Sunday, January 8, 2012

Dinos were diverse, too

God creates dinosaurs.
God destroys dinosaurs. 
God creates man. 
Man destroys God. 
Man creates dinosaurs. 
Dinosaurs eat man … 
...woman inherits the earth.*

I never really did think of dinosaurs as actual living creatures.  Maybe it was their starring role in works of obvious fiction*, or perhaps an intervening 65 million years of dino-free history that just did not lend them the same reality as, say, an elephant.  But then I read a new research paper that looked at some of the same patterns for dinosaurs that I, as an ecologist, study in modern-day plants and animals.  And it clicked; dinosaurs are animals too, with their own ecology and hotspots of diversity.

The enlightening paper was by Phillip Mannion and group of European scientists who wanted to know whether dinosaurs had similar patterns of diversity to present-day animals.  Many groups of plants and animals, including dinosaurs’ contemporary descendents, the birds, reach their greatest diversity in the tropics and have fewer species farther away from the equator.  This pattern, called the latitudinal diversity gradient, is so pervasive that it is almost considered a ‘rule’ in biogeography- groups that don’t follow the pattern (see previous post) are regarded as interesting, but anomalous.  The question is, did the dinosaurs that lived a hundred million years ago follow the modern latitudinal diversity rule?

The short answer, that Mannion and colleagues discovered after analyzing a massive database of locations of fossilized dino remains, is ‘no’, they didn’t-  dinosaur diversity peaked between 30-60° latitude in both the northern and southern hemisphere, but was generally lower in the tropics.  This actually isn’t that surprising, given that the earth the dinosaurs inhabited looked nothing like it does today. 

The modern latitudinal peak in diversity around the equator is thought to be caused by a mixture of climate and evolutionary history.  Groups of organisms appear to diversify more rapidly in the tropics, and because of the less stressful climate and lack of glaciers repeatedly plowing across the land (as has happened in the temperate zone for the past 2.5 million years), species are also less likely to go extinct.

The earth the dinosaurs inhabited for 160 million years probably did not have as strong of a change in temperature from the equator to the poles, nor did it have glaciers, or even polar ice caps.  For this reason, the latitudinal climate gradient probably did not influence dinosaur evolution to the same extent that it affected organisms living in the more recent past.  What may have played a stronger role, the authors of this paper hypothesized, was the distribution of land on the Earth’s surface.  The land that the dinosaurs lived on was divided into two large landmasses on either side of the equator- Laurasia and Gondwana.  The reason that there was lower diversity in the tropics may have been because there wasn’t much land there; it is a well-established ecological rule that larger areas have more species.

Regardless of the true reason for higher dinosaur diversity at higher latitudes, its deviation from the modern pattern is a reminder of something invisibly obvious.  The earth now is not how it used to be, nor how it will exist in the distant future.  Which ‘rules’ are based on assumptions of Earth’s current geography and which will remain true throughout time?

*Jurrasic Park. dir. Steven Spielberg. 1993.

You can find this paper at:

ResearchBlogging.orgMannion, P., Benson, R., Upchurch, P., Butler, R., Carrano, M., & Barrett, P. (2012). A temperate palaeodiversity peak in Mesozoic dinosaurs and evidence for Late Cretaceous geographical partitioning Global Ecology and Biogeography DOI: 10.1111/j.1466-8238.2011.00735.x

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