A New History of Life

by Peter Ward

  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.

  The carnivores that evolved to exploit this resource were mainly ammonites, but in shapes very different from the traditional and ancestral planispirals, which marked the species living just above the sea bottoms. The mid-water ammonites had shells that let them float for their entire lives; they had bizarre shells that would not have allowed any kind of rapid swimming speed. But once in the thick oceanic water mass or liquid stratum inhabited by the denizens of the deep scattering layer, food would have been so abundant that they only had to stay in the layer, rising when it rose, slowly sinking with it during the day, to have a food-rich and predator-free existence. They thus lived a slow, floating existence; in essence these strange creatures acted like hot air balloons, with a large flotation device above, attached to a small passengers’ basket suspended below the flotation device.

  The ammonites of the mid-water5 needed to efficiently control their buoyancy. We know that the buoyancy system of the living nautiluses is quite crude and slow to operate. But it may be that ammonites in general used their complex septa with the beautiful sutures as part of a far better buoyancy ap
paratus, one that could pump water out or let it flood back with great rapidity into otherwise air-filled chambers as ballast. These then-new Late Cretaceous ammonites are informally called heteromorphic ammonites, because of their shells’ departure from the original, traditionally coiled design of ammonites from their first appearance in the Devonian period until their final demise at the end of the Cretaceous. They were body plans never found before or since their approximately 60-million-year-long existence, and they lived right up to the day that the Chicxulub asteroid fried all ammonites out of existence.

  Some of the heteromorphic ammonites looked like giant snail shells, but snail shells filled with chambers of air, with one last, longer shell portion hanging beneath and containing the soft parts of the ammonites, tentacles and all. Others looked like gigantic paper clips, and some were simply huge hooks. But the most common were long, straight cones. The pointed end of the cone was the first formed chamber, and by adulthood, these long, thin cones could reach up to six feet in length. They hung vertically in the water, their tentacle heads hanging straight down and beneath the chambered flotation portions of the shell. There were named Baculites, and in the Late Cretaceous they may have been the most common carnivores on the planet.

  Vast schools of these Baculites filled the mid-water depths.6 They are often illustrated in the many murals and paintings of the Cretaceous oceans, and invariably they are wrongfully depicted as long, arrow-like shapes living horizontally in the water, much as fish and squids do. But this would have been impossible. They were vertical in orientation, with their tiny, earliest shell portions up, and their heavy head and tentacles hanging downward. They were never able to swim sideways or even float in a sideways orientation. Everything was up and down to them. They probably were amazingly rapid, shooting upward using jet propulsion and then sinking slowly back downward. The predators of the Baculites, probably fish and sharks, would have been mystified again and again in attempting the normal predatory attack, in which the prey tries to escape by swimming ahead of the predator. But the attackers would have seen a rapid upward movement of the long vertical creature, like a puppet on a string, as they swam helplessly forward in the direction that all self-respecting prey were supposed to flee.

  THE MARINE MESOZOIC REVOLUTION

  During the later Mesozoic era, a revolutionary changing of the guard took place in the seas. Paleobiologist Gary Vermeij of the University of California at Davis called it, simply enough, the Mesozoic marine revolution.7 It was nothing less than a world where the marine predators ran wild in terms of evolution.

  Watching our friend and colleague Gary Vermeij, blind since early boyhood, “see” (as he describes the process) the intricate adaptations for the strengthening of post-Paleozoic mollusk shells is to watch a concert pianist’s fingers: rapid, complex movements of fingers seemingly gone boneless as they “play” the many morphological keys of a snail shell, from the spines of the turret to the fat-lip-like callus on the snail’s aperture; the gentle discovery of a calcareous filling of the otherwise dangerous umbilical region of the shell, to the rapid trilling of the tiny yet strength-giving teeth on the outside of the shell’s apertural lip, also thickened. We guide him to museum drawers, and then he guides us to insights coming to him from a single sense—touch.

  Touch has memory, but touch also visualizes, and it was with touch only that Vermeij’s agile mind took his “vision” of the ever-increasing offensive, shell-breaking capabilities of the newly evolved, post-Permian predators, coupled to the coevolving and ever-better calcareous suits of armor of the invertebrate herbivores, and smaller predators into a generalization we now know as the Mesozoic marine revolution.

  At first the concept was only that predation of the post-Permian mass extinction shifted toward shell breaking—toward new ways of gaining the rich meat to be found in formerly impregnable fortresses such as those of the Paleozoic clams, snails, echinoderms, and brachiopods. But the concept has expanded.

