A New History of Life

by Peter Ward

  Today we are at 21 percent atmospheric oxygen. But studies on alligators and insects shows that optimal development takes place at 27 percent. Eggs incubated in either higher or lower values of oxygen take longer to develop and hatch. In lower levels of oxygen—not coincidentally, perhaps, the 10–12 percent that occurred in the latest Triassic—many or most eggs never hatch at all, or do so only after such a long time that their probability of not being eaten by egg-eating predators becomes low indeed. Adding heat to the equation makes survivability even lower, because the eggs need holes to let oxygen in. But water escapes from these and causes a higher chance of death of the embryo. The worst combination would have been an atmospheric oxygen level of 10–12 percent in a world both hotter and drier than now. We know of such a time. It was the late Triassic. Those creatures laying eggs in the late Triassic period were in for trouble.

  The problem is that reptiles were first evolved in a relatively high oxygen world: the Carboniferous, when oxygen was above 27 percent. These early reptiles pioneered the amniotic egg. But as oxygen levels dropped and temperature levels increased globally, the original reptilian eggs may have become death traps: not enough oxygen could diffuse in from outside the egg, while too much water was diffusing out. Seemingly a better response to heat and low oxygen (which is further magnified by the heat) would be live birth. The evolution of live birth thus may have come about in response to lowering global oxygen values in the late Permian. In spite of the enormous number of therapsid bones found in South Africa, Russia, and South America, there has never been an egg or nest found from these rocks. Therapsids may have already evolved live birth by this time, a trait carried on by their descendants, the true mammals that are first found at about the same time as the first dinosaurs appeared on the scene.

  It may be that many lineages of dinosaurs evolved the calcareous egg in the Late Jurassic as a response to rising oxygen, and that the formation of calcareous eggs that are then buried was not viable in the late Permian through middle Jurassic environments of lower atmospheric oxygen.

  The low oxygen–high heat conditions of the late Permian into the Triassic perhaps stimulated the evolution of live birth and of soft eggs that would have been effective at allowing oxygen movement into eggs and carbon dioxide out. On the other hand, the higher oxygen levels (and continued high temperatures) of the late Jurassic–Cretaceous interval stimulated the evolution of rigid dinosaur eggs and egg burial in complex nests.

  Like characteristic metabolism, the contrasting patterns of live birth vs. egg laying is one of the most fundamentally important of all biological traits—and one that has received surprisingly scant attention by evolutionary biologists. Solving this problem by learning the time of origin and the distribution of one kind of birth strategy or the other should be a major research topic of the near future, but sadly may prove to be intractable because of the nonpreservation of parchment eggs.

  CHAPTER XV

  * * *

  The Greenhouse Oceans: 200–65 MA

  * * *

  Most discussion about the Mesozoic world (Triassic, Jurassic, and Cretaceous) concentrates on its land animals, especially the dinosaurs. But great changes were taking place in the marine world as well. The Mesozoic oceans progressively became more and more modern as the Mesozoic wore on in the shallow waters, but the mid-water to deepwater faunas remained very different from those of today. A transect going from shallow to deep water illustrates this, even near the very end of the Mesozoic era, in this case in the Late Cretaceous. Here is what such a dive might look like, a trip that can summarize a great deal about our current understanding of what we can call Mesozoic “greenhouse” oceans.1

  The atmosphere that the greenhouse ocean sits under and interacts with importantly affects the chemistry and physical environment of any ocean.2 The temperature of the atmosphere, the differences in temperatures from pole to equator, and the chemistry of seawater—including how much dissolved oxygen it contained—all dictated ocean conditions and the creatures the oceans contained. A crucial physical fact is that warm water holds less oxygen than cold water. During all of the Mesozoic, except the last 5 million years of the Cretaceous period, the global atmosphere from pole to equator was hot and humid. But the heat alone caused a lower overall oxygen content than we find in the ocean today. Coupling that with less oxygen in the air leads us to understand how different and less amicable to life the Mesozoic oceans would have been. The life that was there was, not surprisingly, evolved in many ways to deal with this low-oxygen world ocean.

  While the Mesozoic world was different from now, in one way it might have seemed familiar. Just as the lower altitudes of our world’s atmosphere are quite populated with a wide diversity and abundance of flying creatures, from insects to birds to bats, so too was the Mesozoic sky a place of movement and life. The air would have been filled with an assortment of flying organisms, including insects, but also with two very different groups than found today: the enormous pterosaurs (reptiles) as well as smaller reptilian pterodactyls, and many kinds of birds, with the latter composed of forms quite different from most birds of today, with and without teeth, and others with or without wings.

  A wide lagoon of some sort would have fronted most Cretaceous oceans. Lagoons are formed when some kind of reef walls off an inner water body. Usually such lagoons are both hotter and of lower oxygen than the open ocean itself. The shallows of these lagoons would have been inhabited by clams and snails that were quite similar and in many cases of the same taxonomic group (such as genus) as those found in tropical lagoons and near shore environments of modern oceans.

