Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere (2006)

The Ordovician Period followed the Cambrian and it is recognized as a separate time interval because of differences in the kinds of animals found in each. As we saw in Chapter 3, the Cambrian came to an end because of a mass extinction. After any mass extinction there is usually a time of rapid evolution to fill empty ecological niches, and so it was with the Ordovician. Many kinds of animals were present in the Ordovician that had not yet evolved in the Cambrian, and many of these appeared soon after the end of the Cambrian mass extinction. The result was an assemblage of animals that were markedly different than most of the Cambrian faunas. Trilobites were still there, but they were swamped out by what might be called a “shelly fauna” of invertebrates, dominated by the bivalved brachiopods, with lots of corals, bryozoans, echinoderms, and mollusks thrown in. It is here too that the first coral reefs appeared as well as the first skeletonized fishes. Let’s go back and look at how the ancient Ordovician world might have been.

We have dusted off our time machine for a look at the early Ordovician. As in the previous chapter’s voyage, this machine is part airplane

and part submarine—and entirely imaginary. What we might see, however, comes from two centuries of paleontological work.

We sweep down over the land surface and notice immediately that it is greener. There are still no trees or even commonly rooted plants, but mosses have spread and among them are primitive vascular plants. There is also the occasional movement of animals on the land surface, not vertebrates, but a diversity of scorpion and centipede-like arthropods. There is not yet a high diversity of them, but there are enough to convince us that an invasion has started, a beachhead has been established, and more troops are on the way. As we pass over the landscape, much is familiar: mountains and plains, great glaciers in the high latitudes, sandy beaches with sand dunes and ripples, the spray of the sea on the land. True, the lack of trees and bushes, flowers and grass, birds and even flying insects is curious enough, and there is much more bare rock than we are used to outside the desert, high mountain, or glacial regions of our own world. But there is something else about this landscape that nags, something also seen on our previous look at the land in the Cambrian world 50 million years previously. As during the Cambrian, the rivers are still braided, not meandering. There are no riverbanks, just a vast wide complex of shallow rushing water on its journey from the distant snow-capped mountains to the sea.

We return to the seashore and power downward into the sea, expecting changes and not being disappointed in this. At first, as we settle onto a shallow, warm bottom, things look superficially the same because the most common bottom dwellers are sponges. In the clear blue tropical water, we first think we are in the shallows off Jamaica or the Florida Keys—there are sponges everywhere, in myriad colors and shapes. Many are glass sponges, but the more familiar demosponges are there too. Amid them are untold numbers of bivalved creatures, looking a bit like clams of our world. But closer inspection shows them to be brachiopods, forms with articulated shells that are more advanced than the small, inarticulate brachiopods of the Cambrian world. They coat the bottom, and amid them is an animal not seen in the Cambrian—the colonial bryozoan, which builds calcareous skeletons, each being a tiny miniature replicate of the solitary brachiopods. The brachiopods look like clams from our world, but this is but

a trick of evolution, where successful body designs can be seen in unrelated stocks. Both bryozoans and brachiopods show a large feathery device used for feeding—the lophophore. But while acknowledged as a feeding organ, it is evident that it is used as a respiratory organ as well. Both the solitary brachiopods and the colonial bryozoans are seen to be actively moving water across these lophophores in impressive fashion. For the brachiopods, water is sucked into the shell and then blown out after crossing the lophophore. The colonial bryozoans produce more complex but equally effective water currents by positioning the tiny zooids, thousands of which make up medium-sized colonies, in such a position as to produce an area on the colony where water enters the forest of tiny lophophores and then different areas where this water is blown away from the colony. For all such animals, once the water has been mined of its twin treasures, food in the form of microscopic plankton and bacteria and life-ensuring oxygen, it must be sent away so that it is not recirculated.

We continue onward, and a great stony city comes into view. It is a reef, and it looks, at least at first glance, surprisingly familiar. We are 475 million years in the past, hovering over a reef that would make a Club Med proud. The size and shape of this reef are something that we did not see in the Cambrian. There were reefs back then, but they were made up of the remarkable archeocyathids, sponges that build cylindrical and vase-shaped skeletons and are very much smaller in size. This one is immense, a great stone edifice of life. Here we see three dominant kinds of reef builders—colonial corals, the tabulates; large calcareous sponges called stromotoporoids; and solitary corals with horn-shaped skeletons, the rugose corals. It is remarkable—when we look at the whole, the shape of the reef through squinted eyes is so familiar. The same coral-rich walls in the fore-reef areas, the same reef flats, the back-reef lagoon areas filled with animals—colonial and solitary. But then we look at what is making this structure, and it is all different. The reef community here is made up of an entirely different assemblage of builders and binders than what will be the bricks and mortar of the reefs in our time, yet the overall reefs look much the same, just different bricks. Is there a respiratory function to forming these big colonies? Possibly. In our own world the very shape of the

reefs themselves (not just the coral that makes them) maximizes water movement over the millions of small animals living there—and in so doing brings in more oxygenated water than would otherwise be present. Back in the Ordovician the same may apply, for the reef shapes (but not the corals making them) are very familiar indeed.

But is it really the same? It takes time to record all the impressions, to make the connections of similarity and difference. There is oddness here—and then it hits home: no fish.

