Transitional Vertebrate Fossils FAQ
Part 1B

Part 1A

Contents

Part 2A

Transition from amphibians to amniotes (first reptiles)

The major functional difference between the ancient, large amphibians and the first little reptiles is the amniotic egg. Additional differences include stronger legs and girdles, different vertebrae, and stronger jaw muscles. For more info, see Carroll (1988) and Gauthier et al. (in Benton, 1988)

• Proterogyrinus or another early anthracosaur (late Mississippian) -- Classic labyrinthodont-amphibian skull and teeth, but with reptilian vertebrae, pelvis, humerus, and digits. Still has fish skull hinge. Amphibian ankle. 5-toed hand and a 2-3-4-5-3 (almost reptilian) phalangeal count.
• Limnoscelis, Tseajaia (late Carboniferous) -- Amphibians apparently derived from the early anthracosaurs, but with additional reptilian features: structure of braincase, reptilian jaw muscle, expanded neural arches.
• Solenodonsaurus (mid-Pennsylvanian) -- An incomplete fossil, apparently between the anthracosaurs and the cotylosaurs. Loss of palatal fangs, loss of lateral line on head, etc. Still just a single sacral vertebra, though.
• Hylonomus, Paleothyris (early Pennsylvanian) -- These are protorothyrids, very early cotylosaurs (primitive reptiles). They were quite little, lizard-sized animals with amphibian-like skulls (amphibian pineal opening, dermal bone, etc.), shoulder, pelvis, & limbs, and intermediate teeth and vertebrae. Rest of skeleton reptilian, with reptilian jaw muscle, no palatal fangs, and spool-shaped vertebral centra. Probably no eardrum yet. Many of these new "reptilian" features are also seen in little amphibians (which also sometimes have direct-developing eggs laid on land), so perhaps these features just came along with the small body size of the first reptiles.

The ancestral amphibians had a rather weak skull and paired "aortas" (systemic arches). The first reptiles immediately split into two major lines which modified these traits in different ways. One line developed an aorta on the right side and strengthened the skull by swinging the quadrate bone down and forward, resulting in an enormous otic notch (and allowed the later development of good hearing without much further modification). This group further split into three major groups, easily recognizable by the number of holes or "fenestrae" in the side of the skull: the anapsids (no fenestrae), which produced the turtles; the diapsids (two fenestrae), which produced the dinosaurs and birds; and an offshoot group, the eurapsids (two fenestrae fused into one), which produced the ichthyosaurs.

The other major line of reptiles developed an aorta on left side only, and strengthened the skull by moving the quadrate bone up and back, obliterating the otic notch (making involvement of the jaw essential in the later development of good hearing). They developed a single fenestra per side. This group was the synapsid reptiles. They took a radically different path than the other reptiles, involving homeothermy, a larger brain, better hearing and more efficient teeth. One group of synapsids called the "therapsids" took these changes particularly far, and apparently produced the mammals.

Some transitions among reptiles

I will review just a couple of the reptile phylogenies, since there are so many.... Early reptiles to turtles: (Also see Gaffney & Meylan, in Benton 1988)

• Captorhinus (early-mid Permain) -- Immediate descendent of the protorothryids.

Here we come to a controversy; there are two related groups of early anapsids, both descended from the captorhinids, that could have been ancestral to turtles. Reisz & Laurin (1991, 1993) believe the turtles descended from procolophonids, late Permian anapsids that had various turtle-like skull features. Others, particularly Lee (1993) think the turtle ancestors are pareiasaurs:

• Scutosaurus and other pareiasaurs (mid-Permian) -- Large bulky herbivorous reptiles with turtle-like skull features. Several genera had bony plates in the skin, possibly the first signs of a turtle shell.
• Deltavjatia vjatkensis (Permian) -- A recently discovered pareiasaur with numerous turtle-like skull features (e.g., a very high palate), limbs, and girdles, and lateral projections flaring out some of the vertebrae in a very shell-like way. (Lee, 1993)
• Proganochelys (late Triassic) -- a primitive turtle, with a fully turtle-like skull, beak, and shell, but with some primitive traits such as rows of little palatal teeth, a still-recognizable clavicle, a simple captorhinid-type jaw musculature, a primitive captorhinid- type ear, a non-retractable neck, etc..
• Recently discovered turtles from the early Jurassic, not yet described.

