Embryos and Ancestors
Phil
Donoghue (Department of Earth Sciences, University of Bristol, Bristol,
Britain)
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
The
fossil record is traditionally perceived to be biased against embryological
remains. Since the Zhang & Pratt embryo paper, several finds have
been made in various places, especially in the Doushantuo Formation of South
China - has produced animal & plant/algae embryos. A growth series of
fossil embryos is now available.
Parapandorina - has a well known embryological series. No
later stages, unfortunately, for the most part (but see Megaclonophycus).
The
Doushantuo Formation is packed with embryos.
Tianzhushania/Megasphaera
(senior synonym) - Megasphaera
was originally considered an acritarch.
It is a resting cyst/cleaving embryo with an inside envelope. It's
identification is unclear - is it a pteropod?
The
South China fossil embryos occur in cherts & phosphorites.
The
Dengying Formation is Ediacaran in age, but with earliest Cambrian
dolostone having fossil embryos, too.
The
Dengying Fm. overlies the Doushantuo Fm. in the Yangtze Platform.
Dengying has cleaving embryos present.
Olivooides - attributed to a series of cleaving embryos.
Described by Bengtson. cf. loricae of some modern organisms.
Possibly cnidarian/conulariid affinities.
Pseudooides - Steiner
has recently well defined an embryological series. Has a well-defined
pinched germ band (lobopodan/onychophoran). Some later stages of
something lobopod-like - but related to Pseudooides?
Michael
Steiner has found an embryo with a membrane and Onychodictyon-like
sclerites poking through.
Markuelia has been pinned down, phylogenetically. It is
now identified as a metazoan embryo by Stefan Bengtson. Markuelia
was a direct developer. Its head is poorly preserved in Stefan’s sample,
though. The body has surface annulations - it's metameric. Stefan
suggested a lobopod affinity or an annelid affinity. Simon Conway Morris
suggested that Markuelia may be a halkieriid. Halkieriids include
the last common ancestor of molluscs, annelids, brachiopods. But, these
three groups are primitively indirect developers.
Dong
Xiping has dissolved lots of rock - now has lots of new Markuelia
samples (Markuelia hunanensis from Middle Cambrian of China). Some
have heads preserved. The Markuelia head has a terminal donut
mouth & three circlets of radially arrayed spines. ~30 mm long, ~130
segments. This information allows us to reject lobopod hypothesis -
lobopods have a subterminal mouth. This information also allows us to
reject the annelid hypothesis and halkieriid hypothesis (ideas based on
associated fossils).
New
hypothesis - Markuelia has affinities with the Introverta (nematodes,
priapulids, nematomorphs, kinorhynchs, loriciferans). Markuelia
appears to be a sister group to the Kinorhyncha, Loricifera, Priapulida
(KLP). This is cool information, but Markuelia is a direct
developer, while KLP are all indirect developers. This means that the KLP
(= Scalidophora) common ancestor was a direct developer.
So,
embryos do have phylogenetic relevance. But, we need more data/samples to
prevent fossil embryos from being curiosities.
Late
Neoproterozoic & Cambrian - shouldn’t there be a lot of indirect developing
embryos? Is this apparent bias toward the presence of direct developing
fossil embryos due to them having higher preservation potential? Someone
should do taphonomic experiments with modern embryos to find this out.
There's
only one planktotrophic larva fossil known (an indirect developer).
We
can squeeze more out of the data we have already.
It’s
only been 10-11 years since the first embryos were found and identified.
But,
dissolution of rock & microfossil fishing has gone on for a long
time, for biostratigraphic purposes. So, we're not sure if we have more
potential.
The
Aldan River basin has 5 sites with paleoembryos.
Northern
Australia has 4 sites with paleoembryos.
The
Lower Ordovician of Nevada has 2 sites with paleoembryos.
These
are all Markuelia - it disappears after the Lower Ordovician.
Dolf Seilacher: What
about this embryo taphonomic window? Is it the same taphonomic window as
for other exceptionally preserved fossils? Note the stranded jellyfish
taphonomic window (Wisconsin, Spain, India) - only in Cambrian beaches - lots
of them.
