We may have had a few bad assumptions about how fossils form, a new study suggests.

The prevailing wisdom among paleontologists today is that fossils form in the absence of oxygen. It isn’t a simple guess: some of the best-preserved fossils ever found formed in oxygen-poor conditions in ancient oceans.

However, new research from the University of Texas at Austin found that this assumption wasn’t wrong, but rather incomplete. Fossil formation is jump started in anoxic (‘without oxygen’) conditions, but oxygen is eventually required to complete the process.

Not too little, not too much

“The traditional thinking about these exceptionally preserved fossil sites is wrong,” said lead author Drew Muscente. “It is not the absence of oxygen that allows them to be preserved and fossilized. It is the presence of oxygen under the right circumstances.”

The best-preserved fossils tend to come from Lagerstätte (German for ‘storage place’), rare sedimentary deposits that make up an anoxic, relatively bacteria-free environment. The unique conditions in Lagerstätten support tissue conservation. Fossils in these structures can even preserve soft tissues like flesh or fur alongside hard tissues like bone. This makes them a unique (and uniquely valuable) window into ancient ecosystems.

The new paper examined the fossilization processes at an exceptional fossil site (Ya Ha Tinda Ranch in Canada’s Banff National Park). The site is known for its unique collection of delicate marine life from the Early Jurassic trapped in slabs of black shale. Lobsters have been found in very good condition here, as well as some vampire squids with ink sacks still intact, the team explains.

At the time these animals died (about 183 million years ago), the world’s ocean water had lower levels of oxygen than it does today, consistent with our understanding of fossilization processes. In order to determine whether the fossils started forming in an oxygen-deprived environment, the team analyzed their mineral makeup using a scanning electron microscope (different minerals form in different chemical conditions).

“When you look at lagerstätten, what’s so interesting about them is everybody is there,” said Jackson School undergraduate Brooke Bogan, paper co-author. “You get a more complete picture of the animal and the environment, and those living in it.”

“The cool thing about this work is that we can now understand the modes of formation of these different minerals as this organism fossilizes,” adds Rowan Martindale, Jackson School Assistant Professor and co-author. “A particular pathway can tell you about the oxygen conditions.”

The vast majority of the fossils analyzed were formed of apatite, a phosphate-based mineral. Most importantly, however, we know that apatite needs oxygen to form — so the fossils were at some point in their formation exposed to this gas. Factor in that animals need oxygen to live, and it becomes pretty obvious that at least some of it was floating around dissolved in the ancient ocean water. That being said, however, the team also found that the climatic conditions of a low-oxygen environment helped set the stage for fossilization once oxygen became available.

Periods of low ocean oxygen are linked to high global mean temperatures, which causes sea levels to rise (yes, what we’re causing today). Higher seas come into contact with fresh rocks on the coast and start eroding them, which generates a large influx of phosphate. But, if this low-oxygen state persists, this phosphate simply escapes the sediment (created by rock erosion) and dissolves into the water. The team explains that oxygen is needed to fix this phosphate in sediment, where it can take part in fossilization processes. Muscente said that the apatite fossils of Ya Ha Tinda point to this mechanism.

The researchers plan to continue their work by analyzing specimens from exceptional fossil sites in Germany and the United Kingdom that were preserved around the same time as the Ya Ha Tinda specimens and compare their fossilization histories.

The paper “Taphonomy of the lower Jurassic Konservat-Lagerstätte at Ya Ha Tinda (Alberta, Canada) and its significance for exceptional fossil preservation during oceanic anoxic events” has been published in the journal PALAIOS.