Mid-Jurassic turtles had already divided into the two main groups of modern turtles, the side-necked turtles and the arch-necked turtles. Obviously these two groups developed neck retraction separately, and came up with totally different solutions. In fact the first known arch-necked turtles, from the Late Jurassic, could not retract their necks, and only later did their descendents develop the archable neck. Early reptiles to diapsids: (see Evans, in Benton 1988, for more info)

• Hylonomus, Paleothyris (early Penn.) -- The primitive amniotes described above
• Petrolacosaurus, Araeoscelis (late Pennsylvanian) -- First known diapsids. Both temporal fenestra now present. No significant change in jaw muscles. Have Hylonomus-style teeth, with many small marginal teeth & two slightly larger canines. Still no eardrum.
• Apsisaurus (early Permian) -- A more typical diapsid. Lost canines. (Laurin, 1991)

GAP: no diapsid fossils from the mid-Permian.

• Claudiosaurus (late Permian) -- An early diapsid with several neodiapsid traits, but still had primitive cervical vertebrae & unossified sternum. probably close to the ancestry of all diapsides (the lizards & snakes & crocs & birds).
• Planocephalosaurus(early Triassic) -- Further along the line that produced the lizards and snakes. Loss of some skull bones, teeth, toe bones.
• Protorosaurus, Prolacerta (early Triassic) -- Possibly among the very first archosaurs, the line that produced dinos, crocs, and birds. May be "cousins" to the archosaurs, though.
• Proterosuchus (early Triassic) -- First known archosaur.
• Hyperodapedon, Trilophosaurus (late Triassic) -- Early archosaurs.

Some species-to-species transitions:

• De Ricqles (in Chaline, 1983) documents several possible cases of gradual evolution (also well as some lineages that showed abrupt appearance or stasis) among the early Permian reptile genera Captorhinus, Protocaptorhinus, Eocaptorhinus, and Romeria.
• Horner et al. (1992) recently found many excellent transitional dinosaur fossils from a site in Montana that was a coastal plain in the late Cretaceous. They include:
  1. Many transitional ceratopsids between Styracosaurus and Pachyrhinosaurus
  2. Many transitional lambeosaurids (50! specimens) between Lambeosaurus and Hypacrosaurus.
  3. A transitional pachycephalosaurid between Stegoceras and Pachycephalosaurus
  4. A transitional tyrannosaurid between Tyrannosaurus and Daspletosaurus.

  All of these transitional animals lived during the same brief 500,000 years. Before this site was studied, these dinosaur groups were known from the much larger Judith River Formation, where the fossils showed 5 million years of evolutionary stasis, following by the apparently abrupt appearance of the new forms. It turns out that the sea level rose during that 500,000 years, temporarily burying the Judith River Formation under water, and forcing the dinosaur populations into smaller areas such as the site in Montana. While the populations were isolated in this smaller area, they underwent rapid evolution. When sea level fell again, the new forms spread out to the re-exposed Judith River landscape, thus appearing "suddenly" in the Judith River fossils, with the transitional fossils only existing in the Montana site. This is an excellent example of punctuated equilibrium (yes, 500,000 years is very brief and counts as a "punctuation"), and is a good example of why transitional fossils may only exist in a small area, with the new species appearing "suddenly" in other areas. (Horner et al., 1992) Also note the discovery of Ianthosaurus, a genus that links the two synapsid families Ophiacodontidae and Edaphosauridae. (see Carroll, 1988, p. 367)

