Paleontology

Trilobites



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It is often said in science that the present is the key to the past. Just as this is true for geologic events such as earthquakes and extraterrestrial impacts, the same can be said for biological and ecological processes as well. One such case is that of predation which can be seen throughout geologic history thanks to the perseveration of fossils. It is obvious of course that fossils do not just come out and tell paleontologists what type of feeding habits they used; it takes analysis and observation of modern, living organisms.
For example, a fossilized track way is found and it contains a long stretch of flat footed heavy dinosaur prints; probably herbivorous. On either side of these tracks are other sets of tracks, smaller, pointed toes and deep indentations which probably indicated marks left by claws; probably carnivorous. Based on the short strides of the predators, they are walking slowly and nimbly, probably stalking the dinosaur herd, which based on its own relatively short strides, is more than likely not aware of the hunting pack trailing it. As the tracks move further across the rocky path, the prints of the herbivores get deeper, they stopped. Ahead of the prints lie the answer, mud cracks and ripples indicated the presents of an ancient lake long since dried up. The hunters' strides now have increased rapidly. It would appear they have been sprinting. The prints of predator and prey become a large conglomerate of motion, and prints of the herbivores can be seen running into where the lake would more than likely have extended into. The predators, stop, the prints circling are a fixed point; the kill. By observing these tracks and looking at living organisms as references, a scientist infers that the predators are raptorial, pack hunters which will stalk a prey until the right moment, in this case, when the herbivores put their heads down at the lake for a drink. The herbivorous dinosaurs, probably not having much natural protection other than size and numbers took to the lake, hoping that the hunters would not be able to cross its depths.
The study of behavior by mixing observations of living organisms with fossils of creatures long since dead does not stop at how they preyed, but continues on to how they defended themselves. One of the most interesting types of defenses that can be found in the fossil record is that which belongs to the trilobites. Trilobites were well diversified in their approximately three hundred million years of existence. One of their most remembered traits were their ability to enroll. Although not all trilobites enrolled, those that used this method of protection found a way to diversify even this one small trait.
Trilobites are an extinct group of Arthropods and are considered to be the first complex organism to have evolved. Trilobites first appeared in the fossil record in the Early Cambrian Period, and became extinct by the end of the Permian. Their bodies were separated into three parts, the cephalon (head region), thorax (midsection), and the pygidium (rear section). The thorax was split into thoracic segments allowing for flexibility. The number of thoracic segments ranged from one or two, to a few dozen, depending on the trilobite species. Likewise, vision, feeding, environments and even body designed varied greatly from species to species. It is this diversification that allowed trilobites to evolve into over 15,000 species which can be separated into nine different orders. It is of no surprise that such diversification also resulted in unique forms of protection such as enrollment.
Trilobites successfully enrolled by bending the exoskeleton which was part of their enrollment protection. The divisions that the segments created caused the exoskeleton to be flexible, therefore allowing bending and enrollment. This process of enrollment is thought to have been used as a source of protection from various sorts of dangers. These dangers fall under two categories; predation and environmental.
Predation on trilobites has been shown throughout the fossil record. Coprolites containing bits of trilobite have been found and fossils with injuries that match up with the claws of a Cambrian predator named Anamolacaris (Christopher Nedin). When a trilobite enrolled, the legs, antennas and other soft parts would have been tucked away and hidden from the opposing attack, therefore making it harder for the predator to be successful. This is similar to what the pill bugs do, the common "rolly polly" bugs found in the backyard.
The environmental dangers can be manifested in various forms. One possibility is that stormy rough waters could cause enough potential danger to force the trilobite to enroll. Another possibility is that the trilobites enrolled when the oxygen content in the water dropped to unlivable levels. The enrollment process would create a temporary environment inside the exoskeleton allowing it to live while it waited for outside conditions to improve. This type of defense has also been recorded in arthropods still living today. In some cases however, the conditions did not improve and the trilobites died. This is evident by the finding of mass enrolled trilobite graves which are often correlated with pyrite, which is usually consistent with low levels of oxygen (Babcock and Speyer).
All the evidence that goes to support these theories comes front not only the fossil record, but observations of modern day organisms. The principles of geology are based upon the ideas of uniformitarianism and this, among other tested theories that geologist use everyday, continue to prove that the processes within geology and other sciences, are the keys to the past.



Bibliography
Gon , Sam M. A Guide to the Orders of Trilobites. .

Loren E. Bacbcock. Stephen E. Speyer, "Enrolled Trilobites from the Alden Pyrite Bed, Ledyard Shale(Middle Devonian) of Western New York"
Journal of Paleontology, Vol 61, No. 3, 539-548

Christopher Nedin, "Anomalocaris predation on nonmineralized and mineralized trilobites" Geology, November 1999, Vol 27, No. 11, 987-990

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