Tetrapods: The Fish Out of Water

A complete fossilized skeleton found in rock
Seymouria (Seymouria baylorensis), a tetrapod from the Early Permian Period found as fossil in North America.

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It's one of the iconic images of evolution: 400 or so million years ago, way back in the prehistoric mists of geologic time, a brave fish crawls laboriously out of the water and onto land, representing the first wave of a vertebrate invasion that leads to dinosaurs, mammals, and human beings. Logically speaking, of course, we don't owe any more thanks to the first tetrapod (Greek for "four feet") than we do to the first bacterium or the first sponge, but something about this plucky critter still tugs at our heartstrings.

As is so often the case, this romantic image doesn't quite match up with evolutionary reality. Between 350 and 400 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. In fact, many of the most celebrated early tetrapods 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.

Origins

The earliest tetrapods evolved from "lobe-finned" fishes, which differed in important ways from "ray-finned" fishes. While ray-finned fishes are the most common type of fish in the ocean 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. 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. Lobe-finned fishes of the Devonian period were already able to breathe air, when necessary, via "spiracles" in their skulls.

Experts differ about the environmental pressures that prompted lobe-finned fish to evolve into walking, breathing tetrapods, but one theory is that the shallow lakes and rivers these fish lived in were subject to drought, favoring species that could survive 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 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. 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 true tetrapods dated from about 385 to 380 million years ago. That has all changed with the recent discovery of tetrapod track marks in Poland that date to 397 million years ago, which would effectively dial back the evolutionary calendar by 12 million years. If confirmed, this discovery will prompt some revision in the evolutionary consensus.

As you can see, tetrapod evolution is far from written in stone—tetrapods evolved numerous times, in different places. Still, there are a few early tetrapod species that are regarded as more-or-less definitive by experts. The most important of these is Tiktaalik, which is thought to have been perched midway between the tetrapod-like lobe-finned fishes and the later, true tetrapods. 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 mix of tetrapod and fish characteristics, Tiktaalik is often referred to as a "fishapod," a name that 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.

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 creature had relatively well-developed 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, Tulerpeton, and Ventastega.

Paleontologists once believed that these late Devonian tetrapods spent significant amounts of their time on dry land, but they are now thought to have been primarily or even totally aquatic, only using their legs and primitive breathing apparatuses when absolutely necessary. The most significant finding about these tetrapods was the number of digits on their front and hind limbs: anywhere from 6 to 8, a strong indication that they couldn't have been the ancestors of later five-toed tetrapods and their mammalian, avian, and reptilian descendants.

Romer's Gap

There's a 20-million-year-long stretch of time in the early Carboniferous period that has yielded very few vertebrate fossils. Known as Romer's Gap, this blank period in the fossil record has been used to support Creationist doubt in the theory of evolution, but it is easily explainable by the fact that fossils only form in very special conditions. Romer's Gap particularly affects our knowledge of tetrapod evolution because, when we pick up the story 20 million years later (about 340 million years ago), there is a profusion of tetrapod species that can be grouped into different families, 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. The diversity of later tetrapods is evidence that a lot must have happened, evolution-wise, during Romer's Gap.

Fortunately, we have been able to fill in some of the blanks of Romer's Gap in recent years. The skeleton of Pederpes was discovered in 1971 and, three decades later, further investigation by tetrapod expert 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. A similar species active during Romer's Gap was the large-tailed Whatcheeria, which seems to have spent most of its time in the water.