One hundred sixty million years ago, an elegant sea monster lay down on its right side and died on the warm, muddy ocean bottom near the present-day town of Lookout, Wyoming. As the creature’s 14-foot-long body was convulsed by one last set of involuntary muscle contractions, its powerful sharklike tail twitched and stirred the bottom mud. A faint trail of bubbles escaped from the corner of its mouth and rose 300 feet to the surface, where tropical sunshine was playing on the waves. Then a bottom current gave the beast a decent burial under a blanket of pale green sand, part of the accumulating sediment layers that would become known as the Redwater Shale Member of the Sundance Formation.
Usually the events leading up to any one individual death in geologic time are obscured, the details lost, the exact time of day not recorded by any sign preserved in the rock record. But from what I know about that ancient Wyoming seabed, I have a mental picture of this one animal in its last hours in the Sundance Sea.
The sea monster was Baptanodon, the fastest, most advanced species among the ichthyosaurs, the fish-lizards, seagoing reptiles with large porpoise-shaped bodies and long, narrow snouts. Baptanodons hunted at dusk. Their eyeballs, as big across as dinner plates, could gather up even the faintest light coming into the upper layer of the water. In my mind, I see my fleshed-out Sundance fossil swimming along in a pod of five or ten individuals, cruising silently 50 feet below the surface, scanning the moonlit water above for prey. The leader of the pod catches the telltale speckled light of armored squid, moving in an immense shoal a thousand strong. The squid rise near the surface every evening at this time to feed on small crustaceans and fish larvae.
Armored squid have big eyes, too--for catching their prey and for detecting their predators. But the baptanodons start their attack from the squid’s blind quarter, behind and below the shoal. Baptanodons also have the deadly advantage of slashing speed. Their bodies have the 40-knot shape preferred by evolution for all its fastest-swimming creations. Mako sharks and albacore, the speediest fishes today, have the same proportions: a teardrop torso, six times longer than it is thick; short, triangular forefins for steering; reduced hind fins; a narrow tail base; and a deep, graceful, crescent-shaped tail fin.
A burst of muscular tail twitches sends the attacking baptanodons up and forward into the shoal’s rear echelons. Squid break formation in every direction. Clouds of camouflaging ink squirt out and swirl in the Sundance water column.
Despite the obfuscatory curtain of ink, the baptanodons’ large eyes catch microsecond glimpses of individual targets. Snouts, ultrathin so water resistance is minimal, swing instantly right and left. Jaws snap open and shut. Squid are impaled, still alive, along the close-packed rows of baptanodon teeth. Though the predators make just one five-second pass, that’s enough for each of them to snag a half-dozen or more squid. The baptanodons swallow their meal as they breach the water’s surface, grabbing quick breaths of air at the rear corners of their jaws. (Baptanodons are true reptiles, after all, descendants of land-living, lizardlike ancestors, and they must breathe air.)
As the fish-lizards churn up the water near the surface, armored mollusks, with large, snaillike shells and dozens of writhing tentacles, move clumsily out of the way. They needn’t bother, really--baptanodon jaws and teeth are too weak to crush such well-protected calamari. However, there are other sea monsters hunting tonight who relish big, hard-shelled servings of seafood.
An ugly triangular head, two feet long with eyes facing upward, darts from below. A mollusk’s coiled shell is shattered in a dozen places by penetrating strikes of long, conical teeth. The jaws release their victim, then grip it again, adding another set of holes to the damage. Again the head strikes, and it now grabs and shakes the stunned victim. The mollusk body, its tentacles writhing helplessly, drifts out of its protective house. The ugly head darts up once more and sucks in the hapless four-pound body.
As soon as the attack began, the pod of baptanodons swerved hard to the right to avoid the scene, while their eyes searched out the body contours of the newly arrived predator. Now they see that the 2-foot head is carried on the end of a 15-foot neck that widens gradually until it merges into a compact 7-foot body. The ugliness of the snout, with its buck-toothed display of crowns sticking out and forward, belies the smooth precision of body movement. Two pairs of backswept flippers beat in syncopated rhythm, the foreflippers on the upbeat when the hind flippers are paused at the downbeat. Like penguins, these animals appear to fly through the water.
