dentary, with coronoid process(for jaw joint), but still lower jaw of more than one bone; reduction to mammalian number of insisors; almost complete secondary palate – before anyone comes in here with the question "How could an almost complete secondary palate work?" - the palate can function quite adequetely by being covered with a fleshy membrane, which it is in reptiles. Thus the underlying bone can form gradually and support the palate more and more, without delateriously affecting the functioning of the palate, until the secondary palate if fully formed, it then becomes important, because it separates the nasal passages from the mouth - this means you can now eat and breath at the same time or more importantly you can breath whilst chomping something that is struggling to get away, or that something else is trying to steal from you); lumbar ribs reduced to small plates - the specialisation of the lumbar area is indicative of the presence of a diaphram, needed for higher O2 intake and homeothermy; the head of the femur is set at a considerable angle to the shaft - this indicates that the limbs were upright and closer to underneath the body that sprawling; adult/baby fossil assemblages have been found - possibly indicating parental care; fossils found curled up - curling usually indicates an attempt to keep body heat, possible homeothermy. C Cynognathus_, Early Triassic, South Africa Enlarged dentary, 90% of lower jaw, teeth differentiating, large canine, molars with cusps; secondary palate well developed; jaw joint quadrate-articular, but bones very small; scapular transverse and turned out - half way to mammal condition; limbs under body; possible evidence for fur in fossil footprints. D Diademodon_, Early Triassic, South Africa Cheek teeth more specialised, with more cusps, occlude together more efficiently; clavical ribs fused. E Probelesodon_, Mid Triassic, South America. Saggital crest for greater muscle attachment; nares separated; lumbar free. F Probainognathus_ Mid Triassic, South America. Additional cusps on cheek teeth; teeth double rooted; 'double' jaw joint, the quadrate-articular and the dentary-squamosal bones articulate, but the quadrate-articular bones are very much reduced and only loosely constrained in a groove in the dentary bone; cervical ribs very short; lumbar free; phalangeal arrangement mammalian - loss of some bones. G _Kayentatherium_, Early Jurassic, world wide. Double occipital condyle; secondary palate; separated nares; dentary bone covers almost all lower jaw; differentiated dentition; double rooted teeth; lumbar free; scapulare with spine; pelvic elements fused; fused clavical ribs; but quadrate-articular although very much reduced, still participate in the jaw joint. This feature classifies the organism as a reptile, even though it has far more mammal characters than reptile ones. Recap: A B C D E F G 1 1 1 1 1 1 1 1 2 * 1 1 1 1 1 1 3 * * 1 1 1 1 1 4 0 * 1 1 1 1 1 5 0 0 * 1 1 1 1 6 0 0 0 * 1 1 1 7 0 0 0 0 0 * 1 8 0 0 0 0 0 * * O = reptilian state * = intermediate 1 = mammalian state This is by no means exhaustive and there have been some changes since I drew up this list. However, the basic principle still holds. There is a diagram showing the difference between mammals and reptiles here: http://www.mun.ca/biology/scarr/QA_vs_DS_jaw.htm A nice tree showing a few therapsid (later mammal-like reptiles) skulls can be found here http://www.kheper.auz.com/gaia/biosphere/vertebrates/therapsida/Therapsida.htm ...also, some detailed skull diagrams and more information on skeletal transformations can be found here http://phylogeny.arizona.edu/tree/eukaryotes/animals/chordata/synapsid_lichen/synapsida_synapomorphies.html After reviewing some of the websites mentioned in the previous post, a forum member named "backel" asked some questions: backel: There were afew other things about the teeth i.e. the canines etc that I noticed, but their focus seemed to be on the splitting of the jawbone and how that could have evolved into the formaiton of the ear, but I have a few questions for you. I don't think it's true that "their focus seemed to be on the splitting of the jawbone and how that could have evolved into the formaiton of the ear" since the jawbone transformation was just one of a large number of changes mentioned on the links I gave, and in the case of the jawbone the focus was on the changes in the jaw itself and not the inner ear...from John McLoughlin's Synapsida: Early tetrapods such as pelycosaurs possessed mandibles (lower jaws) composed of a dentary (tooth-bearing bone) associated with a number of thin plates of bone that originally served to strengthen the jaw as a whole. The hindmost of these bones, the articular, articulated (hence the name), or hinged, with the quadrate bone of the skull. In therapsids this arrangement persisted, but the new life-style of these progressive animals, with its growing emphasis on high consumption of energy for lots of activity, required precise manipulation of food in the mouth in order that it might be better broken down for efficient digestion. In these animals, the act of chewing was forcing changes in the structure of the lower jaws to strengthen them further and increase their biting leverage. In this process, the dentary bone was becoming longer and thicker in response to the increase in stress to which it was subjected. In addition, as the molar teeth came into being and were put to grinding up food, changes in jaw musculature forced