Replies

Transition from primitive jawless fish to sharks, skates, and rays
Late Silurian — first little simple shark-like denticles.
Early Devonian — first recognizable shark teeth, clearly derived from scales.
GAP: Note that these first, very very old traces of shark-like animals are so fragmentary that we can’t get much detailed information. So, we don’t know which jawless fish was the actual ancestor of early sharks.

Cladoselache (late Devonian) — Magnificent early shark fossils, found in Cleveland roadcuts during the construction of the U.S. interstate highways. Probably not directly ancestral to sharks, but gives a remarkable picture of general early shark anatomy, down to the muscle fibers!
Tristychius & similar hybodonts (early Mississippian) — Primitive proto-sharks with broad-based but otherwise shark-like fins.
Ctenacanthus & similar ctenacanthids (late Devonian) — Primitive, slow sharks with broad-based shark-like fins & fin spines. Probably ancestral to all modern sharks, skates, and rays. Fragmentary fin spines (Triassic) — from more advanced sharks.
Paleospinax (early Jurassic) — More advanced features such as detached upper jaw, but retains primitive ctenacanthid features such as two dorsal spines, primitive teeth, etc.
Spathobatis (late Jurassic) — First proto-ray.
Protospinax (late Jurassic) — A very early shark/skate. After this, first heterodonts, hexanchids, & nurse sharks appear (late Jurassic). Other shark groups date from the Cretaceous or Eocene. First true skates known from Upper Cretaceous.
A separate lineage leads from the ctenacanthids through Echinochimaera (late Mississippian) and Similihari (late Pennsylvanian) to the modern ratfish.

Transition from from primitive jawless fish to bony fish
Upper Silurian — first little scales found.
GAP: Once again, the first traces are so fragmentary that the actual ancestor can’t be identified.

Acanthodians(?) (Silurian) — A puzzling group of spiny fish with similarities to early bony fish.
Palaeoniscoids (e.g. Cheirolepis, Mimia; early Devonian) — Primitive bony ray-finned fishes that gave rise to the vast majority of living fish. Heavy acanthodian-type scales, acanthodian-like skull, and big notochord.
Canobius, Aeduella (Carboniferous) — Later paleoniscoids with smaller, more advanced jaws.
Parasemionotus (early Triassic) — “Holostean” fish with modified cheeks but still many primitive features. Almost exactly intermediate between the late paleoniscoids & first teleosts. Note: most of these fish lived in seasonal rivers and had lungs. Repeat: lungs first evolved in fish.
Oreochima & similar pholidophorids (late Triassic) — The most primitive teleosts, with lighter scales (almost cycloid), partially ossified vertebrae, more advanced cheeks & jaws.
Leptolepis & similar leptolepids (Jurassic) — More advanced with fully ossified vertebrae & cycloid scales. The Jurassic leptolepids radiated into the modern teleosts (the massive, successful group of fishes that are almost totally dominant today). Lung transformed into swim bladder.
Eels & sardines date from the late Jurassic, salmonids from the Paleocene & Eocene, carp from the Cretaceous, and the great group of spiny teleosts from the Eocene. The first members of many of these families are known and are in the leptolepid family (note the inherent classification problem!).

Transition from primitive bony fish to amphibians
Few people realize that the fish-amphibian transition was not a transition from water to land. It was a transition from fins to feet that took place in the water. The very first amphibians seem to have developed legs and feet to scud around on the bottom in the water, as some modern fish do, not to walk on land (see Edwards, 1989). This aquatic-feet stage meant the fins didn’t have to change very quickly, the weight-bearing limb musculature didn’t have to be very well developed, and the axial musculature didn’t have to change at all. Recently found fragmented fossils from the middle Upper Devonian, and new discoveries of late Upper Devonian feet (see below), support this idea of an “aquatic feet” stage. Eventually, of course, amphibians did move onto the land. This involved attaching the pelvis more firmly to the spine, and separating the shoulder from the skull. Lungs were not a problem, since lungs are an ancient fish trait and were present already.

