Early Skeletal Fossils
Stefan
Bengtson (Department of Palaeozoology, Swedish Museum of Natural History,
Stockholm, Sweden)
Paleontological Society short course: “Neoproterozoic-Cambrian
Biological Revolutions”, Denver, Colorado, USA
6 November 2004
Looking
at small shelly fossils. Bengtson loathes that term, but it’s
impossible to fight the tide. The term was coined by Matthews in 1975.
Biominerals - at least
60 different minerals are known; only a few are used to make skeletons - opal,
calcite,
magnesian calcite, aragonite,
dahllite, francolite, amorphous calcium phosphate, pyrite,
greigite, magnetite.
The
most common way for organisms to make a mineral is to seal it within a cell,
like sponge spicules (siliceous, calcareous) and octocoral (?) spicules, such
as Microcoryne (a possible fossil octocoral spicule).
Another
simple way to make a skeleton is to make a tube (a mineralized sheath) - Cloudina
was the first to do this (~550 my in China, for example). Holes are known
in Cloudina tubes - only certain-sized tubes have the holes (caused by a
predator seeking out certain-sized prey).
“Clover
animal limestone” of Rosén (1919) - contain triradiate
tubes. These were first discovered by a Swede - they are now known as anabaritids.
Other
types of tubes are conchs - more modern looking. Example: hyoliths.
Example: Archaeospira (a stem-group mollusc).
Cupitheca - has a
sophisticated skeleton; a tube-dwelling animal. It threw off older parts
of the tube & walled off the tube.
Sclerites - includes
jaw-like forms (Cyrtochites) (a predators), but not from a known animal
body plan.
Includes
Scoponodus - mystery sclerites.
Includes
Microdictyon
- made less sense that Scoponodus; Microdictyon was originally
interpreted as the basal skeleton or support for egg laying - all wrong.
Chengjiang showed them to be velvet worm
shoulder pads.
“Tommotian
trilobite” - an enigmatic sclerite was found in the basal Tommotian
(Fedorov et al., 1979). The actual specimen is not trilobite-like
in appearance - is called Tumulduria. Tumulduria is of
unknown affinities - it's a weakly mineralized plate.
Includes
Paracarinachites - have a lamellar structure, have growth
lamellae. It is not chiton-like.
Includes
Allonia, a chancelloriid.
The whole organisms have been found in Burgess Shale/Chengjiang-type
deposits. Chancelloriids have a different kind of sclerite - they have
coelosclerites (hollow sclerites). It’s probably easy to
make hollow sclerites (= phylogenetic problems). Chancelloriids were
discovered by a Swede, one year before Walcott did. Chancelloriids have
platelets in the integument (Burgess Shale, Chengjiang). The
integument is continuous, with sclerites; the only difference between
integument & sclerites is mineralization of sclerites.
Reconstructions
of sclerite-bearing animals often look like loaves of bread with sclerites.
Siphogonuchitids -
sometimes find sclerites fused into scales - scaly sclerites.
Halkieriids - hollow
sclerites. The scleritome was found by Peel and Conway Morris in North
Greenland. Halkieriids were stem-group molluscs or stem-group
lophophorates.
Tommotiids - may not
be a phylogenetic grouping; sclerites are phosphatic, with accretionary growth,
can have sclerite merging. Example: Eccentrotheca displays
very diverse shapes - “guano”; made by a sloppy animal. Example:
Lapworthella, Camenella (has left- & right-handed sclerites).
Tannuolina - a
tommotiid? related to Camenella? This also has left- &
right-handed sclerites. Two mitral sclerites can be fused in Tannuolina.
Were there 4 rows of sclerites on a slug?
Dailyatia - another
tommotiid. Variously reconstructed as loaves with scales.
Roger
Thomas et al. (2000) defined skeletal space. Animals use most available
skeletal space. But, there are combinations that are ridiculous and
animals recognize this and don’t use those.
Spines
& scales - useful & common structures (protection is the primary
function of these) - note a cactus and a pine cone.
The
modern, deep-sea scaly foot gastropod (Crysomallon
squamiferum) - has an aragonite shell with pyrite and greigite scales covering
the foot - may be a modified operculum. Greigite (Fe3S4)
is the sulfide equivalent to magnetite, so it is also magnetic.
Such
structures may have formed with the help of bacteria, but mediated by the
gastropod.
There’s
a choice of minerals for skeletonization - animals use 4 basic groups of
minerals to make skeletons - why the choices are made isn’t always clear.
But,
sometimes the “why” is known - chitons have a magnetite radula - it
is very hard for scraping rocks.
The
Precambrian-Cambrian boundary is a mineralization phenomenon.
Why
mineralization? Detoxification idea - Ca is toxic at high levels - but
can get rid of it or incorporate it into the skeleton for protection or
mechanical improvements in the body.
The
type of minerals used by an organism may be based on where the skeleton
originated.
Massive
phosphate deposits occur near the Precambrian-Cambrian boundary (like
Doushantuo, China). This is near the beginning of everything.
