“THE MEEK INHERITORS
[OR ONE EXTRATERRESTIAL IMPACT CAN RUIN YOUR WHOLE DAY]:
THE DINOSAUR EXTINCTION AND THE RISE OF MAMMALS”
100 million to 10 million years ago
|About 65 million years ago, an extraterrestrial impact wreaked havoc on earth’s life forms. The vestiges of the catastrophe are recorded in the geological strata that is known as the Cretaceous-Tertiary Boundary.
Credit: Illustration by Don Davis, courtesy of NASA
How did mammals and birds come to replace the reptilian dinosaurs as the major inheritors of the earth? The later evolution of the dinosaurs occurred between 100 and 65 million years ago, including such forms as Tyrannosaurus rex and Velociraptor as well as the three-horned Triceratops.
It now appears that 65 million years ago the earth was struck by a large extraterrestrial body (an asteroid or comet) about 5 miles in diameter, about the size of Mt. Everest). This catastrophic impact (now known from a 100 mile-wide crater off the coast of Yucatan, Mexico) would have created a 5 mile-high tidal wave in coastal areas, reaching far into the continents and flooding much of the land. This was followed by a firestorm of countless rock fragments launched into space only to re-enter the earth’s atmosphere as fireballs heating much of the earth to oven-like temperatures. The massive quantities of dust particles created by the impact circled the earth and probably then caused a cold, dark “nuclear winter”, in which much of the earth’s plant life would have perished, starving many of the surviving animals that would have fed on them. Talk about being in the wrong place at the wrong time! Geologically, the strata documenting this catastrophe have been called the “K-T” or Cretaceous-Tertiary Boundary.
About two-thirds of the existing species perished and are not found in the fossil record after this event (that is, above the K-T Boundary), including all of the dinosaurs. This apparently opened vast niches for mammals and birds, as well as a host of other animals, and created conditions for the rapid spread and diversification of new forms. The major orders of mammals emerged during this time, including marsupials (kangaroos, platypus, etc.), insectivores (moles, shrews, hedgehogs), lagomorphs (hares and rabbits), carnivores (dogs, cats, hyenas, bears, raccoons, skunks, otters, etc), rodents (mice, rats, chipmunks, squirrels, hamsters, gerbils, etc.), bats, sea mammals (whales and dolphins), artiodactyls (even-toed giraffes, deer, antelope, cattle, sheep and goats, pigs, hippos, and camels), perissodactyls (odd-toed animals including rhinos, horses, and tapirs), edentates (armadillos, sloths, and anteaters), and hyraxes and dassies. The first representatives of our order, the primates, emerged during this time. The fossil primate record documents the evolution of the first prosimians, monkeys, and apes over the following tens of millions of years.
THE LATER DINOSAURS
95-65 million years ago
A golden age of dinosaur evolution occurred during the later Cretaceous Period between approximately 95 and 65 million years ago. The climate was warm then, with higher sea levels and no ice at the poles. This was the time period of many of the famous dinosaurs, including meat-eaters such as Tyrannosaurus and Velociraptor, (both featured in the somewhat misnamed movie “Jurassic Park”), Oviraptor,and plant-eaters such as the three-horned Triceratops, the duck-billed Hadrosaurus, and the armored and spike-tailed Ankylosaurus. All indications were that dinosaurs would continue to be the dominant animal form for millions of years into the future, but, as we will see in the next event, Fate had other ideas…
“ONE EXTRATERRESTRIAL IMPACT CAN RUIN YOUR WHOLE DAY”:
THE DINOSAUR EXTINCTION
65 million years ago
Imagine the worst day of your life. Multiply that by the largest number you can think of. That’s the kind of day you would have had some 65 million years ago when a six-mile-wide asteroid struck the earth, travelling as fast as 5,000 miles per hour. Its impact created a catastrophic explosion – triggering massive earthquakes, creating gigantic tidal waves, igniting massive forest fires, and releasing huge quantities of acidic gases into the atmosphere to fall to the earth as acid rain. A massive dust cloud would have covered the earth, blocking sunlight and creating a so-called cold “nuclear winter” for several years, killing much of remaining vegetation, followed by a stifling “greenhouse” warming period. This was the second largest extinction event in the history of the earth, with 75 percent of all species going extinct, including all of the large land reptiles (dinosaurs), sea reptiles (plesiosaurs), and air reptiles (pterosaurs). Luckily, many smaller reptiles, birds, fish, and mammals survived this terrible event and continued on to inherit this brave new world.
“A WARM AND FUZZY FEELING”
THE MAMMALIAN RADIATION
60 million years ago
With the extinction of the dinosaurs 65 million years ago, mammals had many new niches available to them. These warm-blooded, hairy forms, with sweat glands modified to feed milk to their young, were particularly able to adapt to the many land and sea niches left open after the dinosaurs died out. Mammals had been around for some time, coexisting with the dominant dinosaurs for millions of years.
