Rhodin and Mittermeier, 1983
Williams' side-necked turtle
This species is most closely related to Phrynops geoffroanus and P. hilarii. Its oval, moderately domed carapace (to 35.5 cm) is broadest at the center, and the entire marginal rim is weakly serrated. A small supracaudal notch is also present. Vertebral 1 is largest; all vertebrals are broader than long. No keel or medial groove occurs along vertebrals 1-4 in adults, but a slight raised area is found posteriorly on the 5th. The cervical scute is about twice as long as broad. The carapace is brown with radiating, narrow black stripes on all scutes, and a narrow yellow or orange border. The unmarked gray to yellowish plastron is well-developed and has an anal notch. Its forelobe is slightly broader than the hindlobe, and the bridge is relatively wide. The plastral formula is: fem > intergul >< an > abd >< pect >< hum > gul; the intergular completely separates the gulars. Unlike that of other members of the subgenus Phrynops, the short, broad intergular is not nearly as long as the combined lengths of the interhumeral and interpectoral seams. The moderately narrow head has a projecting snout, an upper jaw lacking a notch or hook, and two pale chin barbels. Skin on top of the snout, interorbital region, and middle third of the dorsal surface of the head is incompletely divided into scales. The rest of the dorsal surface of the head is covered with small irregularly shaped scales. The head is black dorsally with several narrow reddish stripes in juveniles, and the ventral surface of the chin and neck is usually white but may be reddish yellow or yellow. Three subparallel broad black stripes occur on the sides of the head: (1) a dorsal stripe extending from the nostrils through the orbit and upper part of the tympanum onto the neck; (2) a medial stripe passing posteriorly from the angle of the jaws along the lower edge of the tympanum and onto the ventrolateral surface of the neck; and (3) a ventral stripe forming a posteriorly directed horseshoe-shaped mark on the lower surface of the chin, from which the contrasting pale barbels protrude. The jaws are dark, though the recessed mandible is normally concealed, and the white portion of the chin thus contrasts sharply with the dark mandibular band. The skull differs from other members of the subgenus Phrynops in having a wide parietal roof, a short, thick temporal arch, no exoccipital contact above the foramen magnum, widely divergent trochlear processes, broad anterior maxillary triturating surfaces with short ridges, and a shovel-shaped mandible. The short neck may contain a few well-defined blunt tubercles. The black to brown dorsal surface of the limbs has an indistinct vermicular pattern; the undersides are reddish yellow or yellow, and the toe webbing may be reddish. Prominent pre-tibial spurs help distinguish this turtle from its congeners; these may be an adaptation to climbing slippery substrate in the flowing waters it inhabits (James R. Buskirk, pers. comm.).
Sexual dimorphism is not apparent in the type series (Rhodin and Mittermeier, 1983), but James R. Buskirk (pers. comm.) reports that the tail of the female is narrower than that of the male, with the vent closer to the body. Females probably grow larger than males; the largest male (MZVC-R 3663) in the collection of the Museo de Zoología Vertebratos, Montevideo has a carapace length of 29.4 cm, whereas the largest known specimen, a female (MZVC-R 2337), measures 35.5 cm (James R. Buskirk, pers. comm.).
Phrynops williamsi occurs in Santa Catarina and Rio Grande do Sul in southern Brazil, and in adjacent Uruguay, Paraguay and northeastern Argentina.
This turtle apparently prefers rocky forest streams. Buskirk (1989b) saw no specimens in an oxbow containing Hydromedusa tectifera and Trachemys dorbignyi adjacent to the Yaguaron River on the Uruguay-Brazil border, but found them locally in this river.
Rhodin and Mittermeier (1983) reported that a 25 cm female contained nine white, oval, brittle-shelled oviducal eggs (32.9-34.2 x 26.7-27.6 mm), and thought that nesting probably occurs in either November or December. A 35 mm hatchling (still with caruncle and yolk remnant, and with a laterally compressed carapace) was found walking on sand on 24 June. This neonate may have been en route from its nest to the nearest water (Buskirk, 1989b).
Little is known of the feeding habits of Phrynops williamsi, but its jaw modifications may be adaptations for bottom feeding and for crushing mollusks. A juvenile was observed, against a stiff current, climbing an algae-covered rock in which its snout was imbedded, as if in search of tiny prey. One freshly caught adult expelled two large, dark fecal pellets, containing mostly sand and minute particulate matter, probably insect remains (Buskirk, 1989b).
IUCN Red List Status (1996)