Impression: The adult would have been about 2m in length

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The 95-million-year-old bones are from an adult and two juveniles and were unearthed in a chamber at the end of a 2.1m-long sediment-filled tunnel.

The researchers say the discovery is the first definitive evidence that some dinosaurs dug dens and cared for their young in such structures.

Details are reported in the journal Proceedings of the Royal Society B.

"Burrowing also represents a mechanism by which small dinosaurs may have exploited the extreme environments of polar latitudes, deserts and high mountain areas," Dr David Varricchio and colleagues tell the publication.

The team has been commended for its diligence

The team says the species' snout, shoulder girdle and pelvis have features one would expect to see in an animal that dug into the ground.

Judging from the preserved vertebrae, the adult would have been about 2.1m (6.8ft) from nose to tail, with the major part of that (about 1.2m; 3.9ft) being the tail itself. The estimated width of the animal fits neatly with the size of the tube it was digging (about 30cm;1ft in diameter).

What is left of the tunnel structure is sloping and has two sharp turns before ending in a chamber. The team says its architecture is similar to the dens of modern burrowers, such as the striped hyena, puffin and some rodents.

The dinosaur remains were covered in the coarse-grained sediment from an ancient flood; but Dr Varricchio, of Montana State University, said this did not explain the animals' deaths.

"The bones are disarticulated; they are not in life position," he told BBC News.

The scientists found the dinosaur fossils inside a den

Commenting on the discovery, Professor Kevin Padian from the Museum of Palaeontology, University of California-Berkeley, said the Montana team should be commended for the detailed way in which it went about its work.

Many would have missed the significance of the tunnel, he said

"This discovery is first and foremost a testament to the value of keeping one's eyes open in the field and noticing everything, and it took a special group of scientists to realise the meaning of the discovery that they made," he added.

Professor Padian bemoaned the impact of commercial fossil hunting which, he claimed, sought to get specimens out of the ground as fast as possible, often destroying valuable scientific information in the process.