Video Transcript
A shell can leave a mold
fossil. Which of the following best
describes a mold fossil? (A) A mold fossil is a complete
replica of the shell, formed from when the shell was filled in and surrounded by
sediment. (B) A mold fossil forms when
organic material is replaced by minerals, but the original shape does not
change. (C) A mold fossil is a piece of
evidence left by the organism when it was still alive. (D) A mold fossil is an imprint
left by the shell on the rock that surrounds it.
This question is asking us about a
mold fossil, which is a type of fossil. So what is a fossil anyway?
A fossil is the preserved remains,
such as bones or shells, impression, or trace of an organism that lived many years
ago. There are different types of
fossils, like a petrified fossil, a complete body fossil, a mold or a cast fossil,
and a trace fossil. Since we’re being asked about a
mold fossil of a shelled organism, let’s discuss how this can form.
Millions of years ago, an organism
with a shell died and settled on the bottom of the ocean. A layer of sand then buries the
shell. Over millions of years, multiple
layers of sand settle on top of the buried shell. These layers build up and become
compacted because of the weight of the layers on top. Over time, these layers compact to
form a special type of rock called sedimentary rock. At some point over these millions
of years, the shell breaks down and disintegrates, leaving only the imprint of the
shell in the rock. This preserved imprint of the shell
is now considered a fossil.
Looking back at our answer choices,
the option that best describes a mold fossil is given by answer choice (D). A mold fossil is an imprint left by
the shell on the rock that surrounds it.
