Side-necked turtles, classified under the suborder Pleurodira within the order Testudines, are named for their ability to bend their necks sideways rather than retracting them straight into their shells like hidden-necked turtles. They first appeared in the Late Triassic Period, around 220 million years ago, and today, over 70 species are found primarily in the Southern Hemisphere, specifically in regions like Australia, South America, and Africa.
Though now restricted to freshwater environments, side-necked turtles were once more widespread, inhabiting marine environments globally during the Cretaceous and much of the Cenozoic eras. These reptiles are mostly omnivorous, though herbivorous and carnivorous species also exist.
These turtles exhibit a variety of shell shapes and sizes. The largest, the Arrau turtle (Podocnemis expansa), has a carapace length of up to 3.5 ft (1.07 m). Most African side-neck turtles range between 8 to 12 inches, with females being a little larger than males. In contrast, Austro-South American side-necked turtles typically measure around 6 to 9 inches.
Like all turtles, members of Pleurodira possess eight vertebrae in the neck. These vertebrae are narrow and spool-shaped with biconvex bones called centra, which serve as double joints and help fold the neck onto itself horizontally. These centra also allow the neck to move sideways, unlike in members of Pleurodira, where the neck folds onto itself in the vertical plane because of wide and flat cervical vertebrae.
In all side-necked turtles, a piece of bone called the mesoplastron remains embedded in the lower shell or plastron of their bodies, to which their pelvic girdles are fused.
The total number of scutes (keratinized external plates) on the plastron differs between side-necked and hidden-necked turtles. Hidden-necked turtles have twelve scutes, whereas side-necked turtles have an additional thirteenth scute called the intergular.
Based on the classification scheme by Ferreria et al. 2018, the Pleurodira suborder is divided into three families: Chelidae (Austro-South American side-necked turtles), Pelomedusidae (African mud terrapins), and Podocnemididae (American side-neck river turtles).
Side-necked turtles mainly inhabit freshwater habitats in the Southern Hemisphere, including areas in Africa, Madagascar, South America, Australia, and some islands in the Indian Ocean. They were widespread across the globe in the past and even lived in marine environments during the Cretaceous Period.
Austro-South American side-necked turtles use a unique feeding method called the gape-suck mechanism. Initially, they slowly open their mouth bit by bit. Once the prey is within striking range, the turtles rapidly open their mouth wide and suck in water. The water influx is so fast that it makes escape nearly impossible for the prey, which is engulfed within just 0.004 seconds.
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