The baptanodons relax. They know this shape, and it’s not a threat. It’s only a long-necked plesiosaur--a swan-lizard. Although it weighs as much as five tons, the plesiosaur has a small mouth and can’t inflict a dangerous bite on anything as large as an adult baptanodon. In the Sundance Sea, ichthyosaurs and long-necked plesiosaurs are noncompeting hunters. The ichthyosaurs harvest huge numbers of small squid. The plesiosaurs go for smaller numbers of big-coiled mollusks and medium-size fish.
Reassured, the baptanodon pod maneuvers for another strike at the squid. The lead baptanodon again begins its high-speed run into the shoal. But this time, just as the animal breaks into the squid’s rear guard, it sees a huge, 50-foot dark shape curl its body into a tight U. The baptanodon instantly flinches and dives, its brain switching from attack behavior to lifesaving escape tactics.
It has recognized the one reptile in the Sundance ecosystem feared by Baptanodon and swan-necked plesiosaur alike: Pliosaurus, a short- necked, huge-headed giant kin of the swan-lizards. Pliosaurs are the top predators of the system, the hunters strong enough to kill any other sea creature.
The baptanodon’s crash dive almost works. The pliosaur’s nine- foot-long head lunges toward the fleeing animal; its jaws, four times stronger than a tyrannosaur’s, make a sideways swipe. Though only the front six pliosaur teeth catch the smaller animal, the baptanodon is momentarily hung up on the six-inch fangs. Then it gives a maximum-power twitch of all its body and tail muscles and breaks free.
But blood trails from the wounds. The body wall has been pierced, the lungs skewered. The next day the baptanodon lays itself down on the bottom of the Sundance Sea and dies.
On the evening of August 12, 1992, I walked up a gully in the gently sloping outcrop of the Sundance Formation. I was teaching a dinosaur field course for the Dinamation International Society, a nonprofit group that supplies volunteers for digs (bless their hearts, volunteers make most of the discoveries these days). The six o’clock Wyoming sun was throwing its rays over the dried grass heads, turning everything gold and bronze. And the low-angle light had just hit a ten-foot fossil backbone, half- exposed by wind erosion, lying like a bas-relief in the pale green and gray sandstone. The front flipper bones were there, too, and all the ribs, mapping out the streamlined body form. The back of the skull was just visible.
It was the most beautiful fossil I’d ever seen in the field. A perfect baptanodon. The field party gathered around. They had been a noisy, rambunctious crew, but as they came up to the ichthyosaur, one by one they fell quiet, simply stunned into silence by the extraordinary specimen.
The discovery was paleontological serendipity, the happy result of a half hour’s side trip away from the Morrison Formation, the rock layer lying above the Sundance. We were there to help excavate evidence about extinctions at the end of the Jurassic Period--extinctions on land, that is, at what I call the Dinosaur Mid-life Crisis, as preserved in the Morrison rocks. This was a time, about 140 million years ago, when most but not all of the dominant dinosaur families went extinct. They were replaced by new groups evolving early in the next period, the Cretaceous. But it turns out that big-eyed, seagoing Jurassic ichthyosaurs--and their squid prey, too--have their own tale to tell about the fate of Jurassic dinosaurs.
Success and failure--evolution and extinction--are controlled largely by ecological role. Dinosaurs are a clear-cut example. At the Dinosaur Mid-life Crisis, extinctions proceeded from top to bottom, ecologically. The top of the ecological pyramid on land, the biggest predators and the biggest herbivores, suffered terrible extinctions. This was when meat eaters like Allosaurus disappeared, along with such giant plant eaters as Stegosaurus. But the bottom of the pyramid got away nearly unscathed. Tiny insect-eating mammals and lizards and frogs survived with little change, and so did most groups of land plants.
This top-to-bottom extinction schedule works for all other mass extinction episodes in the terrestrial ecosystem. The most recent episode was the Ice Age die-offs of 11,000 years ago, when such monster meat eaters as saber-toothed cats were exterminated, as were most multiton herbivores, among them mammoths and giant sloths. But the Ice Age event left little mark on moles, voles, frogs, trees, shrubs, and grasses.
