BSCI392
10-15-07
The Gardens of Ediacara

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Having learned about the raw data of paleontology and the constraints that we must apply to our interpretations, we now enter the third phase of the lecture course: a series of detailed case studies, starting just prior to the Phanerozoic Eon with the world's first macroscopic faunas, encountered during the Ediacaran Period, 630-542 mya. NOTE: Many sources refer to the Ediacaran Period by an obsolete name, the Vendian. Don't get confused.

Remember, by the beginning of the Ediacaran:

Regarding animals:

Thus, it's not altogether surprising that animal body fossils should appear at this time. The striking pattern - macroscopic animal fossils appear very quickly after the final snowball-Earth glaciation, almost 595 mya, forming a fauna that persisted until the base of the Cambrian, at which many of its members went extinct.

The Ediacaran Fauna

Components:

Cloudina, tiny (1mm or smaller) calcitic tubes formed from internested cone-shaped shells. Unlike the skeletons of corals, these internested cones have open bottoms, making them as much like the shells of some polychaete worms as like corals. The taxonomy of their maker is unknown, but distribution is worldwide.
Soft bodied forms:A variety of animal fossils preserved as impressions on soft sediment. Many of these creatures were large but flat, with a quilted texture. These make up the classic Ediacaran fauna from Ediacara Hills, Australia. By 570 Mya these creatures occur worldwide. Found in Canada and northern Russia. In Namibia they extend right up to the base of the Phanerozoic.

But what were they? Ediacaran animals seem to fall into two broad categories:

  • Those that seem to be related to living cnidarians
  • One that seems to be related to mollusks.
  • Everything else.
Possible cnidarians: Radially symmetric impressions that probably represent anemone-like polyps but could, in some cases, be medusae (jellyfish).
  • Occupy a considerable size range
  • Are common in Ediacaran rocks.
  • Some forms are clearly colonial and show evidence of having died while reproducing by budding. (Argues that they are sessile polyps and not medusae.)
  • In one case we get what might be cnidarian tentacles (or not.)

Kimberella, an early mollusk?: An oval, soft-bodied form up to 9 cm. long. Well preserved fossils from Russia indicate.
  • There was no obvious segmentation
  • A muscular mollusk-like foot was present
  • The fossil impressions tend to be surrounded by deep grooves made by a relatively hard (but not calcified) "shell" being pressed into the sediment upon burial.
  • Between the foot and the "shell" impressions are the those of organs resembling the gills of primitive living mollusks.
  • An anterior mass may be a head with a mouth.

In all of this, Kimberella agrees with our expectations of mollusks, however there is no positive evidence for a radula - the rasping feeding apparatus found in all proper mollusks. Whatever it is, Kimberella is our best evidence for proper bilaterians with significant muscle mass.

Reconstruction

The enigmas: The remaining Ediacaran fossils are deeply controversial. In body form, they range considerably, with many like Charnia (right) being frond-like. Other body morphs include:

Two schools of thought have debated their affinities:

  • They are early members of better known recent groups
  • The represent a unique and extinct early radiation of animals.


Charnia

In the first instance:

"Vendobionts:" An alternative has been proposed by Adolf Seilacher to the effect that the resemblances between the Ediacaran biota and living taxa is illusory. Instead, this biota represents an early, but largely failed diversification of animals. He notes:

Indeed, the Ediacaran radiation of triradiate fronds certainly has no Phanerozoic equivalent. (Except maybe one).

..

As with many controversies, these positions may not be absolutely mutually exclusive. Quilted organisms like Dickinsonia may truly represent an extinct early animal radiation, but:

Maybe the Ediacaran/Cambrian boundary represents the first major animal extinction event.

Ediacaran ecology: