Over a period of 400 million years, animals in the vertebrate category have evolved to a diverse array of life adaptations on earth. Vertebrates represent animals that are most familiar with people and are used for food, adopted as pets and deployed for labor. They include fish, reptiles, birds, amphibians and mammals. The classification system that groups vertebrates according to each of the classes that have been mentioned above utilizes genetics, anatomy, and the evolutionary relationships. Animals are classified starting with the kingdom which is a large body that encompasses animals that have similar characteristics and end up with Genus and species classifications. Vertebrates have progressive evolutionary adaptations for life on land as well as other ecological niches (McDiarmid, Foster, Guyer, Gibbons and Chernoff, 2001, p.23). These adaptations are mostly found in protective as well as insulating body coverings, improved respiration, efficient reproduction on the land, and muscular and paired appendages. This paper is a critical analysis of the factors that led to the diversity of the reptiles’ class of the vertebrates. It compares and contrasts the lifestyle, diet, distribution and reproduction of the members of the reptiles’ class.
Vertebrates in which the reptile class belongs are characterized by the derived characteristics of having a bilateral symmetry, protective outer covering of a cellular skin and can be modified to form unique structures exemplified by hairs, feathers, and scales, jointed locomotor appendages that are jointed which may include fins such as the anal or dorsal fins, pectoral fins, hind limbs and the forelimbs. Vertebrates also have a well-developed body cavity also known as coelom that is lined with epithelium and is divided into 4 or 2 compartments. The bone and internal cartilage skeleton is well developed and is separated to an axial skeleton which consists of the ribs, vertebrae, sternum and skull as well as the appendicular skeleton that includes the appendages and the girdles. The brains of the vertebrates are highly developed and are enclosed by the head while the nerve cord is usually enclosed by the vertebrae. The skull and vertebrae provide an advanced neural structure which is highly protective of any damage. The respiratory systems for vertebrate include the lungs, gills and are usually located closer to the throat or the pharynx. Vertebrates have a closed circulatory system with medium dorsal artery and the ventral heart. They also have digestive tracts made up of two main digestive glands namely pancreas and the liver which secretes the digestive juices in the food materials.
The reptiles are considered as the first animals that fully escaped a dependence on the availability of open water body for survival. This transition was made possible by having a thick as well as the dry skin that had scales thus they are less prone to water loss typically known as desiccation. They also have the amniotic eggs which are an evolutionary innovation in the reptile class; the reptilian eggs develop in the internal watery compartment, amnion chamber where it is provided with abundant nutritional value supply in yolk sac that is surrounded by a tough outer shell. The transition was also made possible through internal fertilization where shelled egg that is laid on land which is dry is incapable of being fertilized by sperms that were hitherto swimming freely on a large body of water. The internal fertilization had the implication that sperms of the male reptile members swim to female eggs for fertilization to take place in moist interiors of the female body. After fertilization takes place, a hard shell is usually added to the outer part of the egg and later laid on the ground for hatching. Reptiles also have advanced breathing systems from the evolutionary characteristics where the rib cage muscles contract and expand so as to pump large quantities of air to lungs which are extensive and more advanced. From the initial stages when the reptiles started to separate from aquatic environments thus evolving to much stronger adaptations to land habitats, reptiles eventual evolved to muscular adaptations that helped them grow too large sizes such as the dinosaurs. Through most modern reptiles may be smaller than the dinosaurs, they have the same adaptations that enable them to grow and ultimately live on land and only occasionally return to water while aquatic reptiles as sea turtles, they have been established to return still at some point to land to carry out key milestones during their life such as when they are laying eggs. Over a period of 250 million years which is considered as when reptiles were dominant terrestrial vertebrates, the primary forms of reptiles included the dinosaurs, pelycosaurs, thecodonts, and therapsids.
Pelycosaurs
This was ones the most dominant of all land vertebrates and comprised 70% of the whole population of land vertebrates. They were better adapted to land living than amphibians since they had evolved water-tight eggs. These forms of reptile had powerful jaw brought by the skull design as well as the muscle arrangement innovation. Pelycosaurs were considered to be synapsids since their skulls were through to have had a pair of holes that were temporal located at the behind of eyes opening. The jaw muscles of the pelycosaurs were anchored on these holes thus allowing them to bite their food powerfully. They weighed about 200 kilograms and had long; sharp teeth described as steak knife thus were able to kill and devour other animals their sizes. They later became extinct 250 million years ago and were replaced by the therapsids which were their direct descendants (Andreone, Vences, and Randrianirina, 2001, p.237). An example of the pelycosaur form is the Dimetrodon which was a carnivorous pelycosaur and which had the dorsal sail that it used in body heat dissipation or gaining the heat when it would bask.
Therapsids
These groups of reptile class were at least ten times as frequent as pelycosaur which was their ancestors. They are believed to have been endotherms thus were capable of regulating their body temperatures. They took extra amounts of food thus producing necessary amounts of body heat. This extra body heat permitted them to be more active at that time during this period when winters were long and cold. They were later replaced by thecodonts 230 million years ago from being the dominant land vertebrate that was ectothermic and cold blooded. By the time they became extinct 170 million years ago, they had given rise to the mammals which were their descendants (Menegon and Salvidio, 2005, p.209). An example of the therapsid was the megazostrodontid which had fur and is considered by paleontologists as the first mammal.
