During Which Period Do We See The First Land Animal?

Plant and Animal Evolution

The History of Animal Evolution

For many people animals are peradventure the most familiar, and most interesting, of living things. This may be because we are animals ourselves. As such, we accept a number of features in mutual with all the organisms placed in the animal kingdom, and these common features betoken that we have a shared evolutionary history.

All animals and plants are classified as multicellular eukaryotes: their bodies are made upwardly of large numbers of cells, and microscopic inspection of these cells reveals that they incorporate a nucleus and a number of other organelles . Compared to prokaryotic organisms such every bit bacteria, plants and animals have a relatively recent evolutionary origin. DNA prove suggests that the offset eukaryotes evolved from prokaryotes, between 2500 and 1000 million years ago. That is, eukaryotes every bit a taxon engagement from the Proterozoic Era, the last Era of the Precambrian. Fossils of both unproblematic unicellular and more complex multicellular organisms are institute in affluence in rocks from this period of time. In fact, the name "Proterozoic" means "early life".

Plants and animals both owe their origins to endosymbiosis , a process where one prison cell ingests another, but for some reason then fails to digest it. The testify for this lies in the manner their cells function. Both constitute and animal rely on structures called mitochondria to release free energy in their cells, using aerobic respiration to produce the energy-carrying molecule ATP . At that place is considerable show that mitochondria evolved from free-living aerobic leaner: they are the size of bacterial cells; they divide independently of the prison cell by binary fission ; they take their own genome in the course of a single circular Deoxyribonucleic acid molecule; their ribosomes are more like to those of bacteria than to the ribosomes institute in the eukaryote cell's cytoplasm; and like chloroplasts they are enclosed by a double membrane every bit would be expected if they derived from bacterial cells engulfed past another cell.

Like the plants, animals evolved in the sea. And that is where they remained for at to the lowest degree 600 1000000 years. This is because, in the absence of a protective ozone layer, the state was bathed in lethal levels of UV radiation. Once photosynthesis had raised atmospheric oxygen levels loftier enough, the ozone layer formed, meaning that it was then possible for living things to venture onto the country.

The oldest fossil prove of multicellular animals, or metazoans , is burrows that appear to have been made by smoothen, wormlike organisms. Such trace fossils have been found in rocks from China, Canada, and India, but they tell us little virtually the animals that fabricated them autonomously from their basic shape.

The Ediacaran animals
The Cambrian "explosion" and the Burgess Shale
What caused the Cambrian "explosion"?
A foot on the state
The primeval vertebrates
Advent of the fish
The jawless fish
Colonisation of the state
Problems encountered in the movement to land
The development of amphibians
What drove amphibian evolution?
Early reptiles and the amniotic egg
The early mammals
Developments in the dinosaur lineage
Further developments in the early mammals
Taking fly: Archaeopteryx and the origin of the birds
The end of the dinosaur age
The appearance of modern mammal groups

The Ediacaran animals

Betwixt 620 and 550 meg years ago (during the Vendian Catamenia) relatively big, complex, soft-bodied multicellular animals appear in the fossil record for the kickoff time. While establish in several localities around the earth, this item group of animals is by and large known as the Ediacaran animal, later the site in Australia where they were first discovered.

The Ediacaran animals are puzzling in that in that location is little or no prove of whatever skeletal difficult parts i.eastward. they were soft-bodied organisms, and while some of them may have belonged to groups that survive today others don't seem to bear any relationship to animals we know. Although many of the Ediacaran organisms accept been compared to modern-day jellyfish or worms, they have also been described every bit resembling a mattress, with tough outer walls effectually fluid-filled internal cavities - rather like a sponge.

As a group, Ediacaran animals had a flat, quilted appearance and many showed radial symmetry. They ranged in size grade 1cm to >1m, and have been classified into three master groups on the ground of their shape: discoidal, frond-like, or ovate-elongate. The large variety of Ediacaran animals is significant, as it suggests there must have been a lengthy period of evolution prior to their kickoff appearance in the fossil record.

