Australia: The Land Where Time Began

A biography of the Australian continent 

Minmi, southern Queensland, Early Cretaceous marine sediments

Minmi parvavertebra, from this deposit, was an Ankylosaur, armoured dinosaur. It was small for an Ankylosaur at less than 4 m long. It was unusual for an ankylosaur in another way, its back was lightly armoured, but its belly was better armoured. It is assumed that as it lacked strong back armour it probably had few if any dangerous predators, possibly living on an island. It has been suggested that it may have been able to run fast enough to escape predators, in which case the lighter armour would have helped.

The type specimen was found by Alan Bartholomai in the Minmi Member, the Bungil Formation, south of Mack Gallery to the north of Roma, south central Queensland. The genus name derives from Minmi Crossing, near which it was found. There were 11 back vertebrae with some associated ribs, a partial right foot and much of the body armour from the ventral surface. The partially articulated bones were in limey concretions. On Marathon Station, to the east of Richmond, an almost complete skeleton was found in limey shales that is one of the most complete ankylosaur skeletons found in the world, that included a well-preserved skull and much of the body dermal armour. At 2.5-3.5 m long it is believed to be of a newly mature individual. It is believed to possibly have been a mummified carcass that was washed into the sea during a flood.

A unique feature among dinosaurs present in this genus is vertebrae in which paravertebrae (bony elements) occur along side the neural spine of the vertebrae to support the bony armour plates on the back. Many 5 mm wide ossicles (scutes) were present on the belly. It had larger scutes on the back, the largest, 13 x 9 cm, being on the neck, hip and tail base. Over most of the body it had rows of medium-sized scutes, in between which were masses of small scutes. The skull is almost pentagonal in dorsal view, and one of the best of any known ankylosaur. Though incomplete it is the most complete ankylosaur skeletons in the Southern Hemisphere. Anklyosaurs first appeared in the fossil record in the Neocomian, early Cretaceous, and soon after it was apparently widespread around the world. In the Queensland faunas there are believed to be possibly 2 species, 1 in the type locality and 1 in the Toolebuc Formation and the Allaru Mudstone (Molnar). Most of the known ankylosaurs are from the last 10 million years of the Cretaceous from Asia and North America. It has been suggested by Molnar that in essence it is probably an ankylosaur. Being an early form from Gondwana has suggested to some it may be an entirely different group of armoured dinosaurs (Long, 1998).

The paravertebral elements are the most characteristic element of Minmi, the dorsal vertebrae are amphiplatan with no notochordal knobs, it has slender triangular processes, not T-shaped, with a broad neural canal and a shallow posterior intervertebral notch. There are 7-9 well-pronounced denticles on the teeth, of which one side is mostly smooth and the other furrowed. Pavements of small ossicles formed the ventral armour. Among the features Minmi shares with ankylosaurs are snout arches that are higher than the skull. It also has many primitive features that  would be expected in a thyreophoran that was ancestral. Among the primitive features seen in Minmi are a femur that is rounded in section and a skull that is narrower than long. In the ilium, the acetabular and postacetabular regions are long (Molnar, 1994).

Sources & Further reading

Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.

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                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading