Evolutionary history of vertebrates

''This article is about the Evolutionary history of Vertebrates. For general information on Vertebrates, see Vertebrate.''

Vertebrates (formally known as Vertebrata) comprise a a subphylum taxon classified under the phylum of Chordates. Organisms of this taxon are defined by the existence of a spinal cord and backbones, which are constructed with bone and cartilage. All vertebrates are also distinguished from other organisms by developing a cranium, used to protect the central nervous system. Tetrapod vertebrate develop two sets of appendages in forms of: legs, arms, wings, flippers, and fins; fish species are constructed with varied number of fins, including the tail. Vertebrates do not develop external skeletons, like arthropods, but they have evolved anatomic structures upon their bodies. Reptiles and fishes are characterized by the existences of scales. Mammals develop hairs, while birds adapted feathers upon the majority of their body, with scales covering their feets. These anatomic structures all are constructed out of proteins called keratin.

Currently more than 40,000 species of vertebrates exist and are organized into the modern classes of: birds, amphibians, reptiles, mammals, and four classes of fish.

This article will mainly cover the evolution process of the major classes of vertebrates, and a few major orders.

The first vertebrates appeared about 500-450 million years ago, during the duration of the Ordovician Period. The first member of this diverse group was represented by a species of jawless fish. This jawless fish, being the most basal species of this group, can be considered the ancestor of all vertebrates. Other fish classes emerged in the Silurian and Devonian period. During the late Devonian period, they underwent a transition to terrestrial life, evolving into amphibians. Amphibians radiated and dominated the terrestrial niche during the Carboniferous period, and then giving rise to reptiles. Reptiles increased in diversity in the Permian and the Mesozoic Era, which is often refered to as the Age of Reptiles. Here they evolved into mammals in the Late Triassic and birds in the Jurassic.

Origin
No fossil evidence is currently avaliable today about the ancestors of the first and therefore, most basal species of vertebrates. However, from various studies, scientist have generally an idea of what the ancestors of all vertebrates are. Most presumably, the ancestors of the basal vertebrates would have to be a species of aquatic invertebrate, since all life begin within the ocean. These invertebrates would have no jaws, or any other internal skeletons made all of bones or cartilage as most invertebrates developed soft-bodies during that time. Hard-bodied invertebrates, like arthropods were constructed with exoskeletons made out of protein chemicals. Because of this, most early invertebrates adapted flexible mouthparts that enabled them to swallow food. Others developed a method known as filter feeding. Filter feeding requires the presence of empty body parts that is connected to the mouth, capable of filtering any waste materials within the mouth. To account for this issue, early invertebrates perhaps developed gill slits to filter the unneccessary materials within the mouth. These gill slits were evolved primary for that purpose, not for breathing like basal vertebrates. Invertebrates were capable of respiring through their skins. During the process of invertebrate evolution, more organisms started to evolve hard-bodies to enable them protection against large predators of that time. As exoskeletons cover the bodies, less skins were exposed. Gills then, were probably evolved to account for the respiration issue invertebrates would of had. The majority of the hard-bodied organisms during that period of time were mainly arthropods. With their high diversity and relativity successful evolution history during the Cambrian and Ordovician period, arthropods are possibly the most likely ancestor for the first vertebrates.

First vertebrates
The most basal species of organisms that would be classified as a vertebrate was a species of fish. From fossil evidence, these early species of fish were believed to have no jaws. This characteristics allow the scientist to classify these fishes into the superclass of Agnatha, which includes modern hagfishes and lampreys. Additionally, these species of fish also did not develop any pairs of fins, along with the development of a short tail. This suggested that they probably unable to perform any rapid locomotion and were generally low in activity. To received their nutrients, these species of jawless fish perhaps relied on the method of filter feeding, using their gills to release any unneccessary material. These early representive of vertebrates had armoured plates covering their bodies. The development of armoured plates was probably used to protect themselves against the dominate predators of sea scorpions of the Ordivican seas. These "armours", however, could of also provide resistance against the water pressure that would be forced upon them, since their adaptation of filter feeding would of caused them to live close or upon the sea floor.

