Cynodonts possessed many mammalian features, including the reduction or complete absence of lumbar ribs implying the presence of a diaphragm; well-developed canine teeth, the development of a bony secondary palate so that air and food had separate passages to the back of the throat; increased size of the dentary - the main bone in the lower jaw; and holes for nerves and blood vessels in the lower jaw, suggesting the presence of whiskers.
By million years ago the mammals had already become a diverse group of organisms. Some of them would have resembled today's monotremes e.
Until recently it was thought that placental mammals the group to which most living mammals belong had a much later evolutionary origin. However, recent fossil finds and DNA evidence suggest that the placental mammals are much older, perhaps evolving more than million years ago. Note that the marsupial and placental mammals provide some excellent examples of convergent evolution , where organisms that are not particularly closely related have evolved similar body forms in response to similar environmental pressures.
However, despite the fact that the mammals had what many people regard as "advanced" features, they were still only minor players on the world stage. As the world entered the Jurassic period - million years ago , the dominant animals on land, in the sea, and in the air, were the reptiles. Dinosaurs, more numerous and more extraordinary than those of the Triassic, were the chief land animals; crocodiles, ichthyosaurs, and plesiosaurs ruled the sea, while the air was inhabited by the pterosaurs.
Taking wing: Archaeopteryx and the origins of the birds. In an intriguing fossil was found in the Jurassic Solnhofen Limestone of southern Germany, a source of rare but exceptionally well-preserved fossils. Given the name Archeopteryx lithographica the fossil appeared to combine features of both birds and reptiles: a reptilian skeleton, accompanied by the clear impression of feathers. This made the find highly significant as it had the potential to support the Darwinians in the debate that was raging following the publication of "On the origin of species".
While it was originally described as simply a feathered reptile, Archaeopteryx has long been regarded as a transitional form between birds and reptiles, making it one of the most important fossils ever discovered. Until relatively recently it was also the earliest known bird. Lately, scientists have realised that Archaeopteryx bears even more resemblance to the Maniraptora , a group of dinosaurs that includes the infamous velociraptors of "Jurassic Park", than to modern birds.
Thus the Archaeopteryx provides a strong phylogenetic link between the two groups. Fossil birds have been discovered in China that are even older than Archaeopteryx, and other discoveries of feathered dinosaurs support the theory that theropods evolved feathers for insulation and thermo-regulation before birds used them for flight.
This is an example of an exaptation. Closer examination of the early history of birds provides a good example of the concept that evolution is neither linear nor progressive.
Not all achieved powered flight, and some looked quite unlike modern birds e. Microraptor gui , which appears to have been a gliding animal and had asymmetric flight feathers on all four limbs, while its skeleton is essentially that of a small dromaeosaur.
Archaeopteryx itself did not belong to the lineage from which modern birds Neornithes have evolved, but was a member of the now-extinct Enantiornithes. A reconstruction of the avian family tree would show a many-branched bush, not a single straight trunk. Dinosaurs spread throughout the world - including New Zealand, which had its own dinosaur fauna - during the Jurassic, but during the subsequent Cretaceous period - 65 million years ago they were declining in species diversity.
In fact, many of the typically Mesozoic organisms - such as ammonites, belemnites, gymnosperms, ichthyosaurs, plesiosaurs, and pterosaurs - were in decline at this time, despite the fact that they were still giving rise to new species. The origin of flowering plants the angiosperms during the early Cretaceous triggered a major adaptive radiation among the insects: new groups, such as butterflies, moths, ants and bees arose and flourished.
These insects drank the nectar from the flowers and acted as pollinating agents in the process. The mass extinction at the end of the Cretaceous period, 65 million years ago, wiped out the dinosaurs along with every other land animal that weighed much more than 25 kg.
This cleared the way for the expansion of the mammals on land. In the sea at this time, the fish again became the dominant vertebrate taxon. At the beginning of the Palaeocene epoch 65 - This unique situation was the starting point for the great evolutionary diversification of the mammals, which up until then had been nocturnal animals the size of small rodents.
By the end of the epoch, mammals occupied many of the vacant ecological niches. While mammal fossils from this period of time are scarce, and often consist largely of their characteristic teeth, we know that small, rodent-like insectivorous mammals roamed the forests, the first large herbivorous mammals were browsing on the abundant vegetation, and carnivorous mammals were stalking their prey. The oldest confirmed primate fossils date to about 60 million years ago, in the mid-Palaeocene.
The early primates evolved from archaic nocturnal insectivores, something like shrews, and resembled lemurs or tarsiers the prosimians. They were probably arboreal , living in tropical or subtropical forests. Many of their characteristic features are well suited for this habitat: hands specialised for grasping, rotating shoulder joints, and stereoscopic vision.
They also have a relatively large brain size and nails on their digits, instead of claws. The earliest known fossils of most of the modern orders of mammals appear in a brief period during the early Eocene Both groups of modern hoofed animals, the Artiodactyla "even-toed" taxa such as cows and pigs and Perrisodactyla "odd-toed" taxa, including the horses , became widespread throughout North America and Europe.
The evolutionary history of the horses is particularly well understood: Stephen Jay Gould provides an excellent discussion of it in his book "Hens' teeth and horses' toes". At the same time as the mammals were diversifying on land, they were also returning to the sea. The evolutionary transitions that led to the whales have been closely studied in recent years, with extensive fossil finds from India, Pakistan, and the Middle East.