The Cambrian "explosion" and the Burgess Shale

The Ediacaran animals disappear from the fossil record at the end of the Vendian (544 million years ago). In their place nosotros find representatives of almost all the modernistic phyla recognised today: sponges, jellyfish and corals, flatworms, molluscs, annelid worms, insects, echinoderms and chordates, plus many "bottom" phyla such as nemertean worms. These "mod" organisms appear relatively chop-chop in the geological time scale, and their abrupt appearance is often described every bit the "Cambrian explosion" however, bear in mind that the fossil record of the "explosion" is spread over about 30 1000000 years. I keep taking things out of brackets because it is interesting relevant and memorable

Ane of the most famous assemblages of Cambrian fossils comes from the Burgess Shale of British Republic of colombia. The rocks of the Burgess Shale were laid downwardly in the heart Cambrian, when the "explosion" had already been underway for several million years. They incorporate familiar animals such as trilobites, molluscs and echinoderms, but also the first appearance of brachiopods, and some odd animals, e.g. Opabinia, that may have belonged to extinct phyla. Even an early on chordate, Pikaia, has been establish in this fossil aggregation.

The Burgess Shale fossils are important, not only for their evidence of early multifariousness amidst animal forms, only also because both soft parts of animals and their difficult bodies (i.east. the whole brute) is preserved, and animals that were entirely soft-bodied. Preservation of soft-bodied organisms is rare, and in this case seems to accept occurred when the animals were rapidly buried in a mudslide down into deep, anaerobic waters, where there was little bacterial decay. Prior to the discovery of this fossil assemblage, early on in the 20th century, there was no evidence of soft-bodied animals from the Cambrian (remember that this is before the Ediacaran animal were establish).

These fossils too provide good evidence of predatory animals (due east.g. Anomalocaris ), and therefore of complex predator-prey relationships. They too give insights into how evolution might have progressed relatively early on in the history of multicellular animals, and in fact some authors view the Cambrian as a menstruation of extreme "experimentation" and variety.

What acquired the Cambrian "explosion"?

The cause of the proliferation of fauna forms in the Cambrian is a thing of considerable debate amidst scientists. Some point to the increase in atmospheric oxygen levels that began around 2000 1000000 years agone, supporting a higher metabolic rate and assuasive the evolution of larger organisms and more circuitous body structures. Inverse sea chemistry would take played a part here, allowing for the first fourth dimension the evolution of hard torso parts such every bit teeth and supporting skeletons based on calcium carbonate (CaCO₃), and also supporting higher levels of principal production as a result of increased concentrations of phosphates and nitrates. The mass extinction that marked the finish of the Vendian period would accept opened up ecological niches that the new animals exploited, equally would habitat changes wrought by continental drift.

Genetic factors were likewise crucial. Recent enquiry suggests that the period prior to the Cambrian explosion saw the gradual evolution of a "genetic tool kit" of genes (the homeobox or "hox" genes ) that govern developmental processes. Once assembled, this genetic tool kit enabled an unprecedented flow of evolutionary experimentation -- and competition. Many forms seen in the fossil record of the Cambrian disappeared without trace. Future evolutionary change was so limited to acting on the body plans that remained in existence.

Recently many scientists have begun to question whether the Cambrian explosion was a real outcome, or a reflection of the patchiness of this ancient fossil record. Genetic data advise that multicellular animals evolved effectually 1000 million years ago; this is supported past fossil embryos from rocks in China that date dorsum 600 million years. These embryos are more complex than those of simple organisms such every bit sponges and jellyfish, which suggests that multicellular animals must have evolved much further dorsum in time. In improver, trilobites were a very diverse group even early in the Cambrian, and some scientists suggest that this indicates that the arthropod group must have had a much before evolutionary origin.

