by Brian Switek
The world was not in stasis during the Permian. Toward its close, the large amount of CO2 in the atmosphere caused the global climate to become much warmer and more arid. Lakes turned into seasonal floodplains and deserts began to spread. Even so, these changing conditions caused populations of organisms to continue to evolve, and while climate change can trigger extinction for some species, the gradual warming trend of the Permian is not enough to explain the mass extinction. Something else must have transpired.
There are several competing hypotheses for the trigger of the extinction. Some have argued that a massive asteroid or other extraterrestrial body must have struck the earth. Such an event is correlated with the mass extinction at the end of the Cretaceous, but so far no concrete evidence of such an impact has been found. Instead, it appears more likely that the planet’s climate and atmosphere underwent some drastic changes that many organisms could not be adapted to. Just how such a shift might have happened, though, is open for debate. Some have argued that massive eruptions from the volcanoes known as the Siberian Traps would have choked the atmosphere with toxic gases. We know from violent eruptions in recent time, such as Mt. Saint Helens and Krakatoa, that eruptions can affect the global ecosystem by altering the atmosphere, but the eruptions alone do not seem to be sufficient for the scale of extinction that occurred.
Instead, the eruption of the Siberian Traps may have itself been a trigger. The intense volcanic activity may have caused the release of large amounts of carbon dioxide and methane stored beneath the earth’s crust. The effects would have been disastrous. These greenhouse gases not only would have caused drastic changes to the global climate, but would have turned the seas into toxic, acidic pools in which most life would perish. Organisms on land did not suffer any less. The drastic change in the atmospheric content would have reduced the amount of available oxygen in the air. According to a 2003 paper by researchers Gregory Retallack, Roger Smith, and Peter Ward, after such an event the air in the Permian Karoo ecosystem would have been as thin as the air 14,000 feet above sea level today. This would make land-dwelling vertebrates susceptible to “nausea, headache, hypertension and pulmonary edema,” similar to the maladies suffered by those who go up mountains too far, too fast.
If this hypothesis is correct, then the cause of the end-Permian extinction would have been more subtle than something like an asteroid impact. An asteroid or comet slamming into the earth would be like a gunshot, an instantaneous event that left horrendous damage in its aftermath. The release of large amounts of methane gas, instead, would strangle much of life on land by reducing the amount of available oxygen while toxifying the seas. The animals that survived would have been those who, simply by chance, had traits that allowed them to survive in the oxygen-deprived world. This hypothesis is not without problems, but so far it is the most consistent with the global pattern of extinction. Ideas about the cause of the crisis will continue to be debated, but we do know that some kinds of therapsids survived.
The recovery from the Permian mass extinction was relatively slow, but as the surviving groups continued to evolve reptiles began to outcompete the remaining synapsids. Early archosaurs, relatives of both dinosaurs and crocodylians, became the dominant vertebrates. Just like the therapsids, these creatures were independently evolving a posture with limbs held straight beneath their bodies. Some were even able to walk on two legs, and it was from one lineage of bipedal archosaurs that the first dinosaurs, similar to the small 230-million-year-old predator Eoraptor, evolved. Some Triassic archosaur groups would become extinct during yet another mass extinction that marked the end of the period, but dinosaurs would emerge in the Jurassic bigger and fiercer than anything that had come before.
The cynodonts continued to evolve alongside the dominant archosaurs, remaining small but persisting nonetheless. They were more efficient at obtaining and consuming food than their therocephalian ancestors, and at about the same time that the first dinosaurs evolved the first mammals evolved from cynodonts. In these creatures the teeth were fully differentiated. They had an enlarged dentary bone that articulated with the skull and possessed a larger secondary palate. They were also probably covered in hair, even if they may have still laid eggs to reproduce. Just why the dinosaurs and other archosaurs dominated these synapsids is not fully known. (It has most often been thought of in terms of direct competition.) For the next 150 million years, mammals would not be larger than a small dog.