Transition from synapsid reptiles to mammals

This is the best-documented transition between vertebrate classes. So far this series is known only as a series of genera or families; the transitions from species to species are not known. But the family sequence is quite complete. Each group is clearly related to both the group that came before, and the group that came after, and yet the sequence is so long that the fossils at the end are astoundingly different from those at the beginning. As Rowe recently said about this transition (in Szalay et al., 1993), "When sampling artifact is removed and all available character data analyzed [with computer phylogeny programs that do not assume anything about evolution], a highly corroborated, stable phylogeny remains, which is largely consistent with the temporal distributions of taxa recorded in the fossil record." Similarly, Gingerich has stated (1977) "While living mammals are well separated from other groups of animals today, the fossil record clearly shows their origin from a reptilian stock and permits one to trace the origin and radiation of mammals in considerable detail." For more details, see Kermack's superb and readable little book (1984), Kemp's more detailed but older book (1982), and read Szalay et al.'s recent collection of review articles (1993, vol. 1).

This list starts with pelycosaurs (early synapsid reptiles) and continues with therapsids and cynodonts up to the first unarguable "mammal". Most of the changes in this transition involved elaborate repackaging of an expanded brain and special sense organs, remodeling of the jaws & teeth for more efficient eating, and changes in the limbs & vertebrae related to active, legs-under-the-body locomotion. Here are some differences to keep an eye on:

(*) The presence of a dentary-squamosal jaw joint has been arbitrarily selected as the defining trait of a mammal.

