Before the Dinosaurs: Walking With Monsters (DVD Review)

Probably the best reconstructions of extinct genera as living animals are those in the BBC/Discovery channel Walking With Dinosaurs series. This was the last of the trilogy made, but the first in terms of time sequence, extending from the formation of the moon (4,533 million years ago) to the early Triassic, with the first ancestors of the dinosaurs. Most of the DVD is taken up with the period from the early Cambrian (530 million years ago) to the early Triassic (248 million years ago) and from the evolution of eyes, predation and hard parts which could be fossilized to the earliest known ancestor of the dinosaurs.

Given this time span, it is somewhat disappointing that the DVD covers only three half-hour segments and focuses only on those forms which gave rise to land vertebrates. Surely there is enough material on the evolution of plants and fish, and the dramatic change in our atmosphere produced by photosynthesis, to have made this DVD at least as long as the other two in the series!

As usual, there are details that most people will recognize are pure guesswork (colors—the fist preserved pigments had not been found at the time this DVD was produced, and as far as I know they are still limited to feathers.) Others, such as behavior, are also highly problematic, and I would like to have seen the arguments addressed in a separate discussion, if not in the main DVD. One that particularly interested me was the gorgonopsid (a synapsid or mammal-like reptile) who was shown cantering—a gait I tend to associate with animals who flex their bodies vertically (as do mammals) rather than horizontally (as do most reptiles.) The DVD does include a “making of” program that covers the entire trilogy of life, but it could easily have been longer.

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Back to the Water

As far as we can tell the fin-to-limb transition, the movement of fish onto land, took place just once. Movement of land animals back to water, on the other had, took place too many times to count, especially if we include early transitional forms like the polar bear.

The final OLLI class on evolutionary transitions focused on cetaceans (whales and dolphins) but we started out with a brief survey of land animals that have gone back to sea—obviously a far easier transition than the movement of fish onto land.

Some of the animals that went back to the oceans are extinct today—the plesiosaurs, icthyosaurs, and  mosasaurs of the Mesozoic. Other reptiles that went back to the ocean are alive today: sea turtles and sea snakes, for instance. If birds are counted as reptiles, penguins also have to be included in this group.

Mammals have returned to the sea several times, and the sirenians, the pinnipeds and the cetacians are the most obvious as they have adapted to water so much that life on land would be difficult. Others that are in the process of adapting include the sea otters and polar bears.

Interestingly, the process of adaptation has generally involved several stages, none of which include the ability to breathe water. Even the deepest-diving of whales, though they can hold their breaths for up to an hour, must come to the surface to breathe.

The first stage of adaptation to the water is simply streamlining and swimming ability. In cold water, insulation in the form of waterproof fur and/or a thick fat layer is also necessary. The polar bear is at this stage. Breeding takes place on sea ice, and the cubs are born on land.

The sea otter can hold its breath a little longer (up to five minutes) and breed and pup in water Its rear legs have been modified for swimming to such an extent that the sea otter finds locomotion on land difficult, though not impossible.

The pinnipeds (seals, sea lions and walrus) are even more clumsy on land—their limbs have been greatly modified into flippers, and their lungs are highly adapted for long periods of breath-holding and recoverable collapse under water pressure. However, they must leave the water to breed and pup, and the pups are not born knowing how to swim.

The sirenians (manatees and dugongs) are unusual in being herbivores. They have lost their hind legs, but their tails have been modified into paddles. The surviving species require shallow, warm water and are totally aquatic. They are also highly endangered due to habitat loss and coastal development. The recently extinct Stellar’s sea cow, however, must have been able to tolerate cold water.

The cetaceans are all carnivores. Some (baleen whales) are filter feeders, consuming large amounts of very small prey. Others (toothed whales, including dolphins and narwhals) hunt larger prey.

Surprisingly, their closest modern relatives are artiodactyls—a group that today includes cows, sheep, moose and hippos. Most of these modern animals are herbivores or in some cases (pigs) omnivores. So the transition from land animals to whales involved a number of changes: herbivory to carnivory; land to water, and fresh water to salt water.

Transitional fossils are still being found, though the most fruitful sites (Pakistan and Afghanistan) are not very hospitable for fossil-hunters these days. It does seem that the transition to water was via wading, then swimming and finally to estuarine systems and the deep ocean. The first (Eocene) episode of the DVD, Walking With Prehistoric Beasts, shows an early transitional form called Ambulocetus (walking whale.) I’m not sure about how well it could hold its breath, but the reconstruction is interesting.

Whales were fully adapted to marine conditions by the late Eocene. Again, Walking with Prehistoric Beasts does a good job of fleshing out Basilosaurus and Dorudon, early forms of whales. Basilosaurus in particular was a far more efficient carnivore than its surviving kin, which are more likely to be descendants of some cousin of Dorudon.

How did we Learn to Walk?

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?

Chased by Dinosaurs (DVD Review)

If you enjoy seeing extinct animals in the flesh, thanks to animatronics and computer animation, you’ll probably enjoy this. The addition of people and artifacts such as an ultralight aircraft, a jeep and a sailboat really puts the dinosaurs and sea monsters (ones that really existed) in scale. They are impressive.

On the negative side, the plot (such as it is) is pretty inane. And the explorer, Nigel Marvin …. Well, all I can say is his writers gave him a death wish.

Any reasonable explorer takes precautions. Even a modern camper in bear country knows not to leave food out if you don’t want a tent broken into. I can only suppose that the writers put excitement above being reasonable. I have to say that I was left feeling that this guy was a grade A idiot.

As a bonus, there is a program about the finding of fossils of Argentinosaurus and Giganotosaurus and the argument on whether carnivorous dinosaurs may have hunted in packs. Personally I found some of the against arguments rather doubtful. Carnivores simply do not move as a group the same way a herd of herbivores do. They split up, attack from different directions, and explore. Consequently, a group of carnivores will not leave the kind of trackways a herd does.

This was a BBC science program and overall is well produced, but I have to admit there are very few natural science programs that I do not find errors in. Overall, though, I probably yelled at the screen less often in this than in many other DVDs.

That said, it’s still worth watching for the dinosaurs.

Again, the DVD is Chased By Dinosaurs.