Why are index fossils useful for dating rocks


19-Oct-2016 05:57

Kimberella may show a similarity to molluscs, and other organisms have been thought to possess bilateral symmetry, although this is controversial.

Most macroscopic fossils are morphologically distinct from later life-forms: they resemble discs, tubes, mud-filled bags or quilted mattresses.

If these enigmatic organisms left no descendants, their strange forms might be seen as a "failed experiment" in multicellular life, with later multicellular life evolving independently from unrelated single-celled organisms.

The first Ediacaran fossils discovered were the disc-shaped Aspidella terranovica in 1868.

There was something very different about the Ediacaran Period that permitted these delicate creatures to be left behind and it is thought the fossils were preserved by virtue of rapid covering by ash or sand, trapping them against the mud or microbial mats on which they lived.

One palaeontologist proposed a separate kingdom level category Vendozoa (now renamed Vendobionta) in the Linnaean hierarchy for the Ediacaran biota.

Palaeontologist Martin Glaessner finally, in 1959, made the connection between this and the earlier finds All specimens discovered until 1967 were in coarse-grained sandstone that prevented preservation of fine details, making interpretation difficult. Poor communication, combined with the difficulty in correlating globally distinct formations, led to a plethora of different names for the biota.

In 1960 the French name "Ediacarien" – after the Ediacaran Hills in South Australia, which take their name from aborigine Idiyakra, "water is present" – was added to the competing terms "Sinian" and "Vendian" for terminal-Precambrian rocks, and these names were also applied to the life-forms.

In 1946, Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges It was not until the British discovery of the iconic Charnia in 1957 that the pre-Cambrian was seriously considered as containing life.

This frond-shaped fossil was found in England's Charnwood Forest, and due to the detailed geological mapping of the British Geological Survey there was no doubt these fossils sat in Precambrian rocks. Misra's discovery of fossiliferous ash-beds at the Mistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously undiscernible.; formerly Vendian) biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms that lived during the Ediacaran Period (ca. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.