Most animals and plants never fossilize. For those that do, it’s usually only hard parts such as bones and shells that preserve. However, in some exceptional cases, soft tissues such as muscles and gills survive the fossilization process and can present a wealth of information about the biology and ecology of ancient organisms.

In a recently published paper in Palaeontologia Electronica, Dr. Adiel Klompmaker (UA Museums’ Curator of Paleontology) and colleagues reported on a remarkable crab with multiple mineralized soft tissues preserved. This crab lived ~75 million years ago during the Cretaceous in South Dakota in an ancient sea known as the Western Interior Seaway.

The crab fossil was collected between 2012–2018, but its importance only became clear when Dr. Klompmaker studied the specimen in July 2019. “I could not believe my eyes when I saw the specimen for the first time,” Klompmaker said. “I grabbed my hand lens to check and, indeed, the crab showed mineralized gills through the broken shell of the crab! I had reviewed a paper on fossil gills in a crab a couple years earlier, so I recognized the gills immediately.” Prior to the new paper, only 4 other fossil crab specimens had gills preserved.

After the gill discovery, co-author Dr. Peter Kloess (Whittier College and University of California Museum of Paleontology) put the crab into a microCT scanner. “I’ve used microCT scans to pick up microscopic details in previous specimens and we anticipated to see these gills in higher resolution than the naked eye,” says Kloess. The results of the scan were more than surprising. “While the gills did not show up, we identified other soft tissues such as the foodpipe leading to a stomach,” Klompmaker mentioned. The crab jaws—or mandibles—also showed up. A 3D-animation of the internal structures can be viewed here. The overall shape and location of soft tissues within the crab is not unlike modern crabs. Stomach contents have not been recovered thus far.

Despite the presence of very rare soft tissues, the team could not identify the species of crab. “We can only identify the genus, Secretanella, because diagnostic features of the shell such as spines are not preserved,” Klompmaker mentioned. “For taxonomy, the preservation is not that good actually.”

A third and final surprise came when the team was writing the scientific paper. The crab was found in limestones representing an ancient methane cold seep. At seep sites in the Western Interior Seaway, methane bubbles up from the sea bottom and often supports a diverse ecosystem of clams, ammonites, snails, worms, sea stars, crustaceans, corals, moss animals, and fishes. “When we searched the literature for preserved soft tissues from any animal living in ancient methane seeps, we found exactly none,” Klompmaker said. “Now that we know soft tissues can preserve in these environments, we would expect to find more fossils with mineralized soft tissues there.”

Dr. Matúš Hyžný (Comenius University, Slovakia), who was not involved in the study, said: “Fossil soft tissues are often considered to be limited to the so called Konservat-Lagerstätten, localities with exceptional preservation due to specific conditions during the fossilization. Any occurrence outside such localities deserves attention as it helps to understand the fossilization process better.”

The remarkable crab specimen with soft tissue preservation is reposited in the Alabama Museum of Natural History collection, where it will remain available for subsequent studies.

Members of the same research team previously published on evidence of predatory octopuses living in fossil cold seeps and provided an overview of crustaceans found in these seeps globally. More research on crustaceans from methane cold seeps in the Western Interior Seaway is underway.

Research into fossil cold seep crustaceans is supported in part by an American Museum of Natural History Lerner-Gray grant and a Western Interior Paleontological Society grant.

Reference:

Klompmaker, A.A., Kloess, P.A., Jauvion, C., Brezina, J., and Landman, N.H. 2023. Internal anatomy of a brachyuran crab from a Late Cretaceous methane seep and an overview of internal soft tissues in fossil decapod crustaceans. Palaeontologia Electronica, 26(3): a44. https://doi.org/10.26879/1277