"If they could squeal, I'm sure they would have done."

Palaeontologist Tim Ewin is standing in a quarry, recalling the calamity that's written in the rocks under his mud-caked boots.

"They tried to protect themselves, adopting the stress position of pulling their arms in," he continues. "But it was all in vain; you can see where their arms got snagged open, right up to the crown. They were pushed into the sediment and buried alive."

There's a little smile creeping across Tim's face, and he's got reason to be happy.

The misfortune that struck this place 167 million years ago has delivered to him an extraordinary collection of fossil animals in what is unquestionably one of the most important Jurassic dig sites ever discovered in the UK.

We can't be precise about the location of the excavation for security reasons, but you'll recognise from the gorgeous, honey-coloured limestone that we're somewhere in Cotswold country.

Things have changed a bit since Jurassic times, though.

No quaint villages and dry-stone walls back then; these parts were covered by a shallow sea, maybe 20-40m deep. And it was a damn sight warmer than your traditional English summer. The movement of tectonic plates means Britain was roughly where North Africa is today.

So you can imagine the types of creatures that would have been living on this ancient, near-tropical seafloor.

The fossils are in clay layers that intersperse the Cotswold limestone

Stalked animals called sea lilies were tethered to the bed in great "meadows". Their free-floating cousins, the feather stars, were ambling by, looking to grab the same particles of food. And down in the sediment, starfish and brittle stars were feeling their way across the bottom with their fives arms, no doubt bumping into the occasional passing sea urchin or sea cucumber.

It's exactly this scene that's preserved in the rocks of our mystery quarry.

The quantities involved are astonishing. Not hundreds, not thousands, but perhaps tens of thousands of these animals that scientists collectively call "the echinoderms". It's a great name, derived from the Greek for "hedgehog", or "spiny", "skin". What is a sea urchin, if not an "underwater hedgehog"?

• Some may look like plants but they are all animals
• Skeletons are made from calcite (calcium carbonate)
• They display radial symmetry, in multiples of five
• They have no brain but do have a nervous system
• Arms and tube feet are moved by pumping seawater
• Lost parts can be regrown, much like a gecko's tail

Most of what we know about the deep history of echinoderms from British fossils comes from the few specimens that emerged from railway cuttings and quarrying in the Victorian age.

The individual calcite plates, or ossicles, that made up the skeletal frames are preserved

Tim Ewin's institution, the Natural History Museum in London, has these items tucked away in a small space that will now be utterly inadequate to accommodate the truckload of new examples that is coming.

"In this age of rock from the Middle Jurassic, only two species of starfish were known, represented by five specimens," he says. "In just a few days of collecting here, we've got 12 starfish specimens, and expect to find many more.

"And it's the same for the comatulids, or stemless crinoids (feather stars) - 200 years' worth of collecting is represented at the museum by about 25 specimens. Here, we've probably got 25 specimens just under our feet, and we've collected over 1,000."

The NHM was given only a few days in the private quarry to collect the specimens

But it's also the quality of the preservation that's jaw-dropping.

Lean in close to a slab of rock that's just been cleaned up and you'll observe what, at first sight, reminds you of a plate of noodles. It is in fact a great mass of fossil arms from who knows how many sea lilies.

You can clearly discern the individual calcite plates, or ossicles, that made up the skeletal frames of these animals when they were alive. What's more, the specimens are fully articulated. That's to say, all parts are still intact. Everything is captured in three dimensions.

"We talk about the fives (radial symmetry) in echinoderms. They're all there; you can see them," says NHM senior fossil preparator Mark Graham.

The arms of another crinoid are imprinted on a piece of Jurassic wood

Specialists in fossil echinoderms believe the Cotswold quarry will help them better categorise the species' different life stages, their ecology and their proper position in evolutionary history.

To paraphrase that old cliche: the textbooks might not need to be rewritten but some extensive notes will almost certainly have to be added to the margins.

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