Monday, January 23, 2017

Why “The Extinction of the Dinosaurs” is Misleading

It’s well-known that about 65 million years ago, something happened. An event so big that it caused a global mass extinction, dramatically changing ecosystems all over the world. This left such a clear mark in the geologic record that scientists knew about the extinction long before they found evidence of possible causes (in the form of an asteroid crater or huge amounts of volcanic rock).

The most famous effect of this event – and the reason most people have heard of it – was the disappearance of the dinosaurs. Scientists may call it the end-Cretaceous mass extinction (since it happened at the end of the Cretaceous Period), or the K-Pg mass extinction (“K” for Cretaceous and “Pg” for Paleogene, the Period that came next), but for most of the general public, it’s known as “the Extinction of the Dinosaurs.”

But this is a misleading name. Here’s why:

#1. Most dinosaurs did not go extinct during “the extinction of the dinosaurs.”

The very earliest dinosaurs evolved around 230 million years ago, near the beginning of the Mesozoic Era (the “Age of Dinosaurs”). That’s 165 million years before the end-Cretaceous event. Dinosaurs spent those 165 million years continuously changing: species would rise, thrive, and eventually go extinct as newly-evolving species replaced them. Extinction is a pretty regular thing.

On average, each dinosaur species may have lasted about one million years before disappearing for one reason or another*. That means about 99% of all Mesozoic dinosaurs had already gone extinct by the time T. rex and Triceratops showed up at the end. Only the very latest species were around for “the extinction of the dinosaurs.”

In fact, the end-Cretaceous event wasn’t the only mass extinction that hit the dinosaurs. There were several throughout the Triassic, Jurassic, and Cretaceous, and they all took dinosaur casualties. The last one is famous because it was the worst of them, and because it was the final straw that ultimately ended the reign of the dinosaurs.

Except for the fact that…

#2. Some dinosaurs survived “the extinction of the dinosaurs.”

Through their history, dinosaurs have come in an incredible variety. Some were huge with long necks and tails; some were covered in armor; some had big heads and tiny arms; and quite a few were small and covered in feathers. One of those families of small, two-legged, hollow-boned, feathery dinosaurs would ultimately give rise to the birds.

Birds were around long before the end-Cretaceous event. The lived alongside Microraptor 120 million years ago, Velociraptor 75 million years ago, and T. rex 65 million years ago. And when the big extinction came, the birds were hit, too. In fact, nearly all birds went extinct along with the other dinosaurs. But a small number made it through – these were the only dinosaurs to survive “the extinction of the dinosaurs.”  

A glimpse of dinosaur evolution. Time moves from the bottom to the top here,
and dinosaur families arise, thrive, and disappear.
"Evolution of dinosaurs" by Zureks: Derivative work: Woudloper. Wikimedia
#3. Most of the species that went extinct during “the extinction of the dinosaurs” weren’t dinosaurs.

Sometimes we focus on dinosaurs so much you would think they were the only life-forms during the Mesozoic Era. But this is far from the truth. In fact, dinosaurs made up a relatively small part of their own ecosystems.

When the end-Cretaceous event happened, what other species were around?
On land: ancient species of lizards, snakes, turtles, mammals, frogs, salamanders, insects, spiders, snails, tons of plants, and much more.
In water: fish, sharks, aquatic turtles, crocodilians, the famous mosasaurs and plesiosaurs, the spiral-shelled ammonites and the straight-shelled belemnites, micro-organisms, and much more.
Up above: the famous flying pterosaurs – not dinosaurs at all!

In all of these groups, species went extinct during “the extinction of the dinosaurs.” Some groups disappeared completely, such as the pterosaurs, mosasaurs, belemnites and ammonites. The end-Cretaceous extinction wasn’t just a dinosaur problem, it was a global catastrophe. One of the worst mass extinctions the world has ever seen – for all living things.

*How long does a species last? Fossil studies have found that mammals through the Cenozoic Era had an average species-lifespan of around 1 million years, while various invertebrates lasted longer (this is explored in Principles of Paleontology). Mammals, being vertebrates of varying size and habits, are a decent comparison for dinosaurs. 

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