Late Neoproterozoic Metazoans: Weird, Wonderful and
Ghostly
Jere
Lipps (and
see) (Department of Integrative Biology & University of California
Museum of Paleontology, University of California at Berkeley, Berkeley,
California, USA)
Paleontological Society short course:
"Neoproterozoic-Cambrian Biological Revolutions", Denver, Colorado,
USA
6 November 2004
What
are late Neoproterozoic organisms?
Baldauf
(2003) - there are 8 supergroups of eucaryotes.
Opisthokonts = fungi +
animals + choanoflagellates
Choanoflagellates
are 1-celled eucaryotes with a collar and flagella. There are colonials
ones too.
Let’s
focus on choanoflagellates and animals.
Based
on molecular interpretations, the 1st animals were around from 1200
to 670 m.y. Such dates are not reliable.
Opisthokonts
have been sequenced - where do choanoflagellates go in relation to metazoans
(the 1st are sponges)?
metazoans
choanoflagellates fungi
\
+
mesomycetozoans
/
\________________/
/
\
/
\___________________/
\
Choanoflagellates
and sponges have long been considered to be related to one another (not a new
observation/idea), since the late 1800s.
Molecular
sequencing shows that this relationship holds true.
Choanoflagellates
have certain genes and gene sets that only occur in metazoans, as well.
So,
there is molecular, genetic, and morphological evidence for choanoflagellates
and metazoans sharing a common ancestor.
No
fossil choanoflagellates are known, not even in the Neoproterozoic-aged
Doushantuo Formation of China (599 m.y.).
Molecular
sequences are an historical record, but just not very well dated.
Ediacarans/Vendozoans
- part of the metazoan crown group? Are they a stem group? Are they
animals at all?
There
are many hypotheses about their identity - fungi, bacterial colonies, lichens,
sister groups to animals, etc.
Most
Ediacarans are demonstrably metazoans, but there are some exceptions.
China’s
Doushantuo (599 my) has metazoan embryos and sponge pieces.
There’s
plenty of evidence that animals were around during the Ediacaran.
Metazoans
in stromatolite horizons? They may have been there, but if so, they were
probably tiny (meiofauna), but possibly recognizable if they were disrupting
sediments.
Metazoans
did inhabit microbial ecologies during the Ediacaran/Vendian.
Example: elephant skin
texture - microbes + eucaryotes (small ones).
Stromatolite
horizons were probably a rich area for animals to develop (rich food supply).
There
are 4 major kinds of early fossils - discoidal/spherical forms, fronds, body
impressions, tracks/trails.
Nemiana - a circular fossil that is 0.5 to >1 cm in size
(it's variable). Is it a burrowing sea anemone? Is that the proper
interpretation? Nemiana has concentric folds at the edges.
If it is a burrowing sea anemone, shouldn’t we see a puckered structure at the
center? Nemiana doesn’t have that. Andy Knoll suggests that
they are compressed algal balls, like Ventricaria - sailor’s eye ball
(green algae). That interpretation fits Nemiana’s
preservation. So, Nemiana is apparently not an animal at all?
Palaeophragmodictya - medusoids? upside down jellyfish?
anemone or anemone-like? holdfasts? A few specimens have fronds
stuck on. Can get circular pods of bacterial cultures/colonies with
stringy things coming off. But, Palaeophragmodictya is not
variable like that. It is probably an animal.
Hiemalora - there are many hypotheses concerning its
identification.
Eoporpita - from the White Sea; 10 to 15 (20?) cm in size.
Cassiopeia-like? Cassiopeia is a modern upside-down
jellyfish. Modern lagoonal burrowing worms in burrows make structures
identical to this. But, Eoporpita’s central hole doesn’t go into
the rock.
Dickinsonia-types. They range from a few cm to 1 m long.
They are detrito-herbivores that digested microbial surfaces. Example:
a slab of the dickinsoniid Yorgia
of the White Sea has 3 body impressions + the actual Yorgia - this slab
records the Yorgia moving from one place to another. The
underside of Yorgia has pathways (digestive juices or transportation
system was present). Yorgia seems to be an animal.
The
White Sea deposits have fecal trails or grazing trails (the latter identified
as Xenophyophora by Dolf Seilacher).
Kimberella - a slug? Fossils of this are depressions with
a mantle. It was a detrito-herbivore scraping substrates and mats.
It had no hard shell but had a muscular mantle. Kimberella
specimens are known with scratch marks in front (made by a proboscis - scraping
a mat). There could be other hypotheses that account for such specimens,
but the mat scraping hypothesis is best.
Frond-like
fossils (Charnia).
See Guy Narbonne’s new publications. Are they high-suspension feeders
with zooids? They are tiered in a partial way - some were at intermediate
levels, some were higher up.
Mistaken Point
spindle forms - lay on the bottom. Mistaken Point fossils are
preserved in event beds, buried by ash beds.
There
is a misconception about soft-bodied organisms and soft-bodied fossils.
Non-dehydrated
jellyfish are rigid and turgid. Jere Lipp’s son tossed a
non-dehyrdated jellyfish once, and nearly knocked Jere Lipps over.
Event
bed deposition - soft-bodied organisms have enough rigidity to get
preserved.
Ventogyrus (and see)-
a predator? See Mikhail Fedonkin’s paper. Ventogyrus has 3-D
preservation - specimens got filled up quickly by silt. There is no
evidence of a stem, despite Fedonkin’s reconstruction. Ventogyrus
was an unattached benthic rhodaliid siphonophore that was hand-grenade sized.
Tribrachidium - who knows what it is? Possibly a low-tiered
filter feeder (of phytoplankton or detritus that gets kicked up), in microbial
mat communities.
The
only Ediacaran-aged shelled form is Cloudina
- it's known with holes in shells (predatory holes? maybe cyanobacterial
holes? - cyanobacteria can erode foram tests & corrode carbonate grains;
are cyanobacteria etching surfaces simply because they live there?)
Soft-bodied
predation - the Ediacaran is often viewed as a time with no predation going on
(Mark McMenamin's Garden of Ediacara). Note Palau’s
Jellyfish Lake - a 400 x 100 meter sinkhole full of jellyfish that migrate
during the day from one side to another, following sunlight, staying out of
tall cliff shadows. Entacmaea anemones in the lake eat Mastigias
jellyfish. This interaction leaves no record, not even partially
eaten carcasses (if any). Jellyfish are eaten whole by sea
anemones. Note cannibalism in the ctenophore Beroe - also leaves
no evidence of predation.
Communities
during the Ediacaran/Vendian (544 to 580 my) - probably had some predation
going on.
Ediacaran
communities are fairly homogeneous in all global occurrences. Why?
It may be due to the late Neoproterozoic plate tectonic configuration - there
was a large southern hemisphere supercontinent, resulting in a latitudinal
distribution of coasts, not longitudinally-oriented coastlines (as
today). Longitudinally-oriented coastlines result in extreme
biogeographic differentiation.
The
Ediacaran communities had normal, but short, trophic structures, with ~rampant
predation. Predators were tentaculate.
Did
Snowball Earth
(and
see) give rise to animals? Afterward, released meiofaunal animals
would have radiated. The end of Snowball Earth would also have changed
oceanographic circulation (both surface & deep waters), changing nutrient
supply & increasing primary productivity.
“Cambrian
Radiation” is a better term, compared with "Cambrian
Explosion". More than just metazoans were involved.
There
are ~200 species of Ediacaran/Vendian critters - not all those names are
valid. Not all are metazoans. Those that are metazoans weren’t in
the crown groups.
