No impact: Asteroids had nothing to do with explosion in marine life diversity

A popular theory about a major evolutionary step for life on Earth has been ruled out.

A massive meteorite bombardment was suspected to have played a role in the Great Ordovician Biodiversification Event (GOBE), an explosion in the diversity of marine life, by causing just the right amount of environmental disturbance to stimulate the event. A new study in Nature Communications rules this out. Anders Lindskog of Lund University and colleagues discovered that the start of the GOBE actually preceded the intense meteorite bombardment by at least two million years.

We spoke to Lindskog about the work.

ResearchGate: What is the Great Ordovician Biodiversification Event (GOBE)?

Anders Lindskog: Most animal groups – such as arthropods, mollusks, and echinoderms – appeared early in the Cambrian time period (around 542–488 million years ago), during the so-called Cambrian Explosion, but it would still take tens of millions of years before life reached any real diversity.

The Great Ordovician Biodiversification Event (GOBE) denotes a time interval in the succeeding Ordovician time period where marine biodiversity suddenly – in a geologic perspective – started to increase rapidly at the global scale.

While the GOBE can be argued to have started at slightly different times for different animal groups and in different parts of the world, the most spectacular rise in diversity occurred in the middlemost part of the Ordovician (470–465 million years ago). This is the time interval that our study focuses on.

RG: Why is the focus on marine life?

Lindskog: Life on land during the Ordovician was not particularly well developed, and the conditions on land were much less beneficial for fossilization, so we know little about diversity in that case.

RG: Can you tell us what was previously suspected to have caused this?

Lindskog: The reasons for the GOBE have been debated for some time, especially during the last two decades. Several different reasons for the rise in diversity have been proposed including a very high sea level (which means more living area for marine organisms), global climate change, the presence of many small continents (which provided more shallow-marine areas), extremely active volcanism giving the sea nutrients and influencing its climate, and ecological factors like competition.

However, perhaps the most spectacular hypothesis is that the GOBE was started by the meteorite bombardment of Earth during the mid-Ordovician. This was said to give just the right amount of environmental disturbance to “stimulate” the diversification processes.

Indeed, a remarkable number of meteorites and meteorite minerals have been found in mid-Ordovician sedimentary rocks. This was likely the result of a massive asteroid collision in the solar system and because the meteorite-bearing rocks broadly coincide with the onset of the GOBE, it was suggested that there is a connection between the asteroid breakup and corresponding meteorite bombardment and the GOBE.

RG: What did you find?

Lindskog: We came across a limestone bed with an exceptional amount of mineral zircon grains – tiny, sand-sized crystals – from right in the middle of the GOBE interval. These beds also contained many meteorites. This opened up solutions for several issues:

First, the zircon grains, which clearly stem from a volcanic eruption, allowed us the unique opportunity to pinpoint the age of the meteorite-bearing rocks in terms of actual years. The mineral zircon basically acts like a clock, because isotopes within it change ratio through time, and by analyzing ratios between uranium and lead we were able to calculate the age of the zircon-rich rocks to 467.5±0.28 million years.

Second, the zircon date allowed us to refine the overall geologic time scale for the Ordovician time period.

Finally, another “geologic clock” that uses noble gasses found in meteorites made it possible to calculate when the meteorites were first ejected into space during the disruption of their parent asteroid. Our calculations give 468±0.3 million years, a number that is much more precise than the previously known 470±6 million years.

This allowed us to confidently dismiss the asteroid breakup and subsequent meteorite bombardment as significant “players” in the GOBE story. The GOBE started well before the asteroid collision even took place.

RG: How did you discover this?

Lindskog: Well, it was quite a lucky find, actually! We have been studying GOBE-spanning rocks intensely over the last couple of years, mainly to try and figure out the environment during the Ordovician. The zircons were found during a routine examination, while looking through dozens of samples treated in acids. Usually, I am on the hunt for small fossils and such, and this is the first and only time I have come across volcanic zircon in this particular rock interval.

Realizing the potential importance of the zircon grains, Mats Eriksson and I promptly did some reorganizing of our to-do-list and got in touch with our colleagues and co-authors in Copenhagen (Christian Rasmussen, Mafalda Costa and James Connelly). They have facilities capable of doing truly state-of-the-art age analyses of individual zircon grains. Putting our heads together, we ended up with this Nature Communications paper, which is a nice example of cross-disciplinary geoscience.

RG: Do you have any thoughts on what the actual cause of the GOBE was?

Lindskog: I believe that the GOBE was the collective result of a number of abiotic and biotic processes and events during the Ordovician and preceding Cambrian time periods.

It is reasonable that the very high sea levels at the time contributed to the development, as the large shelf areas simply gave more space for life to thrive. Combined with the presence of many small continents and beneficial cooling climate change, we have a pretty nice grand recipe for biodiversification.

Personally, I think that biological and ecological processes should not be underestimated and that these may be the most important of all. Life has an inherent drive to evolve and there was so much opportunity and space for “experimentation” during the early days of evolution, both in terms of form and function. The GOBE is clearly associated with an overall expansion of organisms into new niches and life modes, so not all of the evolution and development during this time interval can be seen in the number of genera and species.

Much remains to be learned about the geologic past, however.

Article: Nature Communications, "Refined Ordovician time scale reveals no link between asteroid breakup and biodiversification".

Featured image courtesy of flickr.