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Natural Science Forum / Biology / Paleontology / September 2004Pikaia... Earlier Cordates?
Question follows Wikipedia excerpt... ***** Pikaia From Wikipedia, the free encyclopedia. Pikaia is a Middle Cambrian fossil found near Mount Pika in the Burgess Shale by Walcott. Described by him in 1911 and classified as a Polychaete worm, based on the obvious and regular segmentation of the body. Conway Morris placed Pikaia gracilens in the chordates in 1979, which probably makes it one of the oldest ancestors of the modern vertebrates. Retrieved from "http://en.wikipedia.org/wiki/Pikaia" ***** Can anyone point me to chordates (or chordate candidates) predating Pikaia (~550MYA)? TIA. Regards, Brett Aubrey. > Can anyone point me to chordates (or chordate candidates) predating > Pikaia (~550MYA)? TIA. Chen J Y, Dzik J, Edgecombe G D, Ramskold L, Zhou G Q, A possible early Cambrain chordate, Nature 1995; 377: 720-22 Shu D G, Conway Morris S, Zhang X L, A Pikaia-like chordate from the Lower Cambrian of China, Nature 1996; 384: 157-58 (Claimed Yunnanozoon was a Chordate) The above two claims were disputed. See : Shu D, Zhang X, Chen L, Reinterpretation of Yunnanozoon as the earliest known hemichordate, Nature 1996; 380: 428-30 Shu D, Luo H, Conway Morris S, Zhang X, Hu S, Chen L, Han J, Zhu M, Li Y, Chen L, Lower Cambrian vertebrates from south China, Nature 1999; 402: 42-46 (Description of two species from Early Cambrian - Myllokunmingia & Haikouichthys) Janvier P, Catching the first fish, Nature 1999; 402: 21-22 (Review of the Cambrian chordates) Xian-guang H, Aldridge R J, Siveter D J, Siveter D J, Xiang-hong F, New evidence on the anatomy and phylogeny of the earliest vertebrates, Proceedings of The Royal Society Biological Sciences 2002; 269: 1865-69 (Two previously described species from lower Cambrian - Myllokunmingia & Haikouichthys - were shown to be the same species) Sheep farmer finds oldest fossil, http://news.bbc.co.uk/1/hi/sci/tech/3208583.stm (This claim of a Chordate from Precambrian is disputed - other palaeontologists think it is a kimberella - a basal mollusc. This one has not been published in a peer-reviewed journal yet) HTH  SignatureGautam Majumdar Please send e-mails to gmajumdar@freeuk.com > Question follows Wikipedia excerpt... > [quoted text clipped - 13 lines] > Can anyone point me to chordates (or chordate candidates) predating Pikaia > (~550MYA)? TIA. Gautam Majumdar has given you the list of references I would have if I'd been quicker. I'll content myself with some quibbles. Pikaia (and the Burgess Shale) are considerably younger than you say, somewhere around 520ma or a bit younger. Dates in the Cambrian have recently been moved up a bit. The period now starts at 543ma and ends at 490ma. The other chordate candidates are mostly from the Chengjiang fauna, perhaps as much as 10ma earlier than the Burgess. > > Question follows Wikipedia excerpt... > > ***** < snip > "http://en.wikipedia.org/wiki/Pikaia" > > ***** > > Can anyone point me to chordates (or chordate candidates) predating Pikaia [quoted text clipped - 7 lines] > chordate candidates are mostly from the Chengjiang fauna, perhaps as > much as 10ma earlier than the Burgess. Thanks, Gautham and John. >> Question follows Wikipedia excerpt... >> [quoted text clipped - 21 lines] >chordate candidates are mostly from the Chengjiang fauna, perhaps as >much as 10ma earlier than the Burgess. As you say, Cambrian dates have been in a state of flux for a couple of decades. The difference between Chengjiang and the Mt Stephens beds may be even more than 10ma. A couple of years ago, the Lower-Cambrian Middle Cambrian boundary in New Brunswick was radiometrically dated at 511ma. Which would make the Burgess Shale even younger if it doesn't turn out that the Lower-Middle Cambrian boundary in Western North America isn't the same as in New Brunswick. And I believe that the Chengjiang beds are very close to the classical start of the Lower Cambrian -- first trilobites. I've sort of lost track of when that would be, and even if I knew, it's probably changed. ... But maybe around 525ma? So the Burgess might be 20 million years or more younger than the Maotianshan Shales. And one other possible early chordate. Emmonaspis from the Lower Cambrian of Vermont. There are articles on both the Maotianshan chordates and Emmonaspis in the Wikipedia. http://en.wikipedia.org/wiki/Maotianshan_shales http://en.wikipedia.org/wiki/Emmonaspis <snip> > >Gautam Majumdar has given you the list of references I would have if I'd > >been quicker. I'll content myself with some quibbles. Pikaia (and the [quoted text clipped - 7 lines] > of decades. The difference between Chengjiang and the Mt Stephens > beds Your Mt. Stephen comment struck me... Mt Stephen is on the south side of Highway 1 while Mt. Burgess (Walcott's initial find) is on the north. Is the Burgess Shale on both mountains? Others as well (Mt. Field and Wapta Mountain, e.g., both closer than Mt. Stephen)? TIA. Regards, Brett Aubrey. <snip> ><snip> >> >Gautam Majumdar has given you the list of references I would have if I'd [quoted text clipped - 15 lines] > ><snip> I believe that the Stephen Formation is exposed in a number of places along a front of -- as I recall -- 35 km or so in Yoho National Park. As I rememberl the story, Desmond Collins had trouble getting permits to dig at Walcott's original quarry a few decades ago, so he asked for permits to look for additional sites, then when out and found a number of them. You could try a Google search on Desmond Collins. He used to have a very interesting paper posted somewhere on the Internet that described -- as I recall -- six different faunas with at least some soft bodied members from the Stephen Formation -- of which the Burgess is only one member. >>>Question follows Wikipedia excerpt... >>> [quoted text clipped - 27 lines] > Lower-Cambrian Middle Cambrian boundary in New Brunswick was > radiometrically dated at 511ma. You may be right. But do you have a reference for the 511ma date? I was going by memory. However, Bowring & Martin in Briggs & Crowther's Palaeobiology II (a great general reference, by the way) are actually a bit more precise than I had remembered. They put the Upper-Middle boundary at 500ma, Lower-Middle boundary at 514ma, and the lower boundary of the Atdabanian (first trilobites) at 525ma. Also according to Palaeobiology II, the Chengjiang is near the top of the Tommotian (just before those trilobites appear), at about 525ma. Assuming all that, it's possible that there's as much as 25 million years between the Chengjiang and Burgess, or as little as 11. Given the strong similarity of the biotas, I would guess that they were closer in time than the maximum. > Which would make the Burgess Shale > even younger if it doesn't turn out that the Lower-Middle Cambrian [quoted text clipped - 14 lines] > http://en.wikipedia.org/wiki/Maotianshan_shales > http://en.wikipedia.org/wiki/Emmonaspis >>>>Question follows Wikipedia excerpt... >>>> [quoted text clipped - 29 lines] > >You may be right. But do you have a reference for the 511ma date? I've seen it in several places. This one looks credible (to me anyway): http://www.uni-wuerzburg.de/palaeontologie/Stuff/casu6.htm > I was >going by memory. However, Bowring & Martin in Briggs & Crowther's [quoted text clipped - 26 lines] >> http://en.wikipedia.org/wiki/Maotianshan_shales >> http://en.wikipedia.org/wiki/Emmonaspis |
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