Paleoniscoids again (e.g. Cheirolepis) — These ancient bony fish probably gave rise both to modern ray-finned fish (mentioned above), and also to the lobe-finned fish.
Osteolepis (mid-Devonian) — One of the earliest crossopterygian lobe-finned fishes, still sharing some characters with the lungfish (the other lobe-finned fishes). Had paired fins with a leg-like arrangement of major limb bones, capable of flexing at the “elbow”, and had an early-amphibian-like skull and teeth.
Eusthenopteron, Sterropterygion (mid-late Devonian) — Early rhipidistian lobe-finned fish roughly intermediate between early crossopterygian fish and the earliest amphibians. Eusthenopteron is best known, from an unusually complete fossil first found in 1881. Skull very amphibian-like. Strong amphibian- like backbone. Fins very like early amphibian feet in the overall layout of the major bones, muscle attachments, and bone processes, with tetrapod-like tetrahedral humerus, and tetrapod-like elbow and knee joints. But there are no perceptible “toes”, just a set of identical fin rays. Body & skull proportions rather fishlike.
Panderichthys, Elpistostege (mid-late Devonian, about 370 Ma) — These “panderichthyids” are very tetrapod-like lobe-finned fish. Unlike Eusthenopteron, these fish actually look like tetrapods in overall proportions (flattened bodies, dorsally placed orbits, frontal bones! in the skull, straight tails, etc.) and have remarkably foot-like fins.
Fragmented limbs and teeth from the middle Late Devonian (about 370 Ma), possibly belonging to Obruchevichthys — Discovered in 1991 in Scotland, these are the earliest known tetrapod remains. The humerus is mostly tetrapod-like but retains some fish features. The discoverer, Ahlberg (1991), said: “It [the humerus] is more tetrapod-like than any fish humerus, but lacks the characteristic early tetrapod ‘L-shape’...this seems to be a primitive, fish-like character....although the tibia clearly belongs to a leg, the humerus differs enough from the early tetrapod pattern to make it uncertain whether the appendage carried digits or a fin. At first sight the combination of two such extremities in the same animal seems highly unlikely on functional grounds. If, however, tetrapod limbs evolved for aquatic rather than terrestrial locomotion, as recently suggested, such a morphology might be perfectly workable.”
GAP: Ideally, of course, we want an entire skeleton from the middle Late Devonian, not just limb fragments. Nobody’s found one yet.

Hynerpeton, Acanthostega, and Ichthyostega (late Devonian) — A little later, the fin-to-foot transition was almost complete, and we have a set of early tetrapod fossils that clearly did have feet. The most complete are Ichthyostega, Acanthostega gunnari, and the newly described Hynerpeton bassetti (Daeschler et al., 1994). (There are also other genera known from more fragmentary fossils.) Hynerpeton is the earliest of these three genera (365 Ma), but is more advanced in some ways; the other two genera retained more fish- like characters longer than the Hynerpeton lineage did.
Labyrinthodonts (eg Pholidogaster, Pteroplax) (late Dev./early Miss.) — These larger amphibians still have some icthyostegid fish features, such as skull bone patterns, labyrinthine tooth dentine, presence & pattern of large palatal tusks, the fish skull hinge, pieces of gill structure between cheek & shoulder, and the vertebral structure. But they have lost several other fish features: the fin rays in the tail are gone, the vertebrae are stronger and interlocking, the nasal passage for air intake is well defined, etc.
More info on those first known Late Devonian amphibians: Acanthostega gunnari was very fish-like, and recently Coates & Clack (1991) found that it still had internal gills! They said: “Acanthostega seems to have retained fish-like internal gills and an open opercular chamber for use in aquatic respiration, implying that the earliest tetrapods were not fully terrestrial....Retention of fish-like internal gills by a Devonian tetrapod blurs the traditional distinction between tetrapods and fishes...this adds further support to the suggestion that unique tetrapod characters such as limbs with digits evolved first for use in water rather than for walking on land.” Acanthostega also had a remarkably fish-like shoulder and forelimb. Ichthyostega was also very fishlike, retaining a fish-like finned tail, permanent lateral line system, and notochord. Neither of these two animals could have survived long on land.

