Deep Time Diversity: Decoding 375 Million Years of Life on Land

By: Emma Dunne (@emmadnn)

Across the world today we can see a tremendous amount of biodiversity. Animals occupy every corner of the globe, from the lush rainforests at the equator to the vast icy expanses at the poles and the plethora of grasslands, deserts, and forests in between. Nature is outstanding in its variation of animal forms; animals have mastered flight, can tolerate extreme environments, demonstrate complex behaviours, and some can even use tools. But exactly how life on land became so diverse remains largely uncertain.

 

Chameloeon

Chameleons are a distinctive group of reptiles which contains many different species that vary greatly in colour. Image: Pixabay.

Life has been around for an extremely long time – 3.8 billion years to be exact. Now, that’s a very long time indeed, but for the first 3.795 or so billion years life was microscopic. It wasn’t until 542 million years ago that animals became a little more complex – during the ‘Cambrian Explosion’ when most major groups, such as arthropods, first evolved. To put things into perspective, wherever you are right now stick both of your arms out straight to the side (don’t be shy!). The very tip of your left index finger represents the present day, and the tip of your right index finger represents the point about 542 million years in the past. Moving from right to left, the first fish appear somewhere in the middle of your right forearm just after the Cambrian Explosion. Plants emerged on land around 425 million years ago, a little closer to your right elbow. It wasn’t until the point just before your right shoulder that vertebrates first ventured onto land, beginning the process of evolving into the beasts we are all familiar with today. At the point in the middle of your body, the continents were all squashed together in a landmass known as Pangaea, while reptiles, such as the sailbacked Dimetrodon, ruled the hot and arid lands around the equator. Dinosaurs first appear somewhere on your left shoulder (about 240 million years ago), followed very closely by the first mammals. Dinosaurs are wiped out just before we reach your left wrist (66 million years ago), paving the way for mammals to begin ruling the land. And now to make you really feel like a big fish in a small pond: Humans did not appear until the very tip of your left index finger, occupying a slice of your makeshift timescale no thicker than your fingernail. So, our species really hasn’t been around for long at all!

2 Dimetrodon

Dimetrodon grandis, an extinct reptile that lived 295-272 million years ago during the Permian period in the wetlands of the supercontinent Euramerica. Illustration: Scott Hartman (www.skeletaldrawing.com)

 

With all of these different animals evolving and going extinct at different points throughout Earth’s history, biodiversity has fluctuated, with increases in diversity punctuated by significant decreases known as extinction events, some more severe than others.

Over the last 50 years palaeobiologists have been trying to quantify exactly how significant these rises and falls in diversity have been using computational methods.

Typically, these analyses involve tallying the number of fossil families for specific time intervals and comparing the totals between neighbouring intervals. Previous studies using this method estimate that diversity on land has risen exponentially, or continued to rise faster and faster over time. A number of reasons have been given for this pattern, including the availability of suitable niches and favourable climatic conditions allowing species to thrive and diversify further.

Sounds simple, right? Not quite…

jlkg

The currently accepted pattern of changes in diversity on land constructed using counts of fossil tetrapod (four-limbed vertebrates) families through time. This pattern shows an “exponential rise” in diversity and more and more families appear on land as time goes on. From Sahney et al. (2010) Biol. Lett. (Numbered 1-3 are the end-Permian, end-Triassic and the Cretaceous/Paleogene boundary mass extinctions)

The problem is the fossil record is inherently biased. When you think of a fossil I could almost be certain that you would think of a skeleton in a piece of rock. And that’s not wrong! Hard parts, such as bones, shells, and teeth, are much easier to preserve than soft squishy bits – bias number one. Luckily for vertebrate palaeontologists, like myself, we don’t usually run into this issue as our study subjects have bones. But we do unfortunately encounter other biases. Some groups of animals contain many more individuals than others, and are therefore more likely to leave fossils behind (think huge herds of wildebeest vs. a pride of lions). Similarly, different habitats allow more diversity than others (for example the Siberian Tundra vs. the African savannah). These ‘biological factors’ come in to play even before the fossilisation process even begins!

