What would a massive extinction event look like in the fossil record?

When I started getting into in paleontology, I was puzzled for a long time about how a mass extinction event, like the one that obliterated the dinosaurs, would manifest in the fossil sequence. I didn’t look it up, because I wanted to think logically about it for myself.

I did, however, come across a text that mentioned that extinctions occur at the point when a species or group is no longer found in younger rock layers. This makes complete sense, as we should not find traces of an animal later than the time it ceased to exist. An organism is extinct when it stops forming fossils, and thus disappears from the fossil record.

This answer was simple enough to silence my wonders about mass extinctions. I figured that mass extinction events must be points where there are lots of organisms disappearing at the same time – i.e. lots of extinctions at the same time. I didn’t think much more about it; the simple answer was satisfying.

Also, I was glad to note that it is not quite as straightforward. If we find traces of the organism from time a to time b, we are confident that that organisms existed between those times. However, because of the nature of the fossil record, i.e. because organisms are not necessarily preserved (and found!) throughout their existence, we cannot be sure that time a marks the precise emergence and time b represents the exact disappearance of the organism. Because a new species is always rare when it has just appeared, and similarly are typically rare when they are near extinction (although, extinction could probably happen suddenly to an abundant species during a truly catastrophic event – e.g. a rapid mass extinction event?), it is less likely that species are preserved as fossils at their dawn and demise. Therefore, paleontologists always have in mind that times a and b are not the definite boundaries of the organism, but rather the time interval when we can be certain that this organism did exist, representing its minimum life span.

As a well-known example of how the earliest time we find fossils might completely miss the time of true extinction, we have the coelacanth fish Latimeria. The coelacanths were thought to have gone extinct in the Late Cretaceous because there have been no fossil evidence of their presence since. However, since the late mid-1990s, live specimens of two species Latimeria have been fished out of the deep oceans – unambiguous evidence that this lineage did indeed survive for 80 million years longer than shown by the fossil record.

So, I was happy with that explanation: Extinctions are manifested as disappearance in the fossil record of younger rocks, but there is some uncertainty about the timing, due to the fossil record being incomplete by nature. I liked the simplicity, coupled with some inherent difficulties due to the nature of the subject.

However, this morning I was struck again with wonder. How would the mass extinction events look in the precise moment, rather than after the event? How could we recognise a mass extinction without looking at what went missing later?

Mass extinction certainly implies mass death, and mass death should also mean mass fossilisation. I understand that more deaths do not mean more fossilisation events, as the conditions for preservation depend primarily on physical and chemical factors. However, while the number of preservation events probably was unaffected, each of those events ought to have involved more organisms, since more organisms were dead. So, I expect that there would be a relative increase in the abundance of fossils in each locality – while not necessarily an increase in the number of fossil localities – during a mass extinction event.

As a counter argument, I guess one could claim that during such disastrous times, maybe there will be fewer occurences where the conditions are right for fossilisation. Maybe the sheer chaos prevents proper fossilisation?

However, considering that many mass assemblages of organisms, especially large ones, are interpreted as having occurred during storm floods and similar catastrophic events. Many others are thought to have been formed through long-term accumulation of organisms in a calm setting, such as a lagoon, or quicksand swamp, and these tend to give much better-preserved fossils. So, maybe we could extract a pattern from this?

I am tempted to predict that mass extinctions would, at the precise event, contain an increase in the number of fossils found, but a potential decrease in the number of localities and in the quality of preservation. Hopefully, I will find an opportunity in the future to see for real what a mass extinction sequence looks like.