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 away from the water. It also meant that in contrast to the amphibians the reptiles could produce fewer eggs at any one time, because there was less risk of predation on the eggs. Reptiles don't go through a larval food-seeking stage, but undergo direct development into a miniature adult form while in the egg, and fertilisation is internal.
The earliest date for development of the amniotic egg is about million years ago. However, reptiles didn't undergo any major adaptive radiation for another 20 million years. Current thinking is that these early amniotes were still spending time in the water and came ashore mainly to lay their eggs, rather than to feed. It wasn't until the evolution of herbivory that new reptile groups appeared, able to take advantage of the abundant plant life of the Carboniferous.
Early reptiles belonged to a group called the cotylosaurs. Hylonomus and Paleothyris were two members of this group.
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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 little, modern, amphibians which may also have direct-developing eggs laid on land e. New Zealand's leiopelmid frogs , so perhaps these features were simply associated with the small body size of the first reptiles.
A major transition in the evolution of life occurred when mammals evolved from one lineage of reptiles. This transition began during the Permian - million years ago , when the reptile group that included Dimetrodon gave rise to the "beast-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 e.
Thrinaxodon during the Triassic period. This lineage provides an excellent series of transitional fossils.
The development of a key mammalian trait, the presence of only a single bone in the lower jaw compared to several in reptiles can be traced in the fossil history of this group. It includes the excellent transitional fossils, Diarthrognathus and Morganucodon , whose lower jaws have both reptilian and mammalian articulations with the upper. Other novel features found in this lineage include the development of different kinds of teeth a feature known as heterodonty , the beginnings of a secondary palate, and enlargement of the dentary bone in the lower jaw. Legs are held directly underneath the body, an evolutionary advance that occurred independently in the ancestors of the dinosaurs.
The end of the Permian was marked by perhaps the greatest mass extinction ever to occur. Recent research has suggested that this event, like the better-known end-Cretaceous event, was caused by the impact of an asteroid. During the subsequent Triassic period - million years ago , the survivors of that event radiated into the large number of now-vacant ecological niches. However, at the end of the Permian it was the dinosaurs, not the mammal-like reptiles, which took advantage of the newly available terrestrial niches to diversify into the dominant land vertebrates.
In the sea, the ray-finned fish began the major adaptive radiation that would see them become the most species-rich of all vertebrate classes. One major change, in the group 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 erect posture, with the limbs held directly under the body. This had major implications for locomotion, as it allowed much more energy-efficient movement.
The dinosaurs , or "terrible lizards", fall into two major groups on the basis of their hip structure : the saurischians or "lizard-hipped" dinosaurs and the ornithischians misleadingly known as the "bird-hipped" dinosaurs.
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Ornithischians include Triceratops , Iguanodon , Hadrosaurus , and Stegosaurus. Saurischians are further subdivided into theropods such as Coelophysis and Tyrannosaurus rex and sauropods e. Most scientists agree that birds evolved from theropod dinosaurs. Although the dinosaurs and their immediate ancestors dominated the world's terrestrial ecosystems during the Triassic, mammals continued to evolve during this time. Mammals are advanced synapsids.
Synapsida is one of two great branches of the amniote family tree. Amniotes are the group of animals that produce an amniotic egg i. 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 by having a pair of extra openings in the skull behind the eyes.
This opening gave the synapsids and similarly the diapsids, which have two pairs of openings stronger jaw muscles and better biting ability 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. Later 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 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.
Go to this interactive website to get a more in-depth view of the Charophytes. As organisms adapted to life on land, they had to contend with several challenges in the terrestrial environment. Desiccation, or drying out, is a constant danger for an organism exposed to air. Even when parts of a plant are close to a source of water, the aerial structures are likely to dry out.