Mammals vs dinosaurs | University of Cambridge
Dinosaurs, as every schoolchild knows, were not just the most terrifying it's high time other prehistoric animals stepped out from the shadows. Among the animals, cold-blooded forms like dinosaurs would have been affected For a long time people found it hard to believe in such spectacular collisions. The so-called Age of Mammals—which began when the non-avian Less time separates us from Tyrannosaurus rex than separated T. rex.
The first of these occurred about one-third of the way through the period, during the early Jurassic. At the close of the Jurassic, about million years ago, a second minor mass extinction happened, affecting mostly the ammonoids, marine reptiles, bivalves and certain dinosaur groups such as the stegosaurs and the giant sauropods. Only one stegosaur escaped this extinction, Dravidosaurus, a small stegosaurid restricted to the Cretaceous of India which was an island at that time.
The causes of both of these extinctions are not clear. The Cretaceous Back to Top The Cretaceous period, spanning the time interval from to 65 million years ago, saw the final phases of the opening of the Atlantic Ocean, as well as the northward migration of India toward its collision with Asia during the Tertiary period of the Cenozoic era. The last of the major forms of plant life, the angiosperms, appeared near the beginning of the period.
Dinosaurs continued their diversification, with the increasing dominance of the herbivorous ornithischians. The Cretaceous ended with the famous mass extinction that resulted in the extinction of the non-avian dinosaurs, the swimming reptiles, pterosaurs, ammonites and other creatures, nearly three-fourths of all life on Earth in fact.
Life in Water The bivalves recovered from the Jurassic extinctions and again became major reef-formers in the numerous shallow marginal seas that encroached onto the continents during the Cretaceous. Reconstruction of a Cretaceous seafloor. Note the large ammonite on the right, the belemnites in the center, and the gastropods and bivalves on the seafloor.
In this view we see the top of one of the shells. The lower shell was usually quite different in shape. Ammonites continued their dominance, as did their relatives the straight-shelled belemnites. Modern teleost fish appeared during the Cretaceous and may have competed for the same prey as the ammonites. The teleost fish were apparently stronger and swifter swimmers than the fish of the Jurassic. Some paleontologists speculate that the extinction of ichthyosaurs during the Cretaceous may have been hastened by the rise of these new faster fish that would have been difficult for the ichthyosaurs to catch and eat.
The specimen is 2. Baculites, a genus of straight-shelled cephalopods, was particularly abundant in the Cretaceous seas. Note the elaborate suture patterns in the fossil specimen below. Left image Baculites was a straight-shelled cephalopod, about two feet in length, that presumably scavenged the bottom in search of food. Here, a small cephalopod becomes dinner image from http: Notice the extremely intricate suturing between septa.
The plesiosaurs from the Jurassic continued their diversification. Ichthyosaurs, however, declined during the early Cretaceous before becoming extinct well before the end-of-the-Cretaceous extinction. Both the long-necked plesiosaurs, and the short-necked pliosaurs were present during the Cretaceous. Despite inevitable competition and changes in their diets as new prey evolved, plesiosaurs were doing well up until the extinction at the close of the Cretaceous. To the list of swimming reptiles we add the mosasaurs, essentially swimming "lizards".
These ferocious predators reached lengths to 30 feet 9 meters. Mosasaurs are usually considered relatives of the monitor lizards, although some recent analyses suggest a closer link to snakes is a distinct possibility. Mosasaurs were predators of the Cretaceous seas.
Many ammonite fossils have been found that have tooth marks suggesting mosasaurs actively sought and ate ammonites. Some paleontologoists believe that mosasaurs became so successful because they were able to utilize the role in the marine environment that was opened once the ichthyosaurs went extinct. Mosasaurs apparently gave birth to live young and perhaps had some level of parental care for their young.
A mosasaur from the Cretaceous approximately million years old of southern Georgia. The specimen is 26 feet long and almost completely original material. Mosasaur dentition was well-adapted for catching fish, ammonites, and other free-swimming prey. The recurved teeth prevented prey from escaping once caught.
