Shorewood Neighbors Baby Gear

Here are Todays Amazing Carrier Skulls Deals!

More Great Information on Carrier Skulls:

Evolution of mammals by jojo

Definition of "mammal"
Living mammal species can be identified by the presence in females of mammary glands, which produce milk.
Mammalian and non-mammalian jaws. In the mammal configuration, the quadrate and articular bones are much smaller and form part of the middle ear. Note that in mammals the lower jaw consists of only the dentary bone.
Other features are required when classifying fossils, since mammary glands and other soft-tissue features are not visible in fossils. Paleontologists therefore use a distinguishing feature that is shared by all living mammals (including monotremes) but is not present in any of the early Triassic therapsids ("mammal-like reptiles"): Mammals use two bones for hearing that all other amniotes use for eating. The earliest amniotes had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). All non-mammalian amniotes use this system including lizards, crocodilians, dinosaurs (and their descendants the birds), and therapsids. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone, which carries the teeth) and the squamosal (another small skull bone). And in mammals the quadrate and articular bones have become the incus and malleus bones in the middle ear.
Mammals also have a double occipital condyle; they have two knobs at the base of the skull that fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. But paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, as it would be confusing if they found a fossil that had one feature but not the other (e.g. a mammalian jaw and ear but a non-mammalian single occipital condyle).
Due to the incremental changes in transitional fossils, it has been said
We may again ask the question, What is a mammal? Where we draw the line between reptile and mammal has no biological significance. It is purely a matter of convenience. There are two obvious choices, both immediately following a period of rapid evolution that make as definite a break as we can hope to find.
The ancestry of mammals
Here is a very simplified "family tree" the text below describes some of the uncertainties and areas of debate.
 Tetrapods 
Amphibians
 Amniotes 
Sauropsids (including dinosaurs)
 Synapsids 
Pelycosaurs
 Therapsids 
Mammals
Amniotes
The first fully terrestrial vertebrates were amniotes their eggs had internal membranes that allowed the developing embryo to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water (a few amphibians, such as the Surinam toad, have evolved other ways of getting round this limitation). The first amniotes apparently arose in the late Carboniferous from the ancestral reptiliomorphs.
Within a few million years two important amniote lineages became distinct: mammals' synapsid ancestors and the sauropsids, from which lizards, snakes, crocodilians, dinosaurs and birds are descended. The earliest known fossils of synapsids and sauropsids (such as Archaeothyris and Hylonomus resp.) date from about 320 to 315 million years ago. Unfortunately it is difficult to be sure about when each of them evolved, since vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier.
Synapsids
The original synapsid skull structure has one hole behind each eye, in a fairly low position on the skull (lower right in this image).
Synapsid skulls are identified by the distinctive pattern of the holes behind each eye, which served the following purposes:
made the skull lighter without sacrificing strength.
saved energy by using less bone.
probably provided attachment points for jaw muscles. Having attachment points further away from the jaw made it possible for the muscles to be longer and therefore to exert a strong pull over a wide range of jaw movement without being stretched or contracted beyond their optimum range.
Early Permian terrestrial fossils indicate that one synapsid group, the pelycosaurs, were the most common land vertebrates of their time and included the largest land animals of the time.
Therapsids
Therapsids descended from pelycosaurs in the middle Permian and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including larger temporal fenestrae and incisors that are equal in size.
The therapsids went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with some that could easily be mistaken for mammals:
gradual development of a bony secondary palate. Most books and articles interpret this as a prequisite for the evolution of mammals' high metabolic rate, because it enabled these animals to eat and breathe at the same time. But some scientists point out that some modern ectotherms use a fleshy secondary palate to separate the mouth from the airway, and that a bony palate provides a surface on which the tongue can manipulate food, facilitating chewing rather than breathing. The interpretation of the bony secondary palate as an aid to chewing also suggests the development of a faster metabolism, since chewing makes it possible to digest food more quickly. In mammals the palate is formed by two specific bones, but various Permian therapsids had other combinations of bones in the right places to function as a palate.
the dentary gradually becomes the main bone of the lower jaw.
progress towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was erratic and very slow for example: all herbivorous therapsids retained sprawling limbs (some late forms may have had semi-erect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semi-sprawling hindlimbs. In fact modern monotremes still have semi-sprawling limbs.
in the Triassic, progress towards the mammalian jaw and middle ear.
there is plausible evidence of hair in Triassic therapsids, but none for Permian therapsids (see below).
some scientists have argued that some Triassic therapsids show signs of lactation (see below).
Therapsid family tree
(simplified from ; only those that are most relevant to the evolution of mammals are described below)
Therapsids

|

+--Biarmosuchia

|

'--+--Dinocephalia

|

+--Neotherapsida

|

+--Anomodonts

| |

| '--Dicynodonts

|

'--+--Theriodontia

|

+--Gorgonopsia

|

'--+--Therocephalia

|

'--Cynodontia

.

