125 Million-Year-Old Crocodile Fossil Yields Skin, Sensory Organs and Color Under UV Light

A 125-million-year-old fossil unearthed more than a century ago in northeastern Spain is revealing new details after researchers examined it under ultraviolet light, a technique that makes fossilized soft tissue fluoresce differently from surrounding rock. The findings, published in the Zoological Journal of the Linnean Society, give the first detailed description of the soft tissues of Montsecosuchus depereti, a small early crocodylomorph — an ancient relative of today's crocodiles.

The fossil, cataloged MGB-512 and about 50 centimeters (20 inches) long, was found at the Pedrera de Meià site in Catalonia and is held at Barcelona's Museum of Natural Sciences. Paleontologists Oscar Castillo and Jesús Serrano spotted unusual structures on it under UV light while building a fossil database. "UV light allows us to see details that would otherwise remain completely hidden in the rock," said Oscar Castillo-Visa, the study's first author.

The UV images revealed epidermal scales varying widely in size and shape, and showed the animal lacked the tall tail fin seen in modern crocodiles. Researchers also found sensory organs within some scales, concentrated around the neck, limbs and tail — structures that in living crocodiles detect touch, water pressure, temperature and chemical signals. Their limited spread suggests these structures evolved first in small regions before spreading across the body in later crocodilians.

Cartilage detected in the chest points to a well-developed respiratory system similar to that of modern crocodiles. "These traits indicate that, despite being a primitive animal, it was already very well adapted to a semi-aquatic lifestyle," Castillo-Visa said.

The UV images also revealed alternating light and dark bands on the tail scales, which researchers believe show the animal's original color pattern and may have camouflaged it by breaking up its outline; if confirmed, Montsecosuchus would be the oldest known crocodylomorph with preserved coloring. Co-author Albert G. Sellés said the exact color cannot be confirmed yet, but it was likely not very different from the varied patterns seen in crocodile species today.