The Reign of Dinosaurs – Part 2
Ornithischians started slowly, beginning to become common in the late Jurassic, just when the greatest biological innovation in the past 300 million years began: the appearance of flowering plants, which first bloomed about 160 mya. Until that time, plant strategies involved avoiding being eaten by animals, whether it was bark, height, poisonous foliage, etc. Flowering plants adopted a different strategy: laying out a banquet for animals. The primary benefit for plants was spending less energy to reproduce, as well as attracting animals that did not seek to eat the plants and even ended up protecting them. The advantage for animals was an easily acquired tasty meal. It was the greatest direct symbiosis between plants and animals ever, other than plants providing the oxygen that animals breathe, which is inadvertent. The two primary aspirations that seed plants achieve for successful reproduction are becoming fertilized via pollination and placing seeds were they can become viable offspring (and feces fertilizer could only help). Flowering plants, also called angiosperms, did not invent animal assistance from whole cloth. Some Jurassic insects have been found in association with gymnosperm (conifer) cones, and were likely doing the work that the wind previously performed. Similar to the enzyme example of a key rattling around in a room, attracting animals to plants, to eat the pollen and nectar, was like a reproductive enzyme, where animals carried the key to the lock to initiate reproduction. Other animals ate the fruit and thereby spread the seeds. That relationship did not become significant until the mid-Cretaceous. Angiosperms mature faster and produce more seeds than gymnosperms do. By the Cretaceous’s end, angiosperms dominated tropical biomes where ferns and cycads used to thrive, and they pushed conifers to the high latitudes, just as they have today. That tropical dominance is likely related to the insect population, which prefers warm climates. Angiosperms became Earth’s dominant plants after the end-Cretaceous extinction, and comprise more than 90% of plant species today.
There is speculation that dinosaurs invented flowering plants in a coevolutionary dance, as low-browsing ornithischians put pressure on plants to grow and reproduce quickly, and angiosperms are far more effective at those activities than all plants preceding them. The spread of angiosperms in the mid-Cretaceous coincided with the ornithischians’ rising dominance, and by the end-Cretaceous extinction, they were the most numerous herbivores by far. Stegosaurs appeared in the late Jurassic and went extinct by the late Cretaceous.
In the late Jurassic, as ornithischians began to become plentiful, a theropod innovation would lead to the only dinosaurs to survive the end-Cretaceous extinction: birds. As with synapsid sails, stegosaur plates, and a Triceratops’s horns and frill, feathers had a display function as well as thermoregulation, long before they were used to fly. Ever since scientists realized that dinosaurs were closely related to birds, they have watched for feathers, and have found more than 20 genera of dinosaurs that sported feathers. That famous Archaeopteryx fossil discovered in 1860-1861 began the speculation that birds evolved from dinosaurs, and was considered one of the first confirmations of Darwin’s theory of evolution. Today, scientists are far from certain that Archaeopteryx flew, and it is not considered a direct ancestor of today’s birds. Feathered dinosaurs existed before Archaeopteryx’s 155 mya appearance, and they are in the clade that led to today’s birds. Birds probably did not fly much, if at all, until the Cretaceous, and the first beaked birds appeared in the early Cretaceous.
When birds began to fly, their energy requirements skyrocketed. Today’s bats, for instance, burn several times as many calories as similarly-sized non-flying mammals, and live several times longer than similarly-sized mammals, just as birds live far longer than similarly-sized mammals. Mammalian life-expectancy follows a curve where size, metabolism, and longevity are all closely related. The general rule is that all mammals have about the same number of heartbeats in a lifetime. A mouse’s heart beats about 20 times as fast as an elephant’s, and an elephant lives about 20 times as long as a mouse. Larger bodies mean slower metabolisms, or less energy burned per unit of time per cell. Birds have the same kind of size/metabolism/life-expectancy curve, but it sits on a higher level than mammals. A pigeon lives for about 35 years, or ten times as long as a similarly-sized rat. On average, birds live three-to-four times as long as similarly-sized mammals.
