The brown or black carapace is rather oblong (to 44.9 cm), with a straight (rather than rounded) anterior rim, almost parallel sides, and a heavily serrated, rounded posterior rim. Lateral marginals are somewhat serrated and downturned. A small cervical scute is present. Vertebrals 1-4 are broader than long in juveniles, and the 5th is longer than broad; however, with age the 2nd to 4th lengthen to become about as long as broad or slightly longer than broad. Vertebral 1 is anteriorly expanded; the 5th is expanded in the rear. Beneath the vertebrals are a series of seven (sometimes eight) neurals, and the 8th pair of costals meet medially behind the neurals. Three knobby longitudinal keels are present; one extending along each row of pleural scutes and a medial keel along the vertebrals. The highest knobs on these are those most posterior. All carapacial scutes are rugose with growth annuli, providing good places of attachment for algal filaments. The hingeless, narrow plastron is somewhat truncated anteriorly and deeply notched posteriorly. Its bridge is relatively narrow, giving the plastron a crosslike appearance; however, both the axillary and inguinal buttresses are well-developed. The intergular scute does not always separate the gulars, which often meet behind it. If the intergular is elongated and separates the shortened gulars, the plastral formula is: fem > pect > abd > hum > an >< gul > intergul, or intergul > hum > an > gul. The axillary and inguinal scutes are indistinct. Plastron and bridge are cream to yellow or brown. The triangular-shaped head is large and extremely flattened with numerous tubercles and cutaneous flaps, a long, tubular snout, and an upper jaw lacking either a medial notch or hook. Nasal bones are absent, so prefrontals form much of the rims of the external nares. The prefrontals do not meet at the midline. The temporal arch is moderate and formed by the squamosal and parietal bones; there is no supraoccipital or quadrate-parietal contact. Medial parts of the jugal and postorbital are entirely on the surface of the skull, and face more laterally than posteriorly. The tympanic cavity is extended laterally. The reduced jaws are weak and ridgeless, but there is a slight ridge on the palatine. The anterior pterygoid process prevents the palatine from reaching the vomer. Two conical chin barbels are present, and two other filamentous barbels occur at the angles of the jaws. Head, neck, limbs, and tail of adults are grayish brown. The neck is longer than the vertebral column, and there is a lateral fringe of small cutaneous flaps on each side. Each forefoot has five claws and all toes are webbed.
The karyotype is 2n = 50 (22 macrochromosomes, 28 microchromosomes). Pairs 1-3 are biarmed and there are 3 acrocentric and 5 biarmed chromosomes among pairs 4-11 (Barros et al., 1976; Bull and Legler, 1980).
Males have concave plastra and longer, thicker tails than do females.
Chelus fimbriatus occurs in most major drainages in northern Bolivia, eastern Peru, Ecuador, eastern Colombia, Venezuela, the Guianas, (probably) Surinam, and northern and central Brazil; unconfirmed reports from the Lake Maracaibo basin (Venezuela) are known (Pritchard and Trebbau, 1984). It has also been found on Trinidad, apparently washed there by mainland floods.
Color variation exists between populations, and Schmidt (1966) thought that various morphological characters varied enough over the range to designate separate subspecies, but has not published adequate descriptions nor given names to these forms. Sanchez-Villagra et al. (1995) found much variation in the plastral formula and in the shape and size of the intergular scute, and suggested that Amazon matamatas differ from those from the Orinoco watershed in the shape of the carapace, plastral pigmentation, coloration on the underside of the neck, and allometric growth of the carapace in relation to the plastron.
Chelus fimbriatus prefers slow-moving, blackwater streams, oxbows, muddy lakes, stagnant pools, marshes, and swamps where it remains at shallow depths which enable it to push its snout just above the surface for a breath of air.
Carpenter and Ferguson (1977) reported the male courtship act consists of a rapid forward lunging of the head toward the female with a gaping and opening and closing of the large mouth, a moving of the lateral cutaneous flaps on the head, and a hyperextension of the legs from the shell.
Nesting occurs from October through December in the Upper Amazon, and 12 to 28 eggs may be deposited in one clutch. Eggs are almost spherical, about 35 mm (34-37.5) in diameter, with brittle shells. Hatchlings are more colorful than adults; their carapace is light brown with a black line along the medial keel, an orange to pink blotch at the outer edge of each marginal scute, and a dark-brown to black spot on each pleural at the posterior seam. The surface of each pleural and vertebral is quite wrinkled. The plastron, bridge, and undersides of the marginals are pink with dark seams. The head is tan with three narrow, dark, dorsal stripes extending onto the neck, the medial being the shortest. Three wide pink to red stripes extend from the neck along the venter almost to the chin. The barbels at the corner of the mouth are brown, while those on the chin are pink. The limbs are tan on the outside and pink with small black flecks on the inner surface. The limb sockets are tan with some dark flecking, and a large dark blotch may occur between the neck and foreleg. The tail is tan dorsally and dark brown ventrally.
Chelus fimbriatus is carnivorous, feeding on aquatic invertebrates and fish. Its bizarre structure and the ease with which algae attach to the shell allow it to occupy the role of a wait-and-ambush predator. The loose flaps of skin on its head and neck sway like tufts of aquatic vegetation, attracting small animals within reach; then the head and neck are thrust forward accompanied by a rapid distension of the throat and a gaping of the huge mouth. The prey is sucked into the mouth by the low pressure created. Excess water is then expelled, but the prey swallowed. The jaws are weak and the mandibles are not attached medially, so the turtle has difficulty chewing prey. The hyoid apparatus and well-developed neck musculature play an active role in this gape-and-suck method of feeding. However, this may not be the only feeding strategy used by some individuals. Holmstrom (1978) observed captive adult C. fimbriatus slowly herd fish into shallow water, trap and eat them. This may be an important alternate feeding strategy, especially for juveniles; however, Wise et al. (1989) failed to find evidence of such behavior during a thorough laboratory study of matamata feeding habits. Perhaps herding behavior is an individualized trait.
The matamata is able to hold its breath for long periods, allowing it to remain motionless on the bottom, mimicking an algal-covered rock. When it does move, Chelus fimbriatus creeps slowly about the bottom rather than actively swimming. Juveniles can swim poorly, but possibly adults are too ponderous to swim. It apparently seldom, if ever, basks.
IUCN Red List Status (1996)