Late Neoproterozoic Metazoans: Weird, Wonderful and Ghostly
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.
Letís 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.
Chinaís Doushantuo (599 my) has metazoan embryos and sponge pieces.
Thereís 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, shouldnít we see a puckered structure at the center? Nemiana doesnít have that. Andy Knoll suggests that they are compressed algal balls, like Ventricaria - sailorís eye ball (green algae). That interpretation fits Nemianaís 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, Eoporpitaís central hole doesnít 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 Narbonneís 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 Lippís 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 Fedonkinís paper. Ventogyrus has 3-D preservation - specimens got filled up quickly by silt. There is no evidence of a stem, despite Fedonkinís 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 Palauís 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.
ďCambrian RadiationĒ 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 werenít in the crown groups.