No scientist has yet confirmed of which organisms were the first vertebrates to exist. The most popular candiate for the first vertebrate is probably the Haikouichthys. This organism seen to contain a backbone, along with the presence of distinct heads, suggesting that they were could possibly be classified as a craniate. Since all vertebrates today are classified into the informal group of craniates, some Paleontologist have been considering these organisms as the first fish, though more fossil evidence is needed to confirm this.

Fishes of the silurian period
Silurian period is the third time period existing in the Paleozoic Era in the geological time scale. Lasting for about 35 millions years from 443-417 million years, the Silurian period was an important period for the evolution of vertebrates. During its duration, the Silurian period hosted numerous evolution processes for the early vertebrates. The development of jaws in fish organisms were present in this period, allowing fish species to adapt to many different niches within their ecosystems. The first Cartilaginous fishes, represented by modern species of sharks; rays; and skates, evolved in this period as well.

Jawed fish
The Silurian period marked the existence of the first jawed fish. Jawed fishes were classified into a superclass of Gnathostomes, which includes the majority of the modern fish species. Jawed fish, as its name suggest, were a group of fish species that developed jaws.

Developing the presence of jaws enabled the early aquatic vertebrates to be able to adapt to other numerous niches, other than filter feeding, within their ecosystems that were previously unavaliable to them. This would then enable more evolution process to be undergo, introducing more diverse organisms. More that, the development of jaws also allowed the evolution of more complex skull structure and perhaps even more advanced senses of hearing. This would allow further development of more complex and advance nervous systems for these early vertebrates, essential for their survival within their ecosystems.

The first of these jawed fish were the Placodermi in the Late Silurian Period. The class of Chondrichthyes soon followed afterwards.

Placodermi
Placodermi(or common name known as Placoderms), was one of the first classes of jawed fish, otherwise considered to be the most basal. Organisms within the Placoderm class were all characterized by containing hard armoured plates covering their head and neck. The rest of the body within most species was mostly naked, or occasionally covered with primitive scales. Often a armoured dorsal fin is also present, located between the head and neck. The purpose of this joint was believe to increase flexibility that enabled the head to move upwards, while the jaws is stretched down, allowing a greater volume of space to be opened up within the mouth perhaps to increase the force of their bite. Placoderms also developed paired pelvic and pectoral fins, being one of the first species of fish to develop fins, suggesting that these organisms were probably capable of moving at fast speeds. Unlike most other fish species, Placoderm also contained very unusual dentition. From the observation of the materials comprised within their teeths, it is believed that Placoderm's teeths were not constructed with calcium, like most modern fish species, but rather from the same material present in their armoured plates. There many other characteristics Placoderms developed that were considered unusual or at least unique to Placoderms. One of these features was that their nasal capsules in Placoderms were not connected to their braincase. Additionally, a fossil find in 1997 discovered pigment cells preserved upon a Placoderm fossil. These pigment cells suggested that these Placoderm organisms were probably capable of viewing colours, using these colours perhaps for mating purposes or to used for hiding from predators or to use for the development of ambush hunting styles.

Nothing is currently known about the ancestors of these plated fishes, except that they develop before the Late Silurian period. Placoderms were generally very successful during the early stages of their evolutionary history, introducing numerous species and adapting to many different niches within their ecosystems. Though near the Devonian period, Placoderm population started to decline in diversity and finally becoming extinct in the Late Devonian period, obtaining an evolutionary history of about 60 million years. Their extinction were perhaps explained by being outcompeted by more advance species of fish that evolved in the Devonian period, or perhaps was due to the mass extinction event that occurred at the near the end of the Devonian period.

