FOSSIL LAB - BROOKLYN COLLEGE - EARTH AND ENVIRONMENTAL SCIENCES

B2. CARBON REPLACEMENT

Replacement of original skeletal material by carbon occurs in situations where organic remains are deposited in an oxygen-deficient environment. Swamp sediments are a good example. When this happens, the remains, including soft tissues, are often converted directly to carbon (eventually to coal if the fossils are abundant), and preserved as black films on rock surfaces, or as coal seams.

Examine the specimen of Neuropteris hirsutus from the late Devonian (310 ma (mille annum, i.e. million years)). Your specimen is a fragment of the trunk of this now extinct fern-tree. As is usual among modern trees, (which, of course, are not ferns), the trunk of Neuropteris was originally cylindrical. However, your specimen was flattened as the sediment enclosing it was compacted and converted into hard rock. The parallel grooves are the remains of the trunk's bark-like surface structure – a distinctive feature of this remarkable plant.

B2.1 Use the picture below and measure the width of the Neuropteris trunk on the picture. Since the specimen is really a flattened cylinder, you will need to relate present width to former diameter. This may be done using the simple expressions given below. Make sure you are using the correct units in your calculations (centimeters (cm) for trunk diameter, meters (m) for tree height). The constant (1.414) in the trunk diameter equation is a factor that accounts for the flattening of the trunk.

Measured Trunk Width = ________________ cm.

Actual Trunk Diameter = (Measured Trunk Width)/ (1.414) = ______________ cm.

You can now use your figure for trunk diameter to estimate the original height of the tree-fern from which your specimen came by using data on the relation between trunk diameter and tree height for modern trees. This approach is reasonable, even though ferns and modern leafy trees are different kinds of plants because the woody structure of the trunk in the two forms is structurally similar.

Data for the relation between trunk diameter and tree height in modern trees is given in the figure below.

To find the height of your tree fern, find the appropriate diameter on the X-axis of the plot, and read across to the Y-axis to obtain a figure for fern-tree height. Make sure you are using the correct units in doing this estimation.

Relation of tree height to trunk diameter for modern trees.

B2.2 What is the approximate height of the tree corresponding to the tree trunk fragment shown in the picture above? (For reference the average height of a man is 1.65 meters)._____________ meters

B2.3 Tree ferns have an extensive fossil record stretching as far back as the Late Devonian (375ma). However, in the modern era there are fewer fern trees species than in the past. Tree ferns were affected by the late Carboniferous glaciation (300 ma) as well as the Cretaceous–Paleogene (K–Pg) extinction event (65.5 ma). The K-Pg event was a sudden mass extinction of three-quarters of the plant and animal species on Earth which opened ecological niches for another period of diversification and led to the appearance of new types of organisms. Can you think about some factors that may have allowed tree ferns to be so successful in the past?

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