Transition from primitive bony fish to amphibians

[Pah]

Transitional Vertebrate Fossils FAQ

General lineage":

This is a sequence of similar genera or families, linking an older group to a very different younger group. Each step in the sequence consists of some fossils that represent a certain genus or family, and the whole sequence often covers a span of tens of millions of years. A lineage like this shows obvious morphological intermediates for every major structural change, and the fossils occur roughly (but often not exactly) in the expected order. Usually there are still gaps between each of the groups -- few or none of the speciation events are preserved. Sometimes the individual specimens are not thought to be directly ancestral to the next-youngest fossils (i.e., they may be "cousins" or "uncles" rather than "parents"). However, they are assumed to be closely related to the actual ancestor, since they have intermediate morphology compared to the next-oldest and next-youngest "links". The major point of these general lineages is that animals with intermediate morphology existed at the appropriate times, and thus that the transitions from the proposed ancestors are fully plausible. General lineages are known for almost all modern groups of vertebrates, and make up the bulk of this FAQ.
"Species-to-species transition":

This is a set of numerous individual fossils that show a change between one species and another. It's a very fine-grained sequence documenting the actual speciation event, usually covering less than a million years. These species-to-species transitions are unmistakable when they are found. Throughout successive strata you see the population averages of teeth, feet, vertebrae, etc., changing from what is typical of the first species to what is typical of the next species. Sometimes, these sequences occur only in a limited geographic area (the place where the speciation actually occurred), with analyses from any other area showing an apparently "sudden" change. Other times, though, the transition can be seen over a very wide geological area. Many "species-to-species transitions" are known, mostly for marine invertebrates and recent mammals (both those groups tend to have good fossil records), though they are not as abundant as the general lineages (see below for why this is so). Part 2 lists numerous species-to-species transitions from the mammals

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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.

n 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.

The last quote was given to show an example of the specific transitions listed. The article is part of a much larger FAQ and has links to the rest.

-pah-

 

[painted wolf]

I find it interesting that the earliest amphibians often had more than 5 toes... one (I'll find the species name and specifics later if wanted.) had ten or more toes per limb.. I've often wondered what led to the specific selection of five toes as opposed to say 7?

wa:do

 

[meogi]

I find it interesting that the earliest amphibians often had more than 5 toes... one (I'll find the species name and specifics later if wanted.) had ten or more toes per limb.. I've often wondered what led to the specific selection of five toes as opposed to say 7?

Well, you say most. That mean`s some at least had 5 toes. What`s to say the 5 toed amphibians were favored through natural selection? I`m not basing this off anything I`ve read (I`ve never heard of the question) but that`s how evolution would explain it, I`m fairly certain. Correct me if I`m wrong, please.