Description
Turtles (Testudines) are an order of reptiles, all species of which are characterized by a sturdy, often spherical shell. Turtles can vary greatly in size, color, and lifestyle, but are easily distinguished from all other reptiles by their external shell. All turtles have a bony shell on both the ventral (plastron) and dorsal (carapace) sides, unlike all other modern reptiles such as crocodiles, lizards, and snakes. The shell is usually covered by a second layer of armor, the scutellum. The dorsal shell is connected to the plastron by a bony bridge on either side of the body. Turtles usually reproduce annually and are, without exception, oviparous. They grow rapidly when young but develop very slowly. Larger turtles take several decades to reach maturity, although such species can also reach great age. Their diet consists of both animal and plant material, depending on the species. There are approximately 330 different species of turtles, divided into 14 families. Turtles are found worldwide in diverse habitats, such as forests, grasslands, swamps, and seas. Dozens of species are critically endangered due to human activities. The main threats are habitat destruction and the collection of wild turtles for consumption or the pet trade. Turtles are clearly distinguished from all other animals by their round to oval, dome-shaped shell, which comprises most of their body. The head and forelimbs protrude through an opening at the front, and the tail and hindlimbs protrude through an opening at the back. The shell consists of a flat underside, the plastron, and a usually convex upper side, the carapace. These two parts are connected by a bony bridge on either side of the shell between the forelimbs and hindlimbs. Some turtles, such as softshell turtles, have a shell lined with a wide strip of skin. The inside of the shell consists of dermal bones, or bony plates, formed from the ribs and projections of the thoracic vertebrae. The bony plates are formed from the bone tissue of the internal skeleton. The keratin plates are composed of keratin, a hard, horny substance that provides protection against external parasites. The scales of turtles (and all other reptiles) are also composed of keratin. The keratin plates lie against each other, with the edges of the keratin plates overlapping those of the bony plates, increasing their strength. The bony plates are fused to the thoracic vertebrae of the spinal column, preventing a turtle from leaving its shell. The outer skin is reinforced with keratin plates or laminae, which originate in the epidermis. The bony plates give the shell its shape. The shape of the shell is often roughly the same across the different sexes. The keratin plates give the shell its color and markings, often specific to the species. Because the shell shape and markings are generally characteristic of the species, they are important identification features. The color and markings often vary slightly between individuals; this is essentially a turtle's fingerprint. A land-dwelling turtle has a spherical shell, while a water-bound turtle has a lighter, flatter shell for better swimming. Sea turtles, in addition to a relatively light shell, have a strongly streamlined shape that tapers at the back. This is due to improved hydrodynamics, which suits their permanently swimming lifestyle.