• Paleothyris (early Pennsylvanian) -- An early captorhinomorph reptile, with no temporal fenestrae at all.
• Protoclepsydrops haplous (early Pennsylvanian) -- The earliest known synapsid reptile. Little temporal fenestra, with all surrounding bones intact. Fragmentary. Had amphibian-type vertebrae with tiny neural processes. (reptiles had only just separated from the amphibians)
• Clepsydrops (early Pennsylvanian) -- The second earliest known synapsid. These early, very primitive synapsids are a primitive group of pelycosaurs collectively called "ophiacodonts".
• Archaeothyris (early-mid Pennsylvanian) -- A slightly later ophiacodont. Small temporal fenestra, now with some reduced bones (supratemporal). Braincase still just loosely attached to skull. Slight hint of different tooth types. Still has some extremely primitive, amphibian/captorhinid features in the jaw, foot, and skull. Limbs, posture, etc. typically reptilian, though the ilium (major hip bone) was slightly enlarged.
• Varanops (early Permian) -- Temporal fenestra further enlarged. Braincase floor shows first mammalian tendencies & first signs of stronger attachment to rest of skull (occiput more strongly attached). Lower jaw shows first changes in jaw musculature (slight coronoid eminence). Body narrower, deeper: vertebral column more strongly constructed. Ilium further enlarged, lower-limb musculature starts to change (prominent fourth trochanter on femur). This animal was more mobile and active. Too late to be a true ancestor, and must be a "cousin".
• Haptodus (late Pennsylvanian) -- One of the first known sphenacodonts, showing the initiation of sphenacodont features while retaining many primitive features of the ophiacodonts. Occiput still more strongly attached to the braincase. Teeth become size-differentiated, with biggest teeth in canine region and fewer teeth overall. Stronger jaw muscles. Vertebrae parts & joints more mammalian. Neural spines on vertebrae longer. Hip strengthened by fusing to three sacral vertebrae instead of just two. Limbs very well developed.
• Dimetrodon, Sphenacodon or a similar sphenacodont (late Pennsylvanian to early Permian, 270 Ma) -- More advanced pelycosaurs, clearly closely related to the first therapsids (next). Dimetrodon is almost definitely a "cousin" and not a direct ancestor, but as it is known from very complete fossils, it's a good model for sphenacodont anatomy. Medium-sized fenestra. 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, but there was no eardrum yet, so these reptiles could only hear ground-borne vibrations (they did have a reptilian middle ear). Vertebrae had still longer neural spines (spectacularly so in Dimetrodon, which had a sail), and longer transverse spines for stronger locomotion muscles.
• Biarmosuchia (late Permian) -- A therocephalian -- one of the earliest, most primitive therapsids. Several primitive, sphenacodontid features retained: jaw muscles inside the skull, platelike occiput, palatal teeth. New features: Temporal fenestra further enlarged, occupying virtually all of the cheek, with the supratemporal bone completely gone. Occipital plate slanted slightly backwards rather than forwards as in pelycosaurs, and attached still more strongly to the braincase. Upper jaw bone (maxillary) expanded to separate lacrymal from nasal bones, intermediate between early reptiles and later mammals. Still no secondary palate, but the vomer bones of the palate developed a backward extension below the palatine bones. This is the first step toward a secondary palate, and with exactly the same pattern seen in cynodonts. 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). The amphibian-like hinged upper jaw finally became immovable. Vertebrae still sphenacodontid-like. Radical alteration in the method of locomotion, with a much more mobile forelimb, more upright hindlimb, & more mammalian femur & pelvis. Primitive sphenacodontid humerus. The toes were approaching equal length, as in mammals, with #toe bones varying from reptilian to mammalian. The neck & tail vertebrae became distinctly different from trunk vertebrae. Probably had an eardrum in the lower jaw, by the jaw hinge.
• Procynosuchus (latest Permian) -- The first known cynodont -- a famous group of very mammal-like therapsid reptiles, sometimes considered to be the first mammals. Probably arose from the therocephalians, judging from the distinctive secondary palate and numerous other skull characters. Enormous temporal fossae for very strong jaw muscles, formed by just one of the reptilian jaw muscles, which has now become the mammalian masseter. The large fossae is now bounded only by the thin zygomatic arch (cheekbone to you & me). Secondary palate now composed mainly of palatine bones (mammalian), rather than vomers and maxilla as in older forms; it's still only a partial bony palate (completed in life with soft tissue). Lower incisor teeth was reduced to four (per side), instead of the previous six (early mammals had three). Dentary now is 3/4 of lower jaw; the other bones are now a small complex near the jaw hinge. Jaw hinge still reptilian. Vertebral column starts to look mammalian: first two vertebrae modified for head movements, and lumbar vertebrae start to lose ribs, the first sign of functional division into thoracic and lumbar regions. Scapula beginning to change shape. Further enlargement of the ilium and reduction of the pubis in the hip. A diaphragm may have been present.