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Groups of animals that exist in large numbers such as wildebeest or antelope, are much more likely to leave behind some fossils for us to find that animals who don’t exist in such large numbers, such as lions. These biological factors affect the fossilisation potential of an organisms waaay before the geological processes kick in!

The chances of an animal becoming a fossil are very slim indeed. Usually, after an animal dies its body rots away or is devoured by predators and scavengers, never to be seen again. But sometimes conditions are just right, and once the body is buried quickly with mud or sand, rock can begin to form and the remains can be fossilised. As we look back further in time our picture of the past gets a little fuzzier, as older rocks get overlain by younger rocks and mashed up by geological forces such as earthquakes and erosion. Fossils also only occur in sedimentary rocks (if you can remember back to your high school geography classes, you might remember that there are three types of rock: igneous, metamorphic, and sedimentary!), and sedimentary rocks are not found uniformly across the globe. So even finding a fossil is an extremely rare occurrence!

Human biases permeate all scientific disciplines, and palaeontology is no exception.

Sometimes it is easy to stumble across a large ‘mass grave’ containing hundreds of fossils, and sometimes these sites can be in very sunny, very beautiful countries worth visiting. Other times fossils have been found in isolation in areas where conditions are harsh, such as the important transitional fossil Acanthostega found in eastern Greenland. So, who’s up for a fun expedition to the wilds of Siberia in search of reptile fossils in the dead of winter? What, no? Yeah, me neither.

All of these factors (biological, geological, and human in origin) contribute to what are known as ‘sampling biases’, or biases that influence the amount and type of fossil data we have available for us to study.

kljg.png

An exquisitely preserved full body fossil of the extinct amphibian Phlegethontia longissima from the Mazon Creek fossil beds in Illinois, USA. Finds like this little fella are very rare indeed. Specimen housed at the Burpee Museum.

With these sampling biases stacked against us, it seems unwise to use simple counts of fossils to illuminate important patterns of diversity through time. This is where my research comes in. We are currently building a shiny new dataset within the publically accessible Paleobiology Database (paleobiodb.org). With this dataset, we are able to apply more sophisticated statistical methods to our analyses and rigorously test the patterns of diversity change on land over the last 375 million years.

My research will allow palaeobiologists to answer the question; are we able to identify genuine patterns of diversity change, or are we simply viewing changes in the number of fossils available to study through time?

So, with so many millions of years to get through, where’s the best place to start? Why, at the beginning of course! My current work surrounds the interval of geological time when the first vertebrates appeared on land and began to diversify over the next 100 million years. Given that the rocks containing these fossils are very old and are poorly surveyed, our ability to identify genuine diversity patterns is significantly distorted. However, the story does begin to improve as we move into the next 100 million years and we begin to see the fossils reflecting the true patterns of diversity.

hjldf

Map of the world from the Paleobiology Database (paleobiodb.org) showing the locations across the world where tetrapod fossils have been found from the time they first appeared approximately 375 million years ago right up to the present day. You can create maps such as this for yourself at: paleobiodb.org/navigator!

My research has just begun to scratch the surface of decoding the diversity of life on land, and there’s still a long way to go! Studies such as ours are becoming increasingly relevant today as we try to anticipate the effects of the current biodiversity crisis happening across the world. Many animals worldwide are currently under threat of extinction, and if this pattern is to continue we might well see ourselves experiencing the terrifying prospect of a 6th major mass extinction.

Research into past extinction events can determine how ecosystems and animal communities responded in the aftermath of dramatic decreases in diversity, and I hope that my research looking into the geological past will give us some hope for the future.

Find out more:

https://www.theguardian.com/environment/2015/jun/21/mass-extinction-science-warning

https://theconversation.com/how-looking-250-million-years-into-the-past-could-save-modern-species-60338

 

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