Sea turtles are large, air-breathing reptiles that inhabit all but the coldest of the world’s oceans, however nest only tropical and sub-tropical beaches, where it is warm enough to incubate their eggs.
Due to turtles being reptiles, they have evolved from amphibians, an even earlier class of vertebrates that lives on both land and in fresh water. Over time, the reptiles came to dominate the Earth. But it was early in the history of reptiles that turtles, members of the order Chelonia split from the main line of reptilian evolution. The origin of chelonians is uncertain, but recognisable turtles are known as far back as the Triassic period, at least 180 million years ago when dinosaurs were becoming the dominant land animal.
It is believed that these sea creatures where once land animals that evolved to spend most of their time in the water in order to survive. This occurred because there is some fossil evidence which shows that some significant changes occurred on earth during that period.
As a consequence to these changes, these turtles developed flippers and also, having been meat eaters, they had to evolve and adapt in order to live on what was found in the marine environment.  ‘ 
Fossil evidence of turtle evolution
Turtles are ideal animals for testing evolutionary ideas because some of their most unique structures, such as their hard shells, preserve extremely well in the fossil record. 
Their body plan is unique among tetrapods, and would require some remarkable changes in the skeleton and internal organs as they evolved from a typical tetrapod to a carapace-plastron clad turtle.  , 
Extensive fossil turtle deposits extending back to the Triassic  have been found throughout the world, including Germany, India, Thailand, South Africa, North America and China.  The earliest known turtle, named Proganochelys and discovered in Germany in the 1880s, was dated by evolutionists to 210-220 million years ago.
The specimens had a shell consisting of 60 plates of various sizes, and a carapace up to 1 m long. Its skeleton was characteristic of turtles-carapace, plastron, scapular girdle inside the rib. This primarily aquatic turtle possessed cervical vertebrae with well developed acuminate, spiny apophyses, making it impossible for the turtle to retract its head to protect itself.  Most known modern turtles can retract their heads  , however this is still an exception for sea turtles.
The abundant turtle fossil record supports the conclusion that turtles have remained unchanged for at least 150 million years. 
Turtle fossils are found more often than other animals of similar size, and the evolutionary history of the modern turtle is fairly well known except the earliest turtle ancestors and, as a result, the exact ancestry of living turtles is disputed among paleontologists. 
Monophyletic and polyphyletic evolution
One theory is that sea turtles evolved from land turtles, requiring significant evolutionary changes to adapt to the sea. For example, sea turtles filter salt from sea water by producing large salty tears. Feet must evolve into flippers, requiring extremely elongated phalanges. Yet not one transitional sea turtle fossil has been found.  , 
Another hypothesis postulates that modern turtles evolved from the Chelidae, a ‘primitive’ side necked turtle unique to Australia and South America. 
Other herpetologists argue for a placodont ancestor, especially a Henodus because of its turtle-like appearance. Since there are many phylogenic problems of postulating turtle evolution from a Henodus, others speculate that the similarity of turtles and Henodus is explained by convergent evolution.  which means that these species have similar structures, however they don’t share the same recent common ancestors. 
For this reason there are doubts to whether or not sea turtles have evolved by monophyletic or polyphyletic evolution. Due to the lack of fossils this hasn’t yet been able to be established.
The anatomy of the sea turtle is unique in that it is one of the few creatures to have both an internal and external skeleton. 
Turtles are reptiles, a class of about 6,000 vertebrate species that have scaly skin, breathe air and use sunlight to heat their bodies (ectotherms). Like all reptiles, turtles reproduce through internal fertilization and, like most reptiles, lay their soft-shelled eggs on land. 
One unique feature of the turtle is its shell. Evolutionists are trying to fill in the gaps where intermediate fossils are lacking. There are a few hypotheses regarding the matter: One hypothesis is that the turtle carapace gradually evolved from elements of the primitive reptilian integument. 
On the other hand, turtles could not evolve by a gradual process. However, most recently, some ideas have been proposed which regard a theoretical embryological model involving movement of the ribs into the dermal layer leading to the evolution of a turtle shell. 
The shell encases the entire turtle except for the flippers and head. There are two main parts to a turtle’s shell. The upper or top part of the shell is the carapace and the bottom or underside portion of the shell is the plastron. 
The shell is composed of hard, bone plates covered by scutes. The scutes are made of keratin and the pigment melanin, present in the scutes, may form intricate designs and brightly coloured patterns in some species. 
However, it isn’t the shell which provides the turtles with support, protection and shape, it is actually the vertebral column, to which the shell is fused with.  It is a way for the muscles of the internal skeleton to fuse to the external skeleton.  The entire shell is composed of small, bony plates and the plastron and carapace are connected on the sides of the turtle by lateral bridges. 
