RE: Question on glide reflections in Ediacaran biota

["Thomas R. Holtz, Jr." <tholtz@umd.edu>]

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] On Behalf Of 
> Erik Boehm
> Subject: Question on glide reflections in Ediacaran biota
> 
> I've seen that many Ediacaran biota are described as having a glide 
> reflection (Spriggina, Pteridinium, Dickinsonia for example) -
yet
> the pictures I see of the fossils are (in my opinion) inconclusive.
> Can anyone comment on this or provide clear photographs conclusively showing 
> this?
> 
> I am unsure how to interpret this. If these disparate forms all share glide 
> reflections, then it seems that one should conclude
they are
> monophyletic.
> 
> Spriggina has been compared to a proto-worm-arthropod - yet I can't see how 
> it fits in with this if it really does have a glide
reflection
> symmetry.
> Perhaps "glide-reflection" forms later evolved true bilateral symmetry? but 
> then:
> 
> If you have Kimberella as a Mollusk, and Arkarua as a echinoderm (assuming 
> these are valid), then bilateria was already around for
a
> while by this point, and it seems unlikely that a creature such as Sprigginia 
> with glide-reflection independently evolved true
bilateral
> symmetry before leading to the ecdysozoa clade. (this would also imply that 
> bilateral symmetry in Deuterostomes was independently
> evolved, which isn't so hard to believe, but we'd also need to postulate it 
> independently evolved in Lophotrochozoa)
> - Or we coud just write off Sprigginia as a dead end, but then we'd be 
> missing a precambrian antecedent to the arthropods in the
> cambrian.

Chances that Sprigginia is a stem-arthropod are near nil: it's wishful thinking.

And don't forget: most non-panarthropod ecydozoans are microscopic. What makes 
you think that stem-ecdysozoans would be big enough
to fossilize in coarse sandstones?

> It would simplify things if this "glide reflection" stuff could be discounted 
> (and also make the Cambrian explosion look like less
of an
> explosion).
> 
> Ok, its not dino related, but I assume anything Paleontology related goes, 
> right?

Glide reflection is unquestionably real in some of these! I had to trace out 
specimens in class in grad school!

Must read literature on Ediacaran biota (our modern understanding thereof):
http://users.unimi.it/paleomag/geo2/Xiao%26Leflamme2008.pdf

and

http://wuos.org/content/334/6059/1091.short (especially the supplementary 
online material)

and

http://www.pnas.org/content/106/34/14438.short


Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216                        
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
http://www.geol.umd.edu/~tholtz/
Fax: 301-314-9661               

Faculty Director, Science & Global Change Program, College Park Scholars
http://www.geol.umd.edu/sgc
Fax: 301-314-9843

Mailing Address:        Thomas R. Holtz, Jr.
                        Department of Geology
                        Building 237, Room 1117
                        University of Maryland
                        College Park, MD 20742 USA