Until recently, obtaining original biological information from ancient fossil remains seemed imaginable only in works of science fiction. Now in a new study, an international team of researchers report using the fossilized skin of three distantly related, multi-million-year-old marine reptiles to successfully determine their skin color.
The research, published in the journal Nature, demonstrates that the fossilized skin samples all contained highly concentrated levels of molecularly preserved eumelanin – a dark pigment that suggests the sea animals’ skin was very dark or even black.
“When the film Jurassic Park came out, it was literally impossible to recover real biological information from any fossil specimen, but suddenly we are there,” said Johan Lindgren, Researcher, Lund University, Sweden, and first author of the paper. “It’s kind of a paradigm shift now that we can actually find original biological remains in fossils.”
The authors suggest that the reptiles’ shared dark pigmentation evolved to meet common needs such as thermoregulation, camouflage, and even UV protection.
“This is really the first pigment in skin tissue to be reported. …It’s pretty fortuitous to find fossils that are preserved like this in the first place,” said Michael Polcyn, Director of the Digital Earth Sciences Laboratory at Southern Methodist University, and coauthor of the study. “There have been a number of papers published on the coloration of feathers and those types of structures, but the ability to start determining pigmentation in these body outlines is significant.”
For Polcyn and Lindgren, the rare fossils – a 55-Myr-old leatherback turtle, an 86-Myr-old Mosasaur, and a 196–190-Myr-old ichthyosaur – provide a key insight into ancient anatomy and evolutionary patterns of secondary aquatic animals that once existed on land, but reentered the sea millions of years ago.
“It underscores the remarkable convergence we are seeing in secondarily adapted marine amniotes, and it illuminates the relatively narrow evolutionary corridor you have when terrestrial amniotes return to the marine realm,” said Polcyn “There are a certain number of adaptations necessary for a marine existence and these different amniote groups are evolving similar mechanisms to deal with these requirements.”
Polcyn says part of his teams’ work was inspired by paleontologist Mary Schweitzer’s discovery of soft tissue from a 68 million-year-old Tyrannosaurus rex fossil specimen.
“I think prior to Mary Schweitzer’s work on soft tissue preservation, the common wisdom was that original biomolecules were not preserved in the fossil record, but here it is,” said Polcyn. “You always have to anticipate that there is going to be a tremendous amount of skepticism when you report these kinds of preservation modes and when you are claiming original organic materials, so great care must be taken in the analysis.”
For their analysis, researchers at SP Technical Research Institute of Sweden and MAX IV Laboratory, Lund University, Sweden, used Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) to determine what structures to sample.
Then, using a high-resolution technique called Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), the team examined the topmost layers of their samples by bombarding their surfaces with primary ions, ejecting secondary ions of the elements present.
The lab’s analysis datasets from these elements ultimately revealed that those specific sample regions closely matched a spectrum obtained from natural eumelanin.
Now, Polcyn and Lindgren are interested in further studying the paleobiology of marine reptiles and looking for other tissue types.
“Hopefully we get a much clearer image of these animals as living beings rather than just some abstract fossils,” said Lindgren.
Lindgren J, Sjövall P, Carney RM, Uvdal P, Gren JA, Dyke G, Schultz BP, Shawkey MD, Barnes KR, Polcyn MJ. Skin pigmentation provides evidence of convergent melanism in extinct marine reptiles. Nature. 2014 Jan 8.