Life on Land Perhaps the greatest change in the terrestrial life of the Cretaceous was the rise and diversification of the flowering plants, the angiosperms. The angiosperms, the last of the seed plant groups to evolve, appeared over million years ago during the the beginning of the Cretaceous. All members of this group produce flowers.
Within the female parts of the flower angiosperms produce a diploid zygote and triploid endosperm. Fertilization is accomplished by a variety of pollinators, including wind, animals, and water. Two sperm are released into the female gametophyte: There are presentlyknown living species.
Whence came the angiosperms? This was Darwin's "abominable mystery". Clearly angiosperms are descended from some group of Mesozoic-aged gymnosperm seed plant Click here to view an online lab exercise in phylogeny and try to figure things out! The flowering plants most likely evolved from a Mesozoic gymnosperm group. At one time the cycadeoids were the prime candidates, although more recent studies suggest another group, the gnetophytes as the sister group to the angiosperms. Three distinctive genera comprise this group: Welwitschia, Gnetum, and Ephedra.
Ephedra occurs in the western United States where it has the common name "Mormon tea". It is a natural source for the chemical ephedrine, although there is no evidence the Mormons in Utah where the plant is extremely common ever used it for tea, the use of stimulants being against their beliefs. Welwitschia is limited to coastal deserts in South Africa, although fossil leaf, cuticle and pollen evidence indicates plants of this type were widespread during the Mesozoic era.
Welwitschia is noted for its two long, prominent leaves. Gnetum has leaves that look remarkably like those in angiosperms, as well as vessels in the xylemgenerally considered an angiosperm characteristic.
Gnetophytes appear in the fossil record before the angiosperms do, and share a great many derived characteristics, making them the current choice as the angiosperm sister group or ancestral group in a noncladistic sense. The classical view of flowering plant evolution suggests early angiosperms were evergreen trees that produced large Magnolia-like flowers. Click here to view an illustration of suggested paths of floral evolution. However, this view has recently been contradicted by the oldest fossil yet found, a million year old plant found by paleobotanist David Dilcher and his associates at the Florida Museum of Natural History.
Fossil birch leaf from the Cretaceous Dakota Formation in Kansas. Image modified from the Internet. Part and counterpart of a Cretaceous fossil angiosperm leaf.
The angiosperms underwent an adaptive radiation during the Cretaceous, and for the most part escaped the major extinctions at the end of the Cretaceous. One group of angiosperms that did suffer from the extinction was the unknown groups of plants that produced triprojectate pollen. My dissertation dealt with the systematics of this fossil group. The pollen is quite odd-shaped and quite striking to see. Aquilapollenites, from the Cretaceous Judith River Group.Dinosaur Myth Busted: Dinosaurs and mammals
This palynomorph occurs in sediments from 74 to 76 million years ago. This grain is about 50 mm in length. Crocodiles, which had been around since the Triassic, produced a massive form, Deinosuchus, during the Cretaceous. This marine crocodile was over 20 feet long, and dwarfs the largest modern crocodile.
Comparison of the skull of Deinosuchus and a modern crocodile. The ornithischian dinosaurs during the Cretaceous evolved into a number of interesting groups. From bipedal ornithischians during the Jurassic, like Camptosaurus, the larger and more nimble ornithopods of the Cretaceous evolved. One of the first dinosaurs to be scientifically described was Iguanodon, a bipedal herbivore thought to have moved about in herds.
Iguanodon was between 6 and 10 meters long and weighed around 5 tons. Reconstruction of Iguanodon, an early Cretaceous ornithopod dinosaur. The hadrosaurs, commonly known as the duck-billed dinosaurs, are another group of ornithopods. These animals had hundreds of teeth in a bill reminiscent of a duck's bill although ducks lack teeth.
