to recll E. Why must vertebrate paleontologists often excavate large volumes of
ID: 213889 • Letter: T
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to recll E. Why must vertebrate paleontologists often excavate large volumes of rock over a wide area remains of skeletal material? and molds are common in certain depositional environments. Examine the following diagrams and follow and brachiopods in terrestrial and nearshore settings. 2. Casts up of Carboniferous club mosses were trees that grew in coastal swamps. These trees pro- duced very little wood; most of the tree's diameter was bark. Fossil forests composed of these trees often are found above and between coals (A). In the Asturias coalfield of Spain, fossil trees of 6 meters or more are often found buried in fine-grained mudstones (B). As the trees died, the upper parts that remained above the sediments rotted and fell over onto the mud. This exposed the base of the tree to decay and it completely rotted, leaving a only hollowed cylinder (C). Subsequent sedimentation infilled this void, and covered and infilled the most resistant plant parts (D). When these fossils are exposed, paleontologists can collect either the infilled void or the rock that surrounds the tree (E). Indicate on the following diagram if the fossils in the processes responsible for the preservation of trees A. One major gro E are external molds, internal molds, or casts. B. Shelled invertebrates may live either on the sediment surface at the bottom of the ocean (epifaunal habit) or (infaunal habit: Exercise 13). The example below is of a shelled invertebrate living at- tached to the ocean bottom at the sediment-water interface (Á). When this brachiopod died (B), the muscles holding the two shells relax and then decay, creating an opening. Following decay the shell is covered with agenesis, as does the sediment surrounding the shell. During diagenesis, the original shell dissolves (D). more sediment (C), some of which fills the interior of the shell. This sediment is lithified and undergoes di- When the fossil is collected millions of years later, two different preservational modes are found. On the di- agram, indicate if these two modes are external molds, internal molds (steinkerns), or casts. FOSSIL PRESERVATION AND TAPHONOMYExplanation / Answer
2) A) External mold leaves an empty cavity which will then be imprinted with the external details of the organism. Here, in the diagram, the carboniferous club moss left an empty cavity that was filled with and sedimentation infilled this void. The external structures were clearly imprinted in this sediment. So, this is an example of external mold.
B) Internal molds are formed from hollowed structures. When the shell dissolves, it leaves an impression of interior surface of the shells on the material. So, the example shown here is an internal mold.
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