A new study led by the University of Leicester has given an insight into our planet’s climate over half a billion years ago.
Using chemical analysis of small fossilised shells, combined with climate model simulations, the team of scientists from the University of Leicester (UK), British Geological Survey (BGS; UK), and CEREGE (France) found evidence that suggests that early animals lived and diversified within a climate similar to that in which the dinosaurs lived.
This time in Earth’s history, known as the Cambrian period, began 541 million years ago (mya) and lasted 55.6 million years up until the beginning of the Ordovician Period – 485.4 mya, and it is from this time when most of the major animal groups first appear in the fossil record. These include the first animals to produce shells, samples of which were analysed in the study.
Until now, scientists have only had a sense of what the Cambrian climate was like because of the types of rock that were deposited at this time and while it has long been hypothesised that the climate was warm, specific details have largely remained unknown.
Sea temperatures, a key component of the early Cambrian marine environment, have remained a mystery to scientists due to the substantial time gap in the stable oxygen isotope (δ18O) record during this time, analysis of which is used for determining the environmental temperature of the time period.
However in this study, the team showed that previously overlooked sources of fossil biogenic phosphate have the potential to fill this gap. Pristine phosphatic microfossils from the Comley Limestones, UK, yield a robust δ18O signature, allowing for an alternative source of δ18O analysis.
Thomas Hearing, a PhD student from the University of Leicester’s School of Geography, Geology and the Environment, explained: “Because scientists cannot directly measure sea temperatures from half a billion years ago, they have to use proxy data — these are measurable quantities that respond in a predictable way to changing climate variables like temperature. In this study, we used oxygen isotope ratios, which is a commonly used palaeothermometer.
“We then used acid to extract fossils about 1mm long from blocks of limestone from Shropshire, UK, dated to between 515 — 510 million years old. Careful examination of these tiny fossils revealed that some of them have exceptionally well-preserved shell chemistry which has not changed since they grew on the Cambrian sea floor.“
Dr Tom Harvey, from the School of Geography, Geology and the Environment, added: “Many marine animals incorporate chemical traces of seawater into their shells as they grow. That chemical signature is often lost over geological time, so it’s remarkable that we can identify it in such ancient fossils.”
Analyses of the oxygen isotopes of these fossils suggested very warm temperatures for high latitude seas (~65 °S), probably between 20 °C to 25 °C. To see if these were feasible sea temperatures, the scientists then ran climate model simulations for the early Cambrian. The climate model simulations also suggest that Earth’s climate was in a ‘typical’ greenhouse state, with temperatures similar to more recent, and better understood, greenhouse intervals in Earth’s climate history, like the late Mesozoic and early Cenozoic eras.
The Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized—hence readily fossilized—organisms became common. The rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
The team’s findings help to expand our knowledge of the early animals of the period and the environment in which they lived.
Thomas Hearing said: “We hope that this approach can be used by other researchers to build up a clearer picture of ancient climates where conventional climate proxy data are not available.”
Thomas W. Hearing, Thomas H. P. Harvey, Mark Williams, Melanie J. Leng, Angela L. Lamb, Philip R. Wilby, Sarah E. Gabbott, Alexandre Pohl, Yannick Donnadieu. An early Cambrian greenhouse climate. Science Advances, 2018; 4 (5): eaar5690 DOI: 10.1126/sciadv.aar5690
Trilobites. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (),(Homotelus bromidensis) (Credit: Visuals Unlimited/naturepl.com)