This is not what happened to the ichthyosaurs and their fellow swimmers. These extinctions proceeded from bottom to top, the reverse of the dinosaur extinctions. The baptanodonts (meaning not just Baptanodon alone but all the similar, related species) were the most advanced ichthyosaurs, featuring the largest eyes, smallest teeth, and fastest body design--adaptations for exploiting small prey near the bottom of the ecological pyramid. And it was the baptanodonts that went extinct at the end of the Jurassic. More-primitive ichthyosaurs, like Grendelius, retained bigger teeth, stronger jaws, and longer bodies, fed on larger prey higher in the food chain, and survived.
At the top of the pyramid were the giant, short-necked pliosaurs. If pliosaurs obeyed the extinction rules we see in the land ecosystem, then these huge, speedy carnivores should have had the highest vulnerability to complete extermination. Just the opposite turns out to be true. Giant pliosaurs survived the massive Jurassic-Cretaceous boundary extinctions that terminated the baptanodonts. When another partial extinction struck halfway through the Cretaceous, giant pliosaurs survived this crisis too, with little visible effect. The giant pliosaurs of the Late Cretaceous are in fact only slightly modified versions of the Jurassic ancestor Pliosaurus, who had hunted Baptanodon in the Sundance Sea nearly 100 million years earlier.
While the top-predator pliosaurs thumbed their noses at major extinction events, the ichthyosaurs displayed much greater ecological fragility. Those that managed to survive into the Early Cretaceous hunted bigger prey than the baptanodonts did, but they were still much farther down the food chain than the giant pliosaurs. When the Mid-Cretaceous extinction event hit, all the remaining ichthyosaurs died out, while the top-predator guild was left unshaken.
So the extinction schedules for ichthyosaurs and pliosaurs were the reverse of those for land ecosystems--the top predators were least vulnerable. If we imagine these sea-creature schedules working on land, then during the Ice Age, foxes and weasels would have died out while saber- toothed cats survived. Or at the end of the Cretaceous, Tyrannosaurus and its kin would have survived while birds died out. That obviously didn’t happen. That never happens.
There is a way to make sense of this topsy-turvy world of sea- monster extinctions: we have to put them in a box--a guild box, to be precise. A guild comprises species living together and performing the same ecological function. A guild box is a simple three-dimensional graph that locates each creature along ecological axes that define key elements of habitat, food choice, and behavior.
All our Sundance creatures fit snugly into a guild box as defined by these axes: axis 1 (left to right), how far from shore the animals hunted; axis 2 (bottom to top), swimming speed; axis 3 (front to back), the size of the average prey. Giant pliosaurs are at the back of the box, occupying the top predator spot (offshore, fast, big prey). Long-necked plesiosaurs, the swan-lizards, grew nearly as big and swam nearly as fast, but they took relatively small prey, so they would be farther forward, at the right-middle-middle (offshore, fast, small prey). And the 40-knot baptanodons would fit at the extreme right-front-top. Far to the left are the long-bodied sea crocodiles, shore-hugging reptilian predators that filled the top-predator spot in the shallower habitats of the Sundance Sea.
Mass extinction opens gaping ecological holes in the guild box, opportunities for some surviving group to expand its adaptive diversity. Tyrannosaur extinction ultimately opened the way for lions and tigers and bears. Oceanic extinctions evidently followed inverted rules, but nonetheless gaps were opened. How did the sea-monster menagerie respond when the food chain was disturbed by ichthyosaur extinction? Did some other reptile group play the role of evolutionary carpetbagger, rushing in to exploit the resources near the bottom of the ecological pyramid?
Yes.
On a shelf at the University of Colorado Museum is the delicately snouted skull of a small Late Cretaceous pliosaur, dug from near Red Bird, Wyoming, and known appropriately as Dolichorhynchops, or long-beaked face. Unlike the giant pliosaur species, the long-faced pliosaurs had lightly built heads. When I first saw that jet black fossil face in Boulder, I had a feeling of osteological déjà vu. I’d seen that exact face design before, in the Jurassic, tens of millions of years earlier: the ultrathin snout, the long rows of very small, tightly packed teeth, the huge eye sockets, the reduced jaw muscles that would permit a weak but very quick bite.
This delicate Cretaceous pliosaur had the face of a baptanodont. The basic adaptive geometry of pliosaurs had been reshaped into a form fit for chasing small, swift squid. And the body size was a perfect match, too. In fact, Dolichorhynchops was small for a pliosaur--only 10 to 20 feet--but its body-size range overlapped perfectly with that of the baptanodonts.
