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| Introduction | Last Updated • April 5, 2019 | ||||||||||||||
The next two chapters (Beecher’s Trilobite Bed and the Bertie Waterlime) are relatively complex in relation to what we know at this point in the semester. Because of this, I will ask you to only read part of them and I will combine both of chapters in one single quiz. I am summarizing themes important points of these chapters in the notes below, and I am also adding some extra information. In the end you should study the Background section of both chapters, and integrate that with what you find on this page. While of course you are encouraged to read the two chapters in their entirety, I will only ask you questions out of the Background part of the two chapters and the notes that I am adding below. | Beecher's Trilobite Bed
| Last Updated • April 5, 2019 |
The Cambrian is followed in time by the Ordovician and the Silurian. While there are no major Lagerstätten in North America between the Early-Middle Cambrian Burgess Shale and the Late Silurian Bertie Waterlime, the Ordovician does have one thin horizon, called the Beecher’s Trilobite Bed, which is famous for its richness in trilobites. The trilobites at this location have been fossilized in their entirety, including soft and delicate parts such legs and antennae, and are exquisitely preserved in pyrite.
The Trilobite Bed is very thin (4 cm, or 1.5 in) and is part of the Upper Ordovician Utica Basin shales outcropping in upper New York state. Visually, it is not distinguishable from the rocks above or below, except for its abundance in pyritized trilobites. The point of this chapter is to describe both the evolution of the faunas (mostly trilobites, which by the way, as you should remember, were also present in the Burgess Shale) and the unusual fossilization of soft parts in pyrite. We will not get into trilobite evolution, but a basic description of these arthropods, accompanied by a discussion pyritization, will be at the heart of this quiz. We have already seen that life in the Cambrian ocean was different from that of today. Most of the phyla of the Burgess Shale are still around, but not most of the species that belong to those phyla. The relative abundance of the phyla was also different from that of today. Because of this, J. Sepkoski spoke of a “Cambrian Fauna”: He noticed how many of the Cambrian animals either became extinct or were much less diverse at the end of the period. New organisms drove them out or restricted their existence to specific, more confined environments (for instance, sponges were outplaced by corals and echinoderms right after the Cambrian). What are trilobites? Trilobites are marine arthropods (for instance, today’s arthropods include crabs and lobsters) that appeared in the fossil record during the Early Cambrian and diversified into nearly 20,000 species over their 270 million year existence on Earth. The Early Cambrian fossils show significant differences within this group, indicating that they must have existed and had already evolved in the Precambrian. Trilobites peaked in diversity during the Late Cambrian and stayed diverse through the Ordovician; they started to slowly decline right after and through the Paleozoic, finally becoming extinct at the end of the Permian. The word trilobite means of course “three lobes”. In the figure below we observe a central axial lobe and two side pleural lobes. But it is also possible to break the body of a trilobite into a head (cephalon), a middle zone (thorax) and a back end (pygidium).
 The three "lobes" of a trilobite [1] While trilobites are extremely common in Paleozoic rocks, their relative importance in the fossil record may be exaggerated by the fact that they are more easily preserved in rocks than other animals without an exoskeleton, or with a non-mineralized exoskeleton. An exoskeleton is an external supportive covering of an animal (as the system covering the body of an insect), as opposed to an endoskeleton, which is an inner supportive structure (like our own skeleton). An exoskeleton is usually made of chitin (an organic material) but in trilobites (and lobsters too) chitin is reinforced by calcite, which makes that structure more easily fossilized. What is pyritization Pyritization is a permineralization process involving sulfur and iron. As a rule, pyritized fossils are not particularly uncommon, but usually we have preservation of hard, mineralized parts where pyrite has replaced inorganic structures; very rarely the soft parts can be preserved in pyrite, but in these rare occurrences we can observe exquisite fossils and soft-tissue preservation. Organisms are pyritized when they are preserved in marine sediments saturated with iron sulfides. Pyrite is iron sulfide (FeS2). As organic matter decays, it releases sulfide which reacts with dissolved iron in the surrounding waters. Pyrite replaces carbonate shell material due to an undersaturation of carbonate in the surrounding waters. Some plants are also pyritized when they are in a clay terrain, but to a lesser extent than in a marine environment.
  Left: crystals of pyrite [2]; right: pyritized fossils from Beecher's Bed [3] We are now ready to move on to the next Lagerstätten, which is the Bertie Waterline, also in New York state. | the Bertie Waterlime
| Last Updated • April 5, 2019 |
The Ordovician period was an era of extensive diversification and expansion of numerous marine organisms. Although organisms also present in the Cambrian were numerous in the Ordovician, a variety of new types including cephalopods, corals (including rugose and tabulate forms), bryozoans, crinoids, graptolites, gastropods, and bivalves flourished. Ordovician communities typically displayed a higher ecological complexity than Cambrian communities due to the greater diversity of organisms. However, as in the Cambrian, life in the Ordovician continued to be restricted to the seas.
A major extinction occurred at the end of the Ordovician period, about 440-450 million years ago. This extinction, cited as the second most devastating extinction to marine communities in earth history, caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, and graptolites. Much of the reef-building fauna was also decimated. In total, more than one hundred families of marine invertebrates perished in this extinction. The Ordovician mass extinction has been theorized by paleontologists to be the result of a single event; the glaciation of the continent Gondwana at the end of the period. Because of the widespread glaciation, large amounts of water became tied up in ice sheets. This also caused a lowering of sea level worldwide. A combination of this lowering of sea-level, reducing ecospace on continental shelves, in conjunction with the cooling caused by the glaciation itself are likely driving agents for the Ordovician mass extinction. A drop in sea level exposes more land at the edge of a continent. The exposed rocks are eroded and the record is removed. Upon a subsequent rise in sea level, new rocks form on top of this eroded surface. This surface is called an unconformity. In the Silurian, sea level started to rise again (a rise in sea level is called a transgression), invading many different areas and creating a high variety of sedimentary environments, each represented today by different rock types in the rock record. As a general rule, when sea level rises and covers previously exposed land (transgresses), sand is deposited first (a beach), followed by shales (deeper water conditions). In the area of the Bertie Waterlime during Mid-Silurian the shales were finally covered by limestones, indicating that conditions got even deeper. For a moment, mountain growth again shed sand and shales over the limestones but eventually in Late Silurian quiet water conditions prevailed again and carbonates, in association with evaporites, were deposited. Dolomites and evaporites indicate shallow-water basins where water circulation is not unrestricted: like a landlocked sea with an arid climate. Halite, anhydrite, and gypsum are deposited. On top of these rocks, the Camillus Shale were deposited. These shales also include halite and gypsum crystals. Above these shales is finally the Bertie Waterlime. The rocks of this group contain the remains of unusual animals and of land plants, indicating the input of a river (land plants do not live in the ocean, so they are carried somehow to the ocean). The importance of the Bertie Waterlime lies in the abundant well-preserved remains of eurypterids and other arthropods. Eurypterids arose in the Ordovician but they also became extinct in the Permian. They were the largest arthropod that ever lived: some of these reached 2 m in size and were the top predator on Earth for some 100 million years. [1] from Foster, J. (2014), Cambrian Ocean World, Indiana University Press [2] from Dave's Rock Shop [3] from Hudson Valley Geologist
© Alessandro Grippo, since 1994 | |||||||||