Chondrichthyans
Chondrichthyans, common name known as Cartilaginous fish, were the second class of jawed fish to evolve. Cartilaginous fish, like their common name suggest, are a group of fishes that developed skeletons made up of cartilage, represented by modern species of sharks, skates, and rays. The development of cartilage for the construction of their skeleton was probably adapted to reduce the weight of these organisms to enabled them to move a lot faster, while using less energy. The first cartilaginous fish were believed to have first existed and evolved in the Late Silurian. However, recent fossil finds discovered a fossilized scales that resembles those of cartilaginous fish. The rock layers where this fossil was found were believe to be formed in the Ordivican period, suggesting that cartilaginous fish may have existed a lot earlier in the Earth's evolutionary history. More complete fossils have been found in the Devonian period.

The most confirmed basal cartilaginous fish found today was in the Devonian period, represented by a species of shark. These Devonian sharks were much similar to those of today. The development of the pelvic, pectoral, and caudal(or the tail)fins were present in these early sharks, suggesting that they were able to move quite efficently, and perhaps adapted active hunting styles. Early sharks were also discovered to have the ability to give birth to their youths, being the first organisms to do so. This was probably adapted because cartilaginous fishes require the development of high amount of urea to enable them to survive in their marine environment, which is not developed by youths until older. Rays and skates did not evolved until the Mesozoic Era.

Devonian vertebrates
The Devonian period hosted some of the most important successes in vertebrate evolution. The superclass of Osteichthyes first evolved in this period along with its two classes of Actinopterygii(Ray-finned fish) and Sarcopterygii(Lobe-finned fish). The Ozone layer that exist today, begin to strengthen and develop in the Devonian period, blocking ultra-violent radiation from the Sun. This decrease the temperatures of the Earth's crust by dramatic rates and it made cool enough to allow life to develop, influencing organisms of the Devonian period begin to make transition from aquatic to terrestial lifestyle. The first Tetrapods, represented by early Amphibians, evolved in this period. However, despite this success, the fish class of Placodermi started to decline in diversity at the beginning of this time period and eventually into extinction at the Late Devonian. The superclass of Agnatha also begin to decline in this period, but survived.

A mass extinction event also occurred at the end of the Devonian period.

Osteichthyes
The superclass of Osteichthyes is a group of vertebrates commonly refered to as bony fishes. Bony fishes, as their common name suggest, are group of fishes classified by the construction of their skeleton. Their skeletons are constructed out of bone, as oppose cartilaginous fishes of cartilage skeletons. Bony fishes are characterized by presence of a caninal bone that is involved in the construction of the skull. Mandible muscles are located on the lower jaw of most bony fishes used to strengthen and increase flexibility of their jaws. Bony fishes also adapted scales that cover the bodies of these fishes. Their scales are defined for their flat and thin properties, different from the rough scales of cartilaginous fish. Sensory organs of bony fishes include the existence of lateral lines that serves the purposes of detecting differentials within the pressure and forces of the waters. This is useful in various situations that allows these fishes to sense the presence of predators and preys. Swim bladders have been developed by most bony fishes. Containing gas particles, these swim bladders are responsible for enabling these organisms to control their buoyancy. Like many other vertebrates of the Early Paleozoic Era, bony fishes develop gills for respiration. Unlike cartilaginous fishes, however, whose gills are present by their gill slit, bony fishes do not have gill slits, rather they develop a flap of skin that covers their gills called gill cover.

Bony fishes are believe to have first existed in the Early Devonian period, although certain fossilized body parts that resemble those of bony fishes have been found in the Late Silurian period. Bony fishes are classified by two classes of: Actinopterygii and Sarcopterygii.

