by As a scientist, laboratory work can become monotonous at times. But in 2017, and Ph.D. paleobiology student at the University of Bristol in the United Kingdom, I heard a gleeful exclamation from across the room. Kirsty Penkman, head of the North East Amino Acid Racemization laboratory at York University, had just read the data printed from the chromatograms and was practically jumping up and down.
The mimicry signature tool of ancient amino acids was detected in an eggshell. Amino acids are the building blocks that make up protein sequences in living organisms. But this was just another eggshell; it was a fossil from a titanosaur, a huge herbivorous dinosaur that lived about 70 million years ago.
Not much organic matter survives over millions of years, limiting scientists’ ability to study the biology of extinct organisms compared to modern ones, whose proteins and DNA can be sequenced. As Penkman’s enthusiasm suggested, these amino acids were unusual.
In fact, this result came as a surprise amid our team’s efforts to test claims of near-pristine protein preservation in dinosaur bone. I brought various fossil bones to Penkman, but the results indicated that no original amino acids were preserved, and that they were even contaminated with microbes from the environment in which they were buried.
Eggshell fossil testing was not even in our original research plan.
Orphan fossil fragments
However, I had just seen that my colleague Beatrice Demarchi, Penkman and their team had detected short protein sequences in a 3.8 million year old bird eggshell. I predicted that if dinosaur eggshells didn’t preserve any original proteins, their bones probably wouldn’t either, and they wanted to find out if that was the case. Fortunately, we had a source of dinosaur eggshell.
Around the year 2000, many eggshell fragments were illegally exported from Argentina into the commercial market. As a fossil-obsessed child, I was even given a coin-sized fragment of a US mineral treasure. Penkman and I tested that fragment, along with another fragment from a European museum gift shop.
These fossil fragments gained scientific value in some ways because they did not belong to any museum collections. We didn’t have to worry about damaging them during the analysis. To our surprise we came upon the rare opportunity to study the organic remains of ancient dinosaurs.
Amino acids in eggshell
Encouraged by this initial discovery, our large international team analyzed more dinosaur eggshells from Argentina, Spain and China, using a wide range of techniques. Although some eggshells preserved amino acids much better than others, the evidence indicated that these molecules were ancient and original, perhaps between 66 million and 86 million years old.
Throughout life, proteins that helped calcify the eggshell were trapped within the mineral crystals. The remaining amino acids we detected, however, were free molecules that had broken off from their protein chains by reacting with water. We found only a few of the most stable amino acids. Less stable ones were absent, because they were degraded.
The amino acids that were still being preserved were what chemists call it. Amino acids can exist in either left-handed or right-handed configurations. Living organisms arrange their amino acids so that they appear almost exclusively in the left-handed configuration. After the organism dies, amino acids can be converted between handedness until they reach a 50-50 combination of the two configurations.
A 50-50 mixture is called racemic and indicates that the amino acids were separated from their protein chains a long time ago.
Calcite, an amino acid archive
Our dinosaur eggshell results showed more degradation than seen in younger bird eggshell and mollusk shell fossils. Our results were consistent with the results from experiments in which eggshells are exposed to heat in the laboratory, simulating degradation over thousands or millions of years.
Organisms strengthen these shells with a type of calcium carbonate mineral called calcite. Unlike the calcium phosphate that makes up bone, calcite can act as a closed system by trapping the products of proteins involved in calcification as they break down, including free amino acids that are separated from each other. protein. This closed system allowed us to look at the amino acids in our analyses.
Even bird eggshell is among the best materials for finding preserved protein sequences in fossils, not to mention free amino acids. Demarchi’s team has found short, intact sequences of amino acids that are still chained together from a bird’s eggshell that is at least 6.5 million years old.
Other researchers claimed to have found more ancient amino acids, as well as more extreme and less likely claims for conserved protein sequences. But, our study uses a wider range of methods and reports the best signal for stable molecules in tissue that we now know conserves molecules well.
Our dinosaur amino acids may hold the record for the oldest protein material yet found for which the evidence is very strong, and the first clear evidence from a Mesozoic dinosaur.
Using calcite to look back in time
The genetic sequence in DNA that is ultimately expressed in proteins provides the source code for organisms that scientists can study. But if only a subset of amino acids are preserved in the fossil, it’s like removing all but five letters from a book – very little literary analysis is possible. So what messages from the ancient world might still be in these calcite time capsules?
One biologically informative signal could be stable isotopes, which are atoms of the same element with different masses. Scientists can look at the stable isotope ratios of carbon, oxygen or nitrogen to learn about their source, such as the animal’s diet. Since eggshell calcite is a closed system, stable isotope ratios in its amino acids are more likely to come directly from the dinosaur, rather than from outside contamination.
In future research, our team will use fossils to search further back in time. Organisms other than dinosaurs laid eggs, they strengthened their tissues with calcite. For example, marine arthropods called trilobites that lived more than half a billion years ago had calcite in their eyes.
Studying older remains could help scientists understand the molecular changes that occur in fossils over long periods of time. Fossil calcite, Earth’s molecular time capsule, can send researchers faint tales of long-gone life to better understand their biology.
The orphan eggshells used in our initial analysis had a happy ending. They finally found a new home at Argentina’s Provincial Museum of Patagonico de Ciencias Naturales, a natural sciences museum in Patagonia, which has returned to the only province known to produce that type of microscopic eggshell structure.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Evan Thomas Saitta, University of Chicago
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This research was supported by the University of Bristol Bob Savage Memorial Fund and the Leverhulme Trust (PLP-2012-116).