Because of the stupendous energy demands of flight, birds not only have the superior air sac system for breathing, but their mitochondria, the cell’s energy-generation centers, are far more efficient than mammalian mitochondria. Avian mitochondria are also probably the leading reason why birds live so long. Parrots in captivity can live to be 80, and scientists have noted an albatross in the wild reproducing at more than 60, and scientists may discover that wild albatrosses live to be a hundred or more, when their tagging programs get that old. The mitochondrial theory of aging likely explains bird longevity, with the efficient mitochondria of birds producing fewer free radicals. While the theory is controversial and will be for many years, I think that an engine analogy can help. A bird is a piece of high-performance biological technology, and when operating at peak output it puts all land-bound animals to shame. But a bird’s metabolism is usually in its slack state, only maximized during flight. Simply put, a bird has a great energy capacity that is rarely used to its fullest. It is like a high-performance engine that rarely runs near its redline. Such engines will last far longer than those regularly running near redline. High performance technology that usually “loafs” in its slack state and is rarely taxed is expensive and long-lasting. The investment in superior technology allows for high performance and long life. High quality technology is more economical in the long run, if the initial investment can be afforded.
Recognizably modern birds existed by the end-Cretaceous, and modern birds were the only dinosaurs to survive the end-Cretaceous extinction. Small pterosaurs called pterodactyls today first flew about 150 mya, about the time that birds appeared. The skies were getting crowded by the late-Cretaceous, although birds and pterosaurs seem to have inhabited different niches. Modern birds likely survived the end-Cretaceous extinction because they found refugia, likely in swampy margins, burrows, and holes in trees, such as those that woodpeckers can create.
Another energy-related activity likely first appeared on a large scale during the reign of dinosaurs: territoriality. While territoriality can be observed in insects, fish, amphibians, and reptiles today, it is most common among birds and mammals. Territoriality is primarily about preserving an animal’s energy base against competitors, and it is usually a behavior oriented toward others of the same species, which would eat the same food resources and mate with the same potential partners. Just as what seems to be consciousness appeared with the earliest animals, territorial behavior may go all the way back to the Cambrian Explosion. But the social behaviors apparent in dinosaurs likely also meant territorial behavior, and likely on a scale never experienced before on Earth. Even the suspected display function of synapsid sails implies territorial behavior. All great apes are territorial, and human political units such as nations are nothing more than ape territoriality writ large, as peoples protect their energy and mating bases. With display being common in today’s birds (with its apotheosis in the peacock, although, as usual, there are competing hypotheses), with the phenomenon likely going all the way back to synapsids, with dinosaurian mass nesting sites, herd behaviors, and the like being discovered, many dinosaurs were likely territorial.
In the late Jurassic, armored stegosaurs and ankylosaurs first appeared, using an ornithischian defensive strategy that ceratopsians also developed in the early Cretaceous, which reached its peak with Triceratops in the late Cretaceous. Today’s rhinoceros is the mammalian equivalent of Triceratops, but today’s rhinos don’t have to face anything as fearsome as Tyrannosaurus Rex, although the most successful predators in Earth’s history, humans, are driving rhinos to extinction.
The Tethys Ocean was fully formed in the Jurassic, and the continents began to break up in earnest, which led to rising sea levels. The shallow seas that began to reappear in the Triassic became widespread in the Jurassic as continental shelves were submerged. The Atlantic Ocean began forming in the Jurassic, as South America and Africa split, and the world-circling Panthalassic Ocean became the Pacific Ocean about the same time, although it is more of a convention among geologists than any dramatic change. Australia began to split from Antarctica during the Jurassic. Mountain-building events along the west coast of North America continued unabated, and the Andes Mountains, which began forming in the Triassic, continued developing in the Jurassic.
The mid-Jurassic marked the beginning of a 140-million-year period of anoxic events that produced most of Earth’s oil deposits, with them finally ending in the Eocene. The anoxia of post-Triassic Mesozoic oceans seems to be at least partly the result of increased runoff from land spurred by volcanic events, combined with warm, stagnant, stratified surface waters. Low atmospheric oxygen, combined with high nutrient runoff and warm waters that absorb less oxygen than cold water, provided the conditions for those anoxic events, with atmospheric oxygen levels only increasing toward modern levels in the Cretaceous. Also, changing currents (including upwelling, which usually brings nutrients to the surface) and rising sea levels (which can make the seafloor anoxic) may have contributed to the unprecedented and never reproduced anoxia of those times. Until the current low-oxygen events that humans are inducing, anoxic events, and hence oil formation, have not occurred much in the past 40 million years.
