Breathtaking Cretaceous Fossil Mammal Preserves Soft Tissue

A mammal fossil from Spain perfectly preserves fur and internal organs, but is said to be 125 million years old.

Believe it or not: “Breathtaking fossil of tiny mammal preserves fur and internal organs,” Sid Perkins writes in Science Magazine. You can almost hear the gasps in his coverage:

Most of our knowledge of very ancient life comes from fossilized remains of hard tissues—bones, shells, and teeth. Now, the exquisitely preserved fossil of a tiny mammal from the time of the dinosaurs reveals a variety of soft tissues, including skin, fur, and spines; even remnants of its external ear were fossilized. The find pushes back the earliest record of mammalian internal organs and well-preserved fur by more than 60 million years, and shows that ancient fur and spines formed just as they do in today’s mammals.

“Finding complete fossils like this raises the bar for the rest of us,” says Richard Cifelli, a vertebrate paleontologist at the University of Oklahoma, Norman, who was not involved with the new study. “My breath is taken away.”

The fossil was announced in Nature a day before official publication: Martin et al., “A Cretaceous eutriconodont and integument evolution in early mammals.” But is the tissue original material, or just mineralized soft-tissue impressions?  It appears from the paper that some of the fossil contains the real stuff, but other statements are not clear:

Soft tissues in the thorax and abdomen (alveolar lungs and liver) suggest the presence of a muscular diaphragm. The eutriconodont has molariform tooth replacement, ossified Meckel’s cartilage of the middle ear….

The new Spanish gobiconodontid (Eutriconodonta, Mammalia) combines a complete and articulated skeleton, with extraordinary preservation of skin, hair, keratinous dermal scutes, and remnants of visceral organs…

The Las Hoyas Konservat-Lagerstätte occurs in finely laminated limestones deriving from a freshwater wetland. Fossils are usually preserved fully articulated, including soft tissues such as mineralized muscle and skin. Potential mechanisms for exquisite preservation are microbial mats, anoxia, and rapid burial by sediments.

Under the section on “Integumentary and soft-tissue structures” there is detail about hair and fur, and mention of “fossilized soft tissues,” that may or may not refer to mineral replacement. The color, however, which is clearly evident in the photos, seems to indicate preservation of original biological material:

The specimen also shows a unique preservation of several organs. In the area of the scalp, the left outer ear (external pinna) is perfectly preserved (Fig. 2a and Extended Data Fig. 7). Within the thoracic ribcage of Spinolestes, a patch of fossilized soft tissues contains tubular structures with a branching pattern (Extended Data Fig. 9). From the position and distribution in the rib cage, this most probably represents fossilized lung tissue, and the branching structures probably represent the bronchioles of the lung. Posteriorly to the lung tissue, a large oval area of reddish-brown soft-tissue (Extended Data Fig. 9a) is interpreted as residues of the liver according to its anatomical position and colour. Liver tissue is rich in iron, and provides a reddish colour, as has been reported for the theropod Scipionyx from the late Early Cretaceous (Albian) of Italy.

One of the photo captions points to “compound hair follicles (FO), epidermal cells (keratinocytes), and pores (P).”  The BBC News has a photo of the fossil slab and artist reconstructions of the mammal.

In Science, Sid Perkins uses the word “preserved” but does it mean preserved as rock, or preserved as original biological material?

The fossil also includes internal organs. 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. Farther down in the abdomen is a large oval region of reddish brown material—likely the remnants of the creature’s liver, Luo says. 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.

There is no mention of protein or collagen, but keratin (the protein in hair) is mentioned numerous times. It’s also possible that original material was lost during handling of the fossil:

Upon discovery, the rock containing the specimen was split into two slabs, each containing original bone, fossilized soft tissues, and natural moulds of bones. One slab (MCCMLH30000A) was transferred to a matrix of epoxy resin to expose undisturbed bone surface hidden in the rock and to obtain casts of the natural moulds of bone impressions. After embedding in epoxy resin, the limestone matrix was removed by a formic acid bath that dissolved the calcium carbonate but not the hydroxyapatite of bone and the phosphatized soft tissue.