The long digits in the limbs of the turtle are fused together to form the flipper. 
They use the front ones to gracefully move around in the water. They also use them to move around on land.  The rear flippers act as rudders, providing both direction and stability to the turtles’ motion. They are also used by females to dig the egg cavity during nesting. 
There are back flippers as well and they help a sea turtle to stay balanced. They also help them to be able to navigate in different directions both on land and in the water. 
The mouths of sea turtles contain no teeth. Instead, they are sharp and beak-like and are well-suited for crushing or tearing their food. 
The large-sized body cavity of turtles, especially sea turtles, contains bulky intestines needed for digesting vegetation and small sea creatures.  They’re digestive system functions very much like humans’. in addition to using their lungs for breathing, turtles have developed supplemental forma of respiration. Some aquatic turtle species bring water in through their nasal passages and into their mouths and throat where oxygen is extracted by the pharynx. 
Sea turtles also have the ability to take in oxygen through their cloaca. This special feature is used in extreme situation typically to keep the turtle alive when oxygen levels are low (i.e. deep sea diving, hibernation). 
Sea turtles have specialized adaptations for their marine existence. Their shells are greatly reduced in weight and streamlined in shape to reduce water friction. Front and rear flippers replace the stumpy legs of land turtles and contain well-developed muscles for swift long distance travel. 
Species of sea turtles
There are different species of sea turtles that have adapted over the years in many different ways, in order to survive in their environment.
- Green Sea Turtle (Chelonia mydas)
Green sea turtles have a heart shaped shell, single-clawed flippers.  Their flesh is a light color with a hint of green and they have small heads relative to the size of their body. 
The adult carapace is smooth, keel less and light to dark brown with dark mottling; the plastron is whitish to light yellow. Adult heads are light brown with yellow markings. Identifying characteristics include four pairs of costal scutes, none of which borders the nuchal scute, and only one pair of prefrontal scales between the eyes. 
- Hawksbill Sea Turtle (Eretmochelys imbricata)
While young, their carapace, or upper shell, is heart-shaped, and as they mature it elongates. Their strikingly colored carapace is serrated and has overlapping scutes, or thick bony plates. 
An elongated and pointed beak and a serrated rear margin of the carapace are distinguishing features.  People and planet
Male hawksbills have longer claws, thicker tails, and somewhat brighter coloring than females. 
- Kemp’s Ridley Sea Turtle (Lepidochelys kempii)
The Kemp’s ridley turtle is one of the smallest of the sea turtles.  Their upper shell, or carapace, is a greenish-grey color, and their bellies are off-white to yellowish.  The carapace has five pairs of costal scutes. In each bridge adjoining the plastron to the carapace, there are four inframarginal scutes, each of which is perforated by a pore. The head has two pairs of prefrontal scales. Hatchlings are black on both sides. The Kemp’s ridley has a triangular-shaped head with a somewhat hooked beak with large crushing surfaces. 
- Leatherback Sea Turtle (Dermochelys coriacea
Leatherbacks are the largest  deepest diving, and most migratory and wide ranging of all sea turtles.  The leatherback’s carapace is slightly flexible and has a rubbery texture. No sharp angle is formed between the carapace and the under-belly (plastron) so a leatherback is somewhat barrel-shaped.  A toothlike cusp is located on each side of the gray upper jaw; the lower jaw is hooked anteriorly. The paddle-like clawless limbs are black with white margins and pale spotting. 
- Loggerhead Sea Turtle (Caretta caretta)
The loggerhead is characterized by a large head with blunt jaws. The carapace and flippers are a reddish-brown colour; the plastron is yellow. The carapace has five pairs of costal scutes with the first touching the nuchal scute. There are three large inframarginal scutes on each of the bridges between the plastron and carapace. 
- Olive Ridley Sea Turtle (Lepidochelys olivacea)
The olive ridley was named for the olive color of its heart-shaped shell and is one of the smallest of the sea turtles.  The carapace of this turtle is olive coloured and relatively heart-shaped, whilst the undersurface is a greenish white  . It can be distinguished from the closely related Kemp’s ridley turtle (Lepidochelys kempii) by the possession of more than five bony plates, or scutes, running the length of the carapace; Kemp’s ridley has only five. 
To conclude, after all the research I did, I understood that herpetologists are still unsure of the sea turtles evolution fro land turtles as there is no evidence of a transitional period.
Nowadays, scientists still cannot work out a common potential ancestor – this is because Turtles are so different from any other reptile that their peculiarities are practically useless as a guide for distinguishing among potential ancestors, and the origin of turtles remains one of the great unanswered questions of evolutionary biology. 
Therefore, the evolution of sea turtles remains a mystery to this day.