Download Discuss-the-evolution-of-the-sea

Discuss the evolution of the sea turtles
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.
They have large flippers and streamlined bodies, in order for them to adapt to
the marine environment [1] ’ [2] .
Although sea turtles live most of their lives in the ocean, adult females must
come ashore to lay their eggs in the sand [3] .
There is still ongoing research on the evolution of the sea turtles, in order to
establish exactly their provenience.
In this essay I will discuss the whole life history of sea turtles and how there is
a distinction between the various types of sea turtles and how this is reflected
in their anatomy.
History and evolution
It is believed that the first sea turtles roamed Earth more than 250 million
years ago and that they were part of the late Jurassic Period based on fossil
records.
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. [4] ’ [5]
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. [6]
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. [7] , [8]
Extensive fossil turtle deposits extending back to the Triassic [9] have been
found throughout the world, including Germany, India, Thailand, South Africa,
North America and China. [10] 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. [11] Most known
modern turtles can retract their heads [12] , 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. [13]
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. [14]
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. [15] , [16]
Another hypothesis postulates that modern turtles evolved from the Chelidae,
a ‘primitive’ side necked turtle unique to Australia and South America. [17]
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. [18] which means that these species have similar structures,
however they don’t share the same recent common ancestors. [19]
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.
Turtle anatomy
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. [20]
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. [21]
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. [22]
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. [23]
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. [24]
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. [25]
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. [26] It is a way for the muscles of the internal skeleton to fuse to the
external skeleton. [27] 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. [28]
The long digits in the limbs of the turtle are fused together to form the
flipper. [29]
They use the front ones to gracefully move around in the water. They also
use them to move around on land. [30] 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. [31]
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. [32]
The mouths of sea turtles contain no teeth. Instead, they are sharp and beaklike and are well-suited for crushing or tearing their food. [33]
The large-sized body cavity of turtles, especially sea turtles, contains bulky
intestines needed for digesting vegetation and small sea
creatures. [34] 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. [35]
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). [36]
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. [37]
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)
38
Green sea turtles have a heart shaped shell, single-clawed flippers. [39] Their
flesh is a light color with a hint of green and they have small heads relative to
the size of their body. [40]
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. [41]
Hawksbill Sea Turtle (Eretmochelys imbricata)
42
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. [43]
An elongated and pointed beak and a serrated rear margin of the carapace
are distinguishing features. [44] People and planet
Male hawksbills have longer claws, thicker tails, and somewhat brighter
coloring than females. [45]
46
The Kemp’s ridley turtle is one of the smallest of the sea turtles. [47] Their
upper shell, or carapace, is a greenish-grey color, and their bellies are offwhite to yellowish. [48] 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. [49]
Leatherback Sea Turtle (Dermochelys coriacea)
50
Leatherbacks are the largest [51] deepest diving, and most migratory and
wide ranging of all sea turtles. [52] 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 barrelshaped. [53] 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. [54]
Loggerhead Sea Turtle (Caretta caretta)
55
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. [56]
Olive Ridley Sea Turtle (Lepidochelys olivacea)
57
The olive ridley was named for the olive color of its heart-shaped shell and is
one of the smallest of the sea turtles. [58] The carapace of this turtle is olive
coloured and relatively heart-shaped, whilst the undersurface is a greenish
white [59] . 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. [60]
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. [61]
Therefore, the evolution of sea turtles remains a mystery to this day.