Video Transcript
Dead organisms can form petrified
fossils. Which of the following best
describes a petrified fossil? (A) A complete replica of the
organism, formed from when the bones were filled in and surrounded by sediment. (B) A fossil that forms when
organic material is replaced by minerals but the original shape does not change,
e.g., dinosaur eggs. (C) An imprint left by the bones or
shells of the organism on the rock that surrounds it. (D) A piece of evidence left by the
organism when it was still alive.
This question is asking us about
fossils. So what is a fossil exactly? 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 petrified fossils, complete-body fossils, mold or cast fossils, and
trace fossils. Each of these form in different
ways. Since this question is asking us
about petrified fossils, let’s look at how that type of fossil forms. We’ll use an example of a dinosaur
egg to explain the process.
In order for any fossil to form, it
needs to be buried under a lot of material. Let’s pretend that this egg is
buried under a lot of sand. This preserves the egg and prevents
it from decaying too quickly. Then, one day many years later, a
volcano explodes and volcanic ash fills the sky. It falls down on top of our
dinosaur egg and forms another layer of material. Over the years, more layers
accumulate due to different events. And as time goes on and these
layers accumulate, the weight of the overlying layers gets very heavy. This compacts the layers below
it. Over millions of years, it becomes
so compacted that it forms layers of rock that we call sedimentary rock.
Over the millions of years it took
to form this sedimentary rock, the dinosaur egg inside became slowly replaced by
rock in a process called petrification. If we zoom in on this section of
the egg here, we can look at this process in detail. Notice how the material around this
dinosaur egg has little streams of water all over. These streams of water can contain
bacteria or other microorganisms that can cause the egg to decay. So now this section of the dinosaur
egg has decayed and has filled up with water. This water isn’t pure water and
contains minerals like salts. You can see these minerals here as
orange dots in the diagram. Over time, this water can evaporate
and leave these minerals behind.
This process can repeat, until this
whole section of the decayed egg is filled up with minerals. This can happen again with other
sections of the egg, where parts of the egg decay and get replaced by minerals. Over millions of years, this whole
egg can be replaced by minerals and is now completely petrified. This is what petrification is: when
organic material, such as this egg, is replaced by minerals but the original shape
doesn’t change. Therefore, looking back at our
options, we can see that the best description of a petrified fossil is given by
answer choice (B). A fossil that forms when organic
material is replaced by minerals but the original shape does not change, e.g.,
dinosaur eggs.
