The OLLI lecture last week was on the origin of tetrapods (that includes all amphibians, reptiles including birds, and mammals) and on the fin-to-limb vs. water-to land transition.
The first land-dwelling animals were arthropods. Millipedes, scorpions, spiders and other creepy-crawlies. No great problems for them—their external skeletons could handle gravity in air, and many could get oxygen from the air with no problem. But the transition from fish to amphibians was a little harder, and was for many years a missing link in evolution.
To start with, there are two kinds of fish. Most of those around today have fins stiffened by bony rays, the ray-finned fish. A very few, the lobe-finned fish, have fleshy structures at the base of their fins. These fleshy structures have bones that are homologous to our own arms and legs. Our arm and hand bones, in turn, are homologous to the wings of birds, bats and pterosaurs as well as the forelegs of every four-legged animal. This group is called the Sarcopterygii, and it includes not only the coelacanths and the first amphibians, but us.
But how did the lobe-finned fish come onto land? Learn to breathe air? Why did lobed fins develop on the first place? The ray-finned fish certainly seem to outcompete the ones which have stayed in the water today. And why did it seem there was a missing link between the development of lobe-finned fish and the first amphibians?
The lobe-finned fish probably had the advantage in waters where there was something other than water to push on. In terms of today’s habitats, think vegetation-choked estuaries and mangrove swamps. (Mangroves hadn’t evolved yet, but there were probably similar habitats.) Think also warm water—warm water can hold less oxygen than cold water, so these fish (and any fish that lives in warm water) probably gulped air at times to get oxygen. Those lobed fins would have been useful in getting around in something like mangrove swamps, and in climbing to the surface to gulp air when oxygen levels were low.
And fish then, as now, probably ate small arthropods. Some of them had moved to land, and the lobe-finned fish probably followed them.
As to why the transition was a missing link for so long, it turns out that tropical coastal sediments of the right age are not easy to find. The equator at that time passed through land masses now in Greenland, the Canadian Arctic islands, and northern Alaska—not easy places to hunt fossils. For a long time here just weren’t any fossils from the right habitats through about a 50 million year time gap. That’s a long time—dinosaurs died out (except for birds) only about 65.5 million years ago. As fossil hunters have moved into the right areas, this gap has been filling in, and at this point only about 10 million years are still missing.
Some potential areas for finding more about this transition have hardly been touched. One of them is even in my own back yard—the Brooks Range of northern Alaska. Wouldn’t it be fun if fossil hunters found the transitional forms here?