• Dvinia [also "Permocynodon"] (latest Permian) -- Another early cynodont. First signs of teeth that are more than simple stabbing points -- cheek teeth develop a tiny cusp. The temporal fenestra increased still further. Various changes in the floor of the braincase; enlarged brain. 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. Single occipital condyle splitting into two surfaces. The postcranial skeleton of Dvinia is virtually unknown and it is not therefore certain whether the typical features found at the next level had already evolved by this one. Metabolic rate was probably increased, at least approaching homeothermy.
• Thrinaxodon (early Triassic) -- A more advanced "galesaurid" cynodont. Further development of several of the cynodont features seen already. Temporal fenestra still larger, larger jaw muscle attachments. Bony secondary palate almost complete. 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. Dentary still larger, with the little quadrate and articular bones were loosely attached. The stapes now touched the inner side of the quadrate. First sign of the mammalian jaw hinge, a ligamentous connection between the lower jaw and the squamosal bone of the skull. The occipital condyle is now two slightly separated surfaces, though not separated as far as the mammalian double condyles. Vertebral connections more mammalian, and lumbar ribs reduced. Scapula shows development of a new mammalian shoulder muscle. Ilium increased again, and all four legs fully upright, not sprawling. Tail short, as is necessary for agile quadrupedal locomotion. The whole locomotion was more agile. Number of toe bones is 2.3.4.4.3, intermediate between reptile number (2.3.4.5.4) and mammalian (2.3.3.3.3), and the "extra" toe bones were tiny. Nearly complete skeletons of these animals have been found curled up - a possible reaction to conserve heat, indicating possible endothermy? Adults and juveniles have been found together, possibly a sign of parental care. 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. NOTE on hearing: 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. These animals could now hear airborne sound, transmitted through the eardrum to two small lower jaw bones, the articular and the quadrate, which contacted the stapes in the skull, which contacted the cochlea. Rather a roundabout system and sensitive to low-frequency sound only, but better than no eardrum at all! 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. All early mammals from the Lower Jurassic have this low-frequency ear and a double jaw joint. By the middle Jurassic, mammals lost the reptilian joint (though it still occurs briefly in embryos) and the two bones moved into the nearby middle ear, became smaller, and became much more sensitive to high-frequency sounds.
• Cynognathus (early Triassic, 240 Ma; suspected to have existed even earlier) -- We're now at advanced cynodont level. Temporal fenestra larger. Teeth differentiating further; cheek teeth with cusps met in true occlusion for slicing up food, rate of replacement reduced, with mammalian-style tooth roots (though single roots). Dentary still larger, forming 90% of the muscle-bearing part of the lower jaw. TWO JAW JOINTS in place, mammalian and reptilian: A new bony jaw joint existed between the squamosal (skull) and the surangular bone (lower jaw), while the other jaw joint bones were reduced to a compound rod lying in a trough in the dentary, close to the middle ear. Ribs more mammalian. Scapula halfway to the mammalian condition. Limbs were held under body. There is possible evidence for fur in fossil pawprints.
• Diademodon (early Triassic, 240 Ma; same strata as Cynognathus) -- Temporal fenestra larger still, for still stronger jaw muscles. True bony secondary palate formed exactly as in mammals, but didn't extend quite as far back. Turbinate bones possibly present in the nose (warm-blooded?). Dental changes continue: rate of tooth replacement had decreased, cheek teeth have better cusps & consistent wear facets (better occlusion). Lower jaw almost entirely dentary, with tiny articular at the hinge. Still a double jaw joint. Ribs shorten suddenly in lumbar region, probably improving diaphragm function & locomotion. Mammalian toe bones (2.3.3.3.3), with closely related species still showing variable numbers.
• Probelesodon (mid-Triassic; South America) -- Fenestra very large, still separate from eyesocket (with postorbital bar). Secondary palate longer, but still not complete. Teeth double-rooted, as in mammals. Nares separated. Second jaw joint stronger. Lumbar ribs totally lost; thoracic ribs more mammalian, vertebral connections very mammalian. Hip & femur more mammalian.
• Probainognathus (mid-Triassic, 239-235 Ma, Argentina) -- Larger brain with various skull changes: pineal foramen ("third eye") closes, fusion of some skull plates. Cheekbone slender, low down on the side of the eye socket. Postorbital bar still there. Additional cusps on cheek teeth. Still two jaw joints. Still had cervical ribs & lumbar ribs, but they were very short. Reptilian "costal plates" on thoracic ribs mostly lost. Mammalian #toe bones.
• Exaeretodon (mid-late Triassic, 239Ma, South America) -- (Formerly lumped with the herbivorous gomphodont cynodonts.) Mammalian jaw prong forms, related to eardrum support. Three incisors only (mammalian). Costal plates completely lost. More mammalian hip related to having limbs under the body. Possibly the first steps toward coupling of locomotion & breathing. This is probably a "cousin" fossil not directly ancestral, as it has several new but non-mammalian teeth traits.

