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 than 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.