This is not to say that the dinosaurs were inherently “superior” to mammals. In fact, mammals survived and continued to evolve throughout the Mesozoic. They may have been small, but they carved out ecological niches small mammals occupy today. One of the most prolific groups during the Jurassic and Cretaceous, the multituberculates, showed adaptations to digging in their skeletons. Others, like a few members of an early group known as docodonts, had beaverlike tails and adaptations similar to those seen in semi-aquatic mammals. There were even Mesozoic precursors to anteaters and flying squirrels among the early mammals of the time, and not all mammals avoided confrontation with dinosaurs. In 2005, a mammal named Repenomanus, from the Cretaceous of Mongolia, was found with baby dinosaur bones in its stomach.
FIGURE 51 - An overview of mammal evolution during the Mesozoic. The larger diagram illustrates how many groups radiated and became extinct while the smaller summary shows the more general patterns of mammal diversity.
All three major groups of mammals alive today also evolved during the reign of the dinosaurs. The egg-laying monotremes, the pouch-bearing marsupials, and the widespread placental mammals all trace their earliest ancestry deep into the Mesozoic. At first they might seem to represent three stages of recent mammal evolution, from odd egg-laying forms through marsupials to placentals, but the fossil record tells a different story.
Marsupial and placental mammals are more closely related to each other than either is to the monotremes. There is a vast evolutionary gulf between the two groups and the living archaic mammals like the duck-billed platypus. While all living mammals shared a common ancestor during the Early Jurassic, not all the descendant lineages survived until today. Living monotremes are the remnants of an early radiation of mammals, one that does not appear to have been as prolific as the mid-Jurassic explosion of multituberculates and other groups. The common ancestor of the marsupial and placental mammals, however, did not evolve until the Early Cretaceous. Thus monotremes, marsupials, and placental mammals do not represent a stepwise pattern of direct evolution, but represent varying outcomes of mammalian evolution that can be traced back to different times.
FIGURE 52 - A line drawing of the skeleton, preserved with fur, of Eomaia, as well as a restoration of the skeleton. This fossil mammal was a 125-million-year-old close relative of early placental mammals.
Then came another great extinction. Sixty-five million years ago, the impact of an asteroid in the area of what is today the Yucatan Peninsula threw scalding air, chunks of the earth’s crust, and other debris into the atmosphere, triggering earthquakes and tidal waves in the immediate area of impact. From a distance it would have looked like the detonation of an extremely powerful bomb, but the worst effects of the event were felt over a longer span of time. The asteroid just happened to strike an area rich in sulfur, and the amount of the element the collision cast into the atmosphere caused the global temperature to drop up to ten degrees Celsius for several decades.48
The most famous victims of this extinction were the non-avian dinosaurs, but mammals had their numbers severely reduced by the extinction, too, and some groups disappeared altogether. Contrary to popular perception, mammals were hit almost as hard by the mass extinction as other groups of vertebrates, and only a few survived. The groups that fared better included the ancestors of living monotremes, marsupials, and placental mammals.
The extinction of the dinosaurs brought synapsids full circle. Nearly wiped out during the Permian extinction, the group diversified in a world ecologically dominated by dinosaurs. Many of the
ir kind perished during the same extinction that wiped out the non-avian dinosaurs, but the survivors thrived just the way the archosaurs had. Things might have turned out differently had the asteroid missed. There is no reason to believe that if the extinction of the non-avian dinosaurs had been postponed or canceled, mammalian evolution would have proceeded in the same manner as we see it today. Despite what we might like to believe, the dinosaurs were not stifling some internal, directing potential that would have ultimately culminated in creatures like us. The mammalian fossil record as we understand it today is a story of contingency, and mass-extinction events weigh heavily on the way evolution unfolds.
