When you think about a fossil, you might think about a towering T-Rex filling the atrium of a museum. Or maybe you think about shells of ancient sea creatures preserved in the rock face. Both of these examples are of hard tissue, like bones or shells, that fossilize and provide some information about what past life may have looked like skeletally. But, sometimes scientists can find fossils of tissues, like skin, when they look in certain rock formations that provide the perfect chemical and mineral concoction for tissue fossilization.
Scientists who study fossils, called paleontologists, have found these special formations all over the world and now they want to know why. Why do these formations exist and why do they do such an amazing job at fossilizing tissue? And the answer, as posited by Sinjini Sinha and colleagues at the University of Texas, may be due to something surprising – climate change.
Often, these special tissue-preserving rocks are from marine environments he researchers determined the plants and animals were living during Oceanic Anoxic Events (OAEs). These are periods in Earth’s history when the oceans were known to have low oxygen levels and some of these low oxygen levels are linked to climate change. During the early Jurassic period (~ 180 million years ago), carbon dioxide levels spiked and the climate warmed by 3 degrees Celsius in the tropics and up to 8 degrees at higher latitudes, similar to changes that we are seeing today (but note, modern climate change is man-made and the Jurassic climate change was due to geologic activity). Higher temperatures and higher humidity led to more precipitation and the rains flushed continental soils and sediments off the land and into the ocean which increased the nutrients like nitrogen and phosphorus that can help plants grow. Eventually the plants and photosynthesizers die and other organisms in the ocean decompose them, using up the available oxygen and the oceans became anoxic (see the figure below for a diagram). All of this contributed to global extinctions and now, if we look in the right place, we can see those fossils today.
Sinha and her colleagues found three known locations of this special, anoxic, tissue-preserving rock from the United Kingdom, Germany, and Canada. Using fossils from these locations, the team tested the theory that anoxic environments lead to better fossil preservation and looked for other environmental factors that could explain why they find amazing fossils in some places but not others.
When Sinha and the team examined the fossils from these locations, they found, as expected, fossilized skeletons but also soft tissue like ink sacs from ancient squid, crustacean claws, fish with intact gills, and eye tissue. Using a scanning electron microscope, they could determine what minerals were found in each part of the fossil. Based on the mineral analysis, the team determined that the fossils from each site were preserved in the same way and one element seemed more important than the rest – phosphorus.
Phosphorus is a common element in bones but they found phosphorus in tissue where it isn’t usually found like crustacean claws and fish soft tissue. Because of this, they knew the phosphorus must have come from the environment and it may be the key to understanding how these fossils are so perfectly preserved.
They hypothesize that the influx of greenhouse gasses during the Jurassic and subsequent changes to precipitation patterns led to heavy rains that stripped phosphorus off the rocks on land (this is where most mineral phosphorus is found even today) and moved it into the water. This, in combination with the anoxic zone due to nutrient runoff and climate change, created the perfect conditions to preserve the fossils.
A similar change in climate is happening now, this time human-caused. Global temperatures are rising, precipitation patterns are changing, and we are suffering through an extinction crisis. But, climate change may also be creating conditions to preserve this period in the fossil record, soft tissue, and all. What paleontologists of the future find from our time is up to us.
Source: Sinha, S., Muscente, A.D., Schiffbauer, J.D. et al. Global controls on phosphatization of fossils during the Toarcian Oceanic Anoxic Event. Sci Rep11, 24087 (2021). https://doi-org.libproxy.sdsu.edu/10.1038/s41598-021-03482-7