  The adaptations of the prey were no less impressive. Clams that used to live only on the ocean bottom, or just beneath, evolved adaptations for deep burrowing. These new clams are called heterodont clams (because of the many “teeth” in their hinge line). They underwent major anatomical revisions by fusing part of their mantle into a pair of siphons (what we call the neck of a clam). These burrowing clams today remain the most diverse group of clams—a whole spectrum of species that are capable of rapid burrowing into sand, mud, or silt. There is but a single reason to do this: to escape predation. Sitting within sediment rather than atop it in no way increases feeding efficiency. But it immensely increases survivability. Others radically changing morphology to allow a burrowing (or wood-boring) lifestyle included snails, new kinds of polychaete worms, some kinds of fish, and totally new kinds of sea urchins.8

  Another group of invertebrates showing radical innovations were the class of echinoderms called crinoids.9 These immense flowerlike invertebrates (they are commonly called sea lilies) were typical of the Paleozoic in that they were attached: they could never move through life after settling from a planktonic larval stage; they attached to the bottom. Driving through the Midwest today provides stark evidence of their former abundance: every road cut is made of rocks that are almost nothing but the tiny round “ossicles” that make up the long stem of the attached crinoids. To produce such abundance, a vast, shallow, extremely clear and warm ocean was needed, whose bottom would have been obscured by the forest of crinoids. It is doubtful that the sun could penetrate to these shallow bottoms, which would have been fine with the crinoids; their food is microscopic plankton, and they live in the “slow lane,” at least metabolically. But once attached, they could never move, and if detached by storm or predator, they would soon die.

  There is nothing like wholesale death to stimulate new evolution. The Permian extinction virtually wiped crinoids off the planet, and in the new rules of the predator-rich Mesozoic, they were quick meals for any predator designed to extract some sort of food out of a crinoid—which would be difficult, as there is probably no other example of life where so much calcium carbonate skeleton protects so little flesh. But the attached crinoids gave way to a new group of crinoids without stalks. These exist to the present day, and are among the most beautiful of all life to be found in modern coral reefs. There are actually capable of swimming—slowly and with great stateliness, using their arms to gently wing themselves through the water.

  The Mesozoic marine revolution was not only about predators and prey but also about ever-greater utilization of new habitats.10 This included the evolution of morphologies allowing ever-deeper burrowing by clams and snails to escape predation, as well as the ever-increasing number of other invertebrates that were using the sediment also for food. These changes are evidenced by an increase in the diversity and abundance of trace fossils, similar to those that we described in the Cambrian explosion chapter. The net result was almost complete bioturbation of Mesozoic sediments.

  It was not just the bottom and beneath the bottom of the Mesozoic oceans where radical change was occurring among animals. For the first time in the existence of animals, a wholesale exploitation of the water column from top to bottom was also taking place. Many of the newly evolved forms were not animals at all, but protozoans and even floating single-celled planktonic plants. Important new groups of microfossils are found in Mesozoic strata, including the evolution of a huge variety of the amoeba-like but skeletonized foraminifera, which lived both on the bottom as well as floating well above it. Other plankton was the siliceous radiolarian. But perhaps the greatest radical change within the plankton of the Mesozoic and onward was the evolution of a group of algae called coccolithophorids—whose tiny skeletons, when accumulated on a sea bottom and turned to stone, is the well-known substance chalk.

  Coccoliths are tiny plants sporting a half dozen to a dozen microscopic calcium carbonate plates bonded to the upper part of their rather spherical bodies. After death these tiny plates would fall to the bott
om and accumulate in untold numbers until such immense sedimentary units as the famous White Cliffs of Dover were formed. The entire northern tier of Europe is lined by such cliffs, from Britain to France, Poland, Belgium, Holland, all over Scandinavia, and through much of the old Soviet Union all the way down to the Black Sea. The coccoliths have played a large part in the planet’s temperature. Coccoliths are white in color, and this whiteness reflects sunlight back into space, which in turn cools the planet.

  Just as in the Cambrian explosion, in which animals were stimulated to produce new kinds of body plans based on respiratory systems, so too did animals of the Triassic seas show a multitude of new adaptations. As we have seen, the land fauna experimented with a variety of lung types. The same kind of exploration took place in the oceans. The bivalve mollusks were one group that evolved a new kind of body plan and even physiology in response to the nearly endless expanse of nutrient-rich but low-oxygen bottoms.

  The very lack of oxygen on the bottoms made them, in one sense, wonderful places to live. Vast quantities of reduced carbon, in the form of dead planktonic and other organisms, fell to the seafloor and were buried there. On an oxygenated bottom this material would be soon consumed by filter- or deposit-feeding organisms and scavengers. But the low-oxygen conditions kept these organisms out, and not even the usual bacteria that decompose dead creatures on the sea bottom were around. As we have seen, this is one reason that oxygen levels plummeted in the Triassic. But the clams figured a way out of this. A few kinds, such as the Inoceramus clams described above, who lived on the seafloor of the bottoms that had at least some oxygen, fed not on the falling organics, but on methane coming up from the some fraction of the organic-rich sediment. Methanogens are a group of bacteria that thrive in low- or no-oxygen conditions, and even several centimeters down into the sediment on a sea bottom with some oxygen would have penetrated into an oxygen-free zone—thus allowing the existence of the methanogens. As they metabolized, they released methane as a by-product. The clams may have had other bacteria in their gills that could exploit the methane and other dissolved organic material, or they may simply have fed on the bacteria. A somewhat similar mechanism is found today in the deep-sea vent faunas, where giant tube worms and clams use these chemicals as food. But the difference is that the modern vent faunas are oxygenated. The animals down there do not even need gills. The clams of the Mesozoic were not so lucky.