Thecodonts
They were disapsids with their skills thought to be having temporal holes that were two paired. Like their amphibians and other early reptiles’ ancestors they were ectotherms. They replaced therapsids 230 million years ago when world's climate warmed (Menegon and Salvidio, 2005, p.207). With such changed climatic conditions persisting in the world, such reptiles required fewer amounts of food and thus the endothermy characteristics never gave the therapsid competitive advantage. Thecodonts are viewed as first land vertebrates that were bipedal meaning they would walk and stand on their two feet. They were mostly dominant in the Triassic age, and they are thought to have survived for at least 15 million years about which they would be replaced by dinosaurs which were their direct descendants. Euparkeria was one of the early thecodonts that had bone plates rows on the backbone side as viewed in modern alligators and crocodiles.
Dinosaurs
Dinosaurs were the decedents of thecodonts and appeared 220 million years ago. They had legs that were directly positioned underneath their bodies. Their body was also significantly improved which enabled their body weight to be placed directly over their legs. This enabled the dinosaurs to run with agility and with high speed. Their fossils are different from those of the thecodont by having a hole on hip socket. Dinosaur legs are positioned underneath sockets thus the force applied by the legs is upward and not inward thus the need for bone on the socket side. They became the dominant and successful land vertebrates for 150 million years until they became extinct years later due to asteroids impact (Andreone, Vences, and Randrianirina, 2001, p.239).
Today’s reptiles that exist include the turtles, tuataras, lizards and snakes and crocodiles. The Turtles are considered to be most ancient reptile lineage that is currently surviving. They have only changed little over a period of 200 million years and have anapsid skulls.
Snakes and lizards are also living reptiles and are second in evolution stage. They branched off during the Permian era from the reptile evolution long before thecodonts appearance. Snakes and Lizards have movable eyelids, legs, external ear openings as well as long tails. Some are limbless and are terrestrial living in a burrow, under rocks, trees, and other foliage. They reproduce through egg laying, but other species bear a live young. The viviparous lizards, the egg's yolk, provide exclusively the nutrients that are needed by developing embryo which is retained in mother’s uterus (Menegon and Salvidio, 2005, p.204). The Suarian are active during the day and regulate their body temperatures through basking in the sunlight early in the morning and hide in tree shade during the hotter periods. Geckos which belong to these group have unique characteristics that allow them to pursue nocturnal lifestyle.
Tuataras are a third surviving lineage of reptiles that appeared before dinosaurs. They were more common during the Jurassic era but began to decline during the Cretaceous since they were unable to compete with lizards. Their two surviving species belong to Rhynchocephalia order and live in a New Zealand island. Their look more like the lizards but can be distinguished by skull morphology, genitalia, and tooth arrangement and type. They are non-ovarian meaning that they do not have specialized sexual organs. Males transfer their sperms to females through the cloaca. They mostly live in burrows which they constructed or abandoned by various other animals. At daybreak, they bask in the sun at their burrow entrance and are mostly active during the night. They feed on bird eggs, young birds and insects. They reproduce through egg lying where newborn emerge after an incubation period that typically takes thirteen months. They reach maturation for sexual reproduction after 20 years. They are slow moving thus making them vulnerable to predation by cats, rats, and pigs.
Crocodiles, on the other hand, look similar to dinosaurs since they are also from thecodonts who are their descendants. They are giant, carnivorous and ambitious reptiles. They have long bodies, muscular tails that are covered by thick armor that have bony plates and also have powerful jaws. Crocodile is aggressive predators with conical teeth as well as short legs that have webbed and clawed toes. All groups of crocodiles have similar characteristics as regarding morphology, behavior, and ecology. They are mainly found in swamps, lakes, and rivers in both subtropical and tropical habitats of Southern and Northern Hemispheres. Most of their time, they spend on water but significantly travel long distances over the land. They can scull in water, belly slide, gallop or large wake when moving. They slowly sit and wait for their predictors while floating on water which they knock using their powerful tail which happens through a swing. Some even clamp their victims using their legs and tearing them apart. They lay their eggs in nests where the mother keep them warm using mud as well as decaying plant materials. This happens until the mother is sure babies will hatch through squeaks signals where it knocks off mud but does not provide any additional care of the young one.
References
Andreone, F., Vences, M. and Randrianirina, J.E., 2001. Patterns of amphibian and reptile diversity at Berara Forest (Sahamalaza Peninsula), NW Madagascar. Italian Journal of Zoology, 68(3), pp.235-241.
McDiarmid, R.W., Foster, M.S., Guyer, C., Gibbons, J.W. and Chernoff, N. eds., 2012. Reptile Biodiversity: Standard methods for inventory and monitoring. Univ of California Press.
Menegon, M., and Salvidio, S., 2005. Amphibian and reptile diversity in the southern Udzungwa Scarp Forest Reserve, south-eastern Tanzania. In African Biodiversity (pp. 205-212). Springer US.
