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| Introduction | Last Updated • April 23, 2019 | ||||||||||||||
The next two chapters (Gilboa and Mazon Creek) discuss the arrival of life on land. Going back in time, we had single celled bacteria for most of the Precambrian, prokaryotic organisms at first and then eukaryotic. The first animals only came around at the very end of the Precambrian, and were all soft-bodied. Shells and other harder, supporting structures developed in the Lower Cambrian. During the Cambrian and the Ordovician animals expanded in the ocean but were not able to expand on land. It is only in the Late Silurian that life finally invades emerged land. I would like to point out again, and have you ponder about this, that while we are now taking life on land for granted, that was not the case at the beginning. Life started in the oceans approximately 3.5 billion years ago, possibly even before that, and there it stayed for more than 3 billion years. It is only around 400 million years ago (or 04 billion years ago) that the first species arrived on land). In these two chapters you will find the description of two different Lagerstätten that testify of the arrival (Gilboa) and the evolution (Mazon Creek) of life on land. You should study the Background section of both chapters, and the paragraphs I outlined in red below. While of course you are encouraged to read the two chapters in their entirety, I will only ask you questions out of said paragraphs and anything else I am adding below. | Gilboa
| Last Updated • April 23, 2019 | These are the main points of this paragraph:
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For this paragraph you should concentrate on the different environments where the organisms were preserved, and how can we tell. The final important point is a possible explanation on the origin of herbivores. There are different localities where fossils have been retrieved, a few where forest tree stumps are present, but no animals, and a locality called brown Mountain where animals are found. In the first case, there is evidence of a fully developed forest, from both tree stumps and foliage. Even if such a forest might recall a present-day forest, the plants found at these location are completely different from those of today. The fossils are buried in sand which was carried in at high speed, so it might be possible that, while the tree stumps are definitely found where they lived because they have been preserved in their living position, the leaves and other vegetation debris might have been carried from somewhere else. At Brown Mountain there are more and more different genera, fossilized in a region of waters moving at a very low speed, maybe even stagnant, within a delta environment. There is also evidence for animals, predators like spiders, but also detritus-feeders. There is no evidence form herbivores. Why are herbivores missing from the fossil record? It could be that herbivores existed but were ALL soft-bodied, so none of them actually got preserved. Geologists sustain that very likely that is not thecae, and that herbivores are missing because they actually did not exist yet. When we look at today's herbivores, cows for instance, we notice that they have fungi and bacteria in their guts that help them digest plant materials. The animal breaks down plant material with their teeth, and let the fungi and bacteria do the work of digestion. Fungi and bacteria are able to break down dead plant materials for their own sake. So, it might be possible that the first detritus-feeders ingested chunks of dead plants that were already covered by fungi and bacteria, and these survived their digestive apparatus. Animals that "imprisoned" microbes in their guts eventually evolved to become the herbivores of today. This seems to have occurred still in the Paleozoic, but much later than the Devonian. While Gilboa is Devonian in age, a record for land colonization appears in the Late Silurian in England and Scotland. These British Lagerstätten are similar in structure, in their lack of herbivores, and in the presence of predators and detritivores. Still, Gilboa shows a more advanced plant assemblage that include mosses, not present in Britain. By the Middle Devonian, evidence from Gilboa shows that full-scale forests had developed. | Mazon Creek
| Last Updated • April 23, 2019 | We have seen in the previous chapter (Gilboa) how life moved onto land at the end of the Silurian (in the British Lagerstätten, while Gilboa is Devonian) In Late Devonian and Mississippian, extensive forests had occupied emerged lands; evidence for this is represented by abundant coal seams in the rock record, which were preserved because in these areas the forest developed on mires*. *mire |mīr| - noun
Anoxic conditions in the mires prevented complete decay of the forest litter and thus favored the formation of peat, which eventually turned to coal. These coals, by the way, were the source of abundant and relatively cheap energy that started the Industrial revolution in Britain and eventually the industrialization process in the western world. In many localities we find Coal Measures. A Coal Measure is a more complex sequence of rocks that represents a series of different coastal environments, from marine bays to brackish lagoons to sand bars, freshwater lakes, levees and swamp forests. As you may remember form our discussion on sedimentary environments, a delta is a sedimentary body formed when running water (such as a river) enters a body of water that is not moving (such as a lake, or the ocean). A delta can have one or more “lobes”, which are the growing, active part where the river is building into the ocean. The areas between different lobes are not actively built up and are subject to flooding. A lobe only lives as long as the river brings new sediment at that location. If the river is diverted, the area is subject to rapid subsidence (that is what is happening in New Orleans and surrounding areas!), so that sea water floods what was previous marshy land. That is why it is not uncommon to find coal seams (representing land) overlain directly by muds with marine fossils. After a while, the river can come back and start to build again, and the cycle is repeated many times. 
An example of different delta lobes (and their time of formation) from present-day Mississippi Delta, Louisiana Some plants from the Devonian are also continuing in the Mississippian but new ones also appear ion the fossil record, such as giant mosses and ferns and the first conifers. Development of the forests had also provided new ecological niches to animals, which developed quickly in response to that. Among them, insects, a new kind of arthropods, and amphibians. These tetrapods amphibians had evolved at the end of the Devonian and established themselves permanently on land by the Mississippian, feeding on the invertebrates that had already moved there in the Devonian. Skip, or simply read (no questions) the following paragraph:
There are only a few concepts that you need to grasp from this short section:
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For this paragraph you will only be quizzed on the materials on page 133. The section about tidal cycles (page 134) is fascinating but we do not have time to discuss it in detail for the quiz.
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© Alessandro Grippo, since 1994 | |||||||||