changes in jaw conformation to permit good chewing. The most primitive therapsids had jaws whose muscles were mainly attached to the inner surfaces of both jaws and skull. In later models, however, the new masseter (chewing) muscles gradually appeared on the outside of the dentary bone, connecting this to the cheekbone. Other muscles arose from a new coronoid (crownlike) process that extended the rear of the dentary bone upward inside the arch of the cheekbone. These muscles inserted along the top and back of the skull, along the temporal bones, and hense are collectively called temporal muscles. Because of the increasing power of these muscles in higher therapsids, the dentary bone was becoming bigger and heavier at the expense of all the other little jawbones. You can see all of this in the sequence of skulls on the mammal-like reptile page at http://www.gcssepm.org/special/fr_cuffey_00.htm (this is an excellent page on the mammal-like reptiles, by the way, definitely worth looking at if you want more detailed info) Backel: Is there more than the jawbones and other dental fossils? I mean skeletally. It seemed that there seemed to be a little bit about ribs for a couple but it seemed mostly to revolve around the skulls. A lot of the most interesting skeletal differences between mammals and reptiles are in the skulls (not just the jawbones as you seem to be saying above) but there are some significant ones in the rest of the skeleton which are also seen transforming in parallel with the skull changes. The biggest one is the change in gait--reptiles walk in a sprawled-out posture, lifting one leg at a time and bending their spine in an s-shape as they move, while mammals carry their legs straight beneath them and flex their spine up and down rather than from side to side. In the mammal-like reptiles we see a series of intermediate gaits, finally leading to a basically modern mammal-like gait. Other differences between mammals and reptiles which can be observed transforming are fused vs. unfused pelvis, differences in the configuration of the phalanges (digits), fused vs. unfused clavical ribs, and the ribless lumbar (lower spine) vertebrae of modern mammals. From the page on "transitional forms" at http://members.aol.com/ps418/tran.htm Next, we'll consider phalanges. In most reptiles, the phalangeal formula is 2-3-4-5-4, whereas the mammaliam formula is 2-3-3-3-3. The early therapsids possessed the reptilian formula. In Thrinaxodon, the phalangeal formula is reduced to 2-3-4-4-3, close to the mammalian count, but still intermediate between the two conditions. The "extra" phalanges in digits 3 and 4 are very small, presaging their later absense. In later cynodonts, the count is reduced to the mammalian formula of 2-3-3-3-3. Whereas reptiles generally display ribs all the way back to the pelvis, mammals lack ribs in the pelvic region. The gradual loss of these pelvic ribs can be seen in the cynodonts or mammal-like reptiles. Probelesodon, for example, displays ribs all the way back to the pelvis, but the last several ribs are greatly attenuated. In other advanced cynodonts, such as Thrinaxodon or the tritylodonts, the pelvic ribs are absent altogether, as in the mammalian configuration. Backel: Do we know anything about their internal organs? I don't know anything about this either way; you'd have to ask a practicing paleontologist. It may be that certain aspects of the skeleton would allow scientists to infer things about their internal organs, but maybe not. Backel: There was something about there being some evidence of fur. What is that? I'm not sure if there's other evidence, but here's something mentioned in Synapsida: In addition to their secondary palates and shearing molar teeth, thrinaxodonts show further advances in food-processing suggested by the presence on the cheek area of their skull of foramina (little holes), through which nerves and blood vessels passed. Such equipment very likely supplied nourishment to active lip and cheek muscles, which in turn may have supported vibrissae (whiskers), sensory structures enabling them to feel their way about at night...the possible presence of vibrissea on these little cynodonts is highly significant, for the embryonic origins of whiskers are similar to those of hairs in general. In primitive ectothermic amniotes such the living reptiles, the skin conducts heat; these animals are dependent on outside temperatures in maintaining their internal temperatures, and must be able to soak up or dissipate heat as fast as possible. Thus their skin is dry and thin, conserving moisture in the body by means of waterproof scales while controlling heat transfer through blood vessels close to the scaly surface. We may justifiably assume (since no trace of skin covering remains in fossils) that the more primitive end of the therapsid line posessed a similar integument serving a similar purpose. No one knows exactly how hair was derived from such skin surfaces. Some students have suggested that it evolved directly from the keratinous (horn) scales of primitive therapsids, much as did the feathers of birds from the scales of primitive archosaurs. However, unlike either scales or feathers, which develop from the epidermis, hair arises from a deeper layer of the skin. The presence of those little holes on the cheek areas of some cynodonts suggests that thick nasal skin may have supported projections similar to those on some living snakes, projections that were perhaps antecedent