GAP of about 30 my in the late Triassic, from about 239-208 Ma. Only one early mammal fossil is known from this time. The next time fossils are found in any abundance, tritylodontids and trithelodontids had already appeared, leading to some very heated controversy about their relative placement in the chain to mammals. Recent discoveries seem to show trithelodontids to be more mammal- like, with tritylodontids possibly being an offshoot group (see Hopson 1991, Rowe 1988, Wible 1991, and Shubin et al. 1991). Bear in mind that both these groups were almost fully mammalian in every feature, lacking only the final changes in the jaw joint and middle ear.

• Oligokyphus, Kayentatherium (early Jurassic, 208 Ma) -- These are tritylodontids, an advanced cynodont group. Face more mammalian, with changes around eyesocket and cheekbone. Full bony secondary palate. Alternate tooth replacement with double-rooted cheek teeth, but without mammalian-style tooth occlusion (which some earlier cynodonts already had). Skeleton strikingly like egg- laying mammals (monotremes). Double jaw joint. More flexible neck, with mammalian atlas & axis and double occipital condyle. Tail vertebrae simpler, like mammals. Scapula is now substantially mammalian, and the forelimb is carried directly under the body. Various changes in the pelvis bones and hind limb muscles; this animal's limb musculature and locomotion were virtually fully mammalian. Probably cousin fossils (?), with Oligokyphus being more primitive than Kayentatherium. Thought to have diverged from the trithelodontids during that gap in the late Triassic. There is disagreement about whether the tritylodontids were ancestral to mammals (presumably during the late Triassic gap) or whether they are a specialized offshoot group not directly ancestral to mammals.
• Pachygenelus, Diarthrognathus (earliest Jurassic, 209 Ma) -- These are trithelodontids, a slightly different advanced cynodont group. New discoveries (Shubin et al., 1991) show that these animals are very close to the ancestry of mammals. Inflation of nasal cavity, establishment of Eustachian tubes between ear and pharynx, loss of postorbital bar. 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, and mammalian tooth occlusion. Double jaw joint, with the second joint now a dentary-squamosal (instead of surangular), fully mammalian. Incipient dentary condyle. Reptilian jaw joint still present but functioning almost entirely in hearing; postdentary bones further reduced to tiny rod of bones in jaw near middle ear; probably could hear high frequencies now. More mammalian neck vertebrae for a flexible neck. Hip more mammalian, with a very mammalian iliac blade & femur. Highly mobile, mammalian-style shoulder. Probably had coupled locomotion & breathing. These are probably "cousin" fossils, not directly ancestral (the true ancestor is thought to have occurred during that late Triassic gap). Pachygenelus is pretty close, though.
• Adelobasileus cromptoni (late Triassic; 225 Ma, west Texas) -- A recently discovered fossil proto-mammal from right in the middle of that late Triassic gap! Currently the oldest known "mammal." Only the skull was found. "Some cranial features of Adelobasileus, such as the incipient promontorium housing the cochlea, represent an intermediate stage of the character transformation from non-mammalian cynodonts to Liassic mammals" (Lucas & Luo, 1993). This fossil was found from a band of strata in the western U.S. that had not previously been studied for early mammals. Also note that this fossil dates from slightly before the known tritylodonts and trithelodonts, though it has long been suspected that tritilodonts and trithelodonts were already around by then. Adelobasileus is thought to have split off from either a trityl. or a trithel., and is either identical to or closely related to the common ancestor of all mammals.
• Sinoconodon (early Jurassic, 208 Ma) -- The next known very ancient proto-mammal. Eyesocket fully mammalian now (closed medial wall). Hindbrain expanded. Permanent cheekteeth, like mammals, but the other teeth were still replaced several times. Mammalian jaw joint stronger, with large dentary condyle fitting into a distinct fossa on the squamosal. This final refinement of the joint automatically makes this animal a true "mammal". Reptilian jaw joint still present, though tiny.
• Kuehneotherium (early Jurassic, about 205 Ma) -- 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 (early Jurassic, ~205 Ma) -- A group of early proto-mammals called "morganucodonts". The restructuring of the secondary palate and the floor of the braincase had continued, and was now very mammalian. Truly mammalian teeth: the cheek teeth were finally differentiated into simple premolars and more complex molars, and teeth were replaced only once. Triangular- cusped molars. Reversal of the previous trend toward reduced incisors, with lower incisors increasing to four. Tiny remnant of the reptilian jaw joint. Once thought to be ancestral to monotremes only, but now thought to be ancestral to all three groups of modern mammals -- monotremes, marsupials, and placentals.
• Peramus (late Jurassic, about 155 Ma) -- A "eupantothere" (more advanced placental-type mammal). The closest known relative of the placentals & marsupials. Triconodont molar has with more defined cusps. This fossil is known only from teeth, but judging from closely related eupantotheres (e.g. Amphitherium) it had finally lost the reptilian jaw joint, attaing a fully mammalian three-boned middle ear with excellent high-frequency hearing. Has only 8 cheek teeth, less than other eupantotheres and close to the 7 of the first placental mammals. Also has a large talonid on its "tribosphenic" molars, almost as large as that of the first placentals -- the first development of grinding capability.
• Endotherium (very latest Jurassic, 147 Ma) -- An advanced eupantothere. Fully tribosphenic molars with a well- developed talonid. Known only from one specimen. From Asia; recent fossil finds in Asia suggest that the tribosphenic molar evolved there.
• Kielantherium and Aegialodon (early Cretaceous) -- More advanced eupantotheres known only from teeth. Kielantherium is from Asia and is known from slightly older strata than the European Aegialodon. Both have the talonid on the lower molars. The wear on it indicates that a major new cusp, the protocone, had evolved on the upper molars. By the Middle Cretaceous, animals with the new tribosphenic molar had spread into North America too (North America was still connected to Europe.)
• Steropodon galmani (early Cretaceous) -- The first known definite monotreme, discovered in 1985.
• Vincelestes neuquenianus (early Cretaceous, 135 Ma) -- A probably-placental mammal with some marsupial traits, known from some nice skulls. Placental-type braincase and coiled cochlea. Its intracranial arteries & veins ran in a composite monotreme/placental pattern derived from homologous extracranial vessels in the cynodonts. (Rougier et al., 1992)
• Pariadens kirklandi (late Cretaceous, about 95 Ma) -- The first definite marsupial. Known only from teeth.
• Kennalestes and Asioryctes (late Cretaceous, Mongolia) -- Small, slender animals; eyesocket open behind; simple ring to support eardrum; primitive placental-type brain with large olfactory bulbs; basic primitive tribosphenic tooth pattern. Canine now double rooted. Still just a trace of a non-dentary bone, the coronoid, on the otherwise all-dentary jaw. "Could have given rise to nearly all subsequent placentals." says Carroll (1988).
• Cimolestes, Procerberus, Gypsonictops (very late Cretaceous) -- Primitive North American placentals with same basic tooth pattern.