All a pliosaur would need to fill a baptanodont hole in the guild box would be the correct combination of body size and jaw armament. Dolichorhynchops had that combination, and that made it a Darwinian carpetbagger, playing the baptanodont role in the Late Cretaceous.
Back in the Late Jurassic, all the pliosaurs had large, widely spaced tooth rows, suitable for subduing large fish and reptiles but not as efficient as the tiny baptanodont teeth for snapping up smaller, quick- turning squid. There’s no anatomical reason Jurassic pliosaurs couldn’t have evolved small teeth, but they didn’t do it. The explanation is probably ecological. Since the small-toothed, fast-swimming guild was already filled by baptanodonts, there was no evolutionary vacancy for pliosaurs here. But the extinction at the Jurassic-Cretaceous boundary opened up to pliosaurs the fast-squid-eater corner of the guild box.
A similar scenario played itself out with the shore-hugging, shallow-water predators (roles played by seals and sea lions today). In the Jurassic seas there were two families of advanced sea crocodiles, 6 to 20 feet long, with elongated, sinuous bodies. Who filled their near-shore top- predator role in the Late Cretaceous? Not the sea crocodiles themselves-- they went extinct during the Mid-Cretaceous event. Instead a totally new group enters the sea-monster guild, the long-bodied sea lizards of the mosasaur family. Mosasaurs have a body form astonishingly similar to that of the sea crocodiles, but they aren’t related at all. Sea lizards evolved from land lizards early in the Cretaceous and were yet another case of evolutionary opportunism, taking advantage of gaps made in guilds by extinction events.
Thinking about all these creatures--pliosaurs and baptanodonts and sea crocodiles and sea lizards--made me suspicious of the long-necked- plesiosaur story that is usually told in textbooks. For their entire history, these swan-lizards certainly were far below giant pliosaurs in the food chain. The upside-down rules of extinction would seem to predict that their fate would be linked to baptanodont fate, since both groups were specialists in small-to-medium prey, and baptanodonts had high ecological vulnerability. But according to the standard textbook view, plesiosaurs were remarkably resistant to die-off events. The swan-lizards supposedly survived through the Late Jurassic extinction and through the Mid- Cretaceous crisis as well.
I didn’t believe it. The case of the Dolichorhynchops-style pliosaurs replacing baptanodonts made me wonder whether the Cretaceous swan-lizards might be carpetbaggers, too. How sure were we that the long- necked plesiosaurs of the Cretaceous were direct descendants of their Jurassic counterparts? All my experience studying extinctions has led me to expect rather rigid ecological rules of survivorship. The swan-lizard scenario didn’t add up.
In body form Cretaceous and Jurassic long-necked plesiosaurs did look identical. Flippers fore and aft, shoulders and hips, and proportions of neck and torso were built the same way. What I needed was a Darwinian marker, some anatomical feature that would expose Cretaceous swan-lizards if they were the product of opportunistic evolution from a non-swan-lizard ancestor. I needed what nineteenth-century anatomists called a heritage character.
In nineteenth-century parlance habitus is the obvious adaptive part of the body design, how the leg shape and tooth shape fit the beast to its particular environment. Heritage is the deeper, more fundamental part of anatomy, those features that evolve early in the history of a group and become fixed, not changing when the family tree branches out into many different ecological roles.
Here’s a good example: from the outside, an African spotted hyena looks like a tall version of the African hyena dog (also called the hunting dog)--both have massive muzzles, long legs, and compact feet, and both hunt in large packs. Early-nineteenth-century zoologists were duped by hyenas and classified them among the true canids. But the heritage details of the skull anatomy around the ear and jaw joint show that these two hunting mammals evolved their similar habitus independently from different ancestors. The hyena dog is a genuine, bona fide member of the dog family, a close kin of the wolf, jackal, and coyote. But the spotted hyena is related to cats, civets, and mongooses, an entirely different branch of the carnivore family tree.
I applied habitus-heritage analysis to swan-lizards of the Jurassic and Cretaceous. I set skulls side by side. I ignored the teeth (habitus features--quick to change in evolution). I focused on the roof of the mouth (the palate), the first two neck bones, and how the snout attached to the back of the skull (heritage features).