These fossils chronicle the change from the land-dwelling mesonychids, which are the likely ancestors of whales, through animals such as Ambulocetus , which was still a tetrapod but which also has such whale-like features as an ear capsule isolated from the rest of its skull, to the primitive whales called the Archaeocetes. The trend towards a cooler global climate that occurred during the Oligocene epoch This change in vegetation drove the evolution of browsing animals, such as more modern horses, with teeth that could deal with the high silica content of the grasses.
The cooling climate trend also affected the oceans, with a decline in the number of marine plankton and invertebrates. While DNA evidence suggests that the great apes evolved during the Oligocene, abundant fossils do not appear until the Miocene.
Hominids, on the evolutionary line leading to humans, first appear in the fossil record in the Pliocene 5. The story of human evolution is covered here - Human Evolution material. New Zealand, by virtue of its isolation and its relatively recent geological development, was not the centre of any novel evolutionary development. However, many of the species that date back to Gondwanaland, or that arrived more recently as migrants, have undergone significant adaptive radiation in their new homeland.
Some of the best examples of this can be related to the major ecological changes that accompanied the Pleistocene Ice Ages. Throughout the Pleistocene there were about twenty cycles of cold glacial "Ice Age" and warm interglacial periods at intervals of about , years. During the Ice Ages glaciers dominated the landscape, snow and ice extended into the lowlands, transporting huge quantities of rock with them.
During these periods the South Island was extensively glaciated, and there were small glaciers on the Tararua Ranges and Central Plateau. Because a lot of water was locked up in ice, the sea levels dropped during the glacials up to m lower than at present.
Extensive land bridges joined the main and many offshore islands, allowing the migration of plants and animals. During the warmer periods large areas became submerged again under water. This suggests that the monkeys are instruments and benefactors of cultural transmission. The findings were made even more potent by those demonstrated in the second paper last week, which focused on humpback whales in Maine.
Usually, humpbacks have been recorded feeding by circling their prey in a ring of bubbles. A group of marine scientists plowed through an enormous Maine database of sightings documenting whale behavior, and which stretched back almost 30 years. Noting instances of observed lobtailing, and the proximity between whales that exhibited the technique, they plugged the information into a computer that showed how lobtail feeding was likely to have spread. A whale lobtailing Photo by A. While the resulting culture is vastly removed from our own, Carel van Schaik, an orangutan researcher who was not associated with the study said to Science that it heralds a new era in animal culture research.
Membership benefits include one year of Audubon magazine and the latest on birds and their habitats. During the mids, this species exhibited a precipitous decline in many locations due to a particularly virulent disease known as white plague. The disease still affects this species and many other corals , but only one colony showed recent tissue loss from this condition.
At the edge of the reef terrace several sergeant majors Abudefduf saxatilis had deposited their purple eggs, each patch forming a perfect circle, cm in diameter. The fish vigorously defended these from predators, including Andy. He had placed his transect line a bit close to one territory, and the angry fish repeatedly nipped at his legs.
While most fish were chased away, one of the fish was unsuccessful in defending his nest; several medium-sized herbivorous parrotfish overwhelmed the fish and rapidly consumed the eggs. Mostly princess parrotfish, these species are supposed to eat algae.
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You can view our complete Privacy Policy here. This species occurs predominately along the southeastern Pacific coasts of Peru and Chile. In contrast to other marine carnivorans, the marine otter spends much of its time out of the water foraging for food along rocky coastlines Fig. Both fossil and molecular evidence indicate that marine mammals did not evolve or descend from one single ancestral group. Although cetaceans, pinnipeds, sirenians, otters, and polar bears are all mammals, they evolved from separate branches of the mammal line Fig.
All of these separate branches evolved from terrestrial mammal groups. The hippopotamus is the closest living ancestor to the cetacean group. Sirenians share a common evolutionary ancestor with modern elephants. Pinnipeds share common ancestry with other carnivorans such as dogs and cats, but are most closely related to the weasels, otters, and skunks. Although many of the adaptations for ocean survival seen in these marine mammals appear similar in form and function, each of these marine mammal groups evolved adaptations for life in an ocean environment independently.
Evolutionary biologists are interested in the physical form of structures on an organism. They are also interested in how these structures function or work to improve the survival and reproduction of the organism. Form and function are so closely tied together that, sometimes, unrelated organisms develop similar structures. We call this phenomenon convergence or convergent evolution.
You can see convergence in very different animals that live in similar habitats or have similar lifestyles. A bat is a mammal, and yet it appears to have flying structures in common with many birds and insects. All of these flyers have wings. If we take a closer look at these structures, though, we will see that they are not as similar as they might appear. The wings of bats are supported by bones that are basically elongated finger bones, whereas the wings of birds are covered with feathers, a structure lacking in bats.
Insects have no bony supports within their wings, and the membranous structures are actually part of the exoskeleton, composed of chitin and proteins. Even though all of these animals use their wings to fly, when we look closely at them, we see that the wings themselves are very different and, taken along with other characteristics of the organisms birds and insects do not have hair, nor do they feed their young with milk , do not indicate a recent common ancestor.
Whales are aquatic mammals that, like fish, propel themselves through the water with fins. Even though whales have fins, they are not fish. If we look at the fins of whales and the fins of fish we will see some substantial differences.
The fins of whales are supported by the same kinds of bones that support the wings of bats. In fact, if we look at these structures closely, they provide evidence for a closer relationship between bats and whales than between bats and birds or between whales and fish.
Fish fins are very different than whale fins. Fish fins are supported by bones, but the origin and structure of the bones supporting the fins are very different than those found in whales. Convergent evolution can make it challenging to determine the evolutionary relationships between animal groups.
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