Biodiversity
How has the diversity of multicellular life
changed over time?
• Multicellular life started from only one kind
and is now many kinds, so, duh. Yes, the
number of different kinds of organism must
have increased over time.
• BUT has it been a steady increase? Is diversity
still increasing now? Or has the earth reached
some kind of limit on the number of kinds of
creatures that can exist?
Look at the numbers
• How many species are there now?
– About 1.8 million species of plants and animals
described
– Is that all there is? How could you know how many
more there might be?
– Estimates range from 5 million to 20+ million
• How many fossil species are there?
– About 300,000 have been named
– Hard to figure out from this how many species have
lived
Why is it hard to figure out past
diversity?
• Level of tabulation:
– Species problem – are we accurately recognizing
species? Some groups probably overestimated,
others underestimated.
– Orders and above are pretty far from natural
groups and are contentious
– Family level is reasonable compromise – close to
natural classification but easier to recognize than
species
Why is it hard to figure out past
diversity?
• Missing groups: unfossilizable taxa
– Perhaps at some times there were many softbodied organisms contributing to diversity
– The insect problem – unfossilizable but most of
the described species
Remember this?
Two thirds of all
described
multicellular life
are insects
Why is it hard to figure out past
diversity?
• Discovered fossil problem – sampling depends
on:
– Exposure of rocks of that age
– Volume of rocks of that age
• Silurian-Devonian transgression means there’s a lot of fossils
of that age, Carboniferous regression means there’s few of
that age
– Interest of paleontologists – different ages have
attracted different amounts of attention from
paleontologists
• Ease of access, economics, coolness factor
Why is it hard to figure out past
diversity?
• “Pull of the Recent”: tops of fossil ranges are
usually underestimates EXCEPT for extant
species.
• So living families count more (e.g. are
represented in more time periods) than
extinct ones.
So what can we do with the data we
have?
• Look at family or genus level data for skeletonized
marine invertebrates
• Eliminates the problem of identifying species
• Helps standardize identification (researchers may
be splitters or lumpers when it comes to species,
but families are universally recognized)
• Eliminates the problem of selective preservation
– we’re only comparing critters with hard parts
across time
• Eliminate the insect bulge
We can recognize three great faunas in the history of life:
Cambrian
Paleozoic
Modern
The three faunas
• Cambrian: Trilobites, lingulate brachiopods,
archeocyathids, primitive echinoderms
• Paleozoic: rhynchonelliform brachiopods,
stony bryozoans, stromotoporoids,
cephalopods, crinoids, blastoids, graptolites
• Modern: bivalves, gastropods, echinoids,
crustaceans, vertebrates
The three faunas ecologically
• Cambrian: benthic organisms living only a few
cm above & below sea floor
• Paleozoic: sessile (attached) benthic
organisms, some rising above sea floor up to a
meter or so, slightly deeper burrows
• Modern: more mobile benthic fauna, deeper
burrowers, swimmers, abundant planktonics
Notice that the faunas exist outside of the time period they are named for, but are
most important during the time period of their name.