The recently discovered remains of a 125-million-year-old mouse-resembling animal are so well preserved that they contain fur, hair follicles, hedgehog-like spines, organs, and even a fungal hair infection, according to a new study.
The find pushes back the earliest record of preserved inner organs and mammal hair structures by more than 60 million years. The fossils, described in the journal Nature and belonging to the Dinosaur Era critter Spinolestes xenarthrosus, also reveal the early evolution of hair and spines in animals.
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"Spinolestes shows remarkable soft-part preservation," lead author Thomas Martin, a paleontologist at the University of Bonn, told Discovery News.
"In the chest, lung tissue has been fossilized that shows the branched pattern of the airways and the fine terminal bulbs typical for lung tissue," he continued. "There is also a brownish-red spot in the abdominal cavity just behind the lung tissue, which derives from the liver (liver tissue is very rich in iron and its fossilized remainders therefore have a reddish-brown color). The lung and liver are in the exact anatomical position, clearly separated from each other, which indicates the presence of a diaphragm in between, like in modern mammals. The breathing pattern of Spinolestes therefore was like in modern mammals."
He added that there is also one external ear (called a "pinna") preserved. It is the first known fossil outer ear of any mammal. It provides evidence that this body part was well-developed and was similar to that of living rodents.
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"Modern small mammals are mostly nocturnal," he explained, "and hearing is very important for detecting predators on the one hand and detecting prey on the other hand."
Affectionately called a "Cretaceous furball" by the researchers, Spinolestes lived in a once swampy environment in what is now Spain. It spent most of its time searching for insects and other small animals, digging and scratching them up with its strong back legs.
During the mammal's lifetime, the region was also home to carnivorous dinosaurs such as 20-foot-long Concavenator, the much smaller bird-like Pelicanimimus, and multiple plant-eating dinos. Martin suspects that Pelicanimimus might have preyed upon the small furry animal. It was too tiny, however, to have been of much interest to Concavenator, he said.
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"Spinolestes looked like a modern spiny mouse (Acomys), although it is not closely related to spiny mice or any other living mammal," Martin shared. "It had spiny fur on its lower back and was a ground-dwelling animal. These spines most probably functioned as protective device."
As for existent spiny mice, the spines easily shed when they were pulled. It is frequently an effective security system, because the predator could take a big bite, and wind up with a nasty mouthful of spines. While the hunter deals with this problem, its intended victim makes a speedy getaway.
This evolved system, along with its other features, reveal that many characteristics of today's mammals were already fully established millions of years before the dinosaurs (aside from birds) went extinct.