Ediacaran Organisms: Relating Form to Function
Jim Gehling (South Australian Museum, Adelaide, Australia)
"Evolving Form and Function, Fossils and Development", a Symposium Honoring Adolf Seilacher for his Contributions to Paleontology, and Celebrating his 80th Birthday, Yale University, New Haven, Connecticut, USA
1 April 2005
Vendobionts have a quilted construction. They are constructed from inflated pneu, filled with fluid. There's no evidence of muscular contraction or dehydration shrinkage. No evidence of internal organs.
Namibian Ediacarans - 540-550 m.y. - also known from western USA; includes the 1st mineralizers (Cloudina).
Vendian Ediacarans - 550-560 m.y. - seen in South Australia, White Sea; this is the time of greatest Ediacaran diversity.
Avalon Ediacarans - 560-575 m.y.
These biozones may be environmental assemblages.
Australia has three zones with Ediacarans, mostly in the Ediacara Member. They are also in the Wonoka Formation & the Uratanna Formation/Rawnsley Quartzite.
Ediacaran diversity in the White Sea area of Russia is high, but probably artificially high - probably many taphospecies.
Taphonomy of Ediacarans - terminology borrowed from ichnology - epirelief, hyporelief, etc.
Evidence of Biomats - wrinkle marks, mistaken for traces. Old rhino hide textures - molds of pustular mats - the hallmark of mature microbial mats - they often occur with Ediacarans. Petee structures - desiccated mat-bound sands. Petee structures are syndepositional, but are not dewatering/degassing features (remember, there is no clay in the South Australian succession).
Ediacarans include recumbent & stalked upright frond forms.
Ediacaran discs have very high preservation potential.
Dickinsonia and Tribrachidium - they appear rather resistant; they hold up well enough to get casts from below. Why no collapse after decay?
Often, there is an iron stained veneer on Dickinsonia fossil - there's never any clay layer that separates the molds from the casts.
Namibian 3-D distorted Pteridinium fossils - they have iron oxide coatings, acting as parting planes.
White Sea fossils - there's pyrite replacement of filamentous mats in this area, but the pyrite oxidizes to limonite by the time samples reach the lab. Pyrite filaments are well preserved & obvious in field. Pyrite death masks are the possible preservation mechanism of Ediacarans.
At Mistaken Point, Newfoundland, there is a richly fossiliferous Ediacaran bedding plane, with volcanic tuff above. Beautiful impressions of Ediacarans (Pompeii Effect). Frondose forms are relatively poorly preserved. Mats of Avalon assemblages - iron oxide stained pustular mat texture occurs below the ash bed.
65% of White Sea Ediacarans are recognized in South Australia.
Australia's Ediacaran Reserve still has lots of small taxa on uncollected slabs - Tribrachidium, Praecambridium, Spriggina, Parvancorina, Rugoconites. So, the idea that Ediacarans are all large is wrong.
Three successive beds in the Ediacara Member have remarkable taxonomic heterogeneity.
New bedding plane excavations are covered with fossils, including lots of Dickinsonia on bedding planes (including down to <1 cm in size).
Also found some Namibian style things (3-D) in new South Australian excavations.
Found tangled Phyllozoon and organic tubes on a slab - a transported assemblage.
Dickinsonia is most important fossil in the area. The smallest Dickinsonia are 6 mm maximum length (5-6 segments). The apparent alternation of Dickinsonia segments across the midline is a taphonomic artifact.
There are probably only three genuine Dickinsonia species (tenuis, costata, rex). brachina and lissa are synonyms of tenuis.
A giant Dickinsonia rex specimen has been found - the largest whole Ediacaran organisms in any collection - 1 meter long (85-90 cm long), 70 cm wide & associated with other Ediacarans.
Dickinsonia is the king of worms.
Dickinsonia elongata is a disturbed Dickinsonia costata.
Concentric wrinkles are often seen in Dickinsonia. No two specimens show exactly the same pattern of concentric wrinkles. They probably represent muscular shrinkage. Maybe dehydration.
The Russians consider the concentric wrinkling in Dickinsonia is evidence of internal organs.
Seeing a contraction history in Dickinsonia is not an uncommon phenomenon.
A torn Dickinsonia specimen has been found - its morphology is still retained (so, it does not have a pneu construction - Dickinsonia is a tissue grade organism).
Isometric growth is seen in Dickinsonia.
No particular orientation is seen on bedding planes in Dickinsonia.
Dickinsonia was tactophobic - no two ever overlap. One sample shows a retreat gap between two Dickinsonia. Gel-like barrier? Chemical repulsion? Agonistic behavior?
“Digestive caecae” are seen in one sample of Dickinsonia costata, possibly? Not sure. Only seen in two Dickinsonia, out of 1000s collected.
Dickinsonia ghosts - pedal impressions near buried specimen. They get progressively clearer close to the organism. They are serial resting traces. Dickinsonia was sitting on microbial mats, not the sea bottom. Dickinsonia moved along the mat & rotted areas of biomats. So, a biomat model - it fed by rotting mats. Analogy - doormat on a lawn. Cestodes/tapeworms - the modern functional analogue.
Kimberella - a cubo-medusoid or a bilaterian?
Dolf Seilacher: this is a nice advance with the excavation of extensive slabs in South Australia, which complement the Mistaken Point planes.
Mistaken Point is clearly deep water. Hummocky cross-stratification is not there, it turns out. So, the Mistaken Point is a slope/basin setting - very deep water. The Ediacarans there are clearly not photosynthetic.
The White Sea & Ediacaran Hills localities are clearly shallow water with sand dump events - they are ~10-100 meters water depth; no clay at all in South Australia. So, there was a transparent ocean. So, Ediacaran fossils in the White Sea area & South Australia area are in photic zone.
Flattened segments occur at Dickinsonia edges.
Sometimes pebbles have been found with Ediacaran casts/molds. So, morphological resolution is controlled by grain size.
The Russians claim Dickinsonia had segment alternation, but the Russians don’t take compaction into account.
There are many cases where segments clearly cross midline.