Monday, January 5, 2015

Cambrian trilobites (Part 1: Where they lived)

This post will come in three parts. First I'll try to give an idea of what the environment was like when trilobites were around, then I'll explain what exactly these things were, and finally I'll tell you how you can go find one for yourself (and how to do so responsibly)!
The head of a trilobite, covered in mud 515 million years ago and hidden until it was pulled out of the rock this fall. What was the world like when these things dominated the waters?

Deep Time

The earth is a dynamic place. From the buzz of an insect's wings to the growth of an acorn into an oak, it seems that there is something interesting happening at every timescale humans are capable of monitoring, yet we know we're missing out on much more when we visit a museum and see the tarnish gathering, like age itself, on ancient weapons and jewelry. Things happening on this scale, centuries to millennia, are already beyond our direct observation, but we can still connect them with the chain of recorded history produced by our ancestors—They are not completely alien to us.

Walk into another wing of the museum however, and you're completely out of your depth. The thread of human history unwinds about 6,000 years ago into frayed, wispy fibers and there is no recorded thought that reaches back more than 40,000 years, to be quite generous. Imagine a race of human-like beings that experienced those 40 millennia as a single year of their impossibly long lives; only then can we even begin to see the real fluidity of our planet. At this scale, the equivalent of a century-old being could witness our full evolution from furry, arboreal apes to thinking, upright humans, while a few generations would be able to notice the motion of the continents. If these long-lived versions of ourselves followed a similar historical arc to scale with ours, the extinction of the dinosaurs would coincide roughly with the founding of their Catholic Church (Which would immediately split into warring factions over the issue of whether the dinosaurs went to Heaven). Within the realm of record, but clouded by age, even to these creatures the dinosaurs would be ancient history!

The oldest scrolls and tablets of their history would record the first mammal-like reptiles' appearance at the dawn of the Triassic period. Their development of agriculture would coincide with the appearance of the first tetrapods, 400 million year-old, newly-legged, near-fishes that would give rise to amphibians and, through them, all reptiles, mammals, and dinosaurs (including birds). Beyond that, this ancient culture would be essentially as devoid of historical reference as we are of life during the last Ice Age, so that even they would have to extract clues to the dawn of multi-cellular life from their equivalent of cave paintings and pottery fragments. The fossil record serves us nearly as well, and so at this point we are finally on near equal-footing. It is this period of time to which the trilobites belong: The ancient history of the ancients.

The Cambrian

The surface of Earth during the Cambrian period. California was, I believe, in the shallow sea north of the westernmost of the three island continents, right on the equator. (Wiki)
The age of the first trilobites, the Cambrian period (541-485.4 mya) was unlike anything we humans are familiar with. Most obviously, the map of the globe is completely unrecognizable—On the scale of hundreds of millions of years even the continents are fluid entities, morphing and flowing as if they were no more substantial than foam in a giant's bathtub. Once you've had a longer look at the map, you may notice that the continents are barren and devoid of the lush greenery with which we are so familiar. That's because, at this great age, land plants had not developed! All life on land was limited to simple fungal and bacterial mats, perhaps a bit like modern lichen, which lived as a crust on the soil—Earth's surface would have appeared only slightly more alive than Mars' does today.

The inhospitable land however, was deceiving. Beneath the waves lived an unbelievably complex and rich assortment of bizarre organisms, some of them unlike anything we see today (follow the links for pictures and more info). Take Opabinia, for starters. This creature, while probably most closely related to arthropods, is so bizarre that it is not classified under any currently-existing major group of animals. It had five eyes and a long, flexible proboscis with a grasping pincer on the end, which it may have used to move food to its backwards-facing mouth underneath the body. It would likely have used its odd optics to avoid coming anywhere near the relatively massive Anomalocaris, a large predator approximately two feet in length with mantis-like appendages for grasping prey and a very spooky-looking mouth for devouring victims (I was lucky enough to help a friend find what I'm pretty sure is a fossil of its mouth, which I'll show in a later part of this post).

With so much time between then and now for evolution to reshape life, even some of the less bizarre-looking animals from this period hold a hidden significance. The worm-like Pikaia may not hold your interest initially, but it's likely that it or something quite like it is the ancestor of all vertebrate animals—That is, everything from sharks, to angler fish, to salamanders, beavers, tyrannosaurs, sperm whales,  you, and me. Believe it or not, a few of the gill slits of these primitive proto-vertebrates would eventually be genetically repurposed to form the jaws that would give their descendants (including us!) such an advantage in catching and devouring food. There are far too many creatures from this period to mention many more, but take a look at the specimens of the Burgess Shale in the Canadian Rockies to see some of the best preserved examples of many of the most interesting Cambrian animals.

Creatures of the Burgess Shale. This is the weird world of the trilobites! Anomalocaris haunts the top left and top right of the image (notice that mouth for later!), while Opabinia sits just below center frame. Pikaia glides along just left of center at the bottom of the frame.
(Painting by D.W. Miller, courtesy of the Smithsonian)
If the extraordinary diversity of the fauna of this period reminds you more of doodles you made once while trying to pass the time in biology class than of realistic animals, well, you may be on to something. At this stage of Earth's history, complex, multi-cellular life was relatively new, and the major groups of animals with which we are familiar hadn't exactly emerged yet. So what we see in the Cambrian is life feeling out all the new niches available to big, specialized organisms. Until the familiar long-term solutions to the problem of how to live were firmly established (by chance or by merit), there were lots of different evolutionary pathways that paid off in the short term, leading to a menagerie of experimental body plans and weird collections of different traits that would eventually hit a dead end. The trilobites belong to one of these experiments, but as we'll see in Part 2 of this series, they were so successful, you may wonder if it's just luck that it is not their descendants writing the history of the Cambrian today.