The differences between mammals and reptiles are considerable. A chief difference is that reptiles have at least four jaw bones and one middle ear bone while mammals have one jaw bone and three middle ear bones. To make matters worse, two bones in the fetal reptile that turn into jaw bones turn into ear bones in developing mammals. Other key differences. Reptile have undifferentiated teeth while mammals have incisors, canines, premolars, and molars. Reptile teeth are continuously replaced, mammals teeth are replaced at most once. Reptile teeth only have a single root while mammal molars have two roots. Reptiles lack a diaphragm. Reptiles have their legs sprawled out to the sides while mammals have their legs underneath. The pelvis of a mammal is fused. They have different numbers of bones in their toes. Reptiles are cold blooded while mammals are warm blooded. A list of transitional animals with limited comments (still long and still a cut and hatchet job but editted to reduce length): Paleothyris - A reptile Protoclepsydrops haplous Clepsydrops Archaeothyris - Showed a slight change in teeth Varanops - Lower jaw shows first changes in jaw musculature...lower-limb musculature starts to change Too late to be a true ancestor, and must be a "cousin". Haptodus - Teeth become size-differentiated, with biggest teeth in canine region and fewer teeth overall...Vertebrae parts & joints more mammalian. Dimetrodon, Sphenacodon or a similar sphenacodont - More advanced pelycosaurs, clearly closely related to the first therapsids (next). Dimetrodon is almost definitely a "cousin" and not a direct ancestor... Teeth further differentiated, with small incisors, two huge deep- rooted upper canines on each side, followed by smaller cheek teeth, all replaced continuously. Fully reptilian jaw hinge. Lower jaw bone made of multiple bones & with first signs of a bony prong later involved in the eardrum.. Biarmosuchia - Upper jaw bone (maxillary) expanded to separate lacrymal from nasal bones, intermediate between early reptiles and later mammals. Canine teeth larger, dominating the dentition. Variable tooth replacement: some therocephalians (e.g Scylacosaurus) had just one canine, like mammals, and stopped replacing the canine after reaching adult size. Jaw hinge more mammalian in position and shape, jaw musculature stronger (especially the mammalian jaw muscle)...more mammalian femur & pelvis. The toes were approaching equal length, as in mammals, with #toe bones varying from reptilian to mammalian. Procynosuchus - The first known cynodont -- a famous group of very mammal-like therapsid reptiles, sometimes considered to be the first mammals. Lower incisor teeth was reduced to four (per side), instead of the previous six (early mammals had three). Jaw hinge still reptilian. Scapula beginning to change shape. A diaphragm may have been present. Dvinia - First signs of teeth that are more than simple stabbing points -- cheek teeth develop a tiny cusp. The dentary bone was now the major bone of the lower jaw. The other jaw bones that had been present in early reptiles were reduced to a complex of smaller bones near the jaw hinge. Thrinaxodon - Functional division of teeth: incisors (four uppers and three lowers), canines, and then 7-9 cheek teeth with cusps for chewing. The cheek teeth were all alike, though (no premolars & molars), did not occlude together, were all single- rooted, and were replaced throughout life in alternate waves. First sign of the mammalian jaw hinge. Scapula shows development of a new mammalian shoulder muscle. All four legs fully upright, not sprawling. Number of toe bones is intermediate between reptile number and mammalian . The specialization of the lumbar area (e.g. reduction of ribs) is indicative of the presence of a diaphragm, needed for higher O2 intake and homeothermy. The eardrum had developed in the only place available for it -- the lower jaw, right near the jaw hinge, supported by a wide prong (reflected lamina) of the angular bone. Cynodonts developed quite loose quadrates and articulars that could vibrate freely for sound transmittal while still functioning as a jaw joint, strengthened by the mammalian jaw joint right next to it. Cynognathus - Teeth differentiating further; rate of replacement reduced, with mammalian-style tooth roots (though single roots). TWO JAW JOINTS in place, mammalian and reptilian. Limbs were held under body. There is possible evidence for fur in fossil pawprints. Diademodon - Mammalian toe bone numbers, with closely related species still showing variable numbers. Probelesodon - Teeth double-rooted, as in mammals. Second jaw joint stronger. Hip & femur more mammalian. Probainognathus - Additional cusps on cheek teeth. Still two jaw joints. Mammalian number of toe bones. Exaeretodon - Mammalian jaw prong forms, related to eardrum support. Three incisors only (mammalian). More mammalian hip related to having limbs under the body. This is probably a "cousin" fossil not directly ancestral, as it has several new but non-mammalian teeth traits. Oligokyphus, Kayentatherium - Alternate tooth replacement with double-rooted cheek teeth, but without mammalian-style tooth occlusion. Skeleton strikingly like egg- laying mammals (monotremes). Double jaw joint. Scapula is now substantially mammalian, and the forelimb is carried directly under the body. Various changes in the pelvis bones...this animal's limb musculature and locomotion were virtually fully mammalian. There is disagreement about whether the tritylodontids were ancestral to mammals or whether they are a specialized offshoot group not directly ancestral to mammals. Pachygenelus, Diarthrognathus - Alternate replacement of mostly single- rooted teeth. This group also began to develop double tooth roots -- in Pachygenelus the single root of the cheek teeth begins to split in two at the base. Pachygenelus also has mammalian tooth enamel. Double jaw joint, with the second joint ...fully mammalian. Reptilian jaw joint still present but functioning almost entirely in hearing. Highly mobile, mammalian-style shoulder. These are probably "cousin" fossils, not directly ancestral. Adelobasileus cromptoni - Currently the oldest known "mammal." Sinoconodon - The next known very ancient proto-mammal. Mammalian jaw joint stronger. This final refinement of the joint automatically makes this animal a true "mammal". Reptilian jaw joint still present, though tiny. Kuehneotherium - A slightly later proto-mammal, sometimes considered the first known pantothere (primitive placental-type mammal). Teeth and skull like a placental mammal. The three major cusps on the upper & lower molars were rotated to form interlocking shearing triangles as in the more advanced placental mammals & marsupials. Still has a double jaw joint, though. Eozostrodon, Morganucodon, Haldanodon - Truly mammalian teeth: the cheek teeth were finally differentiated into simple premolars and more complex molars, and teeth were replaced only once. Tiny remnant of the reptilian jaw joint. Thought to be ancestral to all three groups of modern mammals -- monotremes, marsupials, and placentals. Peramus - A "eupantothere" (more advanced placental-type mammal). The closest known relative of the placentals & marsupials. Endotherium Kielantherium and Aegialodon Steropodon galmani - The first known definite monotreme. Vincelestes neuquenianus - A probably-placental mammal with some marsupial traits. Pariadens kirklandi - The first definite marsupial. Kennalestes and Asioryctes - Canine now double rooted. Cimolestes, Procerberus, Gypsonictops - Primitive North American placentals with same basic tooth pattern. So we have a finely divided set of fossils going from purely reptile to purely mammal with intermediate features seen gradually changing throughout the sequence. To read more see http://www.talkorigins.org/faqs/faq-transitional/part1b.html and http://www.talkorigins.org/faqs/comdesc/section1.html#morphological_intermediates_ex2 The latter has drawings of the jaw in transistion to see what it looked like. The former has the full text of most of what I posted above. Still think there are no transistionals?