'Spectacular' mammal skeleton dating back 125 million years forces 'rethink'
A mammal that lived at the time of the dinosaurs is so well preserved scientists have been able to conduct detailed analysis of its skin and fur.
The find, described as "spectacular" by one expert, shows that the strange creature resembled a cross between a rat and a hedgehog.
Spinolestes zenarthrosis had hairs on its back that were fused into short spines and an unusually strong back that may have helped it forage for insects.
A complete skeleton of one of the animals dating back 125 million years to the Cretaceous period was unearthed from a site near Cuenca, central Spain, which has previously yielded a wealth of bird and reptile fossils.
As well as hair and skin, scientists also identified a large external ear - the oldest in the mammalian fossil record - and soft tissues of the liver, lung and diaphragm.
The researchers were even able to spot signs of a fungal skin disease in the specimen similar to modern ringworm.
Professor Zhe-Xi Luo, from the University of Chicago, US, a member of the team reporting the discovery in the journal Nature, said: "Spinolestes is a spectacular find.
"It is stunning to see almost perfectly preserved skin and hair structures fossilised in microscopic detail in such an old fossil.
"This Cretaceous furball displays the entire structural diversity of modern mammalian skin and hairs."
Like other mammals that survived under the feet of the dinosaurs, S. zenarthrosis was small, measuring just over nine inches long and weighing 50 to 70 grams (1.7 - 2.4 ounces).
Its teeth and skeletal features suggest it was a ground-dweller that fed on insects.
Scientists believe its small spines, similar in structure to those of hedgehogs and African spiny mice, were probably used to ward off predators - perhaps small dinosaurs.
Lead researcher Professor Thomas Martin, from the University of Bonn in Germany, said: "We are familiar with these characteristics in modern spiny mice from Africa and Asia minor.
"If a predator grabs them by the back, the spines detach from the skin.
"The mouse can escape and the attacker is left with nothing more than a mouthful of spines."
Other features include plate-like skin structures called dermal scutes, a more advanced version of which can be seen today in armadillos and pangolins, and a highly strengthened spinal column.
The strong back may have enabled Spinolestes to force apart logs or break off palm fronds to access nests of insects.
Modern armoured shrews use their exceptional vertebral strength in the same way.
Detailed analysis revealed microscopic airways forming part of the lung and iron-rich residues associated with the liver.
A curved boundary separating these areas is believed to be what is left of the diaphragm.
The fossil is the oldest in which mammalian organ systems can be identified.
Prof Martin said the evidence suggested Spinolestes was extremely well adapted to its ecological niche.
He added: "We have to revise our thinking.
"Mammals were indeed very small during the time of the dinosaurs.
"But they were certainly not primitive."
Las Hoyas Quarry, where the skeleton was found, was lush wetland during the early Cretaceous period.
Since 1985, hundreds of fossils have been discovered at the site including important birds and dinosaurs.