Turtles: Order Testudinata
The evolution of the turtles
The design of the turtle is a successful one.
The oldest fossils are dated from the Triassic period, the first period of the Mesozoic era, which was the time of the dinosaurs.
Fossils from slightly later, in the Jurassic period, show that the turtle has barely changed it's design. I
t's conservative form means they have obviously come up with a good design and they're gonna' darn well stick to it.
Turtles have adapted to live in a huge range of environments - they may perch in low trees, float down freshwater creeks, walk across deserts or swim across the oceans; but still their basic form does not change.
Because they appear in the fossil record so early and suddenly, their origins remain a bit of a mystery.
Two more recent molecular studies have come up with two different versions of relationships; one where they are related to the crocodiles, the other where they are related to the lizards (Schwarkopk 2000).
Generally, most biologists place the turtles as quite a distinct group away from the other reptiles.
The shell of the turtle
The shell of the turtle is a unique structure.
It basically encloses the body in a box that is covered with hardened scales.
The top part is called the carapace, and the bottom part the plastron.
The shell is actually an extension of the ribs.
They have expanded outwards and the tips have fused together to form a circle of peripheral bone.
The whole arrangement is then laid over with large hard scales or `scutes'.
It's an unusual arrangement, as it means the ribs contain the shoulder and pelvic girdle.
In all other vertebrates (such as ourselves) the shoulders and hips are outside of, not inside, the ribcage.
Some suggest it evolved in one spectacular mutant leap, one of the `hopeful monsters' that are occasionally suggested when there is no gradual fossil record to explain the appearance of successful and quantum leaps in animal technology. However, some suggest it may have gradually evolved from an ancestor that was essentially ribless, such as the frogs.
There are advantages and disadvantages to the shell. This includes protection against predators and environmental elements.
However, no adaptation can avoid trade offs and the development of the shell is not without it's costs.
Because turtles have a shell that is essentially their expanded ribs, and this is very rigid, they cannot contract any coastal muscles to breathe in and out.
Instead they create a vacuum by relaxing the muscles that hold up the viscera.
This sags down, expanding the internal air space, and sucking in air.
Pulling the muscles that hold that viscera up pushes air out of the lungs.
This is the process in land turtles, and it is expiration that requires the effort. However, for turtles living in water it is the opposite; the surrounding water medium supports the body and it thus requires effort to push the gut contents down and away from the shell for inspiration.
Some aquatic turtles have evolved variations on their respiration.
They suck water in and pass it over a surface that has high vascularisation, with outfoldings of skin (villi) that work they same way as they do in the lung.
By vastly increasing the surface area of the skin they are allowing for gaseous exchange to occur.
Such areas have evolved in both the feeding entrance, the pharnyx, and in the urogenital entrance - the cloaca.
A species called The Fitzroy River turtle (Rheodytes leukops) pulls in water through it's cloaca.
This Australian species uses cloacal respiration, and essentially breathes through it's arse.
The shell of course, is a remarkable adaptation against predation.
But it also has disadvantages -the shell can still be injured and can bleed and the shell may also slow the animal down if it wants to escape.
There has been two general directions in which turtles have evolved to solve this problem.
The first direction is to make the shell even harder and more difficult to penetrate. When threatened, most turtles can pull their limbs and head into the shell. In some species, there has actually evolved closures, such as hinged flaps that close up and completely seal the shell.
The other solution involves evolving in the opposite direction, and making the shell actually softer. This means the shell is more flexible and lighter, as in the soft shelled turtles, and thus making it easier to escape. In the pancake turtle, it means the animal can wedge itself in between boulders, not only to avoid predation, but also to allow it to feed on it's small lizard prey.
Many species have evolved flatter shells, but for those with high domed shells mating presents a challenge.
Generally, males are smaller than females.
All turtles are oviparous; that is, they lay eggs.
For many species this is a laborous process, involving the digging of a pit and the laying of many eggs.
For the turtles that live in water, be it in the ocean or in creeks, the effort involved appears even more intensive as they have to leave their supportive medium.
They have to drag themselves ashore, dig the pit, lay the eggs and drag themselves back to the water.
The temperature that the turtle eggs receive during development determines the sex of the young.
This is called 'temperature dependent sex determination, or TSD for short, and also occurs in other animals.
'Pull your head in': the two different types of turtles
There are two types of turtles. The two different types are based on how they pull their head into their shell, which is based on the morphology.
The suborder Cryptodira contain the 'hidden-necked' turtles that retract their necks, while the suborder Pluerodira contains the `Side-necked turtles' that retract their neck with a sideways motion.
Turtles in Australia
The first group is represented in Australian territory by the marine turtles and the strange 'pig-nosed turtle'.
The second group is represented by the various species of freshwater turtles found in Australian waterways.
There are at least 23 species of turtles in Australia and it's waters (Egerton et al 1997).
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide