Meet the Cretaceous Furball, a 125-million-year-old mouse perfectly preserved

The remains of a 125-million-year-old mammal were discovered in Las Hoyas Quarry in central Spain. The creature appeared similar to a modern mouse and may shed light on the development of early mammals.

A report on the Spinolestes xenarthrosus was published in the journal Nature.

The science community is particularly excited about this find because it is one of the most well preserved fossils ever discovered. The remains include fur, hair follicles, and even internal organs.

“Yes, indeed, it is the best-preserved mammal fossil from the Mesozoic. The discovery of Spinolestes is extremely exciting for me because it provides information on structures that we believed would never be accessible,” said lead author said Prof. Dr. Thomas Martin of the Steinmann Institute of Geology, Mineralogy and Paleontology of the University of Bonn.

The creature also appeared to have hedgehog-like quills along its lower back, probably for protection. This system of spines indicates that the Spinolestes was highly evolved. Researchers are very excited by the prospect Spinolestes presents; the creature’s advanced composition could mean that characteristics of modern mammals were well established before the dinosaurs became extinct.

“Spinolestes looked like a modern spiny mouse (Acomys), although it is not closely related to spiny mice or any other living mammal,” said Dr. Martin. “It had spiny fur on its lower back and was a ground-dwelling animal. These spines most probably functioned as protective device.”

The Spinolestes xenarthrosus has been affectionately dubbed the Cretaceous furball by scientists. Its discovery marks a new record for earliest fossil with preserved inner organs, beating the incumbent by more than 60 million years.

The dead animal was so well kept thanks to a process known as phosphatic fossilization. In this event, a decaying animal is rapidly enveloped in phosphate-emitting microbes, in effect freezing the decay process.

Among the organs scientists were able to examine were lung tissue in the chest with a branched airway pattern and a brownish-red spot in the abdominal cavity that would have been the liver. Since the lung and liver are clearly separated, scientists believe a diaphragm once existed in between. The breathing pattern would have therefore been similar, if not the same, as that of modern mammals.

“We are not able to classify the finding in any of the groups of mammals alive today,” said Dr. Martin. “It displays characteristics which we also find in today’s mammals. However, these are not signs of relatedness but rather they developed independently–throughout the course of evolution, they have been ‘invented’ many times.”

The Spinolestes would have lived its life in the swamp-like environs that once covered modern day Spain. Its powerful back legs were used to dig up insects and smaller animals to eat. Although too small to be of interest to the 20-foot-long carnivorous dinosaurs that roamed the land, the Cretaceous furball would have been a tasty snack for the many smaller dinosaurs and bird-like predators.

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