Glen and some others have been talking about the number of
species of creatures. Glen compared the number of fossil
cephalopods with living species. For Mark and others these are
the squids, octopus and nautiloids (and a few other groups like
the ammonoids - which any fossil collector loves to have in
his/her collection). They all have a prominent head with
tentacles. The subgroup of cephalopods with the longest fossil
record is the nautiloids. These are like an octopus with a
shell. There are lots of different kinds of fossils but only one
of this kind of cephalopod, the chambered nautilus, is still
living. By the way Glen's numbers refer only to cephalopods.
There are over 1.5 millions species of living animals described.
Let me emphasize that this is not the only group for which
there are a lot more fossils than living specimens (obviously
extinct groups always have a higher number of fossils since there
are no living representatives). There are a number of
invertebrate animal groups for which there are loads and loads of
fossils and, in some cases, not as many living representatives.
Three of the more abundant invertebrate animal groups that come
immediately to mind are the echinoderms, brachiopods, and
foraminferins (shelled ameba like critters). The echinoderms
(chaps like starfish, sea urchins, sea cucumbers, crinoids and
others) have 6000 living representatives but three times the
number of fossils. In Mississippian age rocks you can find
limestones that are MAINLY made of bits and pieces of crinoids.
The brachiopods are also VERY abundant, "often being the
commonest and most ubiquitous fossils in any shallow water
deposit." (Clarkson 3rd ed, p. 153). If your kid picks up a
fossil shell, the odds are that it will be one of these. Some of
the habitats (niches) of the fossils are now occupied by bivalves
(Mollusca). Many limestone quarries (especially Paleozoic) have
limestone rocks that are just covered with these. There are
3,000 fossil genera and only 100 living (again from Clarkson).
Lastly many of the planktonic fossils are VERY abundant. One of
these is a shelled ameba like chap called foraminiferins. They
can be so abundant that most of certain rocks like chalks (white
cliffs of Dover) are made largely of shells of these chaps. I
couldn't find figures for number of fossils compared to living
foraminferins, but there are LOADS of fossils.
If you get the impression that I am trying to tell you that
for some fossil groups there loads and loads of fossils, you are
right. But do remember that some things fossilize better than
others. We have lots of fossil plants preserved in Carboniferous
Coals (that I study) and almost no animals in the coal (we do see
some tiny fecal matter). We know a lot about the animals from a
rather unique kind of preservation in some shales (Mazon Creek
nodules). But I will save that for a latter post maybe......
-- :James F. Mahaffy e-mail: mahaffy@dordt.eduBiology Department phone: 712 722-6279Dordt College FAX 712 722-1198Sioux Center, Iowa 51250