There were two subgroups of hadrosaurs. The crested lambeosaurs, and the noncrested hadrosaurines. The latter group is exemplified by Maiasaura and Edmontosaurus. The lambeosaurs are exemplified by Parasaurolophus. Maiasaura, the "good mother lizard" has been extensively studied for its nesting and parental care. Adult Maiasaura and two juveniles.
Evolution of mammals
The skull of Saurolophus showing the crest and duckbill-like snout. Reconstruction of Saurolophus by a Cretaceous lake. Another major group of Cretaceous ornithischians was the ankylosaurs, the armored, quadrupedal herbivores. Unlike the Jurassic stegosaurids, the armor covering on the back was more robust, as were the spikes behind the shoulders.
In some ankylosaurs the tail terminated in a bony knob that might have been used to sweep the legs of predators, possibly even breaking bones. Sauropelta, a Cretaceous ankylosaur. The other great ornithischian group was the ceratopsians. These quadrupedal herbivores were characterized by a beak-like snout, horns above the eyes and possibly on the end of the nose, and some degree of a bony frill around the back of the skull.
Triceratops is perhaps the most famous and well known of this group. Triceratops was about 8 meters long and weighed between 4 and 8 tons. Ceratopsians first evolved, possibly from an ornithopod ancestral group, at the beginning of the Cretaceous, some million years ago. By the end of that geological period, ceratopsians were among the most common dinosaurs in the northern hemispjere.
Triceratops in fact was one of the last of the nonavian dinosaurs. A herd of Triceratops, with palm trees in the background and the large flying reptile Quetzlcoatulus. Triceratops was a rhinoceros-sized herbivore with three bony facial projections and a large neck frill.
The saurischians of the Cretaceous included the ferocious Tyrannosaurus rex literally "tyrant king lizard "as well as numerous smaller predatory dinosaurs such as the "raptors" popularized by the Jurassic Park series. In addition to the carnivores, Cretaceous sauropods existed, although diminished in size and abundance from their Jurassic forebearers.
Perhaps no other dinosaur has inspired as many plush toys, tattoos, and awe as Tyrannosaurus rex. Sporting six inch long serrated teeth T. Tyrannosaurs ranged from meters in length and weighed in at an estimated tons, and were restricted to North America during the latest part of the Cretaceous period. The most famous T. Sue is now on exhibit at that museum. A smaller carnovore that preceeds T. This smaller version of T. It ranged North America in the time interval just prior to the rise of T.
Corythosaurus, a crested hadrosaur, fleeing from a hungry Albertosaurus. A related group of carnivorous Cretaceous dinosaurs is the dromaesaurs, considered by some recent studies to be the sister group to the birds.
Dromaesaurs had been around since the Jurassic or earlier. Sometime during the Jurassic the line leading to birds diverged. The dromaesaurs had a variety of unique features, such as their slashing modified foot claw, as well as an inferred behavior of grasping with their forelimbs. This latter feature may have been important in the evolution of avian dinosaur flight. Until the Cretaceous, dromaesaurs were smallish predatory dinosaurs.
Deinonychus, considered one of the most important dinosaurs ever found, was a small animal about meters long, weighing a mere 75 kg. Utahraptor, discovered from middle Cretaceous rocks in Utah, is the largest dromaesaurid at a weight of one ton and a length of 5 to 7 meters.
Velociraptor, the dromaesaurid so prominent in the Jurassic Park series, was about 2 meters long and had an estimated weight of 7 to 15 kg. A pack of Dromaeosaurus hectoring a doomed Lambeosaurus. The flying reptiles of the Cretaceous were the pterodactyls, short tailed descendants of the long tailed Jurassic forms. The largest of the pterodactyls was Quetzalcoatlus, named after the Aztec feathered serpent god.
Quetzalcoatlus northropii had a wingspan of 11 meters and may have weighed over km. Pteranodon whose scientific name literally means "wing without tooth" was a common Cretaceous form that had a 7 meter wingspan as well as a large crest that presumably served as a navigational aid. Two Quetzalcoatlus flying over a grouyp of ceratopsians by a Cretaceous seashore.
