Those Dang Tetrapod Tracks (Part 1)

This entry is part 1 of 2 in the series Those Dang Tetrapods

Once again, the evidence flies in the face of evolutionary (i.e., neo-Darwinian) theory. Or, does it?

A new discovery in Poland (see here or here) places the oldest tetrapod several million years earlier than the supposed transitional forms that biologists have been touting as proof of fish evolving into land animals. Naturally, this has caused quite a few excited ripples throughout the scientific community, with evolutionists in general insisting that the discovery isn’t really a problem and Darwin-skeptics pointing out that it really is a problem — possibly even a game-changer. But, everyone agrees that this is a significant find which could lead to further, revolutionary discoveries and advancement in knowledge of the era in question.

Before delving into the new find itself, let’s get a little background on the prevalent theory. First, a “tetrapod” is any four-limbed (but, not finned), usually terrestrial, animal with a spine — from salamanders to lizards to buzzards to Joe Sixpack. Evolutionary biology teaches that the first tetrapods evolved from lobe-finned fish, out of brackish and/or fresh water,
in the Devonian Period, which spans 416 to 359.2 million years ago (Mya). Loooong before the dinosaurs ruled the Earth, obviously, since they’re, like, “advanced” tetrapods.

For quite awhile, evolutionary biologists believed that some of these lobe-finned fish started coming up on land to hunt for prey (like today’s mudskippers) or to search for water when their pond started drying up. Those who were able to stay out of the water for longer periods began adapting and eventually developed lungs, legs, etc. In the late 1980s, British paleontologist Jennifer A. Clack discovered the fossil of an Acanthostega, an early “transitional” tetrapod that turned out to be entirely aquatic, thereby forcing a shift in the consensus view of the “fish to tetrapod” transition. That is, it became apparent that the first tetrapods were not land-dwellers, so the reasons for the transition no longer fit. Some have gone with an “escape-from-predator” scenario as the selective pressure for moving to land. Others speculate the limbs were for “negotiating their way through underwater obstacles” and were later “co-opted for terrestrial use.”

The major “players” are/were as follows:

Coelacanth is probably the best-known of the prehistoric, lobe-finned fishes, primarily because several specimens have actually been found alive in modern times (latest in 1998). It first shows up in the fossil record at 410 Mya and until 1938 was believed to have gone extinct roughly 65 Mya. Some of the coelacanth’s unusual features include a three-lobed tailfin; pectoral, pelvic and anal fins on muscular stalks (or lobes) supported by bones; a special electroreceptive device (i.e., a rostral organ) in the front of the skull, thought to have aided with balance and/or in hunting prey. However, while the coelacanth was originally thought to be a “missing link”, it is now understood to not be part of the direct tetrapod lineage. Rather, it is their sister group, the Rhipidistians. These split into lungfishes and Tetrapodomorphs, both of whom are believed to have developed proto-lungs and proto-limbs — in the middle Devonian (397-385 Mya) and late Devonian (385-359 Mya) epochs, respectively.

Lobe-finned fish and early tetrapods

Lobe-finned fish and early tetrapods (borrowed from Wikipedia)

Often featured in textbooks as a “missing link” dating to about 385 Mya, Eusthenopteron was actually a primitive lobe-finned fish. But, it shared certain features with tetrapods — e.g., certain skull rooting bone patterns, internal nostrils, labyrinthodont teeth, a two-part cranium hinged at mid-length along an intracranial joint, and similar patterns of its fin endoskeleton (i.e., there are bones reminiscent of arms & legs). On the other hand, Eusthenopteron appears to have lacked a clear larval stage and metamorphosis, unlike later tetrapods.

Panderichthys, which lived around 380-385 Mya, had a large head like a tetrapod’s and is considered to have many features that demonstrated an early transitional phase between the lobe-finned fish and primitive tetrapods. For example, “its pectoral girdle shows derived characteristics while its pelvic girdle retains ancestral ones,” so it could at least prop itself up on its muscular forelimbs. Research reported on in 2008 discussed a fanlike array of fingerlike extensions “tucked beneath the fin’s skin and bony scales and rays.” Notably, it also had a spiracle (kind of like a whale’s blowhole), which is held by some to have eventually evolved into the stirrup bone of the human middle ear.

Acanthostega dates to about 365 Mya. It was salamander-like in appearance, with four legs & feet and eight webbed digits on the end of each, but it had no wrists (ankles?) to support itself out of the water. It had primitive lungs, but its short ribs could not yet support its chest cavity on land. Its gills were more like a fish’s than like a modern amphibian’s. Research on Acanthostega‘s skull indicates that it took part in “terrestrial-style” feeding (i.e., it bit down on its prey, rather than suction-feeding like most fish). It is the fusions in the creature’s pelvic girdle, a development that allowed it to bear more weight than its ancestors, that is most significant in tetrapod evolution. But, because it still couldn’t survive on land, the legs must have evolved for some reason other than walking.

Known Ichthyostega fossils range from 367-362.5 Mya. It appears to have had a more fish-like skull than its contemporary Acanthostega but likely relied more on its lungs for breathing than its gills. Its more robust girdle, more supportive ribs, and stronger vertebrae would seem to make it more suited for living on land than Acanthostega. It had seven digits on each hind limb (unknown for forelimbs) and fin rays on its tail. While the adult Ichthyostega was able to pull itself onto land and possibly sun itself for awhile, its hind limbs were apparently too weak to allow for much terrestrial locomotion. It had to return to the water to cool down, feed, and reproduce. [Note: Recent studies published in 2009 on the pattern of muscle attachment processes on limb bones suggest Ichthyostega was earlier in the evolutionary chain than Acanthostega. Yet, the latter was entirely aquatic….]

Other candidates for earliest tetrapod are the Elginerpeton and Obruchevichthys, both of which are known by only a few fossil bones (just a jawbone for the latter) dating to around 375 Mya.

That’s where things stood for many years….

Continued at Those Dang Tetrapod Tracks (Part 2).

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