Paleontologists have discovered 'exceptionally' preserved fossils of a dog-sized dinosaur that lived underground in Utah over 99 million years ago.

The new species, called Fona herzogae, was a small, plant-eating creature that featured powerful bicep muscles, a fused pelvis for stability and large hindlegs that were ideal for its underground lifestyle.

Researchers found near complete skeletons of several creatures still in their original death pose, chest down with splayed forelimbs, which suggests they perished below the surface.

The researchers found the Fona in the Cedar Mountain Formation, an area that borders the northwest corner of Colorado and is part of the Mussentuchit Dune - a large sand dune near the Green River.

Researchers at North Carolina State University and the North Carolina Museum of Natural Sciences said they were surprised at how many fossils they'd found of the dinosaur in the area since 2013 and the majority were still positioned in their original death pose.

The formation was once covered in a warm, wet and muddy environment caused by a massive ocean to the east of the area and volcanoes and mountains to the west.

This type of environment would typically cause fossils to deteriorate or scatter over time, but the Fonas were so well preserved that the team believed the creatures may have spent time underground, according to the study published in American Association for Anatomy.

'The best explanation for why we find so many of them and recover them in small bundles of multiple individuals, is that they were living at least part of the time underground,' said Dr Lindsay Zanno, the study's co-author and associate research professor at North Carolina State.

'Essentially, Fona did the hard work for us, by burying itself all over this area.'

The researchers found skulls, legs and pelvic bones, spines, ribs and chevrons - bones that attach to the underside of the tail among many others. 

The Fona had several characteristic that showed it burrowed including larger bicep muscles, strong muscles on their hips and legs, fused bones along the pelvis and hindlimbs that were distinctively bigger than forelimbs.

It had a simple appearance that was very different from other highly-decorated creatures like horned, armored and crested dinosaurs.

The researchers have not yet found the caverns the Fona might have dug but reported that they did find tunnels in Idaho and Montana created by its closest relative, the Oryctodromeus -  further establishing their belief that the dog-like dinosaur also burrowed underground.

This is a new revelation because previous scientific discoveries have only noted that dinosaurs were aquatic, land-based or took to the air, but the Fona adds a fourth possibility that the species also lived underground.

'The bias in the fossil record is toward bigger animals, primarily because in floodplain environments like the Mussentuchit, small bones on the surface will often scatter, rot away, or become scavenged before burial and fossilization,' said Dr Haviv Avrahami, the study's lead author and digital technician at the North Carolina Museum of Natural Sciences.

'But Fona is often found complete, with many of its bones preserved in the original death pose, chest down with splayed forelimbs, and in exceptionally good condition.

'If it had already been underground in a burrow before death, it would have made this type of preservation more likely.'

These fossils may give researchers better insight into the diversity of different dinosaur species and their adaptability to withstand certain environmental conditions or how they escaped predators.

'People tend to have a myopic view of dinosaurs that hasn't kept up with the science,' she continued.

'We now know that dinosaur diversity ran the gamut from tiny arboreal gliders and nocturnal hunters, to sloth-like grazers, and yes, even subterranean shelterers.'