About 183 mya, an extinction event linked to anoxic and volcanic events hit ammonoids hard, as usual. The extinction seems to have been confined to the oceans. Along with the appearance of carbonate hardgrounds, reefs slowly recovered in the Jurassic, and by the Jurassic’s end, coral reefs lined Tethyan shores. Low-oxygen tolerating marine animals proliferated in the Jurassic. Ammonoids, with their superior respirational equipment, developed large, thin-shelled varieties that likely housed their large gills, required to navigate the Jurassic’s low-oxygen waters. Also, a different kind of cephalopod, the ancestor of squids, became plentiful in the Jurassic. The first crabs appeared in the Jurassic, and they also developed a superior respiration system, where they put their gills within their armor and developed a pump gill. As most seashore visitors know, crabs are quite tolerant of being exposed to air, much as nautiloids suffer no ill effects when exposed to air for a short time. Crabs proliferated with the late Jurassic’s reefs, to only collapse with the end-Jurassic reef collapse (called the Tithonian event, or end-Jurassic extinction), which was caused by a sudden drop in sea levels, and the extinction again appeared to be restricted to marine biomes.
The sea level drop quickly reversed in the early Cretaceous, and the Cretaceous (c. 145-66 mya) saw the most dramatic rise in ocean levels during the eon of complex life. At the sea level’s peak, the land’s surface area during the Cretaceous was about two-thirds of today’s (18% versus today’s 29% of surface coverage). By the early Cretaceous, today’s continents were recognizable, and for the first time ever, marked regional differences appeared among the terrestrial animals that inhabited continental biomes. The sauropods generally stayed south, and ornithischians came to dominate the northern continents, and theropods also became quite diverse in the late Cretaceous. The iconic theropod and most famous dinosaur, T-rex, appears to have solely been a North American resident. Earth’s fossil record for dinosaurs is richest in North America (with China and Mongolia coming in second), so the fossil record may be biased toward northern dinosaurs. Today, there are only about one hundred professional dinosaur paleontologists on Earth; not a very large community. To most six-year old boys, those scientists won the lottery, being paid to study dinosaurs and dig their fossils from the ground. One of the uglier disputes in paleontology’s history was a race in the late nineteenth century between two Americans bent on outcompeting each other in finding and describing dinosaur fossils. In T-rex’s northern range, Triceratops was the dominant herbivore, and its confrontations with T-rex may have been Earth’s greatest land battles ever, at least until humans appeared. In T-rex’s southern range, North America’s largest dinosaur, a gigantic sauropod, lived.
As land’s surface area shrank, the continents became wetter, as all land became closer to the oceans. In the late Jurassic, there was a cooling period, the coldest time of the entire Mesozoic, with even some mountainous and polar glaciation, but end-Jurassic volcanism kept carbon dioxide levels high and the climate warmed. Warm-climate plants lived within 15 degrees of the South Pole during the Cretaceous, and forest went all the way within five degrees of the poles, which has fascinated scientists as they try to envision a biome where it was dark for nearly half the year. The Cretaceous was generally a hot, wet time on Earth.
India broke away from Gondwana in the early Cretaceous, and the Gondwana breakup beginning about 150 mya is generally considered the birth of the Indian Ocean. By the Cretaceous’s end, India was alone and swiftly moving toward Southern Asia and a prodigious collision that formed the Himalayan Mountains and Tibetan Plateau. The Andes were uplifted during the Cretaceous, and mountain-building events (1, 2) continued in western North America, and in the late Cretaceous, the Rocky Mountains began their rise and the volcanic hotspot that created the volcanic mountain chain that is currently represented by the Hawaiian Islands first appeared. In the late Cretaceous, the Tethys Ocean connected with the Pacific and created a world-circling tropical current, which helped gentle and warm Earth’s weather systems, which contributed to the anoxic events. The North America’s Great Plains were under a shallow sea in the Cretaceous.
