SYDNEY: Fossils have been found of a new dinosaur that not only had unusually large hip bones, but was likely to have had the largest leg muscles of any known member of the sauropod family, new research suggests.
Named Brontomerus mcintoshi, or ‘thunder-thighs’ after its enormously powerful thigh muscles, the dinosaur is, “part of a flood of new sauropod dinosaurs coming out of the Early Cretaceous of North America – a period that was originally thought to be devoid of sauropods”, said co-author Mathew Wedel, assistant professor of anatomy at Western University of Health Sciences in California.
“What it points out is that sauropods were globally successful for almost the entire Age of Dinosaurs,” said Wedel. “Very early in their evolution they committed to being big, and that just never stopped working for them.”
Used for kicking the raptors away
During the Early Cretaceous Period 110 million years ago, Brontomerus – a member of the long-necked sauropod group which includes Diplodocus and Brachiosaurus – probably had to contend with fierce ‘raptors’ such as Deinonychus and Utahraptor, so may have used its powerful thighs as a weapon to kick predators, or to help travel over rough, hilly terrain.
“When we recognised the weird shape of the hip, we wondered what its significance might be, but we concluded that kicking was the most likely,” said first author Mike Taylor, a researcher in the Department of Earth Sciences at University College London.
“The kick would probably have been used when two males fought over a female, but given that the mechanics were all in place it would be bizarre if it wasn’t also used in predator defense.”
Specimens found of mother and offspring
The fossilised bones of two specimens of Brontomerus mcintoshi – an adult and a juvenile – were rescued from a previously looted and damaged quarry in eastern Utah by researchers from the Sam Noble Museum in Oklahoma.
The team suggest that the larger specimen could be the mother of the younger and would have weighed around 6 tonnes – about the size of a large elephant – and measured 14 m in length, the team naming it in honour of John ‘Jack’ McIntosh, a retired physicist at Wesleyan University, Connecticut, and lifelong avocational palaeontologist.
At a third of the size, the smaller specimen would have weighed about 200 kg – the size of a pony – and been 4.5 m long, according to the paper recently published in the journal Acta Palaeontologica Polonica.
Pushing the envelope with giant hips
The authors classified the new genus based on an incomplete skeleton including bones from the shoulder, hip, ribs, vertebrae and some unidentifiable fragments.
They used the bones to identify Brontomerus’ unique features, primarily the shape of the ilium (hip bone), which, in the case of Brontomerus, is unusually large in comparison to that of similar dinosaurs.
“Sauroposeidon, found back in 2000, was extreme in having the longest neck vertebrae, proportionally, of any sauropod,” said Wedel. “Brontomerus is equally extreme in having the largest hip bones, proportionally, of any sauropod. Both animals have helped push the envelope of what we know about dinosaur anatomy. Brontomerus just pushes that envelope in a more humorous direction.”
The wide, blade-shaped bone projects forward ahead of the hip socket, providing a proportionally massive area for the attachment of muscles. The shape of the bone indicates that the animal would likely have had the largest leg muscles of any dinosaur in the sauropod family.
Fragmentary specimen requires caution
“What this fossil probably reflects are some sort of unusual proportional characteristics, so perhaps the limbs were enlarged and heavier, but you can’t really say with much certainty because it is very fragmentary,” said Steve Salisbury from the the University of Queensland’s Vertebrate Palaeontology and Biomechanics lab.
“With any sort of behavioural hypotheses about fossil animals you have to be quite cautious depending on the material,”
“But it’s an interesting addition to what we know about sauropods in North America during the Early Cretaceous period. This particular location in Utah has produced some diverse fauna, and it’s an interesting snapshot of the time.”
Same Noble Museum of Natural History
Original paper in Acta Palaeontologica Polonica