. . . (Mammals, eventually)
Only the dicynodonts, therocephalians and cynodonts survived into the Triassic.
Biarmosuchia
The Biarmosuchia were the most primitive and pelycosaur-like of the therapsids.
Dinocephalians
Dinocephalians ("terrible heads") were large, some as large as a rhinoceros, and included both carnivores and herbivores. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs. In many ways they were very primitive therapsids, for example they had no secondary palate and their jaws were rather "reptilian".
Anomodonts
Lystrosaurus, one of the few species of dicynodonts that survived the Permian-Triassic extinction event
The anomodonts ("anomalous teeth") were the most successful of the herbivorous therapsids one sub-group, the dicynodonts, survived almost to the end of the Triassic. But anomodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw and it is generally agreed that they had beaks like those of birds or ceratopsians.
Theriodonts
The theriodonts ("beast teeth") and their descendants had jaw joints in which the lower jaw's articular bone tightly gripped the skull's very small quadrate bone. This allowed a much wider gape, and one group, the carnivorous gorgonopsians ("gorgon faces"), took advantage of this to develop "sabre teeth". But the theriodont's jaw hinge had a longer term significance the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear.
The gorgonopsians still had some primitive features: no bony secondary palate (but other bones in the right places to perform the same functions); sprawling forelimbs; hindlimbs that could operate in both sprawling and erect postures. But the therocephalians ("beast heads"), which appear to have arisen at about the same time as the gorgonopsians, had additional mammal-like features, e.g. their finger and toe bones had the same number of phalanges (segments) as in early mammals (and the same number that primates have, including humans).
Cynodonts
Life restoraton of the cynodont Trirachodon within a burrow
The cynodonts, a theriodont group that also arose in the late Permian, include the ancestors of all mammals one sub-group, the trithelodonts, is widely regarded as the most likely to contain mammals' ancestor. Cynodonts' mammal-like features include further reduction in the number of bones in the lower jaw; a secondary bony palate; cheek teeth with a complex pattern in the crowns; the brain filled the endocranial cavity.
Multi-chambered burrows have been found, containing as many as 20 skeletons of the Early Triassic cynodont Trirachodon; the animals are thought to have been drowned by a flash flood. The extensive shared burrows indicate that these animals were capable of complex social behaviors.
Triassic takeover
The catastrophic Permian-Triassic mass extinction killed off about 70 percent of terrestrial vertebrate species, and the majority of land plants. As a result
Ecosystems and food chains collapsed, and the recovery took about 6 million years.
The survivors had to re-start the struggle for dominance of their former ecological niches even the cynodonts, which had seemed on the way to dominance at the end of the Permian.
But the cynodonts lost out to a previously obscure group of sauropsids, the archosaurs (which include the ancestors of crocodilians, dinosaurs and birds). This reversal of fortunes is often called the "Triassic takeover". Several explanations have been offered for it, but the most likely is that the early Triassic was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage (all known sauropsids have glandless skins and excrete uric acid, which requires less water to keep it sufficiently liquid than urea, which mammals excrete and presumably therapsids excreted). The Triassic takeover was gradual in the earliest part of the Triassic cynodonts were the main predators and lystrosaurs were the main herbivores, but by the mid-Triassic archosaurs dominated all the large carnivore and herbivore niches.
But the Triassic takeover may have been a vital factor in the evolution of cynodonts into mammals. The cynodonts' descendants were only able to survive as small, mainly nocturnal insectivores. As a result:
The therapsid trend towards differentiated teeth with precise occlusion accelerated, because of the need to hold captured arthropods and crush their exoskeletons.
Nocturnal life required advances in thermal insulation and temperature regulation to enable the ancestors of mammals to be active in the cool of the night.
Acute senses of hearing and smell became vital.
This accelerated the development of the mammalian middle ear, and therefore of the mammalian jaw since bones that had been part of the jaw joint became part of the middle ear.
The increase in the size of the olfactory and auditory lobes of the brain increased brain weight as a total percentage of body weight. Brain tissue requires a disproportionate amount of energy. The need for more food to support the enlarged brains increased the pressures for improvements in insulation, temperature regulation and feeding.
As a side-effect of the nocturnal life, discerning colors became less important (they lost two out of four opsins), and this is reflected in the fact that most mammals have poor color vision, including the "lower primates" such as lemurs.
From cynodonts to true mammals
Many uncertainties
While the Triassic takeover probably accelerated the evolution of mammals, it made life more difficult for paleontologists because good fossils of the nearly-mammals are extremely rare, mainly because they were mostly smaller than rats:
They were largely restricted to environments that are less likely to provide good fossils. The best terrestrial environments for fossilization are floodplains, where seasonal floods quickly cover dead animals in a protective layer of silt that is later compressed into sedimentary rock. But floodplains are dominated by medium to large animals, and the Triassic therapsids and near-mammals could not compete with archosaurs in the medium to large size range.
Their delicate bones were vulnerable to being destroyed before they could be fossilized by scavengers (including fungi and bacteria) and by being trodden on.
Small fossils are harder to spot and more vulnerable to being destroyed by weathering and other natural stresses before they are discovered.
In fact it was said as recently as the 1980s that all the Mesozoic fossils of mammals and near-mammals could be contained in a few shoeboxes and they were mostly teeth, which are the most durable of all tissues. Since then, the number of Mesozoic fossil mammals has increased, from 116 genera known in 1979 to about 310 in 2007, with an increase in quality such that "at least 18 Mesozoic mammals are represented by nearly complete skeletons".
As a result:
In many cases it is difficult to assign a Mesozoic mammal or near-mammal fossil to a genus.
All the available fossils of a genus seldom add up to a complete skeleton, and hence it is difficult to decide which genera are most like each other and therefore most likely to be closely-related. In other words, it becomes very difficult to classify them by means of cladistics, which is the most reliable and least subjective method currently available.
So the evolution of mammals in the Mesozoic is full of uncertainties, although there is no room for doubt that true mammals did first appear in the Mesozoic.
Mammals or mammaliformes?
One result of these uncertainties has been a change in the paleontologists' definition of "mammal". For a long time a fossil was considered a mammal if it met the jaw-ear criterion (the jaw joint consists only of the squamosal and dentary; and the articular and the quadrate bones have become the middle ear's malleus and incus). But more recently paleontologists have usually defined "mammal" as the last common ancestor of monotremes, marsupials and placentals and all of its descendants. So they had to define another clade mammaliformes to accommodate all the animals that are more mammal-like than cynodonts but less closely related to monotremes, marsupials and placentals. Although this now appears to be the majority approach, some paleontologists have resisted it because it simply moves most of the problems into the new clade without solving them; the clade mammaliformes includes some animals with "mammalian" jaw joints and some with "reptilian" (articular-to-quadrate) jaw joints; and the newer definition of "mammal" and "mammaliformes" depend on last common ancestors of both groups, which have not yet been found. Despite these objections, this article follows the majority approach and treats most of the cynodonts' Mesozoic descendants as mammaliformes.
Family tree cynodonts to mammals
(based on Mammaliformes - Palaeos)
--Cynodonts

