It's one of the iconic images of evolution: way back in the prehistoric mists of time, a brave fish crawls laboriously out of the water and onto dry land, the first wave of a vertebrate invasion that leads directly (hundreds of millions of years later) to dinosaurs, mammals, and human beings. Logically speaking, of course, we don't owe any more to the first tetrapod than to the first bacterium or the first sponge, but something about this plucky critter still tugs at our heartstrings. (See a gallery of tetrapod pictures.)
As is so often the case, though, this romantic image doesn't quite match up with evolutionary reality. The fact is that, 400 to 350 million years ago, various prehistoric fish crawled out of the water at various times, making it nearly impossible to identify the "direct" ancestor of modern vertebrates. Even worse, many of the most celebrated early tetrapods (Greek for "four feet") had seven or eight digits at the end of each limb--and because modern animals adhere strictly to the five-toed body plan, that means these tetrapods represented an evolutionary dead end from the perspective of the prehistoric amphibians that followed them.
The Origin of Tetrapods
What kind of fish did the earliest tetrapods evolve from? Here, there's a solid consensus: the immediate predecessors of tetrapods were "lobe-finned" fishes, which differed in important ways from "ray-finned" fishes (the most common type of fish today). The bottom fins of lobe-finned fishes are arranged in pairs and supported by internal bones--the necessary conditions for these fins to evolve into primitive legs. What's more, the lobe-finned fishes of the Devonian period were already able to breathe air, when necessary, via "spiracles" in their skulls. (Today, the only lobe-finned fish on the planet are lungfish and coelacanths, the latter of which were thought to have gone extinct tens of millions of years ago until a live specimen turned up in 1938.)
Experts differ about the environmental pressures that prompted lobe-finned fish to evolve into walking, breathing tetrapods. One theory is that the shallow lakes and rivers these fish lived in were subject to drought, favoring species that could survive (at least for a while) in dry conditions. Another theory has it that the earliest tetrapods were literally chased out of the water by bigger fish: dry land harbored an abundance of insect and plant food, and a marked absence of dangerous predators. Any lobe-finned fish that blundered onto land would have found itself in (by Devonian terms, at least) a veritable paradise.
In evolutionary terms, it's hard to distinguish between the most advanced lobe-finned fish and the most primitive tetrapods. Three important genera nearer the fish end of the spectrum were Eusthenopteron, Panderichthys and Osteolopis, which spent all of their time in the water yet had latent tetrapod characteristics, which only a trained paleontologist can possibly hope to detect. (Until recently, these tetrapod ancestors nearly all hailed from fossil deposits in the northern Atlantic, but the discovery of Gogonasus in Australia has put the kibosh on the theory that land-dwelling animals originated in the northern hemisphere).
Early Tetrapods and "Fishapods"
Scientists once agreed that the earliest tetrapods (as opposed to the tetrapod-like lobe-finned fish described above) dated from about 385 to 380 million years ago. That has all changed with the recent discovery, in Poland, of tetrapod track marks dating to 397 million years ago, which has had the effect of "dialing back" the entire evolutionary calendar by a whopping 12 million years. If confirmed, this discovery will prompt some revision in the evolutionary consensus (as well as this article)!
The reason I stress this little tidbit is that tetrapod evolution is far from written in stone: as mentioned above, it seems that tetrapods evolved numerous times, in different places. Still, there are a few early species that are regarded as more-or-less definitive by experts. The most important of these is Tiktaalik, which seems to have been perched midway between the tetrapod-like lobe-finned fishes and the later, true tetrapods (about which more below). Tiktaalik was blessed with the primitive equivalent of wrists, which may have helped it to prop itself up on its stubby front fins along the edges of shallow lakes, as well as a true neck, providing it with much-needed flexibility and mobility during its quick jaunts onto dry land.
Because of its startling mix of tetrapod and fish characteristics, Tiktaalik is often referred to as a "fishapod" (though this name is also sometimes applied to advanced lobe-finned fish like Eusthenopteron and Panderichthys). Another important fishapod was Ichthyostega, which lived about five million years after Tiktaalik and achieved similarly respectable sizes--about five feet long and 50 pounds, a far cry from the tiny, flopping, stubby-legged fish most people picture as crawling out of the prehistoric sea.