Actinopterygii
Actinopterygii is a class of fish that belongs into the superclass of Osteichthyes, or bony fishes. Actinopterygii is known as ray-finned fish from its common name. These species of fishes were relatively abundant in the Devonian period. The earliest ray-finned fish evolved in the Middle Devonian period. Ray-finned fishes are mainly characterized and, separated from the other class within the bony fish superclass, by fins. Their fins are extremely thin and contain vertical spines that is perhaps used to support the fins. The spines located within the fins very much resemble rays, thus forming the existence of their common name. Each "rays" used to support their small fins are coordinated by a small set of muscles located at the end of the fin that enable these fins to become very flexible. The purpose of this development was perhaps used to control the balance of these fishes, and would of also provided small assistance to the steering of these fishes. Most early ray-finned fish of this period also developed an extended outgrowth at their pharynx(a connection between the stomach and lungs ) that was believe to serve the purpose of lungs. These lungs were perhaps used as a secondary respiratory method, during periods where the water becomes too warm and as the less solvent were presented into the water that would prevent the gills from functioning. Their kidneys were also adapted to the hypotonic environments they lived in. As these species of fish develop and adapted to new environments and habitats, the lungs that they developed eventually turned into modern day swim bladders.

Sarcopterygii
Sarcopterygii is the second class under the superclass of Osteichthyes. Sarcopterygii are known as lobe-finned fish by their common names. These lobe-finned fish are mainly characterized and known for the structure of their fins. The fins of Sarcopterygii are connected to the bodies of these fishes by a single large bone, in oppose to the tiny set of bones that is present in the thin fins of ray-finned fish. The fins of lobe-finned fish are known to have contained large muscles that is located around the center of their fins. These fins were perhaps strong enough to support the body weight of these fishes upon land. The construction of the bones comprised within lobe-finned fish were also found to be similar to those of primitive Tetrapods. Because of this characteristic, most Paleontologist believe that it was the Sarcopterygii that evolved into the early Tetrapods.

The most basal Sarcopterygii was believe to have existed in the Early Devonian period. These fishes had asymmetical caudal fins, with upper tail fin larger than the bottom, similar to the caudal fins of primitive sharks. This suggested that these early lobe-finned fish were perhaps very efficent swimmers, allow to propell themselves rapidly through the waters. Sarcopterygii were also found to develop enamel upon their teeths, suggesting that the ability to be able to chew.

In addition to the strong fins these fishes developed, there were other features to lobe-finned fish obtained that enable Paleontologist to believe they were the ancestors of the early Tetrapods. Primitive lobe-finned fish developed a swim bladder that is found in all bony fishes. These swim bladders had very similar construction to those of lungs, which perhaps suggest that these swim bladders might have evolved into primitive lungs. These lungs were used as secondary respiratory systems and allow these fishes to respire pure oxygen. Some species of lobe-finned fish of the Devonian period were also found to contain internal openings in their nasals that enabled these fishes to breathe with their mouths close.

Sarcopterygii were generally very successful and diverse throughout the Early-Middle Devonian period. The arrival and emergence of their relatives, the ray-finned fish, decreased their diversity. The transition towards life upon land may also contributed to this.

Primitive amphibians
Amphibians were one of the first(and therefore the most basal) species of Tetrapods to ever exist. Evolving from species of Sarcopterygii, Amphibians first existed in the Late Devonian period in the form of an early Amphibians called the Elginerpeton. Though primitive, Amphibians were very successful during the beginning of their evolutionary history. Early Amphibians, however, were not very adaptive towards life upon land. From fossil records, the characteristics and features these early Amphibians developed suggested that they were still very adapted to aquatic life, rather than upon land unlike the modern species today. The development of a streamlined body suggested that they were very efficent within the waters with the addition to webbed limbs and a long and strong tail used for propelling through the waters. The development of moist skin and the ability of laying liquid eggs, the defining characteristics for Amphibians, also suggest the adaptation to aquatic life. Additionally, these Amphibians were also unable to move too well upon land, due to their heavy bodies that requires the support of water to left.

Perhaps the purpose behind the adaptation to terrestrial life was for these early Amphibians, was to avoid predators that hunted these early Amphibians. The ability to live upon land perhaps also enable these vertebrates to develop ambush hunting styles, being able to hide behind vegetation to tackle any unwary prey. These Amphibians, however were only probably capable of living in freshwater habitats, being that moisture would be lost within their bodies with exposure to salt. Most Amphibians of the Devonian period adapted to carnivorous diets, with the development of teeth. However, they did have relatively strong jaws muscles, so they werent't most likely unable to chew. This perhaps explains their development of a large body, used to contain the large digestive systems they would to develop.

