A New History of Life (43 page)

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.
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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.

The mass appearance of ammonite fossils in lower Jurassic strata suggests that the ammonites were superbly designed to extract maximum oxygen from minimal dissolved volumes of the oh-so-precious gas. Jurassic-through-Cretaceous ammonite body plans thus may have evolved near the Triassic-Jurassic boundary in response to worldwide low oxygen. Their new body plan (compared to the ammonoids that came earlier) involved a much larger body chamber relative to the phragmocone. Because of this, they had to use thinner shells, and this required more complex sutures. The sutures also allowed faster growth by increasing the rate of chamber liquid removal for buoyancy change. Within the large body chamber was an animal that could retract far into the body chamber, and that had very long gills relative to its ancestors.

We do not know if ammonites had four gills (like the
Nautilus
) or two like modern-day squid and octopuses. The lack of streamlined shells of the majority of early Jurassic forms makes it clear that these animals were not fast swimmers. It is far more likely that they slowly floated or gently swam near the surface, using their air-filled shell like a zeppelin.

The ammonites of the Jurassic period changed only in detail right up until the Cretaceous, but then spectacular changes in shell design began to take place. Where there remained many of the original planispiral shell design (like a nautilus shell), other shell shapes came into being in the Cretaceous, and with this, let us return to our dive in a late Cretacous ocean, among the ammonites.

Regardless of shape, most ammonites searched the bottoms for crustaceans or other small food. More than a dozen different kinds of ammonites could exist in the same environments, each with a different shell shape. Some were tiny, no more than an inch in diameter, while others were up to six feet in diameter. Most in the Cretaceous seas had thick, intricately branching ribs or tubercles of some kind, defensive armament that is testament to the abundance and efficiency of the shell-breaking predators present in these greenhouse oceans,
and in all probability the plesiosaurs and mosasaurs were their major predators.

The ammonites would have looked a bit like squids stuck in a nautilus shell. Today’s
Nautilus
has ninety tentacles, while the ammonites would have had either eight or ten. Nautiluses are scavengers, while the squids of today and the ammonites of the Mesozoic were carnivores, needing living organisms for food.

The second mollusk of the greenhouse oceans were clams, not as bizarrely different in shape as the rudists, but certainly different from anything alive today. They were what we call flat clams, known as
Inoceramus
. Related to oysters, they came in a variety of species, all competing on the same muddy bottoms. None could burrow, but instead had to sit on the bottom. Some were veritable giants, with gently ribbed, almond-shaped shells that attained lengths of more than eight feet from their beaks to their broad apertures. Yet unlike any clam today, their shells were almost paper-thin relative to their size, and their gently ornamented upper shells were sometimes encrusted with a diversity of oysters, scallops, bryozoans, barnacles, and tubeworms. Usually, however, the
Inoceramus
clams lived on bottoms and in seawater that had too little oxygen for “normal” mollusks or other invertebrates to live. A great many of our colleagues have used geochemistry to better understand how different these clams were from perhaps any clam living. New work by Neil Landman of the American Museum of Natural History in conjunction with geochemist Kirk Cochran has wonderfully shown the strangeness of these Mesozoic communities.

Just looking at the sizes of the inoceramids compared to other clams tells us how strange they were. In the modern world, the largest clams, the
Tridacna
(giant clams) of the tropics, can be six feet from end to end, thus holding hundreds of pounds of flesh. But the next biggest clams, known as geoducks, are at most a foot in length, with no more than a pound or two of living tissue. Some oysters attain a foot in length as well, but not many. But the inoceramids fill that modern-day gap between the giant
Tridacna
and the far smaller geoducks. An enormous variety of the inoceramids existed from the
Permian to the end of the Cretaceous when they died out, and it was in the greenhouse oceans where they flourished. They contained microbes allowing these big clams to live off methane and other chemicals seeping out of the organic-rich, low-oxygen sea bottoms of the greenhouse oceans rather than filtering food out of seawater, as the modern clams do.

A final region of the greenhouse oceans was the mid-water region,
⁴
the nether regions of the oceans that are too deep for sunlight, but still hundreds to thousands of feet above the stagnant bottoms of the seas. This vast, mid-water environment in today’s oceans is the largest single habitat on the planet, and is colonized by a variety of creatures adapted to a life in which they never encounter the surface, and its sun and atmosphere, nor do they ever come in contact with the sea bottom. Here, life depends on staying “in between,” for to these creatures both the warm shallows and the deeper cold bottoms would be fatal, from predation to temperature and oxygen conditions, or both. Thus, adaptations for the attainment and then maintenance of neutral buoyancy are paramount for existence. In our oceans, the most common of the larger inhabitants in this region are the mid-water squid, animals that have evolved floating tentacles or sacs within their bodies concentrated with fat or other chemicals such as ammonia-rich solutions that renders the entire animal lighter than seawater.

Their prey was individually small but vast in quantity and was composed of a diverse and abundant assemblage of small swimming animals that combined are known as the deep scattering layer (DSL)—based on their discovery by some of the world’s first sonar, used in the 1940s. This DSL is composed of untold numbers of small crustaceans and other arthropods such as amphipods and isopods, as well as a variety of other phyla. By day this enormous layer of life—extending from perhaps eight hundred to six hundred meters deep—extends for hundreds or even thousands of miles in all directions in the ocean regions far from shore. But as daylight fades, the entire layer slowly begins to swim upward toward shallower depths, and with full darkness the untold gigatons of animals making up the DSL arrive in the
shallower, warmer, more nutrient-rich depths—depths, however, that would be fatal during daylight for the tiny, succulent arthropods making up the vast preponderance of the DSL fauna because of visual predators such as fish and squid.

We have good evidence showing that this fundamentally new kind of lifestyle in the sea first appeared in the Cretaceous. Before that there would have been no food resources worth pursuing in the mid-water regions, and hence there were no species making the extensive adaptations that a larger animal would need not only to float throughout its life, but be able to somehow migrate hundreds of meters upward each nightfall and then settle back down into greater depths each morning. But with the appearance of the mid-water arthropods, evolution quickly produced animals capable of feeding on them using new kinds of buoyancy devices, for the most fundamental adaptation was some kind of way of being weightless in the mid-water.