Trilobites, and the evolutionary arms race


For this week’s transitional fossil, I will be talking about trilobites (instead of picking on one specific fossil, as I usually do).  If you didn’t know (shame on you!) trilobites are marine arthropods which lived approx. 540 to 250 million years ago. 

Through the 300 million years that trilobites existed, prior to their extinction in the Permian, there were many opportunities for diversification of form, starting from the presumed primitive morphology exemplified by a species such as Redlichia. This typical primitive morphotype had a small pygidium, well developed eye ridges, a simple, lobed glabella, several thoracic segments, and a rather flattened body form.  The first trilobites were characterized by this primitive form. Among the over 20,000 species of described trilobites there are species in which aspects of morphology have diverged greatly from the primitive state… (http://www.trilobites.info, read more)

Evolution is amazingly well-documented in trilobites, including evolution of the eye, spines, thorax, etc. depending on taxa.  Here’s a Phacops’ eye.

Another interesting fact before we move on.  Having seen the fossils, you may have always thought that trilobites are small.  However, the largest known species of the trilobite is the Isotelus rex, being 72 cm long!

And we have some alien-looking trilobites…

Yes, planet earth.

Plus, tens of thousands of other trilobite species, for example, the trilobites of North America (link).

In the article “Why do cheetahs run so fast?” I talked about the evolutionary arms race.  Now, I shall explain how this very same concept is the driving force behind trilobite evolution, as documented by the fossil record.

Many things drive evolution.  Adaptation to climate and natural environment is a classic.  Polar bears are an example of adaptation to a cold environment.  Maybe this explains why some people have lots of body hair, while others don’t.  But I wouldn’t count on it, I haven’t done the research.

The other driving force is co-evolution.  In the article “Of acacias and ants” I presented an example of co-evolution in which two species work for the benefit of each other; if you like, they become allies.  The other type of co-evolution isn’t so pleasant… cheetahs and gazelles, for example.  As one species evolve, the other needs to as well, to match up with its opponent, otherwise they would go extinct — and I remarked that 99% of all creatures on earth are extinct.

This sort of “evolutionary arms race” is apparent in the fossil history of trilobites.  From very primitive creatures, due to many selective pressures, trilobites needed to evolve defence mechanisms, such as better sight, and body armour (exoskeletons).  Umm… SO COOL, right?  The remarkable thing is that the transition is very clearly laid out in the fossil record; there were so many trilobites back then!  Let’s take a look at some of these awesome war machines.

The Asaphus kowalewskii had periscope eyes.

Some trilobites even rolled up for defence!

Defensive spines.  Can’t touch this!

Unfortunately (umm… fortunately?), trilobites disappeared from the face of the earth some 250 million years ago, during the Permian-Triassic mass extinction event, ie. Noah’s flood.  Creationist win.

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