Transitional Vertebrate Fossils FAQ
Copyright © 1994-1997 by Kathleen Hunt
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:
- Many transitional ceratopsids between Styracosaurus and Pachyrhinosaurus
- Many transitional lambeosaurids (50! specimens) between Lambeosaurus and Hypacrosaurus.
- A transitional pachycephalosaurid between Stegoceras and Pachycephalosaurus
- 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.
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.
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.
[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.]
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