EVOLUTION (11:704-486) – FOSSIL RECORD
MOUSE – SPRING 2011
As I mentioned earlier, it is difficult to see deep into the tree of life,
if we only have the existing (now time-depth) cross-cut of specimens. Imperfect as it is, the fossil
record provides us with the clues that tie it all together. We end up with both locational and
temporal information about the ancestral forms that existed, but to decipher that information, we
need to start with a bit about the geological processes that have operated.
– Rocks formed at the earth’s surface originated as molten material (magma),
extruded from deep within the earth. Under intense heat and pressure, extrusion is explosive,
via volcanoes, but most rock is formed as crustal rock, pushed up from below. Rock formed in
this fashion is called
. If it erodes and is deposited elsewhere, it becomes sediment –
elsewhere. Under pressure, sediment becomes
rock. With enough heat and
pressure, both igneous and sedimentary rock become
Most fossils are found in sedimentary rock, never in igneous or volcanic rocks. A few fossils
are found in other situations, archaic humans in peat deposits, saber tooth tigers in tar pits,
mammoths in frozen ice or tundra, insects in amber, and so on. Over 250,000 fossil species
have been defined, a very tiny fraction of the species that have ever been there.
In other words, those things that either wash down in the runoff and are deposited in the local
catchment, which might be the continental shelf in some areas, or things that sink to the
bottom and get covered in silt, could possibly end up as fossils, but most critters die and get
consumed, being utterly biodegradable. The probability of becoming a fossil is very small,
but we’ve had 3.5 billion years and a planet to work with, and it has happened, now and then.
– Alfred Wegener first posited the idea that the continents drifted around on
the surface of the earth’s core in 1915, but his idea wasn’t taken seriously until the 1960s,
when both definitive evidence for and an explanation of the phenomenon was forthcoming.
The surface crust, the
, includes both the continents and the crust, below the oceans,
is now known to consist of 8 great plates and some minor ones, which move across the planet.
Figure 6.1. Futuyma. Page 128