Rhoetosaurus brownei, a primitive sauropod from the Middle Jurassic of Queensland, is one of the largest and most complete Australian dinosaurs known. It is also one of the oldest sauropods, dating from a time when Australia was at high latitudes and closely connected to other Gondwanan continents. It is the only Australian sauropod from before the Cretaceous period, making Rhoetosaurus key to understanding global relationships of the continent's early dinosaur faunas.
Rhoetosaurus brownei was of average build, up to 15 metres in length, and about 9 tonnes in weight. Named after the Greek giant Rhoetos, Rhoetosaurus would have been about the same size as Wintonotitan or Diamantinasaurus, titanosaurs from the Late Cretaceous Winton Formation of Queensland. Rhoetosaurus might have resembled the Late Jurassic Shunosaurus from China, with a comparatively short, clubbed tail.
Rhoetosaurus had several primitive features and was neither distinctive nor specialized, so it is hard to link it to known sauropod groups. Hollow cavities (pleurocoels) in its bones, however, suggest that Rhoetosaurus had at least some advanced features and that it was more specialized at least in this feature than many Jurassic sauropods. The elongate shape of the neck vertebrae suggests that the neck was relatively long for a sauropod of this period.
The following combination of features is seen only in Rhoetosaurus (from Long 2002): amphicoelous anterior vertebrae with solid, laterally compressed centra and expanded elliptical articular surfaces; elongated prezygapophyses with vertical articular surfaces; no postzygapophyses; well-developed hyposphene; robust neural spines that are not elongated; anterior neural spines that are subrectangular in lateral view, with an oval median recess on the posterior margin above the hyposphene junction ; massive anterior chevrons; a relatively large neural canal in anterior caudals; opisthocoelous dorsal vertebrae with lateral pleurocoels and complex neural arches with bracing laminae, small elevated zygapophyseal articulations and extensive intramural cavities.
During the Middle Jurassic, south-central Queensland was lushly vegetated, with forests of large araucarian conifers, an understorey of cycads, tree-ferns and lycopods, and a ground cover of smaller ferns and mosses. The climate was wet, humid and subtropical to warm, although the Roma area during the Middle Jurassic was at high latitudes (about 50º south). The remains of Rhoetosaurus were found in what had been a large, sandy braided river channel..
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Rhoetosaurus was described from the Middle Jurassic Hutton Sandstone near Roma in south-central Queensland, and the material currently under study comes from this area. However, a vertebral centrum from Western Australia near Geraldton is similar in form to a distal caudal vertebra of Rhoetosaurus and may be either from this species or a related species.
Feeding and diet
Rhoetosaurus, like all sauropods, was probably a browsing herbivore. It may have eaten araucarian conifers, seed ferns and ferns, all known from the Jurassic of Queensland. Rhoetosaurus could have reaching a walking speed of up to 15 km per hour (up to 20-30 km a day) based on calculations using its limb measurements.
The partial skeleton of Rhoetosaurus brownei was collected from Durham Downs, Eurombah Creek, Taloona Station, near the town of Roma in south-central Queensland (Injune Creek Beds; Hutton Sandstone Formation). It was brought to the attention of the Queensland Museum by station manager Arthur Browne and described by Heber Longman of the Queensland Museum in 1926 (who honoured Browne by giving Rhoetosaurus the species name brownei). The type material, collected in 1924 by Longman, includes one and a half neck vertebrae, trunk vertebrae and ribs, a complete hind limb and half of the tail. Further excavations in 1976 unearthed more of the original skeleton, including the lower part of the right hind leg and an almost complete foot skeleton. The skull and front limbs are unknown.
Rhoetosaurus was first described as a cetiosaurid, Jurassic sauropods basal to Eusauropoda known from both Laurasia and Gondwana. Other suggestions include a possible relationship to Shunosaurus from the Jurassic of China, although this does not seem to be well supported. Although Rhoetosaurus is comparatively complete, the lack of definitive, diagnostic characters makes it difficult to place within a known sauropod group. Many of its characters are primitive (‘plesiomorphic’) and are of little use in determining its relationships. Rhoetosaurus has opisthocoelous dorsal vertebrae with well-developed pleurocoels, which may place it closer to Neosauropoda than to cetiosaurids or basal eusauropods. Rhoetosaurus, originally described in 1926 before the development of modern systematics, is currently the subject of a long-overdue revision.
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