20 Sharks: The Animal Answer Guide
deposits 100 to 2.5 million years old (Late Cretaceous through the Tertiary
period). Embedded teeth require careful digging and picking to free them
from the surrounding rock. Individual, isolated teeth of all sizes come from
animals that lived recently to about 25 million years ago and can be found
in land deposits, by beachcombers just above the tide line, or by divers on
the ocean floor. Two good sources of information and photos from many
species and locales are at http://www.blackriverfossils.org and http://www.sharkteethrus
.com. Fossil shark teeth are also a popular item on Internet auction sites.
Shark teeth aren’t the only fossilized remains found in these areas.
Stingray spines and tooth plates, vertebrae, sawfish “teeth,” and shark feces
also fossilize. Shark “coprolites” (fossil shark poop) are identifiable as hav-
ing belonged to sharks because they have a twisted nature, having passed
through the spiral valve intestine of the animal.
How can you tell whether a fossil shark lived in a river,
a lake, or an ocean?
The kind of rock a fossil occurs in is a good indicator of what the ani-
mal’s habitat was. Look in an ichthyology (fish biology) textbook and you
will find statements such as, “In the Late Devonian, sharks were found
primarily in freshwater habitats, in contrast to their primarily marine dis-
tribution today.” Researchers frequently determine the habitats that fossil-
ized aquatic animals lived in by studying the kinds of hardened sediments
in which the fossils occur. Marine mud and sand have chemical and physi-
cal characteristics (particle size, organic matter, rock type) that are differ-
ent from freshwater habitats, and lakes have different sediments from riv-
ers. When sediments become hardened into rock over time, which is the
process that creates fossils, the original differences result in different rock
types.
Additional clues come from the other animals and plants found in the
same deposits as the shark fossil. Some organisms only lived in the ocean
(ammonites, trilobites, sea scorpions), others only in fresh water (horsetail
rushes, amphibians). But sometimes the fossils get moved around and away
from their original deposits, as happens in floods and mudslides. Fossils of
freshwater animals then wind up in the ocean, confusing the picture. De-
termining the habitat type of a fossil is clearly a mystery-solving process,
and the detectives who solve such mysteries (paleontologists, geologists),
like any good detective, must have knowledge in a number of disciplines to
complete the task.
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