to the honest-to-goodness whiskers of the sort we find on cats and mice. The gradual spread of such whiskerlike appendages across the cynodont integument not only augmented their efficiency at detecting insect prey in the dark but also served to trap air near the body surface and thus insulate the little animals against the nighttime cold. Backel: Is there any hypothetical situation, or progression which shows these "freed up" bones "becoming" the bones which are the basis of the mamallian ear, or is it scientific conjecture of what could have happened with those bones. We have a progression in which the articular and quadrate become smaller and smaller and more "freed up" from their previous role in hinging the jaw, but the actual transformation into middle ear bones is not seen—probably past a certain point these bones would become too small to have much chance of being fossilized. The strongest line of evidence for the transformation is embryological--in mammal embryos the malleus and incus start out at a position just behind the dentary as in reptiles, and then migrate to the adult position in the middle ear as the embryo develops. And again from the transitional forms page http://members.aol.com/ps418/tran.htm : There are several lines of evidence which converge in support this conclusion. First, the progressive reduction in size and increase of mobility of the postdentary bones is clearly seen in the cynodont fossil record. For instance, Dimetredon, the therocephalians, Thrinaxodon, Probainognathus and Morganucodont show the postdentary bones in progressively more reduced form, and illustrate the step-wise transformation from the reptilian to the mammalian configuration. Second, "the malleus articulates with the incus in exactly the same way as the articular articulates with the quadrate in advanced therapsids and the quadrate (incus) articulates with the stapes" (Carroll, 1988, p 395). Third, the ontogeny of the incus and the malleus reflects or recapitulates their reptilian derivation. When marsupials are still in the pouch, for instance, "the malleus and the incus maintain the reptilian role of the articular and quadrate. Only when the young leave the pouch do these bones seperate from the lower jaw and enter the middle ear" (Carroll, p. 395; see also Crompton and Jenkins, 1979; McGowan, 1984). Yet again, the evidence from the fossil record, comparative anatomy and developmental biology converge in support of the same evolutionary sequence. In Synapsida, McLoughlin explains how the transformation is believed to have occurred: Now, we recall that in the long-lost days of the Pennsylvanian and early Permian, our ancestral line was represented by the pelycosaurs, a low-slung crew indeed. Because of their posture, these animals spent a lot of time with their heads, and thus their lower jaws, resting on the ground. Which had its advantages. The ears of these early tetrapods appear to have been as yet ineffecient at picking up airborne vibrations such as those we call sound. For these early landlubbers, fresh from the water, the only necessity for hearing was related to detection of the footfalls of potential predators, "sounds" that are transmitted well enough by the bones of the lower jaw from the ground to the balance organs of what we now call the inner ear. These balance organse were (and still are) fluid-filled structures equipped with nerve ends highly sensitive to movement of the fluid. In early tetrapods, extensions of these balance organs came to be associated, in the form of the hyomandibular bone, with part of old tongue-support mechanism in fishes. Originally supporting part of the upper-jaw rims with the tongue, this little bone aided the transmission of vibration to the balance organs. As land vertebrates got better at detecting airborne vibration through the jawbones, the hyomandibular was reduced in size to become the stapes (stirrup), connecting the typanum (eardrum) to the cochlea (snail-shell-shaped inner ear). Air vibrations moving in the tympanum thus moved the stapes, which wiggled the fluid in the cochlea, which alerted the brain. This is the way the ancestors of reptiles, archosaurs, and birds took to hearing, and it served them very well. The therapsids, however, were forced along a different route as their hearing improved. As the subtlety of land-lubbing increased, synapsids continued for a time to keep their jaws to the ground in hopes of picking up noise; in them the tympanum stretched alongthe rear of the jaw in such a way as to be intimately associated with the articular bone. As the dentary bone increased in size, however, a conflict arose: increase in chewing efficiency forced an increase in dentary size at the expense of the little articular bone, which was still necessary for hearing. Increased chewing efficiency threatened hearing acuity: how would it work out? Ultimately, the little jaw-joint bones, the articular and quadrate, receded into the head as cynodont evolution progressed. With the inception of the mammalian jaw joint--that is, one between the dentary mandible and the squamosal cheekbone--these tiny bones were entirely freed from their function with relation to the jaw. They continued to be joined together, however, and their ancient articulation with one another persists as they transmit sound from the eardrum to the cochlea. Here they remain, and here we hear, still listening with parts of our mouths in the manner of our ancestors 300 million years ago.