Toward True Tetrapods
Until the recent discovery of Tiktaalik, the most famous of all the early tetrapods was Acanthostega, which dated to about 365 million years ago. This slender, fish-sized creature had relatively well-developed (but still fin-like) limbs, as well as such "fishy" features as a lateral sensory line running along the length of its body. Other, similar tetrapods of this general time and place included Hynerpeton (which was discovered in Pennsylvania), Tulerpeton and Ventastega.
Paleontologists once (perhaps wishfully) believed that these late Devonian tetrapods spent significant amounts of their time on dry land, but they're now regarded as having been primarily or even totally aquatic, only using their legs (and primitive breathing apparatuses) when absolutely necessary. The most startling thing about these tetrapods, though, was the number of digits on their front and hind limbs: anywhere from 6 to 8, which strongly hints that they couldn't have been ancestral to later tetrapods and their mammalian, avian and reptilian descendants, which adhere strictly to the five-toed body plan.
Romer's Gap - A Tetrapod Roadblock
Here's where the story of tetrapod evolution becomes a bit murky. Frustratingly, there's a 20-million-year-long stretch of time in the early Carboniferous period that has yielded very few vertebrate fossils, anywhere in the world. Creationists like to seize on "Romer's Gap" as evidence that the theory of evolution is half-baked, but you have to remember that fossils only form in very special conditions--so we shouldn't be surprised if global geology occasionally worked against the preservation of individuals.
What makes Romer's Gap maddening, from the perspective of tetrapod evolution, is that when we pick up the story again 20 million years later (about 340 million years ago), there are a profusion of tetrapod species, grouped into different families, and some coming very close to being true amphibians. Among the notable post-gap tetrapods are the tiny Casineria, which had five-toed feet, the eel-like Greererpeton (which may already have "de-evolved" from its more land-oriented tetrapod ancestors), and the salamander-like Eucritta melanolimnetes (otherwise known as "the creature from the Black Lagoon") from Scotland. These later tetrapods are already very diverse, meaning that a lot must have happened, evolution-wise, during Romer's Gap.
Fortunately, in recent years Romer's Gap has become a bit less gaping. Although the skeleton of Pederpes was discovered in 1971, it wasn't until three decades later that further investigation (by the famous tetrapod hunter Jennifer Clack) dated it smack to the middle of Romer's Gap. Significantly, Pederpes had forward-facing feet with five toes and a narrow skull, characteristics seen in later amphibians, reptiles and mammals. Its fellow occupant in Romer's Gap was the similar, but bigger-tailed Whatcheeria, which seems to have spent most of its time in the water.
The following is a list of the most important tetrapods; just click on the links for more information.
Acanthostega One of the most famous early tetrapods.
Casineria Among the first of the five-toed tetrapods.
Cochleosaurus One of the most dangerous Carboniferous tetrapods.
Crassigyrinus This tetrapod looked like a giant tadpole.
Diadectes This tetrapod was the biggest land animal of its time.
Elginerpeton This ancient tetrapod was discovered in Scotland.
Eucritta Better known as the "Creature from the Black Lagoon."
Eusthenopeteron This lobe-finned fish was ancestral to the first tetrapods.
Gogonasus Yet another early fish with tetrapod-like characteristics.
Greererpeton This early tetrapod looked like an eel.
Hynerpeton The only tetrapod to be dug up in Pennsylvania.
Ichthyostega One of the first tetrapods ever to be identified.
Osteolepis This ancient fish had some latent tetrapod characteristics.
Panderichthys Another Devonian fish with tetrapod leanings.
Pederpes This creature fills an important gap in tetrapod evolution.
Pholidogaster A long, slender tetrapod of the Carboniferous period.
Tiktaalik This "fishapod" was halfway between a fish and a tetrapod.
Tulerpeton One of the first "true" tetrapods.
Ventastega An alligator-like tetrapod of the late Devonian.
Whatcheeria Along with Pederpes, an important missing link in tetrapod evolution.