As a scientist, lab work can sometimes become monotonous. But in 2017, while a Ph.D. student of paleobiology at the University of Bristol in Britain, I heard a cheerful exclamation from across the room. Kirsty Penkman, head of the North East Amino Acid Racemization laboratory at the University of York, had just read the data printed on the chromatograms and was practically jumping up and down.
The instrument had detected telltale signatures of ancient amino acids in the eggshell. Amino acids are the building blocks that make up protein sequences in living organisms. But this wasn’t just any eggshell; it was a fossil of a titanosaur, a gigantic herbivorous dinosaur that lived about 70 million years ago.
Not much organic matter survives for millions of years, which limits scientists’ ability to study the biology of extinct organisms compared to modern organisms, whose proteins and DNA can be sequenced. As Penkman’s enthusiasm suggested, these amino acids were extraordinary.
In fact, this result came unexpectedly amid our team’s efforts to test claims of virtually pristine protein preservation in dinosaur bones. I had brought several fossil bones to Penkman, but the results suggested that no original amino acids had been preserved and that they had even been contaminated by microbes from the environment in which they were buried.
Testing eggshell fossils wasn’t even in our original research plan.
Orphaned fossil fragments
However, I had just seen that my colleague Beatrice Demarchi, Penkman and their team had detected short protein sequences in the eggshell of a 3.8 million year old bird. I predicted that if dinosaur eggshells didn’t retain original proteins, their bones probably wouldn’t retain any either, and I wanted to see if that was the case. Luckily we had a source of dinosaur eggshells.
Around 2000, many eggshell fragments were illegally exported from Argentina to the commercial market. As a fossil-obsessed child, I was even gifted a coin-sized fragment from an American mineral store. Penkman and I tested that fragment, as well as another fragment from the gift shop of a European museum.
In a sense, these fossil fragments acquired scientific value because they did not belong to any museum collection. We didn’t have to worry about damaging them during analysis. To our surprise, we had stumbled upon a rare opportunity to study ancient organic remains of a dinosaur.
Amino acids in eggshell
Following 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 overall suggested that these molecules were old and original, possibly ranging from 66 million to 86 million years old.
During life, proteins that helped calcify the eggshell became trapped in the mineral crystals. However, the remaining amino acids we discovered consisted of free molecules that had been broken off their protein chains by reactions with water. We detected only some of the most stable amino acids. Less stable specimens were absent because they had degraded.
The amino acids that were still conserved were what chemists call racemic. Amino acids can occur in left- or right-handed configurations. Living organisms regulate their amino acids so that they occur almost exclusively in the left-handed configuration. After the organism dies, amino acids can be converted between handedness until they reach 50-50 mixtures of both configurations.
A 50-50 mix is known as racemic and suggests that the amino acids were separated from their protein chains a long time ago.
Calcite, an amino acid archive
Our results from dinosaur eggs showed more extreme degradation than we saw in younger fossils from bird eggshells and mollusk shells. Our results were also consistent with those of experiments in which eggshells were 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 capturing the products of proteins involved in calcification as they are broken down, including free amino acids separated from protein sequences. This closed system allowed us to observe the amino acids in our analyses.
In fact, bird eggshells are among the best materials for finding preserved protein sequences in fossils, let alone free amino acids. Demarchi’s team has discovered short, intact sequences of amino acids still linked in a chain of bird eggshells that are at least 6.5 million years old.
Other researchers claim to have found more ancient amino acids, as well as more extreme and less likely claims about conserved protein sequences. But our study uses a broader range of methods and reports the best signal for stable molecules in a tissue that we now know preserves molecules well.
Our dinosaur amino acids could hold the record for the oldest protein-related material found to date, for which the evidence is very strong, and the first clear evidence of a Mesozoic dinosaur.
Use calcite to look back in time
The genetic sequence in DNA that is ultimately expressed in proteins provides a 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—little literary analysis is possible. What messages from ancient life might persist in these calcite time capsules?
One biologically informative signal could involve 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 more about their source, such as the animal’s diet. Because calcite in eggshells is a closed system, it is more likely that stable isotope ratios in their amino acids came directly from the dinosaur, rather than from outside contamination.
In future research, our team will use fossils to search even further back in time. Organisms other than egg-laying dinosaurs 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 vague tales of life long gone, helping researchers better understand its biology.
The orphan eggshells used in our initial analysis met a happy ending. They were eventually given a new home at Argentina’s Museo Provincial Patagonico de Ciencias Naturales, a natural science museum in Patagonia, repatriated to the only province known to produce those kinds of microscopic eggshell structures.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand 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).