Fossilised leg buried by dinosaur-killing asteroid uncovered in North America

Could we have found a dinosaur which saw the asteroid that wiped out its kind?

The leg of a Thescelosaurus, a type of small herbivorous dinosaur, was found lying between layers of sediment set down by the Chicxulub asteroid in Tanis, a fossil site in the US state of North Dakota. Its position, along with its preservation, have been used as evidence to support the suggestion it was killed as a direct result of the asteroid strike.

While the fossilised leg is already significant in itself, having one of the first examples of preserved Thescelosaurus skin, the claim that it came from the last day of the dinosaurs has proven to be controversial. While some scientists have accepted the finding, others say that there isn't yet enough evidence to be able to confidently make this claim.

Prof Paul Barrett, a researcher at the Museum and who has examined the leg as part of an upcoming BBC documentary, says that in any event, the find is an important one.

'It is a stunning fossil, and probably one of the most beautiful Thescelosaurus legs I've seen,' he says. 'Skin preservation like this is still relatively rare, especially to have so much skin on one leg. 

'While it is plausible that this Thescelosaurus was killed on the day of the strike, it's also possible it was exhumed by the asteroid impact, and then mixed together with everything else in the aftermath. 

'But the fact that it is so well-preserved suggests to me that even if the animal didn't die as a result of the events that caused the deposit, it must have died very close in time to it.

'This is still very interesting, as very few dinosaur fossils are found in the uppermost layers of rocks from the Cretaceous Period. To have fossils so close to the boundary, even if it's not the same day, is still removing a gap that has vexed palaeontologists for decades.' 

What is Tanis, and what happened there?

There is no doubt that Tanis is a significant palaeontological site. Since being discovered over a decade ago, mammal bones, dinosaur footprints, and even an extremely rare pterosaur embryo have been found.

They have been dated to the dying days of the Cretaceous Period where fossils are usually scarce.

'The lack of fossils in this period has in the past been used as evidence for dinosaurs being in decline before the asteroid strike,' Paul explains, 'but it's more likely to be related to a palaeontological phenomenon known as the Signor-Lipps effect. 

'It describes how on the approach to a fixed boundary, the sampling opportunity diminishes. This means there is increasingly less chance to find fossils as there is less rock to investigate. This means, in the thousands of years approaching the end of the Cretaceous, there is often a gap in the fossil record.' 

Tanis may help to fill this gap. Over 66 million years ago, the site would have been a floodplain crossed by rivers and streams lying at the northern end of a large body of water known as the Western Interior Seaway. At the time, North America was still forming from the collision of two smaller landmasses, leaving a gap filled by water stretching from North Dakota to the Caribbean.

The seaway was inhabited by a range of fish, which developed growth lines in their bones and teeth each year, not unlike those found in trees. The point at which these growth lines stop, when the fish died, has been used to infer that the dinosaur-killing asteroid fell during springtime and struck the Gulf of Mexico.

Hitting the Earth at a steep angle, the strike caused vast amounts of rock to be thrown into the air, some of it forcefully enough to be ejected into space. It caused vast earthquakes that warped the planet, as well as releasing gigatonnes of dust and greenhouse gases into the atmosphere that caused widespread climate change.

Excavations at Tanis, situated around 3,000 kilometres from the impact, have revealed the area was rocked by earthquakes following the initial asteroid shockwave. This would have caused waves at least 10 metres high to rapidly cover the site, burying any living creatures as they washed back and forth.

Further evidence of the site's asteroid-related origins is found in the bodies of fish strewn throughout the sediment. Trapped in their gills are glass spherules, formed of rock superheated by the strike. These spherules are almost chemically indistinguishable from glass created by the asteroid elsewhere.

As the glass entered the fish's bodies, they were rapidly buried along with the surrounding area before an iridium-rich layer formed above them. This is a tell-tale sign of the asteroid and which is used to mark the end of the Cretaceous.

While the provenance of the fish is not in doubt, suggestions that the dinosaur and other associated finds were killed by the same impact have met firmer resistance. 

Have we found a fossil from the dinosaur-killing asteroid? 

If any dinosaur was at Tanis at the time of the Chicxulub impact, Thescelosaurus is a strong contender. The dinosaur is often found in sandstone rich areas, suggesting that it spent a lot of time near inland bodies of water.

The leg itself was found amongst the asteroid-disturbed sediment, but below the iridium layer, which has been used as evidence that is dates to the impact. The leg's condition also points to it having been deposited rapidly.

However, the placing of the leg in this area of the sediment doesn't necessarily mean the asteroid was responsible. As the earthquakes warped Earth's crust, already buried remains or those lying on the ground could have been mixed into the sediment as it was laid down.

There are also other processes which are known to result in well-preserved remains. Counterintuitively, slow burial and scavenging have been associated with the preserved skin and bone of another dinosaur, Edmontosaurus.

Concerns have also been raised that the claims made at Tanis are yet to be peer-reviewed. This is the process through which scientists provide feedback on each other's work to make sure their findings stand up to scrutiny.

Once this process is complete, the scientific community at large may come to the conclusion that this leg was indeed buried on the last day of the dinosaurs. But at the moment, the claims are yet to be rigorously assessed, so doubts will remain. 

'The quality of the material from Tanis is impressive,' Paul says, 'and the story that it may have come from near to or on the day of the asteroid impact is one that is becoming more convincing as more of the evidence is released. 

'I suspect they will be working on this material for years, as there is a lot of detailed work that is required. It will probably take a couple of decades to fully excavate the site, and arguments surrounding the meaning of the finds will continue for some time to come.'

A documentary looking into the discoveries at Tanis, Dinosaurs: The Final Day with Sir David Attenborough, and featuring Prof Paul Barrett, will be broadcast next week on BBC One on 15 April.