Calcareous plankton appeared in the Mesozoic, and required oxygen to form calcium carbonate. They became so abundant in the high oxygen of the late Cretaceous that the rain of their bodies on ocean floors gave the Cretaceous its name: chalk (the Latin name). Calcium carbonate, the primary constituent of limestone, comes in two forms: calcite and aragonite. The magnesium content in the oceans, as well as the ocean temperature and level, determines which form of calcium carbonate will dominate. The Permian extinction also marked the end of a hundred million year ice age, giving way to about 200 million years of hot times. During the eon of complex life, Earth has vacillated between icehouse and greenhouse conditions, with ice ages separated by hot periods. It also seems to be related to supercontinent dynamics. Hot seas are generally calcite seas, and cold seas are usually aragonite seas. Calcite seas create carbonate hardgrounds, which influence what kind of biome forms. The Ordovician and Silurian periods had vast carbonate hardgrounds, which disappeared during the Karoo Ice Age and returned in the hothouse age of dinosaurs, becoming common in the Jurassic. Today’s ice age has aragonite seas, so organisms that form calcium carbonate shells use aragonite, which is less stable than calcite and its formation is sensitive to temperature and acidity. Coral reefs, key phytoplankton (which help produce Earth’s oxygen), and shellfish use aragonite today to form their shells. There is already strong evidence that the acidification of the oceans, due to humanity’s burning of fossil hydrocarbon deposits to power the industrial age, is interfering with the ability of coral, carbonate-forming phytoplankton, and shellfish to form their shells. That is only one of the industrial age’s many deleterious ecosystem impacts. The current aragonite situation is not a theoretical construct of fearful environmentalists, but an impact that is measurable today.
According to GEOCARBSULF, oxygen levels rose in the Cretaceous and nearly reached modern levels by the end. But anoxic events also dotted the Cretaceous, likely related to rising sea levels. The largest bivalve ever lived in the Cretaceous, reaching three meters in length, but it was not a fearsome predator. It was a deep-water species that probably formed symbiotic relationships with chemosynthetic organisms, along with those other low-oxygen Mesozoic bivalves, and it went extinct as oxygen levels rose in the atmosphere and likely also in the seas.
When sea levels rise dramatically, as they did in the Cretaceous, coral reefs will be buried under rising waters, and the ideal position, for both photosynthesis and oxygenation, is lost, and reefs can die, similar to burying a tree’s roots. About 125 mya, reefs made by rudist bivalves, which thrived on carbonate hardgrounds, began to displace reefs made by stony corals. They may have prevailed because they could tolerate hotter and more saline waters than stony corals could. About 116 mya, an extinction event happened, likely caused by volcanism, which temporarily halted rudist domination, but rudists thrived until the late Cretaceous, when they went extinct, probably due to changing climate. Carbon dioxide levels steadily fell from the early Cretaceous until today, with temperatures falling during the Cretaceous, and hot-climate organisms gradually became extinct during the Cretaceous. Around 93 mya, another anoxic event happened, perhaps caused by underwater volcanism, which again seems to have been confined to marine biomes. It was much more devastating than the previous one, with rudists hit hard, although it was a more regional event. Biomes beyond 60 degrees latitude were barely impacted, while those close to the equator were devastated. Ammonoids seem to have been brought to the brink with nearly all marine mass extinctions during their tenure on Earth, and it was no different with that late Cretaceous extinction. Ammonoids recovered once again, with their largest species ever living in the late Cretaceous, but the end-Cretaceous extinction marked their final appearance, as they went the way of trilobites and other iconic animals.
Sauropods were high grazers, eating tree ferns, cycads, and conifers as their staple, and the dramatic radiation of ornithischians in the late Cretaceous coincided with the spread of angiosperms, and their chewing ability continually improved. Insects also dramatically diversified, as did birds and mammals, in an epochal instance of coevolution between plants and animals. Hive insects (bees, wasps, termites, ants) began their rise when flowering plants did.
Shell-cracking lobsters first appeared in the early Cretaceous, and by the late Cretaceous, mosasaurs became the dominant marine predators, with ichthyosaurs going extinct after 150 million years of existence and plesiosaurs declining. Those apex predators preyed on squids as large as today’s, and sharks and ray-finned fish always seemed to do well, with some substantial sharks appearing in the mid-Cretaceous that even preyed on mosasaurs and plesiosaurs. The largest sea turtles yet recorded lived in the late Cretaceous, at four meters long and two metric tons.
In the nineteenth century, the Jurassic was called the Golden Age of Dinosaurs, but that moniker is arguably most applicable to the late Cretaceous, and was a Golden Age clear up until a bolide impact brought it all to an end. The dinosaur extinction is likely the largest and most contentious controversy in the history of paleontology. Again, the subject of mass extinction was taboo, due to Lyell’s and Darwin’s prevailing uniformitarianism, until my lifetime. The hypothesized bolide event, first proposed in 1980, was a kind of a bolide event inflicted on paleontology, with acrimonious disputes ignited that still burn, but it made studying mass extinctions respectable. Initially attacked and dismissed, the bolide impact hypothesis is by far today’s leading hypothesis for explaining the end-Cretaceous extinction. However, at the same time, India was speeding toward its date with destiny, and it movement is associated with a huge volcanic event that created the Deccan Traps. Also, sea levels seesawed at the end-Cretaceous, so the bolide event has some theoretical competition as the causative agent.