According to The Guardian, one of the paleontologists who studied the fossil, Thomas Martin (U of Bonn), has a theory about how the soft tissues were preserved:

Martin believes the creature was so well preserved because bacterial films grew over the animal within hours of its death. These biofilms can form a protective coating, preserving the soft tissues long enough for them to fossilise over much longer periods of time.

Without mass spectroscopy of the dark parts, it may be impossible to know if original material from the mammal is present. One thing is clear from the sources: this mammal had modern mammal features for a creature in the early Cretaceous. Perkins writes,

“Diaphragm, liver, lungs; it’s amazing to see all that detail,” Cifelli says. Plus, he notes, Spinolestes has all the hair categories seen in modern mammals “and that’s not a small thing.” Only fossils unearthed in recent years provided evidence that ancient mammals had fur that looked like that seen in modern relatives.

The Editor’s Summary makes it clear this mammal shared modern characteristics:

The triconodonts are a type of extinct early mammal, so named for their characteristic three-cusped molars. The extraordinarily well-preserved 125-million-year-old triconodont fossil reported here pushes back the earliest record of mammalian soft-tissue preservation by over 60 million years. Named Spinolestes xenarthrosus, the fossil from Las Hoyas, Spain, exhibits typical mammalian features including fur, an external ear, and skin structures including — on its back — tiny spines resembling those of hedgehogs and spiny mice. Under the skin there is evidence for a liver and alveolar lungs, suggesting the presence of a muscular diaphragm.

To be so well preserved, this mammal had to be buried rapidly. If it’s 125 million years old as claimed, it had to remain undisturbed for all that time despite geological upheavals, bioturbation and other dynamic processes.

This fossil mammal comes from the same area as the fossil bird that flew over the heads of dinosaurs (see 10/07/15), which also displayed exquisite soft tissue and shared many traits of modern birds, such as agile flying ability. It appears that the Lower Cretaceous limestones of Las Hoyas, Spain are poised to reveal many more such breathtaking fossils.

Update 10/15/15: Live Science made some comments worth repeating.

1. The fossilized remains of a furry critter that once roamed the Earth alongside dinosaurs suggests that mammals have been growing hair the same way for at least 125 million years.
2. The new discovery of Spinolestes pushes the fossil record further back by some 65 million years, into the Mesozoic era, proving that mammals have been hairy creatures for a really, really long time.
3. You may think that, over the course of 125 million years, the process by which mammalian hair grows would have changed somehow, but that’s not the case, Luo said. The bones of Spinolestes, which was about the size of a small rat, are proof that ancient mammals grew hair the same way as modern mammals do.
4. The fossilized remains of the fur ball also held evidence of the animal’s soft tissues. Iron-rich residues associated with the creature’s kidney were preserved, as were microscopic bronchiole structures of the lung and an open body cavity that may have once held a muscular diaphragm used for respiration. These fossilized structures represent the earliest known record of mammal organs, the researchers said.
5. Spinolestes is the first-ever mammal fossil found at Las Hoyas Quarry in Spain. The researchers from the Universidad Autónoma de Madrid who discovered the ancient mammal have been digging up vertebrate fossils at the site since 1985. They have previously discovered prehistoric crocodiles, birds, fish and dinosaurs.

Horsing Around?

Another beautifully-preserved mammal fossil was found in Germany’s Messel Pit: a miniature female horse with an embryo still in its uterus. This fossil also has soft tissue, PhysOrg says, but the article claims it was preserved by bacteria. The horse, like the Cretaceous fossils, shows advanced mammal traits and must have been buried quickly:

Applying SEM, the authors discovered a bacterial lawn replacing the soft tissues, as is common with other specimens found in that area. The observable details correspond largely with living mares, which lead the authors to posit that the reproductive system was already highly developed during the Paleocene, and possibly even earlier.

Follow-up questions not addressed include: Why would a “bacterial lawn” form unless original tissue were present?  Why wasn’t the bacteria itself mineralized, if this specimen fossilized millions of years ago?

Rapid burial – modern traits – does that sound like evolution?  We eagerly wait confirmation that original biological material is present in these fossils, as it has been reported in dinosaurs (6/10/15).  Even without it, what has been revealed so far fits abrupt appearance of modern-looking animals, recent creation, and a flood.