A New History of Life (42 page)

Just when these various kinds of eggs first evolved thus remains a mystery. In 2005 it was proposed that calcareous eggs first appeared at the end of the Permian as an adaptation to avoid desiccation in the increasingly dry late Permian through Triassic global climates. Unfortunately, there is no fossil evidence to support this: there are no accepted Permian eggs in spite of the existence at that time of anapsids (the group that would give rise to turtles), diapsids (the group that would give rise to crocodiles and dinosaurs), and synapsids (the group that would give rise to us). Furthermore, only a small number of late Triassic eggs are known that may be from dinosaurs. But this poses a great dilemma: dinosaur eggs preserved commonly in Cretaceous sediments are not found in the same kinds of depositional environments of Permian and Triassic-aged strata. In all likelihood, if archosaurs had used hard eggs in the Permian or Triassic—if any group of reptiles had used hard eggs then—we would have already found them.

The absence of evidence is always a dangerous tool, but eventually numbers must be accepted. All evidence suggests that hard eggs were not commonly produced by pre-Cretaceous terrestrial egg-laying organisms. Even the 2012 discovery of hard dinosaur eggs in South Africa became but a single exception to the rule. It is hard to see how future collecting, no matter how intense, can now overcome this trend.

Only two shapes of dinosaur eggs are known: rounded and elongate. But seven different patterns of crystal arrangements making
up the eggshells are now recognized. This diversity of egg wall morphology would be surprising if all dinosaurs had evolved from a single egg-laying ancestor. But it would be what we would predict if hard-shelled egg laying evolved
numerous
times by separate lineages of dinosaurs. If we add the additional (and different) eggshell morphologies found in extant reptiles and birds, there are a combined twelve separate eggshell microstructures that have evolved during the now-long history of reptile, nonavian dinosaurs, and the real avian dinosaurs: birds.

Perhaps each of these is an adaptation to a different kind of stress an egg belonging to a particular group or species is normally subjected to: a turtle egg in a deep burrow, for instance, has a very different series of challenges facing it than does a robin egg in a nest high in a tree. But another possibility is that the various calcareous eggs are evidence of an independent evolutionary history, in which hard eggs separately evolved in multiple lineages, including dinosaur lineages.

One of the most interesting of new discoveries about the evolution of many of the modern stocks of land animals is that so many came from a fairly narrow time interval—during a time in the late Paleozoic when oxygen was higher than now. This is true for many groups of extant vertebrates, including the first members of families that would go on to be lizards, turtles, crocodiles, and mammals. But it is not just land vertebrates that suggest this trend: many of the terrestrial invertebrates, including the basal stocks of many insects, arachnids, and land snails, also began during the Carboniferous period of more than 300 million years ago. New experiments of the last five years are indicating that there is a “magic” level of oxygen for the fastest rate of embryonic development within both land vertebrate eggs and insect eggs, and that number is 27 percent.

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.

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 are 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.