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 soft- bodied 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.