These small insect-eating animals fed at night to help escape being eaten themselves! After the dinosaurs were gone, the mammals soon spread and became very diverse, becoming the dominant animals on the earth up to today (thus the name for our era, “The Age of Mammals.”
THE FORMATION OF THE HIMALAYAS
55 million years ago
Most mountain ranges were created by the collision of one of the earth’s moving plates with another, producing uplift or “orogeny.” Around 167 million years ago the Indian subcontinent (connected then to Australia and Madagascar) broke off of Africa and subsequently began drifting eastward, then broke off of Australia and started drifting northward, culminating in a massive collision with Asia starting around 55 million years ago. This crunching together of two continental plates, the Indo-Australian Plate and the Eurasian Plate, began slowly but surely to create the highest mountain range in the world today, with the peak of Mount Everest reaching 29,028 feet above sea level. The creation of the Himalayas radically affected the courses of river systems and created a major biogeographical barrier for plants and animals, including our ancestors, and has drastically affected not only Asian climate, but likely that of the entire planet.
“OF THE FIRST ORDER”
THE RADIATION OF THE PRIMATES
55 million years ago
The 17th century Swedish naturalist Carol Linnaeus got it right – When he developed his system of biological classification now called Linnaean taxonomy, he correctly grouped humans, apes and monkeys into the order Primates (meaning “first” or “top”). Compared to other mammals, primates are characterized by having relatively large brains to body size, nails rather than claws, stereoscopic (“3-D”) vision, color vision, and an overall reduction in the snout and organs for smell. They tend to give birth to small litters and spend more time caring for their young as well. The fossil record shows that, as is the case of most mammal orders, primates evolved from a shrew-like ancestor between 65 and 55 million years ago.
“THE LAND DOWN UNDER”
THE ORIGIN OF AUSTRALIA AND ITS ANIMALS
40 million years ago
By 80 million years ago, due to plate tectonics, the Australian landmass (originally containing New Guinea and Tasmania) began to separate from Antarctica (both landmasses containing marsupial mammals, then being much closer to the equator), and by 40 million years ago had no more contact with the rest of the Old World. It then began its unique evolutionary history dominated by marsupial mammals. Naturalist Alfred Wallace (along with Charles Darwin, the co-discoverer of evolution by natural selection) was one of the first to notice a biogeographical barrier separating the marsupial animals of Australia and adjacent islands from the placental mammals of the rest of the Old World. This became known as the “Wallace Line”.
“HEY, HEY, WE’RE THE MONKEYS”
THE RISE OF MONKEYS
35 million years ago
Monkeys differ from more primitive primates (called prosimians or “pre-monkeys”) by having larger brains relative to body size, a different number of teeth in the upper and lower jaws, and often a larger body size. The earliest monkey-like forms, Apidium and Aegyptopithecus emerged around 35 million years ago in the Africa. At around this time it is also believed the ancestors of the New World monkeys migrated to the Americas, possibly living on floating island of vegetation as they drifted across the Atlantic Ocean to South America, which was significantly closer to Africa than today due to plate tectonics and perhaps also a lower sea level some of the time.
“GRAZING IN THE GRASS”
THE SPREAD OF GRASSLANDS AND GRAZING ANIMALS
Beginning 25 million years ago
We tend to take grasses for granted. We mow them in our front yards, we eat them in the form of wheat, rye barley, rice, and corn, and their pollen can give us sneezing fits from hay fever. Yet grasses only started becoming common on earth starting about 25 million years ago and especially widespread in the form of prairies starting about 7 million years ago.
“THE SULTANS OF SWING”
THE RISE OF APES
23 million years ago
We belong to an advanced group of primates called the apes, who evolved in the trees of the Old World during the past 35 million years. What are apes? Apes are primates that have descended from prehistoric monkey ancestors, but differ from monkeys in having larger brains, usually larger bodies, and no tail; the arms of an ape are also highly mobile, allowing them (and us) to rotate the arm a full 360 degrees. Apes, including humans, are members of the superfamily Hominoidea or “hominoids”. These include, in increasing order of relatedness to humans, the so-called “Lesser Apes”, (the gibbons and siamangs of southeast Asia); and the “Great Apes” (the Orangutan of southeast Asia and the African apes: gorillas, chimpanzees, and bonobos or “pygmy chimpanzees”. In the trees, non-human apes are known for their swinging (brachiation) and climbing skills.
“FATHER OF KONG”
THE EARLIEST GORILLA-LIKE FORMS
10 million years ago
Genetic evidence suggests that the last common ancestor of the African apes (chimpanzees, bonobos or pygmy chimpanzees, and gorillas) and humans was around seven million years ago. African ape fossils are not very common, probably because they tended to live in humid, tropical forests with acid soils that do not preserve bones very well. Many anthropologists suspected that earlier African apes might look more similar in size and shape to modern chimpanzees or bonobos than to the much larger gorillas.