A pes on the country

Whatever their origins, animals may have ventured onto country early in the Cambrian. Previously scientists believed that animals did not begin to colonise the land until the Silurian (440 - 410 million years ago). However, the 2002 discovery of the footprints of animals that scuttled about on sand dunes about 530 million years ago has changed this view. These animals were arthropods, and resembled centipedes about the size of crayfish. They probably didn't live on land, instead coming ashore to mate or evade predators. At this time the only land plants appear to have resembled mosses .

The earliest vertebrates

Animals continued to diversify in the Ordovician seas (505 - 440 meg years agone). They were mostly invertebrates, including graptolites , which were stick-like branching colonies of tiny animals, together with brachiopods , trilobites, cephalopods , corals, crinoids and conodonts . We now place the conodonts with the chordates, but for a long fourth dimension they were known only by their tiny, but very common, teeth.

In terms of number of species invertebrates were by far the nearly common Ordovician animals - as they still are today. However, members of another taxon were as well evolving in the Ordovician seas. These were the fish.

Appearance of the fish

Similar the conodonts, fish are members of the chordate phylum because they display certain defining characteristics: a dorsal stiffening rod called the notochord, a dorsal nerve cord, pharyngeal gill slits and a tail that extends beyond the anus. Withal, fish are placed in the subphylum Vertebrata , because they also show the evolution of skeletal features such as a backbone, skull, and limb basic.

Not all the modernistic groups of fish were represented in the Ordovician oceans. At this time only the jawless fish had evolved from a chordate ancestor. The sharks and their relatives and ii extinct groups, the placoderms (which had bony plates roofing their heads) and the acanthodians (the outset known jawed vertebrates, with a skeleton of cartilage) made their appearance in the Silurian. Nonetheless, neither the sharks nor the agnathans became common until the Devonian. The other two living lineages, the ray-finned (e.g. bother and kahawai) and the lobe-finned fish (e.yard. lungfish and the coelacanth), evolved during the Devonian period.

The jawless fish

Agnathans , or jawless fish, were the earliest fish: an excellent fossil of Haikouichthys ercaicunensis dates dorsum well-nigh 530 million years, to the Cambrian. Previously the earliest-known agnathans were dated to around 480 million years agone. Agnathans have traditionally been placed with the vertebrates due to the presence of a skull, although the modernistic forms such as hagfish lack a vertebral column. The primeval agnathans were Ostracoderms. They were bottom-feeders and were near entirely covered in armour plates. When the sharks and bony fish began to evolve, effectually 450 meg years ago, most ostracoderms became extinct. Simply the lineage that produced the modern hagfish and lampreys survived.

Colonisation of the country

Fish continued to evolve during the Silurian period (440 - 410 million years ago). At the same fourth dimension some groups of plants and animals took a major stride as they colonised the land for the commencement time. We are not sure why this advance occurred, only it was probably the result of competition in the marine ecosystems, plus the opportunity to escape predators and the availability of new terrestrial niches.

Arthropods, which had ventured temporarily onto land 100 million years before, were the beginning animals to become more permanent colonists. Fossil footprints made in the sandy flats surrounding temporary lakes dating back about 420 million years have been found in Western Australia.

The arthropods were pre-adjusted to life on land. By the time they moved ashore, they had already evolved lighter bodies and slim, strong legs that could support them against the pull of gravity. Their hard outer exoskeletons provided protection and would assistance to retain water, although the development of a waxy, waterproof cuticle was necessary for efficient water conservation.

Spiders, centipedes and mites were amidst the primeval land animals. Some of them were giants: the largest was Slimonia, the size of a man and a relative of the scorpions. This animal was still besides big and too heavy and the walking legs besides small to venture onto state for any length of time and so they lived in marginal marine (deltaic) environments.

Problems encountered in the motion to state

These early land animals had to solve the same issues that plants faced when they moved to the land: water conservation, gas substitution, reproduction and dispersal, and the fact that water no longer buoyed them up against the pull of gravity. Similar plants, animals evolved waterproof external layers, internal gas commutation systems, ways of reproducing that did non involve water, and strong back up systems ( endoskeletons and exoskeletons) that allowed them to move almost on land. Remember that non all animal taxa were every bit successful in solving these problems.

The evolution of amphibians

By the Devonian flow two major creature groups dominated the state: the tetrapods (4-legged terrestrial vertebrates) and the arthropods, including arachnids and wingless insects. The first tetrapods were amphibians , such as Ichthyostega, and were closely related to a group of fish known as lobe-finned fish e.g. Eusthenopteron . Once thought to be extinct, the coelacanth is a living representative of this group.

Eusthenopteron had a number of exaptations that pre-adapted it to life on country: it had limbs (with digits) that immune information technology to motility around on the bottom of pools, lungs - which meant it could gulp air at the surface, and the beginnings of a neck. This last is important as a terrestrial predator cannot rely on water electric current to bring nutrient into its mouth, but must movement its head to catch prey. And the basic in Eusthenopteron's fins are almost identical to those in the limbs of the earliest amphibians, an example of homology .

Ichthyostega'due south skull was most identical to that of the lobe-finned fish Eusthenopteron, a definite cervix separated its body from its caput, and it retained a deep tail with fins. While Ichthyostega had iv potent limbs, the class of its hind legs suggests that it did not spend all its time on land.

All modern tetrapods have a maximum of five digits on each limb, and are thus said to have a pentadactyl limb. For a long time scientists believed that pentadactyly was the ancestral state for tetrapods. However, conscientious exam of the fossils of early amphibians such as Ichthyostega and Acanthostega has revealed the presence of up to 8 toes on each foot!

In addition, these early amphibians were large-bodied animals with strong bodies and prominent ribs - quite dissimilar in advent from modernistic representatives such every bit frogs and axolotls.

What collection amphibian evolution?

It was originally believed that the tetrapods evolved during periods of drought, when the ability to move between pools would be an advantage. The animals would also have been able to take advantage of terrestrial prey, such as arthropods. Juvenile animals could avoid predation by the land-based adults past living in shallow h2o.

Notwithstanding, fossil and geological prove tells us that the early tetrapods lived in lagoons in tropical regions, so that drought was not an issue. They were unlikely to exist feeding on country: arthropods are modest and fast-moving, unlikely prey for large, sluggish amphibians. But amphibians that laid their eggs on land, rather than in water, would be at a selective reward, avoiding predation by aquatic vertebrates (such as other amphibians and fish) on gametes, eggs and hatchlings.

Fifty-fifty today some amphibians e.g. the Eleutherodactylid frogs of Australia and Indonesia lay their eggs in soil on the land. Notwithstanding, they must nevertheless be in a moist surround, and the size of the egg is restricted to less than 1.5cm in diameter. This is because the egg is dependent on diffusion alone for gas commutation, and ways that the embryo must develop apace into a food-seeking larval class rather than undergo prolonged development within the egg.

In the Devonian seas, brachiopods had become a ascendant invertebrate grouping, while the fish continued to evolve, with sharks becoming the ascendant marine vertebrates. The placoderms and acanthodian fish were quite various during the Devonian, but their numbers then dwindled chop-chop and both groups became extinct by the end of the Carboniferous flow. Lobe-finned fish also peaked in numbers during the Devonian.

Early on reptiles and the amniotic egg

One of the greatest evolutionary innovations of the Carboniferous period (360 - 268 million years ago) was the amniotic egg , which immune early reptiles to move away from waterside habitats and colonise dry regions. The amniotic egg allowed the ancestors of birds, mammals, and reptiles to reproduce on land by preventing the embryo inside from drying out, so eggs could be laid abroad from the water. It likewise meant that in dissimilarity to the amphibians the reptiles could produce fewer eggs at whatever i time, because there was less risk of predation on the eggs. Reptiles don't go through a larval nutrient-seeking stage, but undergo directly development into a miniature adult grade while in the egg, and fertilisation is internal.