Skeleton of a pterodactyl. Cretaceous birds continued to expand their environmental range. Hesperornis was a flightless swimming bird that lived in the shallow seas of the Cretaceous. There were other Cretaceous birds that flew. Large, flightless birds, like the ostrich, would not appear until the Tertiary period.
Hesperornis and a nestling. Cretaceous mammals were still quite small and minor components of the fauna. Their significance would change with the Cretaceous mass extinction, from which they would emerge as co-rulers of the land with the avian dinosaurs.
The Cretaceous World The breakup of Pangaea continued during the Cretaceous, with the Atlantic Ocean finally opening completely by the late Cretaceous. The westward movement of North and South America led to massive mountain building episodes that produced parts of the Rocky Mountains in the north and the Andes Mountains in the south.
During much of the Cretaceous, North America was split into two separate land areas by the north-south tending seaway. This led to the development of two distinct paleofloristic regions: The Aquilapollenites province extended northward through Alaska and Siberia and then southward into eastern Asia.
The normapolles province passed through Greenland and into northern Europe. With the drop in sea-levels at the end of the Cretaceous plants and animals were able to migrate between these and other regions.
India, which had separated from the rest of the former Gondwana earlier, remained an island throughout the Cretaceous and into the Tertiary period. One of the best-known is Gorgonops itself, the dominant predator of its day, which thanks to its pillar-like rear legs probably moved at very fast speeds.
Another, called Lycaenops, had powerful canine teeth in both its upper and lower jaws, which meant that despite measuring about three feet in size, it was capable of stabbing or tearing at much larger animals - although it probably stuck to hunting reptiles and other small prey most of the time. Its leg positioning was such that it was probably much more agile than many contemporary creatures, and able to outrun them to hunt them down. Mammals are not the only interesting, but little-known creatures from around this time for which fossil evidence is growing.
Today, for example, we are familiar with crocodiles and alligators, which are reptiles. Yet these are just two surviving examples of a much larger lineage called Pseudosuchia, which thrived during Triassic times, and in some cases were a much more fearsome prospect.
It was also bigger — probably about four metres 13 feet in length.
On Dinosaur Time | Science | Smithsonian
Not something you want to run into, nor indeed away from, given that it could probably move pretty swiftly, as well. The preeminence of dinosaurs really only began about million years ago, when there was a sudden, devastating extinction event which probably wiped out something like half of the life on Earth. This, however, did not rule out the existence of mammals. For example, scientists have uncovered the remains of a small, squirrel-sized mammal that lived at least million years ago, at a time when dinosaurs were dominant.
Whatever this creature was, it was clearly gliding around the place in exactly the same age as the reptilian Pterosaurs.
Morganucodon, also a contemporary of dinosaurs, was an apparently less-spectacular, nocturnal creature. It lived about million years ago and many remains attesting to its existence have been found in Glamorgan, in Wales.
This small, furry animal had quite a long tail and a skull two to three centimetres in length, lived in a burrow, and might have looked a bit like a vole.
It survived by eating insects, and other small animals. As face-offs go, this appears to be something of a no-brainer.
Most of these animals are extinct. We know about the Pleistocene mammals not only from fossils but also from carcasses, especially of the woolly rhinoceros, which have been found in petroleum deposits in Romania. Mammoths, complete with flesh, hair and stomach contents, have been found frozen in the ice in Siberia. Some members of the Pleistocene megafauna were restricted to certain areas. For example, the woolly rhinoceros, giant deer, the moose-like giraffe shown in the slide, and the cave bear were found only in Eurasia and Africa.
But the Bering land bridge has been present intermittently through the entire Cenozoic period. It allowed the entry of many of the large mammals that were to subsequently dominate the North American fauna, including the woolly mammoth, imperial mammoth, mastodon, bison, deer, sheep, cattle and many large carnivores.