So, by the late Cretaceous the three groups of modern mammals were in place: monotremes, marsupials, and placentals. Placentals appear to have arisen in East Asia and spread to the Americas by the end of the Cretaceous. In the latest Cretaceous, placentals and marsupials had started to diversify a bit, and after the dinosaurs died out, in the Paleocene, this diversification accelerated. For instance, in the mid- Paleocene the placental fossils include a very primitive primate-like animal (Purgatorius - known only from a tooth, though, and may actually be an early ungulate), a herbivore-like jaw with molars that have flatter tops for better grinding (Protungulatum, probably an early ungulate), and an insectivore (Paranyctoides).

The decision as to which was the first mammal is somewhat subjective. We are placing an inflexible classification system on a gradational series. What happened was that an intermediate group evolved from the 'true' reptiles, which gradually acquired mammalian characters until a point was reached where we have artificially drawn a line between reptiles and mammals. For instance, Pachygenulus and Kayentatherium are both far more mammal-like than reptile-like, but they are both called "reptiles".

Transition from diapsid reptiles to birds

In the mid-1800's, this was one of the most significant gaps in vertebrate fossil evolution. No transitional fossils at all were known, and the two groups seemed impossibly different. Then the exciting discovery of Archeopteryx in 1861 showed clearly that the two groups were in fact related. Since then, some other reptile-bird links have been found. On the whole, though, this is still a gappy transition, consisting of a very large-scale series of "cousin" fossils. I have not included Mononychus (as it appears to be a digger, not a flier, well off the line to modern birds). See Feduccia (1980) and Rayner (1989) for more discussion of the evolution of flight, and Chris Nedin's excellent Archeopteryx FAQ for more info on that critter.

