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Minmi paravertebra
Molnar 1980

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MinmiMinmi's head
Colourised Image

    From Minmi Crossing
    Ankylosauria, Thyreophora, Ornithischia
    Early Cretaceous (Aptian\Albian) 100-115 MYA
    Bungil Formation
    Referred specimens: Toolebuc Formation
    and Allaru Mudstone
    3 metres (10 feet) long
    Roma, Queensland

The holotype of Minmi paravertebra was discovered by Alan Bartholomai near Roma, Queensland, in 1964, and described by Ralph Molnar in 1980. The partially articulated remains consisted of eleven thorasic vertebrae with ribs, most of the ventral body armour and an incomplete hind foot. In 1990 another specimen was discovered on Marathon Station, Queensland, which has been designated as a Minmi sp. This specimen was almost complete, lacking only the right ilium, distal (lower) parts of the left limbs, and the distal end of the tail. It also included most of the body armour, consisting of large and small scutes on the body, large scutes on the limbs, plates around the neck, tear-drop shaped spiked plates behind the hips, and triangular plates and large scutes along the tail. The Marathon specimen, as well as a third partial ankylosaur found nearby, are yet to be completely prepared. At least five other individual fragments in the Queensland Museum have also been tentatively referred to Minmi, two of which may come from the same individual animal.

The holotype included two features unusual to ankylosaurs. The vertebrae had bony projections along side the neural spines called paravertebrae (hence the species name) which seem to be unique among dinosaurs. These seem to be ossified tendons rather than bones in their own right, and it may be that such tendons have simply not been preserved in other ankylosaurs, perhaps because only in Minmi were the tendons ossified to form bone. The other unique feature is seen in skin impressions of the belly skin, which show that even the underside was protected by small bony scutes imbedded in the skin. Most thyreophorans ("shield bearers", the group that includes ankylosaurs, nodosaurs, stegosaurs, and primitive ancestoral forms) lacked armour on the belly.

Minmi verterbae

One theory concerning the function of the paravertebrae suggests that they helped to reinforce the back and bear the weight of the armour to allow the creature to run. Given the small size (around 3 metres long) of the specimen, and the relatively long legs for an ankylosaur, this theory would make sense. Most ankylosaurs would have been too large and heavily armoured to have been able to run at any great speed. The structure of the paravertebrae in Minmi is similar to the tendons seen in the vertebrae of crocodilians, which serve to stiffen the croc's back so they can "gallop" like a rabbit when they are young (see the background image on the Australian Dinosaur Fossils page, based on a photo of a fresh water crocodile galloping).

The exact relationship between Minmi and other armored dinosaurs is uncertain. It seems to exhibit characteristics of both ankylosaurs and nodosaurs. Minmi has a long postacetabular region of the illium similar to that of scelidosaurs, whereas in ankylosaurs and stegosaurs it is much shorter. The region of the snout arches higher than the roof of the skull, a feature seen in nodosaurs. The pattern of the bones that make up the skull are more similar again to scelidosaurs than either ankylosaurs or nodosaurs. The femora of most ankylosaurs are flattened fore and aft, whereas the femur of Minmi is rounded in cross section, more like those of the ancestral thyreophorans. Minmi may turn out to be closer to ankylosaurs than to nodosaurs, although if so it seems to have split off from them early in their evolution. Similar Minmi-like material from New Zealand suggests that Minmi itself probably did not have a tail club, which is a feature common to almost all other Ankylosaurs.

If you had to design a creature that had to contend with velociraptors on a regular basis you might come up with something similar to Minmi. The armoured belly may have given them some protection from sharp retractable foot claws. The body armour lay mostly between the ribs, and if this was not an artefact of preservation then they too may have helped to ward off foot-claw attacks. The blade-like cross section of dromaeosaur sickle claws would have been well designed to slice between the ribs of prey to damage the internal organs. But of all the defences the best would have been to run away, since dromaeosaur were probably not as fast as most theropods. This is all speculation on my part of course. The spikes along the hips and tail would have prevented a small predator from leaping onto the hind quarters to slow the Minmi down, and may have discouraged attacks from above by larger predators.

Minmi display
The Minmi display at the Richmond Marine Fossil Museum in Queensland

Molnar, R.E. 1980 An ankylosaur (Ornithischia: Reptilia) from the Lower Cretaceous of southern Queensland. Memoirs of the Queensland Museum 20:65-75.

Molnar, R. E., and Frey, E. 1987 The paravertebral elements of the Australian ankylosaur Minmi (Reptilia: Ornithischia, Cretaceous). Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen 175: 19-37.

Molnar, R.E. 1994 Minmi: all tanked up and ready to grow. Dinonews 7:7-9. Western Australian Museum

Molnar, R.E. 1996 Preliminary report on a new ankylosaur from the Early Cretaceous of Queensland, Australia. Memoirs of the Queensland Museum 39(3):653-668

Molnar, R. E., and Clifford, H. T. 2000 Cut contents of a small ankylosaur. Journal of Vertebrate Paleontology 20: 194-196

Molnar, R. E. 2001(b) Armour of the small ankylosaur Minmi. In K. Carpenter (ed.) The armoured dinosaurs. Indiana University Press, Bloomington. Pp 341-362

Molnar, R. E., and Clifford, H. T. 2001 An ankylosaurian cololite from the Lower Creatceous of Queensland, Australia. In: K. Carpenter (ed.) The armoured dinosaurs. Indiana University Press, Bloomington. Pp 399-412