Earth Detectives: Solving the Planet's Mysteries—Part 2
By Stewart Mittnacht, Staff Writer
The progress of science is often driven by happy accidents. When Nobel Laureate Luis Alvarez set out to date a sample of Mesozoic sediment sent to his lab by his son, Walter, also a renowned scientist, they never expected to become the first scientists to provide solid evidence that Earth had suffered a mass extinction, rattling the biology community to its core and overturning nearly a century of scientific dogma.
It took several decades for the Alvarez hypothesis to gain traction among biologists and geologists. Conventional wisdom held that extinction was gradual and local, with fitter, “more evolved” species slowly replacing older, deficient forms. Even Darwin couldn’t imagine any catastrophe of a large enough scale to cause most species to die off in a relatively short amount of time. Most evolutionary biologists assumed that the rapid death of organisms in the fossil record represented missing sediment layers that had yet to be discovered or were otherwise destroyed.
The Alvarez discovery forced biologists to take a look at the problem from a different angle—the fossil record wasn’t as incomplete as had previously been imagined. Instead, sixty-five million years ago a Mount Everest-sized asteroid struck, triggering an explosion that set all of North America on fire and generated a three-mile-high tsunami large enough to hit every landmass across the globe.
Preceding this cataclysmic event, for 135 million years terrestrial life on Earth had been completely dominated by dinosaurs. Only a small handful would survive this catastrophe—the birds. Why some species survive the worst disasters and others perish comes down to small differences in habitat requirements and feeding behaviors. Understanding these differences is an important step in preserving biodiversity and defending threatened species. With this in mind, let’s take a look at the world 66 million years ago, just before the end of the dinosaurs.
By the time of the Cretaceous, the former supercontinent of Pangaea had been torn apart. A great rift valley had formed, forcing the Americas to split from Asia and Africa. This valley eventually widened until it formed the Atlantic Ocean. Powering this transformation was the longest chain of volcanoes in the solar system, now known as the Mid-Atlantic Ridge. This crack in the Earth’s crust not only broke America away from Asia, but it also separated Africa from Antarctica. The resulting map would dictate the future of each continent’s ecosystem.
Much like the Carboniferous, the Cretaceous epoch was warm and wet. The weather was fed by the near constant volcanic activity churned out by the Mid-Atlantic Ridge. Because volcanoes release tremendous amounts of carbon dioxide, this potent gas raised temperatures around the globe. This heat, in turn, caused the oceans to expand; shallow seas covered North America’s Midwest, and Europe existed as a series of archipelagos in the tepid Tethys Sea.
Marine reptiles thrived in these hot waters, such as long-necked plesiosaurs, mosasaurs (which resembled finned Komodo dragons), sea turtles and Hesperornithiformes, which were flightless, dolphin-sized birds equipped with long, splayed feet that propelled them through the water.
The Cretaceous shallows were also home to reefs, but corals competed alongside rudist clams as the main reef builders. Ammonites, too, developed into new forms—the spiral shell variety now shared the waters with a straight-shelled group.
Flying above these seas like reptilian albatross were the last vestiges of the pterosaurs. Although their ancient lineage had largely died out by this time for uncertain reasons, the remaining pterosaurs grew enormous and fed largely on fish.
In many ways, the Cretaceous was a mixture of ancient and modern species. You might recognize familiar trees, like sycamore and magnolia, and early forms of monocots, such as bamboo and palm. They had developed along with a new partner that was giving them a distinct advantage—bees. Before bees, angiosperms—flowering plants—were rare, dependent on beetles to act as their pollinators. Beetles lacked the pollen-collecting hairs and specialized nectar-seeking nose of a bee, and they were not dedicated pollinators. Bees changed everything. By the end of the Cretaceous, most conifer forests were losing ground to the angiosperms.
Though the gymnosperms still survive today, they tend to live in habitats where soil conditions or temperature are less suitable to their angiosperm rivals, and groups of once cosmopolitan distribution, such as monkey puzzle trees, are now restricted to remote and isolated groves.
As this change in conifers accelerated toward the end of the Cretaceous, it took its toll on the sauropods, the largest animals to ever walk the Earth. These mighty dinosaurs were broad grazers, capable of digesting many plants, but apparently avoiding broad-leafed trees. Their fossil dung contains the remains of conifers, cycads and early forms of bamboo and rice.
Despite their varied diet, the sauropods had already peaked in the Jurassic Period (the epoch preceding the Cretaceous), with only one surviving group remaining—the titanosaurs. These titanic lizards emphasized size above any other defense.
Such large herbivores required vast amounts of plants to sustain their bulk, and sauropod avoidance is one explanation for the size of some surviving conifers, such as the sequoia. With an abundance of such large prey animals, some therapod carnivores began to specialize in hunting larger game. This group contains the familiar tyrannosaurs, but also more unusual forms, including the semi-aquatic spinosaurus, a shark-hunting therapod longer than Tyrannosaurus Rex and equipped with a crocodile-like skull and long vertebrae that supported either a fatty hump or a sail-like fin.
Fossil trackways indicate that many later therapods developed pack-hunting behavior as a response to the increasing size of their prey. Along with pack hunting came larger brains. Tyrannosaurus Rex had a complex, highly developed brain compared to most dinosaurs, which helped it hunt its favored prey, the ceratopsians, horned dinosaurs that include the famous triceratops and styracosaurus.
Other medium-sized herbivores included the ankylosaurids, pachycephalosaurus and hadrosaurs. The dromaeosaurs rounded out the major dinosaur groups of the era. That group included the colloquially known “raptors.”
Though the dinosaurs were diverse and dominated the top tiers of the food web, they shared their world with other reptiles. Though many modern forms were present in the Cretaceous, including crocodilians, lizards, and snakes, the diapsids were more diverse than they are today, with many more ancient family lines still present right up to the end of this “Age of Reptiles.”
A few odd survivors of these reptile lineages remain—the tuatara of New Zealand. Though they resemble lizards, the tuatara are the last vestiges of a formerly diverse and robust group of reptiles collectively known as the sphenodonts. Another reptilian group that lived during the Cretaceous—and lasted well beyond its end—were the champsosaurs. These “lizard crocodiles” proved to be quite resilient and remained one of the top marine predators until the appearance of marine mammals nearly twenty million years after the Cretaceous mass extinction.
Cretaceous mammals, too, are more diversified and prolific than previously assumed. Though many mammals retained primitive features, such as egg-laying, early marsupials and even placental mammals began to outcompete older varieties by the end of the epoch, and though many were specialized insectivores, a few developed wider palates and innovative body types. Early forms of the platypus appeared in the Cretaceous, alongside the badger-like repenomamus, the only known mammalian carnivore of this age.
A pattern begins to emerge when you consider what animals survived the impact of the asteroid. The few large animals that survived, including champsosaurus, crocodilians, and marine turtles, share an aquatic lifestyle. This is also true of the birds—the only surviving group was a close relative of the Hesperornithiformes, likely a gannet-like shorebird.
The platypus also fits this aquatic profile. All of these species occupied habitats that are fed not by photosynthesis, but by decay. Flowing water tends to be low in nutrients as any new minerals get quickly flushed out to sea. This suppresses primary food production within the river itself, and most riverine communities rely on the addition of detritus from the surrounding landscape. After the impact, vast amounts of ash would have drifted, possibly for years, in the upper atmosphere. This perpetual smog shut down photosynthesis, and the food web upon which dinosaurs and other large reptiles depended collapsed. Only animals that were adapted for detritus-based ecosystems, or ones that relied on insect-heavy diets, had a chance at survival.
The dinosaur’s extinction showcases both the vulnerability of stable ecosystems and the resilience of life itself. Large animals such as dinosaurs tend to be slow breeders, and are less adaptable when their environment is stressed. This same problem currently threatens the Earth’s remaining megafauna, as the recent extinction of the northern white rhinoceros showcases.
Our understanding of the complex relationships that develop in food webs might give us an edge in preserving species. For example, to preserve their elephants from poaching, the Botswanan government has installed water pumps in proximity to ranger stations. Such direct, but guided, intervention in an ailing ecosystem may be the key to life’s future.
Earth now has one key adaptation that the dinosaurs lacked—intelligence. In the future, our species might mitigate or even prevent such disasters as asteroid impacts, either by deflecting incoming asteroids with rockets or by constructing impact-proof bunkers to house frozen embryos and seeds as a sort of scientific version of Noah’s Ark.
Eventually, we might even introduce life to other planets, or help other species through genetic modification (such as an ongoing effort to make the now dwindling American chestnut tree immune to blight).
Science gives us the wisdom and knowledge to learn from the disasters of the past and the tools to prepare for the future. As long as we avoid squandering these gifts, both humans and the Earth can share a brighter future.
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