Coates & Clack (1990) also recently found the first really well- preserved feet, from Acanthostega (front foot found) and Ichthyostega (hind foot found). (Hynerpeton’s feet are unknown.) The feet were much more fin-like than anyone expected. It had been assumed that they had five toes on each foot, as do all modern tetrapods. This was a puzzle since the fins of lobe-finned fishes don’t seem to be built on a five-toed plan. It turns out that Acanthostega’s front foot had eight toes, and Ichthyostega’s hind foot had seven toes, giving both feet the look of a short, stout flipper with many “toe rays” similar to fin rays. All you have to do to a lobe- fin to make it into a many-toed foot like this is curl it, wrapping the fin rays forward around the end of the limb. In fact, this is exactly how feet develop in larval amphibians, from a curled limb bud. (Also see Gould’s essay on this subject, “Eight Little Piggies”.) Said the discoverers (Coates & Clack, 1990): “The morphology of the limbs of Acanthostega and Ichthyostega suggest an aquatic mode of life, compatible with a recent assessment of the fish-tetrapod transition. The dorsoventrally compressed lower leg bones of Ichthyostega strongly resemble those of a cetacean [whale] pectoral flipper. A peculiar, poorly ossified mass lies anteriorly adjacent to the digits, and appears to be reinforcement for the leading edge of this paddle-like limb.” Coates & Clack also found that Acanthostega’s front foot couldn’t bend forward at the elbow, and thus couldn’t be brought into a weight-bearing position. In other words this “foot” still functioned as a horizontal fin. Ichthyostega’s hind foot may have functioned this way too, though its front feet could take weight. Functionally, these two animals were not fully amphibian; they lived in an in-between fish/amphibian niche, with their feet still partly functioning as fins. Though they are probably not ancestral to later tetrapods, Acanthostega & Ichthyostega certainly show that the transition from fish to amphibian is feasible!

Hynerpeton, in contrast, probably did not have internal gills and already had a well-developed shoulder girdle; it could elevate and retract its forelimb strongly, and it had strong muscles that attached the shoulder to the rest of the body (Daeschler et al., 1994). Hynerpeton’s discoverers think that since it had the strongest limbs earliest on, it may be the actual ancestor of all subsequent terrestrial tetrapods, while Acanthostega and Ichthyostega may have been a side branch that stayed happily in a mostly-aquatic niche.

In summary, the very first amphibians (presently known only from fragments) were probably almost totally aquatic, had both lungs and internal gills throughout life, and scudded around underwater with flipper-like, many-toed feet that didn’t carry much weight. Different lineages of amphibians began to bend either the hind feet or front feet forward so that the feet carried weight. One line (Hynerpeton) bore weight on all four feet, developed strong limb girdles and muscles, and quickly became more terrestrial.

Transitions among amphibians
Temnospondyls, e.g Pholidogaster (Mississippian, about 330 Ma) — A group of large labrinthodont amphibians, transitional between the early amphibians (the ichthyostegids, described above) and later amphibians such as rhachitomes and anthracosaurs. Probably also gave rise to modern amphibians (the Lissamphibia) via this chain of six temnospondyl genera , showing progressive modification of the palate, dentition, ear, and pectoral girdle, with steady reduction in body size (Milner, in Benton 1988). Notice, though, that the times are out of order, though they are all from the Pennsylvanian and early Permian. Either some of the “Permian” genera arose earlier, in the Pennsylvanian (quite likely), and/or some of these genera are “cousins”, not direct ancestors (also quite likely).
Dendrerpeton acadianum (early Penn.) — 4-toed hand, ribs straight, etc.
Archegosaurus decheni (early Permian) — Intertemporals lost, etc.
Eryops megacephalus (late Penn.) — Occipital condyle splitting in 2, etc.
Trematops spp. (late Permian) — Eardrum like modern amphibians, etc.
Amphibamus lyelli (mid-Penn.) — Double occipital condyles, ribs very small, etc.
Doleserpeton annectens or perhaps Schoenfelderpeton (both early Permian) — First pedicellate teeth! (a classic trait of modern amphibians) etc.
From there we jump to the Mesozoic:

Triadobatrachus (early Triassic) — a proto-frog, with a longer trunk and much less specialized hipbone, and a tail still present (but very short).
Vieraella (early Jurassic) — first known true frog.
Karaurus (early Jurassic) — first known salamander.
Finally, here’s a recently found fossil:

Unnamed proto-anthracosaur — described by Bolt et al., 1988. This animal combines primitive features of palaeostegalians (e.g. temnospondyl-like vertebrae) with new anthracosaur-like features. Anthracosaurs were the group of large amphibians that are thought to have led, eventually, to the reptiles. Found in a new Lower Carboniferous site in Iowa, from about 320 Ma.