In the aftermath of the Cretaceous extinction, new types of mammals flourished and were cut down by extinction. As Darwin rightly knew from his own study of fossil mammals, this pattern did not represent progress but the continued adaptation of the old into the new in the wake of ever-changing conditions. Among the most striking examples of how these contingencies and constraints molded vertebrates can be seen in the early evolution of whales, a transition that until very recently was one of the great mysteries of the fossil record.
As Monstrous as a Whale
“Scarcely anywhere in the animal kingdom do we see so many cases of the persistence of rudimentary and apparently useless organs, those marvellous and suggestive phenomena which at one time seemed hopeless enigmas, causing despair to those who tried to unravel their meaning, looked upon as mere will-of-the-wisps, but now eagerly welcomed as beacons of true light, casting illuminating beams upon the dark and otherwise impenetrable paths through which the organism has travelled on its way to reach the goal of its present condition of existence.”
—WILLIAM HENRY FLOWER, “On Whales, Past and Present, and Their Probable Origin,” 1883
In the summer of 1841 the German-born fossil collector Albert Koch unveiled a monster. Over thirty feet in length and fifteen feet high at the shoulder, the tusked wonder dwarfed the visitors to Koch’s eclectic St. Louis Museum. He called it Missourium theristrocaulodon.
Koch had exhumed the remains of his star attraction not far from the lush banks of the Pomme de Terre River in Benton County, Missouri, the previous year. Among scraps of fossilized swamp moss and cypress trees were the bones of several individual animals, and Koch cobbled together their bones to construct a beast that seemed larger than life. Patrons flocked to the museum to marvel at its enormous tusks and tree-sized limbs.
Koch’s Missourium proved popular enough that he decided to expand his audience. The beast was to go on an east coast tour, and one of the first stops was the Masonic Hall in the bustling port city of Philadelphia. Crowds came to marvel at what Koch sold as the ruler of the antediluvian world, but mixed among the curious members of the public were representatives of the city’s prestigious community of naturalists.
Among them was the anatomist Paul Goddard, who immediately knew something was wrong. Koch’s Missourium was not a new creature but was already familiar to naturalists as a mastodon, an extinct elephant given the name Mammut americanum. Even worse, Koch had erred in his reconstruction by adding extra ribs and vertebrae to inflate the stature of the already-gigantic proboscidean.
Koch’s lack of academic training and his sensational promotion of his specimen did little to help him. Accusations and counter-accusations about Koch’s lack of expertise and motives circulated through Philadelphia’s scientific community, but Richard Harlan, another local anatomist and polymath, took the middle ground. After studying the skeleton himself Harlan could only conclude that Koch had simply made a few honest mistakes. Surely now that the errors had been exposed the extraneous bones would be removed.
If Koch agreed with Harlan’s assessment, he did not let on. When the skeleton was erected in London’s Egyptian Hall later the same year it appeared with every extra bone in place. Now, however, it had some competition. The scaly representatives of the recently described Dinosauria threatened to overshadow the “Missourium.” As a consequence Koch played up the might and size of his ancient pachyderm. A broadside poster proclaimed:This unparalleled Gigantic remains, when its huge frame was clad with its peculiar fibrous integuments, and when moved by its appropriate muscles, was Monarch over all the Animal Creation; the Mammoth, and even the mighty Iguanodon may easily have crept between his legs.
Such fanfare did not deceive British naturalists. Even though the Missourium was greeted with enthusiasm by some members of the London scientific elite, in 1842 the anatomist Richard Owen pointed out the spare ribs and vertebrae his American colleagues had previously noticed. Koch passionately defended his reconstruction before the Geological Society, but the British scholars were not convinced.
In the wake of this controversy Koch took the skeleton on tour elsewhere in Europe, yet the London scientists had not entirely soured on Missourium. Despite Koch’s overblown claims, it was still an impressive and valuable mastodon skeleton. When Koch stopped back in London in November 1843 at the end of the tour, the British Museum purchased it for £1,300. This was enough to enroll the Koch children in a school in Dresden, Germany, while the paleontologist and his wife traveled the European continent.
