RHYTIDOSTEIDAE

Australia: The Land Where Time Began

RHYTIDOSTEIDAE

These are labyrinthodonts that are difficult to characterise. The skull was broad, pointed at the front, with eyes near the front of the skull. They are distinguished from other temnospondyls by a set of anatomical features - a short pterygoid-parasphenoid suture; exoccipitals not underplated by parasphenoid; flattened elliptical articulating surfaces on the condyles; the quadrate ramus of the pterygoid is narrow; a raised dental series on the palatal series, the parasphenoid has a broad, flat cultriform process.

Acerastea wadeae from The Crater, the Arcadia Formation, Rewan Group, 72 km SW of Rolleston, south central Queensland.

The type specimen is poorly preserved, it is estimated to have been about 30 cm long, indicating a size range of 1.5-2 m for the animal. The site it was found in is several metres from the sites where 4 other amphibians were found - Rewana, Parotosuchus rewanensis and P. gunganj. The specimen was reconstructed from several large fragments, showing the outline of the head and the relative sizes of the eyes and some of the body. The presence of some post-cranial remains is a rare occurrence in amphibian palaeontology in Australia. Stones believed to be gastroliths, stomach stones, were associated with the skeleton. Another suggestion is that they may have been used as ballast when the animal was diving.

The characteristic features of the genus are the deeply concave rear margin of the skull, no tabular horns, reduced post-temporal fenestra, and a flange traversing the posteriolateral parts of the tabular and squamosal bones. There are well-developed pleurocentra of the vertebrae and elongated neural arches, and advanced ossification of the scapulocoracoid. Ventral ribs are present across the belly.

Arcadia myriadens from the Arcadia Formation, Rewan Group, sandstone near the headwaters of Duckworth Creek, southwest of the township of Bluff in south-central Queensland.

The skull is 19 cm long by 17 cm wide. The description of this genus led to the a proposed regrouping of the temnospondyl amphibians and a new hypothesis of the relationship of the Triassic temnospondyls. The characteristic features of this genus are - the skull is slightly longer than wide and the anterior position of the eyes, the articular end of the lower jaw is unusually complex, the anterior palatal vacuity with small processes pointing towards the midline. Some have suggested that Arcadia is closely related to Rewana because of unique features in the lower jaw.

Nanolania anatopretia, is described from the Early Triassic Arcadia Formation (Rewan Group) of Queensland. Some characters include the absence of a lacrimal, an untwisted quadrate ramus of the pterygoid, and a shallow otic notch. It is tentatively referred to the Rhytidosteidae. N. anatopretia is the third temnospondyl taxon from the Arcadia Formation to be represented by a skull less than 50 mm long. The others are the basal stereospondyl Lapillopsis nana, and juveniles of the capitosaurid Parotosuchus aliciae. Given that Lapillopsis nana and Nanolania anatopretia are not known from any larger specimens, and that the temnospondyl fauna from the Arcadia Formation has been well sampled, it is suggested that they are species that never grew large. It seems quite likely that a diverse array of small temnospondyls lived alongside the larger temnospondyls of the Triassic elsewhere in the world but have not been discovered due to biases working against the preservation and collection of small temnospondyl specimens. See link 1

Deltasaurus kimberleyensis from the Blina Shale, Erskine Ranges, 90 km east of Derby, Western Australia, and from sites in Tasmania - the Cluan Formation, Poatina Road near Launceston; The Knocklofty Formation, Midway Point & Cunningham localities near Hobart.

The type specimen of the genus was described from a complete left side of the skull. Many bones and fragments of this species have also been found, including well-preserved lower jaws. A partial skull and isolated bones of this species have been found in the Knocklofty Formation in Tasmania, indicating that the species had a large geographic range. The skull was 24 cm long, indicating a body length of about 1.25 m. The head was strongly triangular, moderately deep, and had small eyes. The skull roof bones are typical for labyrinthodonts.

Deltasaurus pustulatus from the Kockatea Shale at the Bore Hole near Geraldton, Western Australia.

This species was described from an imperfect skull found in a drill core near Geraldton, WA, 806 m below the surface, so excavation of the site is not likely. The drill cut section of the head that included most of the skull roof and the right side of the cheek, enough material to recognise a new species.

The species was smaller than D. kimberleyensis, the bone surface was covered with rows of wart-like pustules, and a slightly narrower skull,

Derwentia warreni from The Knocklofty Formation, Midway Point & Cunningham localities near Hobart, Tasmania

The type specimen is an almost complete skull about 9 cm long. It had abrasions along the edges that occurred before burial. It had small teeth, indicating it was probably a fish eater. It had large eyes towards the front of the skull. The skull roof is irregularly bumpy, the individual bones being raised in their centres. The exposed parts of the tabular bones are longer than the exposed parts of the postparietal bones. It is unusual for a rhytidosteid, having its eyes set close to the front of the skull, and the rounded sides of the skull, and larger tabular horns. It has been suggested that it is closely related to Deltasaurus because of a similar skull shape, dermal ornamentation and lateral line canal grooves.

Rewana quadricuneata from The Crater, the Arcadia Formation, Rewan Group, 72 km SW of Rolleston, south central Queensland.

The skull of the type specimen was collected as fragments that form most of the skull. It is about 18 cm long. Along with the skull some post-cranial bones were also found, they include some limb bones, vertebrae, ribs and fragments of the pelvic girdle. It would have been about 1 m long.

The length and width of the skull are about the same, its small round orbits are situated on the skull roof close to the front of the skull near its margins. It is distinguished from other Australian broad-headed labyrinthodonts in having a slightly pointed snout. The unusual feature of the spine is that each vertebra has 6 parts - left and right sides of the neural arch, intercentra and pleurocentra. In most temnospondyls the vertebrae have an intercentrum and 2 pleurocentra plus neural arches.

Sources & Further reading