(Gray, 1841)
Australian big-headed turtle
Recognition
The oval carapace (to 30 cm) is broadest at the center, has a smooth or only slightly serrated posterior rim, and a medial keel which is best developed in the male, but becomes lower with age. Neural bones are absent, but a well-developed, cervical scute is present. Vertebrals 1 and 5 are broader than long with the 5th expanded posteriorly; vertebrals 2-4 may be as long as or longer than broad in adults. Surfaces of the carapacial scutes are often rugose with various longitudinal striations. The carapace is uniformly light to dark brown. The long plastron is narrow and much of the carapacial opening is uncovered. Its forelobe is rounded anteriorly and is broader than the posterior lobe which gradually tapers to the rear and is posteriorly notched. The bridge is narrow. The intergular scute separates the gulars, but is less than twice as long as broad. The plastral formula is: pect > fem > abd > an > intergul > gul > hum. Both plastron and bridge are cream to yellow. The head is broad with a slightly projecting snout and an unnotched upper jaw. The hard palate is very broad and extends backward beyond the maxillae covering over half of the roof of the mouth (the palate is not as extensively developed in other species of Emydura). Also, the diameter of the mandibular symphysis is very broad, much more so than the greatest diameter of the orbit. Only rudimentary chin barbels are present, at best. The dorsal surface of the head is covered with smooth skin, and the neck tubercles are poorly developed. Head, neck, and limbs are gray to olive or brown. On each side of the head are two light stripes: one extends from the orbit to the neck and the other from the corner of the mouth to the neck. In juveniles these stripes are pinkish, but they become more yellow with age.
The karyotype is 2n = 50 (Bull and Legler, 1980).
Males have longer, thicker tails than do females.
Distribution
Emydura australis occurs in northern Australia from Cape York peninsula, Queensland to Arnhem Land, Northern Territory.
Geographic Variation
An allozyme study by Georges and Adams (1996) revealed that Emydura australis as currently recognized actually represents two distinct yellow-faced forms. A population restricted to the Daly River system is related to E. victoriae, but is distinguishable from it morphologically (yellow-faced, a dark marking through the eye, and a median division in the palate) and electrophoretically. Cann (1997b) very recently described this form as Emydura tanybaraga, the northern yellow-faced turtle. The other form, ranging from the Jardine River in northern Queensland westward to the Daly River in northwestern Northern Territory, is related to Emydura subglobosa, and it is indistinguishable from it electrophoretically. E. subglobosa is traditionally regarded as having a shell, neck and limbs suffused with red; such a form is found in Australia only at the tip of Cape York, Queensland. The yellow-faced form lacks the red suffusing, but this may be diet related (Arthur Georges, pers. comm.). Until such time as a morphological study is undertaken, and shows the contrary to be true, Georges regards the second yellow-faced turtle to be E. subglobosa. He states that at the very best, it might be regarded as a subspecies, in which case it would be Emydura subglobosa worrelli. The 'painted' form would be Emydura subglobosa subglobosa, restricted to New Guinea and the tip of Cape York.
Habitat
Emydura australis lives in shallow rivers and streams.
Natural History
Worrell (in Pritchard, 1979) reported that copulation occurs while in a plastron-to-plastron position, very unusual for a turtle. Hatchlings have pinkish red facial stripes and some red on the neck, limbs, and plastron. Their carapaces are peaked with a prominent medial keel and somewhat serrated posteriorly. The broad adult head develops with age.
The well-developed hard palate is apparently an adaptation for crushing molluscan shells; freshwater snails and bivalves form an important part of the diet. Emydura australis also feeds on insects, amphibians, and some fruits.
Remark
Based on incorrect locality data of the holotype, Cogger et al. (1983) proposed Emydura australis to be synonymized with E. macquarrii.
IUCN Red List Status (1996)
Not listed.