  Already present, for example, were burrowing clams, tusk shells, oysters, scallops, mussels, cowries, cone shells, tritons, conchs, whelks, sea urchins (both the globular surface dwellers, the “regular” urchins, and the burrowing or “irregular” urchins such as the sand dollars and “sea biscuits” of today). There would also have been spiny lobsters and crabs. All in all, the “modern” fauna was already well established in the shallows of the Late Cretaceous oceans, and in fact would be relatively little affected by the gigantic era-ending mass extinction that by the Late Cretaceous (from about 90 to 65 million years ago) was coming ever nearer in time.

  In deeper water, the kinds of life would change, just as it does in our ocean, to forms adapted for the finer sediments found in deeper water rather than for species that live in the coarser sand environments of shallower water. There would have been many animals that were buried, including many clams still around today, as well as many other kinds of burrow dwellers. Hiding in the sediment was a major survival tactic, because by the Late Cretaceous many kinds of predators adapted either to break open or to drill into mollusk shells that were present. Also present in the shallow waters of the lagoon would have been hunks of hard limestone formed by reef-forming organisms, such as corals in our modern day. These patches are tiny reefs that then, as now, formed into horseshoe shapes, with the front or arch of the horseshoe growing into the prevailing wind.

  Farther from shore would have been the large barrier reef, which would have grown right up to the sea surface. These enormous walls of limestone would have been hundreds to thousands of miles long, growing right at the edge of large islands or continents at the point where the continental shelf gives way to the continental slope and deep water. Both sides of the barrier reef rampart would have been home to many fish species, including the bony fish and cartilaginous sharks, skates, and rays.

  This inner edge of the barrier reef—in fact its entire overall shape—would have been a direct look-alike of what many of today’s barrier reefs, such as the Great Barrier Reef of Australia, look like. Yet a major difference is that while there are coral species living on reefs today, their main framework builders were not corals at all.

  The three-dimensional wave-resistant structures that we call reefs have been a key community of life since the Ordovician period. All have been and continue to be composed of builders and binders; they a
re akin to brick houses made of coral “bricks,” encrusting algae, flat coral, and carbonate-particle mortar. But perhaps a much better analogy is that they are like some ancient city, where centuries of buildings have been erected, existed for some time, and then have tumbled down or disintegrated but were only partially cleared away before new construction rose atop the older rubble. Over time, the great weight of ever-larger stone buildings often caused the very crust of the Earth beneath the ancient city to slowly but measurably subside.

  Such is the nature of a coral reef: over centuries, the larger and blockier corals attach to an already-existing reef surface and build up, growing upward toward sunshine, in a true life-or-death race to grow faster than one’s neighbors. Corals compete to avoid being grown over or shielded from life-giving sun and open water, as the sun is necessary for the millions of single-celled plants growing in each coral polyp, and open water gives the carnivorous coral polyps their own sustenance. The tiny plants allow the coral animals to build their gigantic skeletons, and in turn the plants, called dinoflagellates, receive nutrients and protection from predators. In this fashion, tiny coral larvae fall out of the plankton to settle onto any nonliving and hard substrate they can find, and then grow up toward the seas’ surface. These microscopic larvae with luck can grow from one polyp into hundreds of thousands in a single gigantic colony and can live for centuries or more, with a vast calcareous skeleton weighing thousands of tons. Although there are single colonies now thousands of years in age or even older, huge colonies eventually die. After death the coral skeleton becomes fragmented and grown upon in turn.

  The reefs of the Cretaceous greenhouse oceans were no different in this process and in the shapes eventually built, but their building materials were not coral reefs at all but clam reefs—created by quite large clams that looked nothing like any clam alive today. They were bizarrely shaped bivalve mollusks called rudists, and most looked like some kind of upright garbage can, complete with a lid that could open or close on the cylindrically shaped shell of the clam. Some approached the size of the modern-day Tridacna, the “giant clam” of today’s tropics. But unlike Tridacna, which are solitary, the rudists grew side by side in gregarious fashion, much as modern-day mussels do, crowding to cover every square inch of substrate, even growing over one another.

  Each large cylinder that is the bottom shell of a single rudist clam is jammed vertically next to others of its kind, all packed into a solid pavement of one- to two-foot-long, sometimes foot-wide cones, each topped with gorgeously colored flesh reaching up toward the light. Like corals, they had tiny symbionts, single-celled plants that need light for photosynthesis and in turn provide the clam with bountiful oxygen, as well as carbon dioxide and waste removal from its tissues. But unlike modern corals, which can take centuries to reach large size, the clams grew very quickly. Within a year after sinking down from the floating plankton onto shallow ocean bottoms (they probably needed light to survive because their flesh contained tiny plants), the small clams grew thick carbonate outer shells to mature size in a year or less. They were born, grew quickly, and more often than not soon died, as others of their kind descended on their hard shells and grew, smothering the immobile yet living real estate they squatted on. A coral skeleton would have needed a century to grow from one individual to a colony several feet high and wide, whereas the rudists could have done the same in five years at most.

  Like all reefs, the rudist reefs grew right to the very surface. On their outer seaward side, water depth dropped off quickly. Outside of the reef lay the vast open oceans of the Mesozoic, and both above and on the bottoms of these oceans there existed other now-extinct creatures.