Slide shows a reconstruction of a Pleistocene scene in North America. Notice the saber-toothed cat with its enormous canine teeth. The Bering land bridge also allowed animals that evolved in North America to colonize Asia. They subsequently 8, years ago for horses became extinct in North America. Horses did not reappear in this country until Columbus' second voyage in See slide of the evolutionary history of the horse - one of the most complete fossil series available, often used as a classical example of evolution.
Mammalian Evolution in South America. At the beginning of the Cenozoic era, there was a land bridge between North and South America, as there is today. This land bridge allowed primitive mammals to colonize South America from the North.
This land link was later during the Eocene broken, and those animals which had settled in South America then evolved in complete isolation from the rest of the world. Over about 40 million years these primitive groups diversified in many unique ways. The placental mammals those that carry their young in a uterus, using a placenta to provide nutriment of South America evolved as herbivores, many of them large and slow moving, like the ground sloth Megatherium which reached up to 29 feet tall when standing upright on its hind legs.
The ground sloth was the largest and heaviest of all land mammals there may have been a sighting in ! Other South American mammals evolved single-digit feet ending in strong hoofs, exactly like those of modern horses, and others possessed a well-developed trunk and massive legs like the elephant's. These are examples of convergent evolution. The marsupials mammals that carry their young in a pouch, like kangaroos evolved in the other direction, becoming carnivores. One of them bore an amazingly close resemblance to the independently evolved saber-toothed cat of North America, which was a placental mammal.
This is another example of convergent evolution. New forms of wildlife continued to arrive in South America even after the continent was cut off from North America. They included primates which gave rise to the New World monkeys, including howler monkey, marmosets, capuchins, woolly monkeys and spider monkeys; and rodents which evolved into a number of families, several of which are found nowhere else in the world. These include the capybara, the agouti, the coypu, the cavy Guinea pig and the chinchilla.
In South America there is a greater variety of rodents than anywhere else in the world. The Invasion of South America. In the upper Pliocene, 3 million years ago, the isthmus of Panama reappeared as a result of changes in the earth's crust.
This was a disaster for many of the animals that had evolved in isolation in South America. South America was invaded by deer, camels, raccoons, tapirs, horses, mastodons, bears, peccaries, rabbits, shrews, cats, dogs, weasels and rodents.
For some reason these animals were able to displace many of the South American species, driving many of them to extinction.
Some of the new arrivals e. Others were very successful, for example the camel family which has given rise to the vicunas, guanacos, alpacas and llamas. The camels as well as the horses subsequently became extinct in North America where they originated. Another group of uniquely South American mammals, the edentates sloths, armadillos and anteaterssurvived the competition with the invaders and are still abundant in South America.
The armadillos, like their primitive ancestors, are armor-plated mammals in which the armor plating is composed of separate shields and hinged bands. But the related species of one extinct group, the glyptodonts, had a single-piece carapace similar to that of tortoises. These glyptodonts, some of which were as big as a Volkswagen and armored like a tank, survived up until quite recent times and may have been hunted by primitive Indian tribes; piles of glyptodont bones have been found alongside various human artifacts.
A few of the animals that had evolved in South America migrated in the reverse direction, becoming established in North as well as South America: South America provides a spectacular example of how evolution can take off in novel directions when a region is isolated for a long enough period of time. It also provides a dramatic lesson in how apparently well adapted species can often be driven to extinction when exotic species those coming from outside are introduced.
Mammalian Evolution in Australia. The mammalian fauna of Australia also evolved in isolation since the early Cenozoic Eocenebut in this case the isolation remained complete. For unknown reasons, Australia was apparently originally populated entirely by marsupials rather than placental mammals.
Today the native mammalian fauna of Australia is made up of marsupials of many different kinds, that occupy ecological niches similar to those occupied by placental mammals in other parts of the world. Evolution produced marsupial mice, a marsupial mole, and, most impressive, a marsupial wolf almost extinct and a marsupial lion extinctall of which bear striking resemblances to the corresponding placental forms. Other Australian marsupials occupy the same ecological niches as certain placental mammals in other continents, but are rather different in appearance.