This straightforward, old-fashioned exercise in anatomizing confirmed my suspicions. Evolutionary carpetbaggery had shaped long-necked plesiosaur history. The swan-lizards of the Cretaceous were not direct descendants of the swan-lizards of the Jurassic seas. In fact, the Cretaceous swan-lizards were taxonomic impostors, long-necked opportunists from the pliosaur family!
The evidence goes like this: the Cretaceous swan-lizards had the bones of the roof of the mouth (pterygoid bones) wrapped around the underside of the braincase bones, exactly as in short-necked pliosaurs. And in the Cretaceous swan-lizards a snout bone and a rear skull bone meet, covering up the forehead bone above the eye--again exactly as in pliosaurs. Finally, the Cretaceous swan-lizards and the pliosaurs displayed the same relative proportions in the first two bones of the neck. The Jurassic swan- lizards were completely different.
Conclusion: Cretaceous long-necked plesiosaurs evolved from Jurassic pliosaurs. And so the Jurassic long-necked plesiosaurs had gone extinct without issue.
I find all this sea-monster history deliciously counterintuitive. It’s intuitively obvious that an ecosystem should collapse from the top down, with the base of the food pyramid surviving longest because the base is where most of the species and individuals--all the small prey and plankton--are located, and the base is closest to the ultimate source of energy, photosynthesis. Intuitively obvious but historically wrong. Giant pliosaurs are the rule in oceanic history, not the exception. In the post- Cretaceous oceans, giant open-water sharks filled the role of fast predator in tropical waters. And these families of sharks, including species akin to the surviving white shark, tiger shark, and mako, ignored the worldwide partial extinctions that terminated families occupying other guild corners.
Does the Jurassic-Cretaceous sea-monster story help solve the general riddle of extinctions on land and water? Yes, definitely. I’ve been persuaded that pioneering paleontologist Henry Osborn’s theory of 90 years ago is the best to explain terrestrial extinctions. He argued that the development of land bridges allowed massive mixing of big-bodied species across different continents, causing extinction through the introduction of new predators, competitors, and diseases. I think the flip side of his argument works in the water.
It’s a venerable idea to see extinctions at sea as being caused by a disturbance of the whole oceanic habitat. There may be a destruction of the rich, shallow-water habitats, as occurred when the Sundance Sea drained off the midcontinent. Or there may be a sudden influx of cold polar water toward the equator. Or there may be severe changes in the upwelling of nutrients in the ocean. Sometimes all three events happen simultaneously. But all such changes should coincide with cycles of land bridges, since widespread land bridges act to impede normal ocean circulation.
Now back to the top-predator paradox. On land, top predators should be hypervulnerable because they are quick travelers, spreading faster than any other guild across land bridges and into all available habitats. During the Ice Age, for example, saber-toothed cats spread quickly from Europe to the southern tip of Patagonia. Fast, complete penetration of new areas guarantees that predators will inflict maximum ecological damage and that they will meet the greatest assortment of new enemies among the competitors and microorganisms native to the lands they’ve invaded.
But in the oceans, the far-ranging habits of the top predator can work in the exact opposite way--minimizing extinction probabilities. If ocean extinctions are caused by physical changes in the oceans, then guilds tied to localized habitat patches are on unstable ecological ground. A baptanodont preying mainly on small, fast-moving squid is exterminated when the squid numbers are cut by planktonic disturbance.
But the top predator is not locked into any one habitat. Giant pliosaurs were already worldwide in distribution before the Late Jurassic and Mid-Cretaceous extinctions began; individual pliosaurs probably had immense hunting ranges and could subsist on prey from nearly any ocean guild structure, just as modern white sharks are catholic in their tastes and cosmopolitan in their species range. As long as some prey populations existed here and there, the top predator could survive the crisis. Only the most profound and prolonged sea crisis could eliminate the giant pliosaur guild corner--a crisis like the apocalypse at the end of the Cretaceous.
The incomplete sea-monster extinctions of the Late Jurassic and Mid-Cretaceous are wonderful opportunities to get at the rules of mass die- offs. But we have to ask the right questions. Extinctions on land and in the sea are not random. No, extinctions have precision. Survival and recovery depend on where each species fits into the network of ecological interdependencies. To appreciate the real meaning of speedy ichthyosaurs, long-necked plesiosaurs, and huge, short-necked pliosaurs, we must first place these marvelous creatures in their proper context--a box, with a slyly shifting pattern of occupancy.