The earliest date for development of the amniotic egg is about 320 1000000 years ago. However, reptiles didn't undergo any major adaptive radiation for another twenty 1000000 years. Current thinking is that these early amniotes were withal spending time in the h2o and came ashore mainly to lay their eggs, rather than to feed. Information technology wasn't until the development of herbivory that new reptile groups appeared, able to take advantage of the arable plant life of the Carboniferous.

Early reptiles belonged to a group called the cotylosaurs. Hylonomus and Paleothyris were ii members of this group. They were small, lizard-sized animals with amphibian-like skulls, shoulders, pelvis and limbs, and intermediate teeth and vertebrae. The rest of the skeleton was reptilian. Many of these new "reptilian" features are also seen in lilliputian, modern, amphibians (which may as well have direct-developing eggs laid on land east.grand. New Zealand's leiopelmid frogs, so perhaps these features were simply associated with the small body size of the first reptiles.

The early mammals

A major transition in the evolution of life occurred when mammals evolved from one lineage of reptiles. This transition began during the Permian (286 - 248 million years ago), when the reptile group that included Dimetrodon gave rise to the "creature-faced" therapsids. (The other major branching, the "lizard-faced" sauropsids, gave rise to birds and modern reptiles). These mammal-like reptiles in turn gave rise to the cynodonts east.g. Thrinaxodon during the Triassic period.

Early adaptive radiation among the reptiles

This lineage provides an excellent series of transitional fossils . The development of a key mammalian trait, the presence of just a unmarried bone in the lower jaw (compared to several in reptiles) tin be traced in the fossil history of this grouping. It includes the excellent transitional fossils, Diarthrognathus and Morganucodon, whose lower jaws have both reptilian and mammalian articulations with the upper. Other novel features constitute in this lineage include the development of different kinds of teeth (a feature known equally heterodonty), the beginnings of a secondary palate, and enlargement of the dentary bone in the lower jaw. Legs are held directly underneath the torso, an evolutionary advance that occurred independently in the ancestors of the dinosaurs.

The cease of the Permian was marked by perchance the greatest mass extinction ever to occur. Some estimates suggest that upward to 90% of the species so living became extinct. (Recent research has suggested that this event, like the better-known end-Cretaceous event, was acquired past the bear on of an asteroid.) During the subsequent Triassic period (248 - 213 million years ago), the survivors of that event radiated into the large number of at present-vacant ecological niches.

However, at the finish of the Permian it was the dinosaurs, not the mammal-like reptiles, which took reward of the newly available terrestrial niches to diversify into the dominant land vertebrates. In the bounding main, the ray-finned fish began the major adaptive radiation that would run into them go the virtually species-rich of all vertebrate classes.

Developments in the dinosaur lineage

One major change, in the grouping of reptiles that gave rise to the dinosaurs, was in the animals' posture. This changed from the usual "sprawling" mode, where the limbs jut sideways, to an cock posture, with the limbs held directly nether the trunk. This had major implications for locomotion, equally it immune much more energy-efficient movement.

The dinosaurs , or "terrible lizards", fall into two major groups on the footing of their hip construction: the saurischians (or "lizard-hipped" dinosaurs) and the ornithischians (misleadingly known as the "bird-hipped" dinosaurs). Ornithischians include Triceratops, Iguanodon, Hadrosaurus, and Stegosaurus). Saurischians are farther subdivided into theropods (such every bit Coelophysis and Tyrannosaurus rex) and sauropods (e.g. Apatosaurus). Virtually scientists hold that birds evolved from theropod dinosaurs.

Although the dinosaurs and their immediate ancestors dominated the world'south terrestrial ecosystems during the Triassic, mammals connected to evolve during this time.

Further developments in the early mammals

Mammals are avant-garde synapsids . Synapsida is one of ii great branches of the amniote family unit tree. Amniotes are the group of animals that produce an amniotic egg i.e. the reptiles, birds, and mammals. The other major amniote group, the Diapsida, includes the birds and all living and extinct reptiles other than the turtles and tortoises. Turtles and tortoises belong in a third group of amniotes, the Anapsida. Members of these groups are classified on the basis of the number of openings in the temporal region of the skull.

