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.
Ediacaran Biostratigraphy:
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.
