What To Do
Inspect the fossil specimens in Sample Set 2 (images available below), paying special attention to the features you are asked to examine. Answer questions (a) to (l) as you work through Sample Set 2.
Note: If the image slide-decks fail to load, click the sample title to open the slide deck in a new window.
Sample Set 2 & Questions
A. Sample SC11GA: Worm Burrows in Cross-Section
Sample SC11GA can be picked up but please treat it with care.
a. What sedimentary structures are visible in this sample?
b. Based on the location of the burrows, did the darker sediments deposit on top of the grey sediments or the opposite? Explain your answer.
c. Looking at a single sequence of light to dark sediments, do you see fining or coarsening upwards? If you have access to Sample SC11GA, use a hand lens to look at the grain sizes. The grain-size differences are also apparent in the last two slides of the slide deck.
Molds, Casts, & Imprints
B. Sample 121-23: Shell Molds and Casts
Sample 121-23 can be picked up but please treat it with care.
This sample shows two ways that a mold can be filled. On one side, the shell filled with grey sediment. On the other side, the shell was initially empty, but then after burial, a new mineral is being precipitated in the mold. Eventually, that side would completely fill with this new mineral.
C. Sample SB2J: Shell Molds and Casts
Sample SB2J can be picked up but please treat it with care.
This sample is a mixture of molds and casts of a shelled organism.
d. The sample has indentations and protrusions. Which (indentation or protrusion) is a cast? Which is a mold?
e. What does the colour of this rock tell you about the depositional environment?
D. Sample LC1
Sample LC1 can be picked up but please treat it with care.
f. Describe the grain size of the sample.
g. Is the sediment well-, moderately-, or poorly-sorted?
h. What does the dark colour combined with (f) and (g) tell you about the depositional environment?
i. The imprint on this sample is from the bark of a tree called Lepidodendron. Is this consistent with your answer from (h) above? Why or why not?
E. Sample FXA25
Sample FXA25 is very fragile. Please do not handle it.
j. What does this organism look like to you (mammal, plant, shell, insect)?
k. Describe the clastic sedimentary rock. If you have access to Sample FXA25, examine it with a hand lens while leaving it on the table, and without touching it. There is a closer image of the rock on the last slide of the slide deck.
l. What two types of preservation are evident in this fossil?
This fossil started from a primitive plant called a horsetail (Equisetum), considered to be one of the earliest forms of terrestrial plants. (It is often referred to as a “living fossil.”) The segmented stalk is characteristic of these plants which grow in damp sandy or semi-aquatic environments. Modern horsetails do not grow to the same size as this sample. Today they typically grow 0.2 to 1.5 m tall but in the past horsetails may have grown as tall as 30 m.
This plant material was originally buried, but after burial the organics rotted away leaving a cavity. Some of the original surrounding sediment must have then been eroded away until a portion of the cavity was exposed. New sediment filled the cavity, creating the cast you see. The remaining surrounding sediment then eroded to leave the cast.
F. Sample 121-25: Coprolite
Sample 121-25 can be picked up but please treat it with care.
Apart from the novelty value, geoscientists can use coprolites to help determine the size of the animal, its geographic range and sometimes its diet. Despite the high frequency of production and therefore large volumes of source material over Earth’s whole history, the easily degraded nature of the original material means a low probability of preservation and hence coprolites are actually quite rare fossils.