|

'--Mammaliformes

|

+--Allotheria

| |

| '--Multituberculates

|

'--+--Morganucodontidae

|

'--+--Docodonta

|

'--+--Hadrocodium

|

'--Symmetrodonta

|

|--Kuehneotheriidae

|

'--crown group Mammals (all descendants of the last common

ancestor of all modern forms)
Multituberculates
Skull of the multituberculate Ptilodus
Multituberculates (named for the multiple tubercles on their "molars") are often called the "rodents of the Mesozoic" but this is an example of convergent evolution rather than meaning that they are closely related to the Rodentia. At first sight they look like mammals: their jaw joints consists of only the dentary and squamosal bones, and the quadrate and articular bones are part of the middle ear; their teeth are differentiated, occlude and have mammal-like cusps; they have a zygomatic arch; the structure of the pelvis suggests that they gave birth to tiny helpless young, like modern marsupials. And they lived for over 120 million years (from mid Jurassic, about 160M years ago, to early Oligocene, about 35M years ago), which in terms of clade longevity would make them the most successful mammaliformes ever. But a closer look shows that they are very different from modern mammals:
Their "molars" have two parallel rows of tubercles, unlike the tribosphenic (three-peaked) molars of early mammals.
The chewing action is completely different. Mammals chew with a side-to-side grinding action, which means that usually the molars occlude on only one side at a time. Multituberculates' jaws were incapable of side-to-side movement and chewed by dragging the lower teeth backwards against the upper ones as the jaw closed.
The anterior (forward) part of the zygomatic arch mostly consists of the maxilla (upper jawbone) rather than the jugal, and the jugal is a small bone in a little slot in the maxillary process (extension).
The squamosal does not form part of the braincase.
The rostrum (snout) is unlike that of mammals, in fact it looks more like that of a pelycosaur such as Dimetrodon. The multituberculate rostrum is box-like, with the large flat maxillae forming the sides, the nasal the top, and the tall premaxilla at the front.
Morganucodontidae and other transitional forms had both types of jaw joint: dentary-squamosal (front) and articular-quadrate (rear).
Morganucodontidae
The Morganucodontidae first appeared in the late Triassic, about 205M years ago. They are an excellent example of transitional fossil

I am China Crafts Suppliers writer, reports some information about oqo model 2 , off lease notebooks.

Article Source: http://www.earticlesonline.com/Article/Evolution-of-mammals/824635