The unkeeled oval carapace (to 114.7 cm) is flattened and only slightly serrated posteriorly in adults (strongly serrated in hatchlings). It is also rather pliable and somewhat slimy to the touch. All vertebrals are broader than long. There are only four pleurals; the 1st is separated from the cervical scute. The lateral marginals may be upturned. The carapace is gray to pale green in adults, darker olive in hatchlings. The wide plastron lacks a hinge and narrows considerably both anteriorly and posteriorly. There is no anal notch, as this area may be filled with one or more postanal scutes. The plastral formula is: fem > pect > an >< gul > abd > intergul > hum > postan. Both plastron and bridge are immaculately cream colored. The head is moderate with a pointed snout. Its lateral lower jaw margins are serrated. As in Chelonia mydas, there is only one pair of elongated prefrontal scales, but in contrast there are only three postocular scales. The upper eyelid is covered with numerous small, irregularly shaped scales, the largest of which is much less than 25% of the width of the adjoining prefrontal scale. The forelimb is short; its distal half is covered with single rows of enlarged scales along the phalanges which are separated by rows of smaller scales or wrinkled skin. Head and neck are olive gray dorsally but cream colored ventrally. The limbs are gray.
Adult females are larger than adult males, but have short tails in contrast to the long, thick tail of the male.
The total range includes the coastal waters of the continental shelf off north and northeastern Australia.
Natator depressus may occur in the open sea, but is more common in shallow coastal waters.
Parmenter and Limpus (1995) calculated the reproductive half life of females as 10.1 years at Peak Island of the east coast of Queensland. Nesting females average 90.7 cm carapace length (Van Buskirk and Crowder, 1994).
The nesting range of Natator depressus is predominantly tropical, extending from northern Western Australia eastward around the Australian coast to Mon Repos, Queensland, at 25°S. Most nesting is in November and December, with mating occurring in the waters off the beaches at that time. Most nests are dug in early evening on the tops of sand dunes, or on the steep seaward slopes (Limpus, 1971; Bustard et al., 1975). A shallow body pit is first excavated, and then the nest is dug. Nests measured by Bustard et al. (1975) were oval: 20-22 cm long, 12-15 cm wide, and 30-36 cm deep.
The round, white eggs are about 52 mm in diameter with parchmentlike shells; a clutch averages 52.8 (7-80) eggs (Van Buskirk and Crowder, 1994). Up to four clutches are laid each season, with a mean of 2.84 at Mon Repos (Limpus, 1971; Limpus et. al., 1984). Incubation takes about six weeks with hatchlings emerging at night. Approximately 75% of the hatchlings emerge from nests (Parmenter and Limpus, 1995). Hatchlings average 60 mm in carapace length (Van Buskirk and Crowder, 1994), much larger than those of C. mydas.
N. depressus is more carnivorous than C. mydas, feeding on sea cucumbers, prawns, and other invertebrates, and consequently its flesh is less palatable than that of its congener. It is rarely eaten by humans.
Bustard (1972) reported that flatbacks spend much of the day floating on the surface basking in the sun, and that it is not uncommon to see birds resting on their backs.
IUCN Red List Status (1996)
Vulnerable (A2cde). N. depressus is more numerous in Australian waters than C. mydas, and is probably in a less critical state. However, though adults are not eaten, the eggs are, and this could eliminate the species from many of its nesting beaches. Strict protection of the nesting beaches is essential.