By 1844, however, Koch was itching to head back into the field to rebuild his collection. A fossil hunting trip through the United States would be just the thing, and as soon as Koch arrived in New England he started prospecting the local outcrops for choice specimens. Shells, shark teeth, and a few bones rewarded Koch’s efforts, but what he was really after was another monster. The Yale professor Benjamin Silliman, a close friend of Koch, would be instrumental in providing him with one.
When Koch stopped in New Haven, Connecticut, to visit Silliman on August 17 he had his entire array of fossil treasures in tow. Silliman was impressed with what his friend had already accumulated, but he knew of another place where there were even more impressive bones to be found. In parts of southern Alabama, local residents had found the abundant remains of an enormous sea serpent, and Silliman knew a woman who could tell Koch where to chisel his own sea monster from the rock. They went off to meet her at once. If Koch could obtain these remains then he would surely have a new attraction.
With directions to the monster graveyard in hand, Koch continued through Pennsylvania, Ohio, Indiana, Kentucky, and Missouri, picking up fossils as he went. He finally reached Alabama on January 20, 1845, but the fossils he sought proved elusive. It would be another month before he would first catch sight of them, and they would not be in the place he expected. On February 14, Koch was on his way to meet an acquaintance in Macon County when he spotted an enormous, charred vertebra lying in a fireplace. It could only have come from the sea serpent.
When he asked about the scorched bone Koch was told that it had been used for nearly three years as a fireplace support. This was only one of the ways in which the plentiful fossils were regularly destroyed. During his travels Koch saw the gigantic vertebrae used to prop up a fence, as a cornerstone in a fireplace, and as a slave’s pillow, and had even heard of a man who thought he could extract lime from the fossils by burning them. (All he got for his trouble were burnt bones that crumbled to pieces.) The bones were so numerous that in some fields they were destroyed because they interfered with cultivating the land. The widespread waste of the petrified treasures troubled Koch. As he wrote in his journal, it was a shame that so many fine specimens were “snatched from science by ignorance.”49
Because these specimens were so lowly valued, however, Koch was able to procure specimens with relative ease. The day after an unproductive sojourn into the field, Koch headed toward Clarksville and spotted a few vertebrae outside an abandoned home. They had not yet been burnt to a crisp, and Koch was able to purchase one from the “ignorant but kindhearted” steward of the plantation.
Koch knew he had finally reached the resting place of the monsters, but he could not just dig anywhere he pleased. The best sites were all on plantations, and he would have to convince the landowners to let him scour their propert
y for the ancient marine deposits on their property.
By March 11, Koch still was starting to feel annoyed by his rotten luck. Not only had his sea serpent failed to emerge but a torrential downpour pinned him inside the home of his friend Colonel Washburn. Nevertheless, there was plenty of work to do. Koch busied himself by organizing some of the other fossils he had collected during his quest, but right in the middle of it all a mail rider arrived with a package for Koch’s host.
Koch never had much patience when he was absorbed in his work, and when the postal carrier asked what he was doing Koch offered a curt reply to satisfy his curiosity and send him on his way. Fortuitously, though, the courier was no stranger to fossils. In fact, he had just heard that an immense stone shark, over ninety feet long, had just been found near the Washington County courthouse.
When Koch heard the mail rider’s description he immediately knew that the bones could not have been from a shark, and he pressed the man for more details. Some vertebrae and ribs had been excavated, but they were too heavy to move, the mail rider said, so most of the bones were still in the ground. Word had gotten around about the find, though, and rumor had it that four vacationers from New York were intent on collecting the fossils. Koch would have to act fast, but even though the messenger offered to take him to the site directly the paleontologist declined. Washington County was a long way to ride for what might turn out to be just a big fish story. Instead, Koch told the mail rider to go to the site, find out everything he could, and return as quickly as possible. The trip was forty miles both ways.