Lower Cretaceous Plesiosaurs

Australia: The Land Where Time Began

Lower Cretaceous Plesiosaurs

In Australian deposits from the Lower Cretaceous there are many plesiosaur fossils that are often very well preserved. Fossils from 5 known families have been found – Rhomaleosauridae, Pliosauridae, Aristonectidae, Elasmosauridae and Polycotylidae. Deposits in the Eromanga Basin in central and eastern Australia of Aptian-Albian age have produced the most complete fossils of this group. Sediments in the Carnarvon Basin, Western Australia have produced the earliest known fossils of the group. In the Barrow Group, Fluviatile deposits of Berriasian age near Exmouth, a small rhomaleosaurid plesiosaur, of uncertain affinity, is represented by an isolated cervical vertebra. The Birdrong Sandstone, of Hauterivian–Barremian age near Kalbarri, Western Australia has also produced Rhomaleosaurids.

Throughout the Hauterivian-Aptian the near-shore marine habitats of Australia were dominated by long-necked primitive pliosauroids, the rhomaleosaurids. Previously fossils of rhomaleosaurids had been assigned to the Leptocleidus, a small-bodied Lower Cretaceous form from England, South Africa and Australia that appears to have had a global distribution. According to the authors^{1^,}recent studies have found that this genus has been used as an artificial ‘waste basket’ to which several genera that are closely related but distinct have been assigned. Leptocleidus fossils from the Birdrong Sandstone – several skeletons that are all fragmentary, have been retained in this genus provisionally as Leptocleidus clemai, as separate species. The authors¹ say this classification is tenuously based only on its larger size of about 3 m.

The Bulldog Shale at Coober Pedy and Andamooka of South Australia are the sites where rhomaleosaurids of Aptian age are best known, where a number of opalised skeletons have been found. Among these was one with a near-complete skull that has been assigned to Umoonasaurus demoscyllus that at 2.5 m was a small-bodied plesiosaur that had a unique feature of thin crests above the orbits along the midline of the skull. Umoonasaurus represents both the last remaining and the most primitive of the rhomaleosaurids. It is surprising that it is so archaic as it is from the Aptian, that is a relatively late age. It has been suggested that as Umoonasaurus was apparently restricted to the near-freezing water of the high-latitude inland seaway of Early Cretaceous Australia it might have had adaptations, such as heightened metabolic levels to cope with lower water temperature. In Australia during the Cretaceous a common phenomenon that is apparent in a number of lineages was survival of ancient lineages, as in Umoonasaurus. It has been suggested by the authors¹ that this might have been a result of isolation in regions of high-latitude low-temperatures.

In the Wonthaggi Formation and the Eumeralla Formation in Victoria, the Griman Creek Formation at Lightening Ridge, New South Wales and Surat, Queensland, isolated remains, mainly teeth, that are Rhomaleosaurid-like, have been described from non-marine strata of Aptian-Albian age. All these deposits were formed in cold high-latitude rivers, further evidence of the presence of plesiosaurs in low-temperature waterways that in the Early Cretaceous were near the South Pole.

In the Australian plesiosaur faunas of Aptian age s distinctive element was the presence of large-bodied pliosaurids that were predatory. From deposits in the Wallumbilla Formation in Queensland and New South Wales and the Bulldog Shale in South Australia many fragmentary remains have been found. In the Toolebuc Formation/Allaru Mudstone in Queensland the remains of large pliosaurids have been found. Kronosaurus is the genus to which this material is currently assigned, the remains from the Aptian remains representing a distinctive species, Kronosaurus queenslandiscus. This was a very large pliosaurids that is believed to have possibly reached a length of more than 10 m. Conical teeth up to 150 mm in height that were coarsely striated, have been found in isolated skulls that were about 2.5 m long. It has been suggested that Kronosaurus may have been an ambush predator that struck from below as the orbits in its characteristically flattened skull were upward-facing. Gut contents that have been found in association with fossil specimens of Kronosaurus indicate a diet of marine reptiles such as elasmosaurid plesiosaurs and large sharks.

In the Bulldog Shale of Coober Pedy and Andamooka, deposits of Aptian age, a rare plesiosaur group has been found that is found only in these deposits, the aristonectids. These were long-necked plesiosaurids that have been allied to both the elasmosaurids and the cryptoclidoids from the Jurassic. From Upper Jurassic deposits aristonectids are also known from Europe (England) and North America, and from high-latitude sediments from the Upper Cretaceous of Antarctica, New Zealand and Patagonia. Aristonectids from the Late Cretaceous have been suggested to have been adapted as low-temperature specialists. Opallionectes andamookaensis is the only known Australian aristonectid. It was named from a single opalised skeleton that was missing the skull. The teeth in the skulls of this species were small and needle-like, leading to the suggestion that they were filter feeders. The characteristics of Opallionectes were a mixture of primitive and derived. This mixture of characteristics places them as intermediate between the aristonectids of the Late Jurassic in the Northern Hemisphere and the advanced southern polar taxa from the late cretaceous.

A distinct reorganisation is believed to have occurred among plesiosaur faunas in the Albian in which the rhomaleosaurids were replaced by the elasmosaurids, plesiosaurs with very long necks from the Cretaceous, as the most ubiquitous group of marine reptiles in Australia. From the Aptian to the Albian elasmosaurids of Australia are extremely widespread, both geographically and stratigraphically, having been found in the Rolling Downs Group, Queensland, Wallumbilla Formation, New South Wales, the Bulldog Shale and the overlying Oodnadatta Formation, South Australia and the Darwin Formation in the Northern Territory. Apart from the skull of Eromangasaurus australis that is almost complete, from the Toolebuc Formation of upper Albian age near Maxwelton, central-northern Queensland, most of the material from the above sites is indeterminable. Several distinctive features of Eromangasaurus indicate possible close relationship with elasmosaurs found in the Americas. Evidence of having been bitten and crushed by a large predator is seen in the holotype skull of Eromangasaurus. The unknown predator has been assumed to be Kronosaurus based on the size and spacing the huge toothmarks.

In the plesiosaur fauna of Australia a rare component is the polycotylids from the Early Cretaceous of Australia. This is considered surprising as they are primarily a Northern Hemisphere group from the Late Cretaceous, Cenomanian-Masstrichtian, that have been found in North America, South America, Europe, North Africa and Japan. An opalised skeleton of a polycotylid from the Wallumbilla Formation at White Cliffs, New South Wales is the oldest known member of the family. In the Toolebuc Formation in central Queensland very well preserved fossil polycotylids of upper Albian age have been found. They represent a new un-named species that is a primitive polycotylid related to Northern Hemisphere taxa from the Late Cretaceous.

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