If the Cambrian Explosion marked the evolution of almost all the modern phyla of animals, and in most of those phyla also marked their development of hard parts, the Ordovician Biodiversity event is more a matter of all those groups filling out and expanding into as many niches as they could. It’s the development of details and, as the name says, diversity. After the Ordovician diversification, there were close to three times as many families of animals as there were in the Cambrian – 400 versus 150, but only one new phylum.
It took place during the early and especially the middle Ordovician, about 485 to 460 million years ago, a span of just 25 million years. Like the time frame of the Cambrian explosion, this is a remarkably short period of time, geologically speaking, for so much to happen.
Some scientists see the Ordovician diversification as a natural outgrowth of the Cambrian explosion, a predictable expansion but one that perhaps was episodic, with the Ordovician event being the greatest pulse.
Factors such as the evolution of jawed animals, like the conodonts, and increases in predation might be part of that way of thinking. And it could be as simple as the idea that once animals began to become complex, and to change, it was an ongoing process and the diversification event was a natural part of it. It could be a 100-million-year-long “event,” but one that had two relatively short pulses.
Others point to external causes, and many of those possible factors are the same or similar as those that are often called on as causes of the Cambrian explosion.
More niches – more places for life to exploit – that’s a common possible explanation. During the early Ordovician, glaciation had just ended, tectonic activity was spreading continents apart more and more, and there was considerable active volcanism. All these things would have put more nutrients and useful trace elements into the oceans, and would have created more coastal areas with diverse environments – tantalizing empty spaces where ecosystems might thrive.
The climate was largely warm, and likely greenhouse conditions prevailed, with high CO2 values that were perhaps initiated by the active volcanism and perpetuated by an ongoing greenhouse effect. The Ordovician saw some of the most intense volcanic activity in the past 600 million years. Sea levels were generally high. One suggestion is that there was an explosion in phytoplankton, the base of the food chain, taking advantage of all those volcanic-derived nutrients. In turn, that might have stimulated the diversification of animals, especially the filter feeders that were expanding dramatically during the Ordovician.
And then there’s the Great Meteorite Shower idea that we discussed yesterday.
So, as with the Cambrian Explosion, it seems likely that the Great Ordovician Biodiversification probably had many causes, and it may be incorrect to think of one, or even multiple specific triggers. I kinda like the idea that the various factors contributed to the changes, and that what we perceive as sharp events might be thought of as life arriving at some kind of threshold that allowed for dramatic expansion. Research continues.
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—Richard I. GibsonReferences and links for further reading:
Photo by Ryan Somma under Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)