Page 78 - Introduction to Paleobiology and The Fossil Record

TAPHONOMY AND THE QUALITY OF THE FOSSIL RECORD 65

looks very like a modern horseshoe crab. The site had been known since 1839 as a source of exquisite
fossils of shallow-water marine organisms such as crocodilians, fishes, ammonites and nautiloids

with beaks and gut contents, crustaceans and other arthropods, as well as well-preserved land plants
washed in from the nearby shore, and pterosaurs that must have fallen in the water.
The specimen was collected during an excavation by the Museum at Stuttgart, and volunteer
excavator, Rolf Hugger, who found the specimen, was amazed when he saw that the major muscles
of the prosoma, the broad head shield, of this horseshoe crab had survived. Chemical analysis showed
that the muscles are preserved as calcium phosphate (apatite). These muscles had a variety of func-
tions: compressing and moving food through the crop, operating the limbs, and bending the body.

Under the scanning electron microscope, all the muscle fibers are clear (Fig. 3.4b), and decay had

highlighted cross-banding on some of the muscle fibers (Fig. 3.4c). At higher magnifi cation, spherical
coccoids (Fig. 3.4d) and spirals could be seen, associated with the preserved muscles. These coccoids
and spirals are actually preserved microbes that were presumably feeding on the muscle tissue after
the animal died, and formed a so-called biofi lm over the carcass.
It is well known that muscle tissue breaks down rapidly after an animal dies. Experiments have
shown that the muscle here must have been mineralized as apatite within a matter of days, or at
most a couple of weeks. The seabed was saturated in calcium carbonate at the time of deposition
(the rock is a limestone), and pH has to be lowered slightly to allow calcium phosphate to precipitate.
Perhaps the carapace of the dead horseshoe crab acted as a protective roof, inside which microbes
began feasting on the muscle tissues and thereby lowered the pH locally enough for apatite to pre-
cipitate. The decaying muscle provided some calcium phosphate, but more must have been derived
from the surrounding sediment.
Find web references about the Nusplingen fossils at http://www.blackwellpublishing.com/
paleobiology/.

snip their way into shelled prey. Much frag- Shells, bones and wood may be abraded by
mentation is caused by physical processes physical grinding and polishing against each
associated with transport: bones and shells other and against other sedimentary grains.
may bang into each other and into rocks as Abrasion removes surface details, and the
they are transported by water or wind. Wave fragments become rounded (Fig. 3.6c). The
action may cause such extensive fragmenta- degree of abrasion is related to the density of
tion that everything is reduced to fi ne-grained the specimen (in general, dense elements
sand. survive physical abrasion better than porous

A

A A
Non-marine AA Marine

cave deposits
fissure filling
Conservation deposits fluvial obrution peat lignitic shales limestone bone bed limestones condensation horizon
amber river bone bed
lake lithographic
Stagnation deposits submarine fissure filling
echinoderm coquina
Obrution deposits deep basin shales
lagoonal shales and
submarine cave filling
Conservation traps Concentration deposits (A)
Figure 3.5 An imaginary cross-section showing possible sites of exceptional fossil preservation, most
of which are conservation deposits, but a few of which are concentration deposits. (Based on Seilacher
et al. 1985.)