  The surface of these oceans would have been patrolled by both large sharks and giant seagoing reptiles. These latter included long- and short-necked plesiosaurs, as well as the lizard-like mosasaurs. They probably lived much as modern-day seals do, diving for food but needing to surface for air. But they were far larger than any seal, larger than any other creature that needs to come out of the water on occasion to rest or breed.

  The deeper bottoms of the greenhouse oceans were also different from those of most oceans. Only the present-day Black Sea is similar to the conditions of the deeper bottoms and even mid-water regions of the greenhouse oceans—warm environments with so little dissolved oxygen that even most fish cannot live there. The bottoms were made up of black mud, as are the bottoms of the Black Sea. The mud trapped great quantities of fine, particulate organic matter that is black in color. There was little oxygen in the seawater at these depths—so little, in fact, that normal decomposition of organic material cannot take place, or does so only at a rate far lower than that on an oxygenated sea bottom. An entirely different community of microbes lived within the first few inches of this muddy bottom sediment, one that lived on sulfur, and a by-product of their particular form of respiration are the compounds hydrogen sulfide and methane.

  Only in a few places on the the Mesozoic ocean bottoms would there have been enough oxygen to support animals that require normal amounts of oxygen.3 But in the greenhouse oceans, two different kinds of mollusks evolved specifically for the characteristic low-oxygen conditions. One, a bivalve mollusk, lived on the bottoms. The other, composed of a vast diversity of cephalopod mollusks, the ammonites, lived in the water column, but fed off the bottom.

  The ammonites of the Cretaceous ocean we are profiling here belonged to a group that first appeared in the earliest Jurassic, and their sudden appearance in rocks of that age suggest that the devastating Triassic-Jurassic mass extinctinon, which took place almost 130 million years prior to the late Cretaceous, opened the door to new kinds of animals, including new designs of ammonites. Finding them is one of the delights of fossil hunting, and because we two coauthors have spent so much time doing research in strata with ammonites over the past two decades, this has been rather a constraint on our great friendship. Coauthor Ward will become totally mesmerized by the least trace of an ammonite fossil. Coauthor Kirschvink would just as soon drill a paleomagnetic core out of even a museum-quality specimen. And has.

  The final group of ammonites, which began in the oldest Jurassic strata and continued to the very greenhouse ocean of this chapter, are of great importance not only to the history of life, but to the very science of geology and using fossils to tell time. There are many places in the world where marine strata of latest Triassic age are overlain by Jurassic strata. At such outcrops one can walk through time, and if the strata are continuous, the dramatic events of the late Triassic and early Jurassic are present for all to see. This interval of time and rock preserves evidence of the great Triassic mass extinction, one of the so-called big five mass extinctions, a dubious honor of species death. As you walk through upper Triassic beds you are first in strata packed with the fossils of the flat clam Halobia, and then you move into younger rocks with the even more abundant Monotis. But then the clams disappear, over only several meters of strata, leaving a long barren interval of rock and time—the last stage of the Triassic, an interval perhaps 3 million years in length known as the Rhaetian stage.

  Finally, after this thickness virtually without fossils, a new group suddenly appears—ammonites. While there were ammonites in the upper Triassic rocks, they are never abundant. But most famously at the beach of Lyme Regis of England, as well as in southern Germany and at many other localities worldwide, the earliest Jurassic ammonites appear in huge numbers, and over only a few short meters of strata they diversify as well. These are not like the Triassic flat clams, where one species is all you get. These ammonites of the first part of the Jurassic are diverse and abundant, which tells us that the great drop of oxygen was finally over and a slow rise was in place. But the ammonites are not telling us that oxygen levels similar to today were suddenly in place. The ammonites appear because the surface of the early Jurassic seas began to have a modicum of oxygen, and the ammonites took full advantage. They did so because they may have been among the best animals on Earth
for low oxygen and could and did seize ecological advantage in the greenhouse oceans of the Jurassic and Cretaceous.

  Because of the overall similarity of the chambered shells in both nautiloids and ammonoids, we presume that they may have had somewhat similar modes of life. Nautiluses today live in highly oxygenated water over most of its range. But here and there they also live in hypoxic bottoms. This was a great curiosity, because conventional wisdom was always that the cephalopods in general need high-oxygen conditions, but not so the one remaining stock of externally shelled, chambered cephalopods, the Nautilus. These latter are very tough and resistant when taken out of the water. They can sit out for ten or fifteen minutes with no ill effects. And when they are in water they gain oxygen through one of the relatively largest and highest-powered pump gills ever evolved, streaming great volumes of oxygen over the gills, thus allowing for sufficient oxygen molecules even in low-oxygen water. If ever an animal was adapted for low oxygen, this is it. British zoologist Martin Wells, who measured oxygen consumption of various captive nautiluses in New Guinea, finally proved this. When a nautilus is confronted with low oxygen, it does two things. First, metabolism slows way down. Second, with its strong swimming ability it can travel vast distances in search of not only food but higher-oxygen water areas.