What lasts four days in the fridge but 380 million years in a rock?
The answer: sterols – a type of lipid or fat that contributes to the cell membranes, hormones, and signaling molecules in animals. Sterols, including the infamous cholesterol, are also part of what makes that holiday turkey taste so good.
Given their soft, decomposable, and tasty nature, it’s not surprising that fats don’t fossilize. That’s what makes the recent article published in Nature Scientific Reports so remarkable. The lead author, PhD candidate Ines Melendez from Curtin University in Perth, Australia and her professors Kliti Grice, and Lorenz Schwark describe a 380 million year old crustacean fossil that still contains cholesterol and a range of and other sterols, or at least their identifiable remnants.
Until this discovery, the oldest known remains of intact sterols came from the late Jurassic, about 165 million years ago. Melendez’s finding from the Devonian extends the length of time we now know sterols can be preserved by 250 million years.
How did a crab keep its cholesterol for 380 million years? The process that preserved the delicate fat was a unique one. The sterols found in the fossil are from the crab, but also from ancient algae, and sulfate reducing bacteria. Melendez, Grice, and Schwark believe the crustacean lived its life in the upper, oxygen rich layer of the ocean which would have been replete with algae. When it died, it sank into the lower, oxygen-starved depths where it was set upon by bacteria that began to eat away at the corpse. This process created hydrogen sulfide, which was consumed by photosynthetic bacteria called Chlorobi. The amalgamation of bacteria began to produce carbonate, eventually encapsulating the crustacean even before they had eaten all of it. The result was a fossil with soft tissue delicately preserved on a molecular level.
There is another reason a carbonate fossil may be difficult to study: “This one is not a beautiful fossil from a paleontological point of view,” says Melendez, “so we were allowed to destroy it. We didn’t know if we were able to get biomolecules or geomolecules [molecular fossils] out but when we tried a lot of organic geochemical approaches a lot was observed.”
The unusual way the fossil was preserved may actually have repercussions for the petroleum sciences. The original point of the study was to better understand the Gogo shale of the oil producing Canning Basin in Western Australia.
“It might change some concepts,” says professor Grice, “particularly in the petroleum industry because some of the molecules which have been found here are actually microbially formed. Traditionally they were thought to be formed by thermal processes.”
Moral of the story: be sure work out after the holiday parties. That extra fat can hang around for a long time.
Source: Melendez, I., Grice, K., & Schwark, L. (2013). Exceptional preservation of Palaeozoic steroids in a diagenetic continuum. Scientific reports, 3.
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