Overberg Geoscientists Group
540 Mya
First animals with hand parts
The period following the appearance of multicellular, but soft-bodied animals brought about a very rapid introduction of animals with hard parts, usually shells or exoskeletons.
The very earliest, were what is now termed the “Small Shelly Fauna”. These, as the name implies were very small, and for the first time had calcareous, or phosphatic hard parts. Some appear to be early shelled creatures, related to our present day molluscs, others were perhaps parts of the ’armour’ of worms and other similar burrowing creatures.
These Shelly remnants may have come from animals such as these reconstructions of known fossils.
The reasons for the comparatively rapid evolution of different body parts and styles are not known with any certainty.
There may, of course, be a combination of factors, and some well-studied occurrences have produced some answers, possibilities, and theories.
There may have been a change in the chemistry of the oceans that enabled the development of Calcareous (containing Calcium Carbonate) Silicic (containing Silica) and Phosphatic hard parts. It is possible that the ending of the “Snowball Earth “glacial period had brought about the erosion of more minerals into the sea making them available to the animals.
The associated die-off of the Stromatolite ‘biomats’ would also have left suitable environments available.
A lot of the animals were ‘burrowers’ and left trace fossils of their tubes in the soft sediments. This may have indicated that they developed this lifestyle in order to escape predators. This then may have become an ‘arms race’ whereby, as predators developed hard mouth parts to catch the prey, the prey developed shells and exoskeletons to protect themselves.
In 1909 Charles Walcott was investigating the outcrop of the Burgess Shale in the Canadian Rockies.
He came across an amazing collection of hitherto unknown fossils. The most important aspect, perhaps, of the occurrence was the incredible degree of preservation of the soft parts of the animals as well as their hard parts. The shale is dated at around 510 mya, so may give some idea of the predators in the arms race.
Ayesheia
Tuzoia
Marella
Anomalocaris
Opabinia
Further Reading:
Gould, S.J. (1990). "A background for the Burgess Shale". Wonderful Life. London: Hutchinson Radius.
Whittington, H.B.; Briggs, D.E.G. (1985). "The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia". Philosophical Transactions of the Royal Society B. 309 (1141): 569–609.
Butterfield, N. J. (1 February 2003). "Exceptional Fossil Preservation and the Cambrian Explosion". Integrative and Comparative Biology. 43
Simon Conway Morris (1998). The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford University Press.
Dzik, J. (1994). "Evolution of 'small shelly fossils' assemblages of the early Paleozoic". Acta Palaeontologica Polonica. 39
Cohen, B.L. (2005). "Not armour, but biomechanics, ecological opportunity and increased fecundity as keys to the origin and expansion of the mineralized benthic metazoan fauna". Biological Journal of the Linnean Society.
​
D J Mourant Aug 2023