The swarming schools of reef fish, the amazing color and diversity in our world’s reefs, are nowhere to be seen back in these Ordovicianaged seas. There are a few swimmers, plenty of trilobites around, and boundless numbers of other arthropods busily going about their business of life. Many bear a cornucopia of upward-pointing spines, bearing witness to the dangers in this world. These are defensive adaptations, all, and we immediately start searching for the familiar top carnivore of the older world, the anomolocarids that so effectively ruled the top of the Cambrian ecological pyramid (producers eaten by grazers, in turn eaten by carnivores) for so long. But they are nowhere to be seen and in fact are by now long extinct. Two things happened. There is now more oxygen in the Ordovician, and a new scourge of a predator has appeared in the seas. We look again into the blue waters, and among the swimming arthropods we readily see an entirely different kind of animal up in the water column. It is a body plan that was not present at all in the early and mid-Cambrian, one that first appeared only at the very end of the Cambrian in fact, at the same time that the mass extinction of trilobites and other Cambrian arthropods was under way. We look more closely at these strange swimming predators and see an animal type that is virtually unknown in our world. These are shelled creatures, but unlike the body armor of the arthropods, which is composed of many segments, here the conelike shells are the most striking feature, some straight, some gently curved, some entirely coiled. Then we see something very similar to the chambered nautilus of our world and realize that we are seeing an amazing variety of chambered cephalopods. In only a few minutes of watching their activities, a new conclusion is reached. These nautiloids have toppled the arthropods as the top carnivores.

While some are but a few inches long, there are also veritable battleships, long straight shells passing backward through the water. Some are immense. Six footers are common, but one is easily double that size, and from the large round opening of its conical tubular shell a great head, with saucer eyes, watches, searches, looks. A mass of tentacles surrounds the head, and we can see that propulsion is from great volumes of water being jetted from a funnel-like fleshy tube situated beneath the mass of tentacles. With shell and flesh, the entire animal must weigh several hundred pounds in air, but in this sea it is weightless. Although invisible, we know the inside of the massive shell is composed of numerous chambers, filled with air, but with a thin tube containing a strand of blood vessels passing through the middle of each chamber. The nautiloids have discovered the secret of achieving neutral buoyancy. But they have done far more than this. They have climbed to the top of the trophic pyramid, and it may be that it was the best respiratory system on the planet that got them there.

We move from the sea and on a whim take one more dive—into the brackish water of a large estuary connected to one of the extensive braided river systems. Here we see in large numbers some familiar shapes. There are fish in this fresh water. But they are fish very foreign to us. They are called ostracoderms, and they have small sucking mouths without jaws. Most have extensive dermal body armor in the form of massive scales and larger bony plates. Their tail is “reversed heterocercal,” like an upside-down shark’s tail. And no wonder, most of these early fish are swimming at the surface, thanks to the action of these tails whose design naturally forces the body upward, and they are eating the prolific algae and vegetable scum at the lagoon’s surface. A few other types are seen as well, hugging the bottom, and these are even more heavily armored than their surface-living brethren.

We look more closely at some of these primitive fish for evidence of a respiratory structure. One of these fish, Arandaspis, shows 15 plates covering a primitive gill system called gill pouches. The gills themselves are relatively large compared to the size of the fish, and swimming seems part of the respiratory cycle. Forward motion passes water through the gills, making this the functional equivalent of a pump gill.

Reconstruction of a fish typical of the Ordovician period, the armored genus Hemicyclopsis. These types of fish, known as ostracoderms, did not have jaws. Their closest living relatives are the lampreys and hagfish.

The Ordovician Period can be regarded as the second half of the two-part initiation of animal diversity on Earth, with the first being the Cambrian Explosion. There is a good reason for this. Like the Cambrian, it was a time when new species as well as new kinds of body plans appeared at a faster rate than was characteristic of more recent times. This high rate was in response to filling up the world with animals for the first time.

While the Cambrian ended with a series of minor extinction events affecting mainly trilobites, this dip in diversity was short lived and was succeeded by a huge increase in the number of animal species in the sea, especially among calcareous shell-forming organisms. As we saw on our trip, large coral and sponge reefs appear, as does the first extensive plankton made up of animals—in this case, floating colonies of tiny filter feeding animals called graptolites, which are now totally extinct. Was there a new trigger to this diversification or was it

but a continuation of the Cambrian Explosion? Here again we can invoke the record of oxygen levels to understand this rise in diversity of animals, as well as the first appearance of communities that were largely composed of calcareous skeleton-forming organisms. Skeletal building (as well as every aspect of animal physiology, as we saw in Chapter 1) works better in air or water that is rich in oxygen and nutrients.

But the Ordovician was a time when oxygen levels were still lower than today, and in the middle part of the period they were markedly so. The extensive appearance of many animals with calcareous shells, some, like the corals, massive in size, indicates that there was significant oxygen in the oceans during some of the period. But there was also a major mass extinction, one of the Big Five. This mass extinction occurred either simultaneous with or soon after a drop in oxygen levels to their lowest levels of the period. While gains in diversity made in the Cambrian Explosion were consolidated over part of the Ordovician, this major drop in diversity may have been related to a profound oxygen drop.

How important was respiration in shaping the kinds of animals of the Ordovician? Very important, in my opinion. It takes an ever-better respiratory system to deal with ever-larger bodies. The Cambrian was a time of mainly small animals, most less than an inch long. The Ordovician was a time of much larger animals. For example, in the Ordovician the brachiopods, all mollusks (most notably the giant nautiloid cephalopods), echinoderms, the chordates (our group), and many arthropods were noticeably different if each group as a whole is compared to Cambrian-aged members. In each case this bigness came about through changes and improvements in respiratory systems.

Let’s now move forward in time, looking at events of the Silurian and Devonian, when a short-lived spike in oxygen to the highest levels ever attained on Earth up to that time caused major events in the history of life. Until now, we have explored how low oxygen shaped the biological world. The next chapter will explore the reverse, looking at the consequences of rising oxygen levels at the time when animals and plants colonized the terrestrial realm in earnest.