The Gardens of Ediacara
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:
- Oxygen had been accumulating in the atmosphere for over a billion years. By this point, its concentration was roughly 10%.
- The ozone layer allowed life to colonize surface waters.
- The Snowball earth episode (830 - 600 Ma) was in its waning phases.
- Single-celled eukaryotes were common.
- Stromatolites and other bacterial mats were common. Indeed, shallow ocean bottoms were often covered in bacterial/algal mats - an environment fundamentally different from that of the Phanerozoic.
- Ambiguous traces of red, brown, and green algae.
- Molecular data suggests many true animal lineages had diverged at around 1 Ga.
- Some controversial trace fossils from c. 800 Ma (some date to 1100 Ma, others think only 570 Ma): only bilteralian animals (with proper mesoderm) can move sediment around in this fashion.
- Phosphatized eggs and embryos of primitive animals in China from 600 Ma
- Definite animal-generated trace fossils by 570 Ma
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
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.
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.
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:
- Spriggina is viewed as an arthropod-relative (albeit soft bodied) close to the ancestry of trilobites:
- Tribrachidium is viewed as an echinoderm-relative (albeit soft bodied) close to the ancestry of edrioasteroids:
- Charnia and other frond-animals are viewed as an early Pennatulaceans - i.e. sea pens:
- A unique (and extinct) radiation of multicellular forms with a quilt-like body
construction: see here and
- May have fed by chemosynthesis (there is no evidence for mouths or anus in these
- Some up to 1 m or more across, but invariably thinner than a slice of bread.
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.
- If molecular-clock based estimates for the divergence dates of major metazoan groups are correct (and they don't have to be) in going back to 1 gya, then the ancestors of major taxa must have been present in the Ediacaran.
- But for Cloudina and the bare beginnings of the primarily Cambrian small shelly fauna (later) at the end of the period, no Ediacaran organism was able to secrete a hard skeleton. When skeletons appear in the Cambrian, they appear roughly simultaneously in many lineages. SO, we would only expect soft-bodied representatives of major lineages in the Ediacaran, and in most cases, their life habits would have to be distinct from their later cousins with skeletons.
- Teh Ediacaran fauna was a shallow-water fauna, ranging from intertidal depositional environments to the upper continental slope.
- None of the Ediacaran fauna apparently burrowed down into the sediment, nor had preservable
hard parts. The only trace fossils are simple and confined to the sediment/water interface.
- Recall that Proterozoic substrates were usually covered by algal or bacterial mats.
This is a fundamentally different environment than the ones that predominate today. We envision the Ediacaran organisms as:
but NOT burrowing significantly into the substrate.
- Rasping or scratching the mat for nutrients. ("Ediacaran mat-scratchers" E.G. Kimberella)
- Lying on sunlit sea floors chemosynthesizing with the aid of photosynthesizing symbionts (zooxanthellae). E.G.: Dickinsonia
- Suspension feeding. E.G.: Charnia
The transition to a modern sea-floor ecology in which infaunal animals figure prominently began with the Cambrian substrate revolution