October 22. Climate and extinctions

You really can’t fully characterize the climate of an entire 50-million-year period like the Jurassic with a few sentences, but we can make some generalities. With the break-up of Pangaea dominating the planet’s tectonics during Jurassic time, there was more volcanism associated with the rifting, and because the supercontinent was fragmenting, there were much longer coastlines. As sea levels rose because new mid-ocean ridges displaced large volumes of water, there were more shallow seas spreading over low-lying areas of the continents. This created many ecological niches for marine and near-shore life.  And with smaller continents, more regions were relatively close to the sea and its moisture, so the desert conditions of the Triassic were much less widespread during the Jurassic.  

That’s the general picture, but it’s punctuated by variations from that general trend at least a few times during the Jurassic. 

There’s no evidence of glaciation anywhere on earth during the Jurassic. It was a warm time, if not so exceedingly hot as the Triassic appears to have been. The atmosphere contained both more oxygen and more carbon dioxide than today, leading more or less to greenhouse conditions – Jurassic plants flourished, and eventually became coal and oil. Many plants were distributed worldwide, suggesting that the climate was probably more uniform than today, both in terms of geographic changes from the poles to the equator and perhaps in terms of seasonal changes as well. With an average temperature as much as 3°C above the present, winters would have been mild and summers hot. There may have been winter snow in the polar regions, but probably nothing like today. Temperate conditions and vegetation extended much further north and south than they do today.  

There were a few relatively minor extinction events during the Jurassic. One, during the Toarcian age toward the end of the Early Jurassic epoch, impacted diversity of brachiopods, crinoids, bivalves, ammonites, and other groups. We talked about this extinction on October 5 in connection with the eruption of the Karoo Volcanics about 183 million years ago.

The Tithonian is the last stage of the Jurassic, with the end of the period at about 145 million years ago. It is marked by an extinction event that appears to have killed off at least 7 of the 11 ammonite families living at the time. About a quarter of the mollusk families in Europe died in this event. There was a regression of the sea at the same time, at least in much of Europe, which could be an important factor in the extinctions. This wasn’t a world-wide sea-level change, and in South America the sea actually transgressed over the land in places. There is actually no evidence for a mass extinction among bivalves in South America at this time, quite a contrast to the situation in Europe. So I think at best, we have to see the end of the Jurassic as a time when regional extinctions happened, but there was nothing huge, nothing of global extent.

—Richard I. Gibson

LINKS:
Smithsonian
Declining Oxygen Levels and Jurassic Extinction
Toarcian biological crises 
Minor extinctions of the Jurassic
Jurassic climate