Adaptations that enabled terrestrial life
Many adaptations and changes within the construction of vertebrates were needed to be made before the existence of terrestrial life was possible. Many of those adaptations Sarcopterygii developed were required to enable the possibility of such evolution process. The following is only some of the important and largely visible changes in terrestrial vertebrates.

Some of the most important adaptations Sarcopterygii had to develop was the development of rib cages. The skeleton construction of aquatic fishes were not strong enough to resist against the extra force of gravity that was impacted upon them. Their organs would of have crushed by their skeletons because their skeletons needed the buoyancy provided by the water to resist against gravity(this is one of the main reason why Cartilaginous fish were unable to develop terrestrial life, their cartilage skeletons were way too soft and light to provide resistance against the forces upon land). The development of rib cages and other supporting bones and extra body fluids enabled these fishes to make transition to terrestrial life. The development of lungs were also required as gills were incapable of inhaling pure oxygen within the atmosphere. This was made possible through the evolution made by the swim bladders. Limbs of legs and arms were needed to perform locomotion upon land. The extension made by the bones comprised within Sarcopterygii fins and the development of extra amount of muscles enabled this. A different skull construction was also required that would enable the adaptation to contain various respiratory and digestive organs required for terrestrial life. A large nervous system was also needed to enable survival upon terrestrial habitats. During this process, the cranial bones the were previously present in Sarcopterygii became absence in early and modern vertebrates. These cranial bones connected to heat directly towards the neck. These cranial bones were replaced by vecral vertebrae and other vertebrae connecting to the spinal cord. The absence of these bones enabled better control of the head, characterized by a mobile neck. This also allowed stability when moving.

Carboniferous period
The Carboniferous period is the fourth time period within the Paleozoic Era. The Carboniferous period is mainly known for its rich and large amount of coal deposits that is presented within it rock layer, which is also the origination of this period's name. This time period is usually divided into two epoches of the Mississippian(lower Carboniferous) and the Pennsylvanian(upper Carboniferous).High levels of oxygen and carbon dioxide were presented within the Carboniferous atmosphere, creating a warm and humid environment that enabled large forests of primitive species of plants to form. The warm and humid environment were also responsible for the massive Arthropod radiation that occurred in this period. Insects reached large sizes that were comparable to the carnivorous birds that exist today. Examples of these insects included the Meganeura, an eagle sized dragonfly. For vertebrate evolution, Amphibians dominated the terrestrial niches, evolving and introducing the numerous species. The development of amniotic eggs(hard-shelled) for reproduction occurred which played an important milestone in Vertebrate evolution. This enabled Reptiles to emerged for the first time within the Carboniferous period, developing into small sizes to avoid being preyed upon by the more developed Amphibians. Within aquatic habitats, bony fishes of Actinopterygii increased in abundance, filling up most of the niches within their ecosystems. Sarcopterygii declined in diversity and made up of only a little of the marine fauna. Cartilaginous fishes were still represented by species of sharks, who had now developed more advanced characteristics and into larger sizes. Jawless fishes remained low in diversity within the waters.

Reptile emergence
Reptiles first evolved and appeared in the Earth's evolutionary history during the Late Carboniferous Period. Fossil evidence of Reptiles during the Carboniferous period is extremely rare, so not much information is currently known about the evolution process that have been undergo to reach the Reptiles. Reptiles were believed to evolve from a land-adapted species of Amphibians. Because of the high quantities of oxygen and moisture presented in the Earth's atmosphere, temperatures upon land were generally favourable for Amphibians to fully adapt to terrestrial life, returning to the waters only when needed. Many Amphibians fossils that were found within the Carboniferous range were found to have similar characteristics and features to those of modern Reptiles. This suggested that evolution processes were already occurring. These Reptile-like Amphibians perhaps give rise to Reptiles after the development of the amniotic eggs.

