(Rhodin, Mittermeier, and McMorris, 1984)
Pantanal swamp turtle

Recognition
This is the largest of the South American Acanthochelys, growing to 23.5 cm in carapace length. It has a broad, oval to moderately elongated, deep carapace with a shallow dorsal groove extending along the 2nd to 4th vertebrals. The 1st and 5th vertebral scutes are very broad, the 2nd through 4th may be slightly longer than broad, and the 5th is laterally expanded. Vertebral and pleural scutes may be rugose with growth annuli. Marginals 1, 2, and 8-10 are slightly expanded but not flared, and 3-7 are often slightly upturned. The carapace is highest just behind the center and broadest at the level of the anterior part of the 8th marginals; its posterior rim may be weakly serrated. The carapace is dark to blackish brown, but may be light brown in some. Juveniles often have lighter brown radiations on their carapacial scutes. The broad plastron and bridge are yellow with some dark pigment extending along the seams (sometimes covering most of a scute, but usually not the areola); this pigment fades with age. The forelobe is broader than the hindlobe, which contains a deep posterior notch. The intergular scute is approximately half as long as the length of the forelobe. The plastral formula is: intergul > fem > abd > hum > an > gul > pect. The head is extremely broad; the carapace length averages only 4.4 times the tympanic head width, and older females may have massive heads. It is dark grayish brown above, yellow or cream below; the area of demarcation is indistinct. The tympanum and posterior part of the lower jaw are yellow with a few gray blotches and orange spots. Jaws are grayish yellow; the iris is brown tan. Dorsally, the head is covered with large distinct scales. There are two chin barbels. The neck is grayish brown dorsally, yellow ventrally, and has a few scattered blunt, conical tubercles on the dorsal surface. Limbs are gray on the outside, yellow beneath, and covered with large scales. Large conical tubercles are present on the inside of each thigh.
The karyotype is 2n = 48 (Rhodin et al., 1984).
Females are larger and more domed; males have slightly concave plastra and longer, thicker tails.

Distribution
Acanthochelys macrocephala is known from the upper Rio Mamoré drainage of central Bolivia, the Pantanal region and other swamplands of the upper Rio Paraguay drainage in southwestern Mato Grosso, Brazil, to the Chaco of Paraguay. Its range in Bolivia may be more extended.

Geographic Variation
Unknown. This species is most closely related to Acanthochelys radiolata; it includes Phrynops chacoensis Fritz and Pauler, 1992b (Fritz and Pauler, 1999).

Habitat
Acanthochelys macrocephala inhabits marshes, swamps, and slow-flowing streams.

Natural History
Nesting starts near the end of the wet season, in April-May, and hatchlings probably emerge between December and March (Cintra and Yamashita, 1989). Clutch size ranges from 4-8 white, rounded (28-32.5 x 25.8-31 mm; 11-20 g), hard-shelled eggs (Cintra and Yamashita, 1989). Incubation takes over six months; hatchlings (about 3.8 mm) have orange-red spots on carapace, plastron, and sides of the neck (Cintra and Yamashita, 1989).
Snails form a large part of this turtle's diet.

IUCN Red List Status (1996)
Lower risk: near threatened. The Chaco population, listed separately as Acanthochelys chacoensis, is considered Vulnerable (B1+2a).