• Coelophysis (late Triassic) -- One of the first theropod dinosaurs. Theropods in general show clear general skeletal affinities with birds (long limbs, hollow bones, foot with 3 toes in front and 1 reversed toe behind, long ilium). Jurassic theropods like Compsognathus are particularly similar to birds.
• Deinonychus, Oviraptor, and other advanced theropods (late Jurassic, Cretaceous) -- Predatory bipedal advanced theropods, larger, with more bird-like skeletal features: semilunate carpal, bony sternum, long arms, reversed pubis. Clearly runners, though, not fliers. These advanced theropods even had clavicles, sometimes fused as in birds. Says Clark (1992): "The detailed similarity between birds and theropod dinosaurs such as Deinonychus is so striking and so pervasive throughout the skeleton that a considerable amount of special pleading is needed to come to any conclusion other than that the sister-group of birds among fossils is one of several theropod dinosaurs." The particular fossils listed here are are not directly ancestral, though, as they occur after Archeopteryx.
• Lisboasaurus estesi & other "troodontid dinosaur-birds" (mid-Jurassic) -- A bird-like theropod reptile with very bird-like teeth (that is, teeth very like those of early toothed birds, since modern birds have no teeth). These really could be ancestral.

GAP: The exact reptilian ancestor of Archeopteryx, and the first development of feathers, are unknown. Early bird evolution seems to have involved little forest climbers and then little forest fliers, both of which are guaranteed to leave very bad fossil records (little animal + acidic forest soil = no remains). Archeopteryx itself is really about the best we could ask for: several specimens has superb feather impressions, it is clearly related to both reptiles and birds, and it clearly shows that the transition is feasible.

• One possible ancestor of Archeopteryx is Protoavis (Triassic, ~225 Ma) -- A highly controversial fossil that may or may not be an extremely early bird. Unfortunately, not enough of the fossil was recovered to determine if it is definitely related to the birds.
• Archeopteryx lithographica (Late Jurassic, 150 Ma) -- The several known specimes of this deservedly famous fossil show a mosaic of reptilian and avian features, with the reptilian features predominating. The skull and skeleton are basically reptilian (skull, teeth, vertebrae, sternum, ribs, pelvis, tail, digits, claws, generally unfused bones). Bird traits are limited to an avian furcula (wishbone, for attachment of flight muscles; recall that at least some dinosaurs had this too), modified forelimbs, and -- the real kicker -- unmistakable lift-producing flight feathers. Archeopteryx could probably flap from tree to tree, but couldn't take off from the ground, since it lacked a keeled breastbone for large flight muscles, and had a weak shoulder compared to modern birds. May not have been the direct ancestor of modern birds. (Wellnhofer, 1993)
• Sinornis santensis ("Chinese bird", early Cretaceous, 138 Ma) -- A recently found little primitive bird. Bird traits: short trunk, claws on the toes, flight-specialized shoulders, stronger flight- feather bones, tightly folding wrist, short hand. (These traits make it a much better flier than Archeopteryx.) Reptilian traits: teeth, stomach ribs, unfused hand bones, reptilian-shaped unfused pelvis. (These remaining reptilian traits wouldn't have interfered with flight.) Intermediate traits: metatarsals partially fused, medium-sized sternal keel, medium-length tail (8 vertebrae) with fused pygostyle at the tip. (Sereno & Rao, 1992).
• "Las Hoyas bird" or "Spanish bird" [not yet named; early Cretaceous, 131 Ma) -- Another recently found "little forest flier". It still has reptilian pelvis & legs, with bird-like shoulder. Tail is medium-length with a fused tip. A fossil down feather was found with the Las Hoyas bird, indicating homeothermy. (Sanz et al., 1992)
• Ambiortus dementjevi (early Cretaceous, 125 Ma) -- The third known "little forest flier", found in 1985. Very fragmentary fossil.
• Hesperornis, Ichthyornis, and other Cretaceous diving birds -- This line of birds became specialized for diving, like modern cormorants. As they lived along saltwater coasts, there are many fossils known. Skeleton further modified for flight (fusion of pelvis bones, fusion of hand bones, short & fused tail). Still had true socketed teeth, a reptilian trait.

[Note: a classic study of chicken embryos showed that chicken bills can be induced to develop teeth, indicating that chickens (and perhaps other modern birds) still retain the genes for making teeth. Also note that molecular data shows that crocodiles are birds' closest living relatives.]

Part 1A

Contents

Part 2A