It is probably safe to say that if the end-Cretaceous extinction had multiple causes, none of the ancient mass extinctions can be attributed to just one cause. However, the sudden disappearance of all non-avian dinosaurs, and what survived, casts a heavy vote for the bolide hypothesis. Also, there may have been multiple impacts, similar to how the Shoemaker-Levy 9 comet fragmented before it plowed into Jupiter. Dinosaurs were all terrestrial and were either herbivores or ate herbivores. The largest bolide impact obviously hit North America the hardest, T-rex would have been among the first casualties, and it would have created an artificial “winter” lasting at least a few months, which might have followed the greatest fires in Earth’s history. All photosynthetic organisms would have been devastated, as well as the food chains that relied on them. That alone can explain the end of non-avian dinosaurs, but it also helps explain what survived. Ammonoids were lightweight versions of nautiloids, living near the ocean’s surface. Nautiloids had retreated to deep waters hundreds of millions of years earlier, and they lay eggs that take a year to hatch, and they lay them in deep water. All ammonoids went extinct in the end-Cretaceous event, ending a 300-million-year-plus tenure on Earth, and all the marine reptiles disappeared, too. Rudist bivalves were in decline before the extinction, likely related to the sea level changes, but it is looking like they lasted until the bolide event. They were all dependent on primary-production food chains that would have been interrupted by the “bolide winter,” for those that survived the initial conflagration, and they all went extinct. However, a year after the disaster, when the smoke and dust had cleared, out hatched nautiloids that had been safe in their shells the entire time, and nautiloids are still with us. Sharks would have feasted on dead beasts; both aquatic animals and carcasses washed into the oceans by tsunamis.
Most plants produce seeds, and seeds would have largely survived the catastrophe, and began growing when conditions improved. Ferns came back first, in what is called a fern spike, as ferns are one of the disaster-taxa. Crocodiles, modern birds (which included ducks at the time), mammals, and amphibians also survived, and all could have found refuge in burrows, swamps, and shoreline havens, lived in tree holes and other crevices that they were small enough to hide in, and all could have eaten the catastrophe’s detritus. In general, freshwater species fared fairly well, especially those that could live on detritus. Also, the low-energy requirements of ectothermic crocodiles would have seen them survive when the mesothermic/endothermic dinosaurs could not hold out. The primary determinants seem to have been what could survive on detritus or energy reserves, and what could not, and what could find refuge from the initial conflagration. While there may have been some evidence of dinosaur decline before the end-Cretaceous extinction (it was gradually growing colder), and the Deccan Traps may have caused at least some local devastation, the complete extinction of non-avian dinosaurs, ammonites, marine reptiles, and others that would have been particularly vulnerable to the bolide event’s aftermath has convinced most dinosaur specialists that the bolide impact alone was sufficient to explain the extinction, and no other hypothesis can seem to explain the pattern of extinction and survival that the bolide hypothesis can. In general, the key to surviving the end-Cretaceous extinction was being a marginal species, and all of those on center-stage paid the ultimate price. The end-Cretaceous toll was nearly 20% of all families, half of all genera, and about 75% of all species, and marked the end of an era, with the Mesozoic ending and making way for the Age of Mammals, also called the Cenozoic, and used to have the Biblically-inspired title of the Tertiary.
In the wake of the success of the end-Cretaceous bolide hypothesis, there was a movement in some circles to explain all mass extinctions with bolide events, particularly the Permian extinction. If bolide events were responsible for all mass extinctions, then the periodic, galactic explanation might still have relevance. Even though an end-Permian bolide event was unveiled with great fanfare and media attention in 2001, it does not appear to be a valid extinction hypothesis today, and invoking bolide impacts to explain every mass extinction seems to have been a passing trend that has seen its best days. The oxygen hypothesis for explaining extinctions, evolutionary novelty, and radiations is similarly being called a current fashion in some circles, and time will tell how the hypothesis fares, although it seems to have a great deal in its favor.
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