Amniotic eggs were believed to help evolved in this process. Basically these transitional forms of Amphibians laid their liquid eggs close towards the bodies of water. During the development of the embryo, moisture and water close towards the egg perhaps evapoured, leaving less amount of moisture around the developing egg. Natural selection or mutation then perhaps enabled these eggs to develop a hard-membrane covering and also more yolk to be presented within the egg itself, producing more food for the embryo. Early forms of these amniotic eggs were perhaps softer and more fragile than those of modern Reptiles. The hard-membrane covering would of protected the developing embryo from being exposed to ultra-violent radiation that would otherwise modify the DNA structure of these organisms. This enabled the egg to be laid upon land, rather than in or close towards bodies of water. The transitional forms that obtained this reproductive advantage changed and adjusted their lifestyle and habits, which allow the evolution of the defining characteristics of Reptiles.

Reptiles were different from Amphibians in numerous and a variety of ways, though previous studies of these vertebrates often consider them to be the same. The defining features of a Reptile is the development of amniotic eggs and scaly skins. Scaly skins were adapted to provide waterproof skins that decrease the amount of moisture loss from their bodies. The resulting of terrestrial life also enabled Reptiles to adapt to other characteristics that were unavailable to Amphibians. One example of this is the changes to skeleton construction. The limbs of Reptiles were grown below their bodies, unlike those of Amphibians whose limbs are developed from the side. This allowed greater mobility to be presented for these species of Reptiles, which is benefitable in various ways. They were capable of escaping predators a lot more efficently, can become more effective hunters, as well as obtain a larger range if should migration to other ecosystems be necessary.

Permian period
The Permian Period is the last time period within the Paleozoic Era. No major classes of vertebrates evolved during the duration of this time period. Climates of the Permian Period became hot and dry throughout the Permian landscape. Arid conditions did not favour the development of life upon land and many species of plants that used to cover the oxygen-enriched environments of the Carboniferous disappeared and were replaced by other species of plants. The first gymnosperms appeared in this period, being the first plants to produce on seeds for reproduction. Large flying arthropods that existed in the previous period were also eliminated from the terrestrial fauna. Vertebrates of this period were dominated by Reptiles. The conditions that were presented in the Permian Period were not suitable for the lifestyles of Amphibians, who requires the existence of high amount of moisture and bodies of water to survive and reproduce. The arid climate of this period were just not conditioned to favour the demands of Amphibians. Reptiles, on the other hand, with scaly skins and the capability of laying amniotic eggs were more tolerate of these conditions. Along Reptile population, most of the niches within the ecosystems were dominated by species of Diapsids and Synapsida. Within marine biodiversity, overall all forms of marine organisms declined in diversity during this period. Many species of organisms that were successful within the previous periods of the Paleozoic Era became extinct here. However, most of the major aquatic vertebrate classes managed to survive throughout this period.

The two supercontinent plates of Laurasia and Gondwana collided during the Late Permian Period. The collisions of these two supercontinent plates formed the famously known supercontinent plate of Pangea. Perhaps the reason behind the arid conditions of the Permian Period was because of the formation of Pangea. Many large bodies of water that were landlocked within the areas of Pangea dried up, producing less water sources for the growth of plants. The dry and hot climates were influenced by Pangea's formation as well, mainly because more concentrated amount of radiation were exposed and presented upon this supercontinent plate. With less vegetation and less bodies of water, less amount of heat can be released, thus causing temperatures to increase at dramatic rates. A mass extinction event also occurred during the end of this period. This mass extinction event presented one of the deadiest event to occur within the Earth's evolutionary history. Marine organisms were the most affected by this crisis. More 90% of all marine species were eliminated and only a few survived to next period. Terrestrial organisms were also affected, but did not suffer as much as the marine organisms did.

