Fostoria: the Australian dinosaur preserved in opal

Fostoria dhimbangunmal is an Australian iguanodont whose fossils were transformed into opal. Material from an underground mine near Lightning Ridge represents at least four animals and includes parts of much of the skeleton. The site is the first recognised ornithopod-dominated bone bed in Australia. It is an unusual fossil discovery, but the bones are still fragmentary and do not amount to one complete articulated dinosaur.

Quick facts

Robert Foster and the Sheepyard field

Opal miner Robert Foster discovered the bones in the 1980s at the Sheepyard opal field in New South Wales. Mechanical mining had broken the material into many fragments, as often happens when opal-bearing seams are excavated. The fossils were shown to the public in Sydney for a time, then transferred and eventually donated to the Australian Opal Centre.

The genus name Fostoria honours Robert Foster. The species name dhimbangunmal combines words meaning “sheep yard” in the Yuwaalaraay, Yuwaalayaay and Gamilaraay languages, preserving the local name of the opal field. Phil Bell and colleagues formally described the dinosaur in 2019. The holotype, LRF 3050.A, is a partial skull roof and braincase. Other bones catalogued with the site include a quadrate, shoulder blades, limb elements, pelvis, vertebrae and foot bones; they do not necessarily all belong to one individual.

At least four animals in one bed

The minimum number of individuals was inferred from repeated shoulder blades of different sizes. Three right scapulae and one left scapula first indicate at least three animals; the smaller left scapula supports the presence of a fourth. Many of the other bones have similar proportions and may represent one larger animal, but the deposit contains repeated elements and a mixed assemblage.

The bone bed is nearly monodominant. Alongside Fostoria, researchers reported one turtle vertebra and isolated theropod bones. The ornithopod remains show little evidence of heavy weathering or prolonged abrasion. Some carcasses may have remained exposed briefly, broken apart and had large bones damaged by trampling before final burial. Those are interpretations of the traces on the fossils, not a witnessed scene.

Several animals of one species buried together can be consistent with herd behaviour, but other processes can also gather bones: water flow, local topography or repeated use of a place. A bone bed demonstrates a shared burial context, not by itself a stable social group in life. The anatomy and depositional evidence have to be considered separately.

How bone became opal

Fostoria came from the upper part of the Cenomanian Griman Creek Formation, roughly 100–96 million years old. At the time, the region lay near the eastern margin of the inland Eromanga Sea. Rivers spread sediment across coastal lowlands where dinosaurs, turtles and crocodylomorphs lived.

After burial, original bone material was dissolved and silica filled the spaces, later forming common opal and, in some cases, precious opal. The fossils are not ordinary bones with a thin decorative coating: substantial portions of their original material have been replaced by mineral while preserving the external anatomical shape. That replacement makes the fossils striking and also changes which laboratory methods can be used.

Because the opalised material cannot be sampled with conventional bone histology in the same way, researchers could not use growth rings to determine each animal’s age precisely. Computed tomography, however, allowed hidden surfaces inside some blocks to be examined. The special preservation creates both a research opportunity and a constraint.

Skull anatomy and classification

The diagnosis of Fostoria relies on several features of the frontal bone and other elements. In top view, the side margin of the frontal has a stepped outline; a stout projection sits at its anterolateral corner. These are features of preserved bone, rather than facial details invented for a reconstruction.

The first phylogenetic analysis placed Fostoria near a group of southern iguanodonts that included Anabisetia, Talenkauen and Muttaburrasaurus. Later classifications discuss it among rhabdodontomorphs. Its position may change as Gondwanan iguanodonts remain unevenly sampled and different character matrices produce different trees. The skull is partial, and some anatomical regions are not preserved well enough to compare.

Fostoria and Muttaburrasaurus are two named Australian iguanodonts from regions near the Eromanga Sea. They come from different formations and have different anatomy. The unusual convex muzzle of Muttaburrasaurus cannot simply be transferred to Fostoria, whose front skull is not preserved.

Growth, movement and feeding

The limb bones indicate a sizeable ornithopod capable of moving on its hind legs and probably using its forelimbs for support when walking slowly. No complete articulated skeleton is known, so length estimates rely on comparison rather than direct measurement. Fostoria was larger than small Australian ornithopods such as Leaellynasaura, but it did not reach the scale of later hadrosaurs.

As an iguanodont, it was herbivorous. A beak cut vegetation and cheek teeth processed it with repeated jaw movements. Specific plants and feeding height have not been established, because gut contents and a complete tooth row are absent. The fossils tell us more securely about the general feeding anatomy than about a precise menu.

A later CT model of part of the brain cavity suggests a relatively broad olfactory region. That may indicate a well-developed sense of smell, but the front, rear and lower portions of the endocast are missing and the skull is compressed. It is not evidence for a particular smell, behaviour, herd hierarchy or intelligence level.

Predators and reconstructed scenes

The Griman Creek Formation also contains bones of large theropods, including megaraptorids. Australovenator lived earlier and in another Australian formation, so a direct encounter between those two genera is not supported. No described bite marks on Fostoria bones identify a particular attacker.

The secure outline is that of a robust ornithopod with strong hind limbs, a tail and a toothless beak at the front of the jaws. The complete muzzle, skin pattern, colour and social behaviour are reconstructed from relatives and general context. Explore the fossils alongside other ornithopods in the dinosaur catalogue. Fostoria stands out because opal preservation, several individuals and partial skull material occur together in one Australian discovery.