125-million-year-old Spinolestes Xenarthrosus Fossil has Hair and Inner Organs

Spinolestes xenarthrosus is the latest in a series of recent discoveries showing that early mammal species were far more diverse and complex than previously believed. Its discovery marks a new record for earliest fossil with preserved inner organs, beating the incumbent by more than 60 million years.

The Spinolestes xenarthrosus has been affectionately dubbed the Cretaceous furball by scientists. Spino refers to the creature’s hedgehoglike spines, and the Greek suffix lestes means “robber”-a term commonly used in the names of ancient mammals thanks to their presumed nocturnal habits”. An astounding finding: The animal may have suffered from a fungal infection of the hair which also strikes mammals nowadays.

The specimen is the first example of a Mesozoic mammal with fossilised soft tissues in the thorax and abdomen.

“It is cute, very cute”, said Prof Martin. It is possible that these structures served a similar objective in the case of Spinolestes. It can be considered as a member of a primitive mammalian group called eutriconodonts that was found nearly 170 million years ago. They met their fate roughly 66 years ago along with the dinosaurs after a massive asteroid changed the fate of life on Earth forever. The scientists are publishing their results in the journal Nature.

Martin and his colleagues can’t be sure exactly how the fossil was so perfectly preserved, but they’ll be doing chemical analysis to find out more.

University of Chicago paleontologist Zhe-Xi Luo added that the fossil provides valuable information about mammals that thrived on the planet during the dinosaur age.

The research team noticed abnormally truncated hairs which could be a sign of fungal infection or dermatophytosis. They also found remains of a large external ear – the earliest known example of such a structure in the mammalian fossil record. “This Cretaceous furball displays the entire structural diversity of modern mammalian skin and hairs”. “Spinolestes is a spectacular find”.

Spinolestes had remarkably modern mammalian hair and skin structures, such as compound follicles in which multiple hairs emerge from the same pore. Within the ribcage, there are patches of soft tissue that contain tubular structures in a branching pattern, which the team interprets as preserved lung tissue. Dr. Thomas Martin from the Steinmann Institute of Geology, Mineralogy and Paleontology of the University of Bonn.

The animal when alive was about 9.5 inches (24 cm) long and weighed 50 to 70 grams, about the size of a modern-day juvenile rat.

Luo goes on to describe, “It would look like small rat, except it has a more pointy nose”. It is thus possible that the prehistoric mammals suffered from diseases similar to those of their modern descendants.

The paleontologists noted microscopic bronchiole structures of the lung, as well as iron-rich residues associated with the liver. The sharp boundary between the two suggests that Spinolestes had a strong muscular diaphragm, which in turn hints at the ability to rapidly breathe and fuel an active lifestyle. “We know, from other mammals, that there was hair on mammals at this time – but you usually only see impressions of the fur in the fossil”.