Umoonasaurus demoscyllus was a small plesiosaur from the Early Cretaceous of South Australia. Its best-known specimen is the opalised holotype AM F99374, nicknamed Eric. The skull bears a tall midline crest and paired crests above the orbits; micro-CT scans later revealed fish vertebrae and rounded stones in the gastric region.
Quick facts
| Type species | Umoonasaurus demoscyllus Kear, Schroeder & Lee, 2006 |
|---|---|
| Age | Upper Aptian, Early Cretaceous |
| Type locality | Zorba Extension opal field, west of Coober Pedy, South Australia |
| Holotype | AM F99374, informally nicknamed Eric |
| Estimated length | About 2.5 m, estimated from the holotype |
| Current placement | Plesiosauria; conservatively placed in Leptocleidia |
Evidence guide
The holotype includes a skull with fragmentary mandible and teeth, articulated posterior vertebrae and parts of the limb girdles and limbs. It is unusually complete for an opalised vertebrate fossil, but it is not a complete skeleton.
These crests are visible on the preserved skull. Their shape is direct evidence; a display or hydrodynamic function remains a hypothesis rather than a result preserved in the bones.
A scan of the gastric region identified 17 vertebrae from an unidentified teleost and about 60 rounded gastroliths averaging around 5 mm. These contents come from one individual and do not define the entire diet of the species.
The 2006 description assigned Umoonasaurus to Rhomaleosauridae. Later analyses proposed Leptocleididae or Polycotylidae; a 2023 checklist used the broader clade Leptocleidia to reflect the uncertainty.
A fossil recovered from an opal mine
Opal miner Joe Vida discovered the bones in 1987 at the Zorba Extension field near Coober Pedy. The fossil came apart into many pieces, which required painstaking preparation. After a public fundraising appeal, the Australian Museum acquired AM F99374 in 1993. Benjamin Kear, Natalie Schroeder and Michael Lee named the new genus and species in 2006.
The genus name combines Umoona, an Antakirinja Indigenous name for the Coober Pedy area, with the Greek sauros, meaning lizard. The species name refers to the public donations that helped keep the holotype in Australia. Eric is an informal nickname, not part of the scientific name.
An opalised skeleton and a crested skull
Holotype AM F99374 preserves a skull with parts of the lower jaw and teeth, an articulated stretch of posterior cervical to caudal vertebrae, parts of the limb girdles, and limb bones. Kear and colleagues estimated the adult at about 2.5 m from this material. Because the skeleton is incomplete, the figure is a comparative estimate rather than a direct measurement of a complete animal.
The skull is short and triangular in outline. A high, blade-like crest runs along the midline, and a pair of arched crests rises above the orbits. These structures are directly preserved, but their function is not. The fossils do not reveal whether they were used for display, stabilisation or another role.
Stomach contents seen with micro-CT
In 2023, researchers scanned two mineral masses from the gastric region of the holotype. The scans revealed 17 vertebrae from an unidentified bony fish and approximately 60 rounded gastroliths with a mean length of about 5 mm. The fish could not be identified to species.
Fish vertebrae are direct evidence that at least one individual had swallowed a teleost. The stones were found in the same gastric region, but one specimen cannot provide a complete prey list or show how representative that meal was of the species as a whole.
Age and changing classification
The type locality is the Zorba Extension opal field west of Coober Pedy. Its fossils come from the Bulldog Shale within the Marree Subgroup of the Rolling Downs Group, in the Eromanga Basin. A 2023 Australian tetrapod checklist assigned the Coober Pedy opal-bearing deposits to the upper Aptian using associated fossils and stratigraphic correlation. This dates the rock unit, not the individual animal.
Kear and colleagues originally placed the genus in Rhomaleosauridae. Later studies have instead discussed Leptocleididae or Polycotylidae. The 2023 checklist used the broader clade Leptocleidia as a conservative placement. The genus Umoonasaurus remains in use, while its more precise relationships remain open to analysis.
What the fossil does not show
The holotype is remarkable, but it does not preserve every body region or soft tissue. It cannot establish the animal's original colour, exact swimming speed or social behaviour. The gastric contents reveal one meal and a set of stones in one individual, not the full ecological role of the species.
Plesiosaurs were marine reptiles, not dinosaurs. The Jurassic Eurycleidus and long-necked Elasmosaurus provide comparisons for plesiosaur diversity, but they do not establish direct ancestry for Umoonasaurus.
Frequently asked questions
Where was Umoonasaurus found?
Holotype AM F99374 came from the Zorba Extension opal field west of Coober Pedy in South Australia.
Why is the holotype called Eric?
Eric is an informal nickname used in museum and popular accounts. The scientific name is Umoonasaurus demoscyllus.
What do the stomach contents show?
Micro-CT revealed 17 vertebrae from an unidentified bony fish and about 60 rounded stones in the gastric region of the holotype. These are direct data from one individual.
How large was Umoonasaurus?
The original description estimated an adult length of about 2.5 m from the incomplete holotype. It is a comparative estimate, not a measurement of a fully preserved skeleton.