Permian reptiles
Reptiles of the Permian Period were classified and organized into three major subclasses of: the Anapsids, Synapsids, and Diapsids. These Permian Reptiles were recognized by the differentials within their skull construction, through observing their temporal openings. Temporal opening can be described as holes that are located at the back of Reptile's skull, within the temporal region of the skull formed by jugal, parietal, squamosal, and postorbital bones behind their eye sockets(also called orbitals). Four different patterns were distinguished and found, described by the quantity and location of these temporal openings. Reptiles classified into the Anapsid subclass were characterized by having no temporal openings behind the orbitals. Synapsids develop one temporal opening, while Diapsid had two, located on each of the skull. The fourth skull pattern was believed to be developed by marine Reptiles that evolved in the Triassic Period, with one temporal opening behind the orbitals and one located upon the upper temporal.

Perhaps the development of these temporal openings were used to contain powerful jaw muscles that would enable these vertebrates to digest and break down food particles more effectively. Other than that, these temporal openings could of also assisted in the reduction of the mass and weight of these Reptile's skulls, making it easier for these Reptile to hold up their heads. This probably allowed the evolution of developing a bipedal stance and the ability to walk on two legs, which is widely known by therapod dinosaurs that existed in the Mesozoic Era.

Synapsids
Synapsids were a subclass of Reptilian vertebrates that first evolved during the Late Carboniferous Period. Synapsids, like the other subclass of Reptiles were characterized by the construction of their skull. Synapsids develop one temporal opening at the temporal area of their skull. In some species of Synapsids, the temporal opening present in their skull were growth into large and massive sizes in comparison to their size. This led some Paleontologist to believe that some species of Synapsids perhaps evolved from Diapsid Reptiles, where the two temporal openings at the temporal area fused together into one large temporal opening, similar to those of Synapsids.

Other than their defining feature of one temporal opening, Synapsids also developed other features that are easily distinguished and found within all, or most species. Most Synapsids of the Permian Period had a different limb construction that other Reptile subclasses. Their limbs were located below their bodies, rather than on the side for other Reptiles subclasses(with the exception of dinosaurs and their ancestors, who were Diapsids). This adaptation enabled more efficent locomotion to be produced that allowed these Reptiles to escape from predators or hunt preys more effectively. Synapsids were also the first Reptiles to develop different sized teeths. The development of different sized teeth enabled these Reptiles to feed on a more wider range of organic materials, as well as increased efficency in chewing. Most Synapsids of this period were also believed to have sharp and acute senses of smell and sight. This enabled these Reptiles to spot preys and predators more effectively, and perhaps also played a role in advancing their nervous systems.

Perhaps the most well-known divisions of Synapsids is the Therapsid order. This order of Synapsids first existed in Middle Permian Period. Therapsids were commonly refer to using the term Mammal-like Reptiles to describe the mammal-like characteristics these Reptiles developed. Indeed, did these Reptiles evolved and give rise to the Mammals, but this happened a bit earlier in the Earth's evolutionary history. The majority of the Synapsids of the were Therapsids, so it is generally acceptable to use their characteristics to characterize all Synapsids. Because Therapsids were believed to have given rise to Mammals, Paleontologists have adapted another way to distinguish Synapsids by their jaw construction. Mammal developed relatively different jaw construction than Reptiles. In Reptiles, the jaws were constructed mainly by two bones of the dentary and the articular. In Mammals, however, the articular and dentary bones have been transported elsewhere and now makes up the construction of the middle ear. Thus, Paleontologists can look for the transitional jaw constructions of Synapsids to classified them.

Synapsid Reptiles were generally very successful throughout the Permian Period, adapting to a wide range of niches. They survived the mass extinction event that occurred at the end of the Paleozoic Era and carried to the Mesozoic Era, where they give rise to the Mammals.

Mesozoic era
The mass extinction event occurred during the end of the Permian Period, also marked the end of the paleozoic Era. After the Paleozoic era came the emergence of the Mesozoic era. The Mesozoic era lasted for about 185 million years, and is organized into three time periods of: the Triassic, Jurassic, and Cretaceous. This time era was mainly characterized by the success in the Reptilian evolutionary history, in which these organisms reached the peak of their diversity. For this reason, the Mesozoic era is also informally known as the Age of Reptiles.