Sauropod dinosaurs were so immense they needed four strong, pillar-like legs to support their bodies, which makes a discovery in Texas all the more bizarre. Fossil footprints found in 2007 that belonged to huge sauropods show they sometimes apparently walked on just their two forelimbs.
This isn鈥檛 the first time we have encountered forefoot-only sauropod footprints, but they are rare, says James Farlow at Purdue University Fort Wayne in Indiana.
He and his colleagues analysed 60 footprints found in a limestone quarry near Austin, Texas, that were seemingly left by bipedal sauropods in rocks roughly 110 million years old. We know that three types of sauropods lived there at the time: Sauroposeidon, Astrophocaudia and Cedarosaurus. In theory, one of these could have made the tracks.
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Some of the footprints are up to 70 centimetres wide. It is difficult to judge a dinosaur鈥檚 size from just its footprints, but estimates suggest that these sauropods weighed anywhere between 15 and 78 tonnes, and may have been 25 metres or more in length.
It is inconceivable that beasts of this size and weight could walk on two legs, says Farlow.
The most obvious explanation, he says, is that the dinosaurs鈥� centre of mass lay closer to the front than the rear of their body. Walking over relatively firm ground, it is possible that their forefeet would leave an impression in the surface while their hindfeet didn鈥檛.
But there are plenty of typical 鈥渇our-foot鈥� sauropod trackways in Texas, and in all of them the hindfeet leave impressions as deep or deeper than the forefeet, he says.
There is also a strangely wide separation between one set of left and right prints, as if a sauropod was splaying its legs outwards. It is possible that the 鈥渂ipedal鈥� tracks were left by sauropods wading through shoulder-deep water, using their front legs to punt along the bottom, says Farlow.
The centre-of-mass explanation would make most intuitive sense, says Steve Brusatte at the University of Edinburgh, UK. 鈥淎s far as we know, no dinosaurs completely left the land and entered the water, the way so many of their reptile cousins did during the Mesozoic 鈥� or whales did more recently,鈥� he says.
Paul Barrett at the Natural History Museum in London says sauropods may occasionally have entered water to cross rivers or even to bathe. But they may have avoided deep water, as they might have been prone to tipping over. 鈥淭he bodies of these animals would have been surprisingly light for their size, due to their large lungs and the presence of many air sacs that invaded many of the bones,鈥� he says.
Until we find more sauropod footprints and skeletons from Texas, it is impossible to be sure how to explain the strange trackways. 鈥淏ut if you ask me in my heart of hearts what I would like to be true, I鈥檇 like them to be punting,鈥� says Farlow. 鈥淚 like to be something of a contrarian I guess.鈥�
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