Synapsids are characterised past having a pair of extra openings in the skull behind the optics. This opening gave the synapsids (and similarly the diapsids, which have two pairs of openings) stronger jaw muscles and better biting power than earlier animals. (The jaw muscles of a synapsid are anchored to the edges of the skull opening). Pelycosaurs (like Dimetrodon and Edaphosaurus) were early synapsids; they were mammal-like reptiles. Afterwards synapsids include the therapsids and the cynodonts , which lived during the Triassic.

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 evolution of a bony secondary palate so that air and food had split up passages to the back of the throat; increased size of the dentary - the chief bone in the lower jaw; and holes for fretfulness and blood vessels in the lower jaw, suggesting the presence of whiskers.

By 125 meg years ago the mammals had already become a diverse group of organisms. Some of them would have resembled today's monotremes (east.g. platypus and echidna), but early marsupials (a group that includes modern kangaroos and possums) were also present. Until recently it was idea that placental mammals (the grouping to which most living mammals belong) had a much later evolutionary origin. All the same, recent fossil finds and Dna evidence advise that the placental mammals are much older, perhaps evolving more than 105 1000000 years ago. Note that the marsupial and placental mammals provide some splendid examples of convergent evolution , where organisms that are not particularly closely related have evolved similar body forms in response to similar environmental pressures.

Yet, despite the fact that the mammals had what many people regard every bit "advanced" features, they were all the same just minor players on the earth stage. Every bit the earth entered the Jurassic period (213 - 145 million years agone), the dominant animals on state, in the body of water, and in the air, were the reptiles. Dinosaurs, more numerous and more extraordinary than those of the Triassic, were the principal land animals; crocodiles, ichthyosaurs, and plesiosaurs ruled the bounding main, while the air was inhabited by the pterosaurs .

Taking fly: Archaeopteryx and the origins of the birds

In 1861 an intriguing fossil was constitute in the Jurassic Solnhofen Limestone of southern Frg, a source of rare merely exceptionally well-preserved fossils. Given the proper noun Archeopteryx lithographica the fossil appeared to combine features of both birds and reptiles: a reptilian skeleton, accompanied by the articulate impression of feathers. This made the find highly significant as information technology had the potential to support the Darwinians in the debate that was raging following the 1859 publication of "On the origin of species".

While information technology was originally described every bit simply a feathered reptile, Archaeopteryx has long been regarded equally a transitional course between birds and reptiles, making it one of the well-nigh of import fossils ever discovered. Until relatively recently information technology was too the earliest known bird. Lately, scientists accept realised that Archaeopteryx bears fifty-fifty 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 take been discovered in Red china that are even older than Archaeopteryx, and other discoveries of feathered dinosaurs back up 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 test of the early on history of birds provides a proficient instance of the concept that evolution is neither linear nor progressive. The bird lineage is messy, with a variety of �experimental� forms appearing. Not all achieved powered flying, and some looked quite unlike modern birds eastward.yard. Microraptor gui, which appears to have been a gliding animal and had asymmetric flight feathers on all four limbs, while its skeleton is substantially that of a minor dromaeosaur. Archaeopteryx itself did not vest 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 prove a many-branched bush-league, not a unmarried straight trunk.

The end of the dinosaur age

Dinosaurs spread throughout the world - including New Zealand, which had its own dinosaur animate being - during the Jurassic, but during the subsequent Cretaceous menstruation (145 - 65 million years ago) they were declining in species diverseness. In fact, many of the typically Mesozoic organisms - such every bit 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 procedure.

The mass extinction at the end of the Cretaceous catamenia, 65 million years ago, wiped out the dinosaurs along with every other land animal that weighed much more than 25 kg. This cleared the fashion for the expansion of the mammals on state. In the sea at this time, the fish once again became the dominant vertebrate taxon.

The advent of modern mammal groups

At the beginning of the Palaeocene epoch (65 - 55.5 million years ago) the globe was without larger-sized terrestrial animals. This unique state of affairs was the starting point for the great evolutionary diversification of the mammals, which upwardly until then had been nocturnal animals the size of small rodents. By the terminate of the epoch, mammals occupied many of the vacant ecological niches. While mammal fossils from this period of time are scarce, and oftentimes consist largely of their characteristic teeth, we know that small-scale, rodent-like insectivorous mammals roamed the forests, the first big herbivorous mammals were browsing on the abundant vegetation, and carnivorous mammals were stalking their prey.

The oldest confirmed primate fossils appointment to about lx 1000000 years agone, in the mid-Palaeocene. The early primates evolved from archaic nocturnal insectivores, something similar 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 nigh of the modern orders of mammals announced in a cursory menstruum during the early Eocene (55.5 - 33.seven million years ago). 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 Due north America and Europe. The evolutionary history of the horses is particularly well understood: Stephen Jay Gould (1983) provides an fantabulous 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 Republic of india, Islamic republic of pakistan, and the Middle Eastward. These fossils chronicle the change from the land-dwelling mesonychids, which are the probable ancestors of whales, through animals such every bit Ambulocetus , which was still a tetrapod but which besides has such whale-like features as an ear capsule isolated from the residuum of its skull, to the primitive whales called the Archaeocetes.

The trend towards a cooler global climate that occurred during the Oligocene epoch (33.vii - 23.8 million years ago) saw the advent of the grasses, which were to extend into vast grasslands during the subsequent Miocene (23.8 - v.3 1000000 years ago). This change in vegetation collection the development of browsing animals, such every bit more modern horses, with teeth that could deal with the loftier silica content of the grasses. The cooling climate trend as well affected the oceans, with a turn down in the number of marine plankton and invertebrates.

While Dna evidence suggests that the great apes evolved during the Oligocene, arable fossils exercise non appear until the Miocene. Hominids, on the evolutionary line leading to humans, outset announced in the fossil record in the Pliocene (5.three - 1.8 meg years agone). The story of man evolution is covered hither - Human Evolution fabric.

New Zealand, past 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, take undergone significant adaptive radiation in their new homeland. Some of the all-time examples of this can be related to the major ecological changes that accompanied the Pleistocene Ice Ages.

Throughout the Pleistocene there were about xx cycles of common cold glacial ("Water ice Age") and warm interglacial periods at intervals of about 100,000 years. During the Ice Ages glaciers dominated the landscape, snowfall and ice extended into the lowlands, transporting huge quantities of rock with them. During these periods the South Isle was extensively glaciated, and at that place were small glaciers on the Tararua Ranges and Key Plateau. Because a lot of water was locked upwardly in water ice, the sea levels dropped during the glacials (up to 135m lower than at present). All-encompassing land bridges joined the main and many offshore islands, allowing the migration of plants and animals. During the warmer periods big areas became submerged again under water. These repeated episodes of environmental fragmentation collection rapid adaptive radiation in many NZ species, especially (but not exclusively) the alpine plants.

For example, speciation patterns in the native Placostylus flax snails of Northland can be related to changes in ocean level. Originally two-3 species were widespread at a time of low ocean levels. Rising seas at the cease of the glacial period isolated these every bit populations on offshore islands, where differential natural option pressures led to the evolution of a greater number of separate species.

The distribution of land snails such as Powelliphanta in Marlborough and the southern North Isle also offers evidence for the presence of land bridges and the possibility of futurity speciation. The aforementioned varieties are establish both north and south of Cook Strait, implying a continuous land bridge in the past as the animals die in salt h2o. The fact that no farther speciation has occurred in this example suggests that the land bridge was recently submerged by rising seas, perhaps only 10,000 years ago.

New Zealand Case

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