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Yes, the concept of "exceptional preservation" is analogous to "high yield" in economic geology, in that there are only a few extremely rich copper deposits, many more that are merely rich, and very many more that are poor but nonetheless viable sources of copper. Similarly, in petroleum geology, there are only a few giant oil fields, many more large fields, and simply oodles of small ones. So too with fossil-bearing deposits, in this case black shales, gray shales, and green shales. Wasn't it Robison who studied a series of "gray shale" deposits, especially in Utah, of about the same age as the Burgess Shale, and bearing fossils that anyone would have called extraordinary if the Burgess Shale did not exist? Priapulid tubes without the priapulids, perfect exoskeletons of trilobites without the appendages, and so on. Fossils to gladden the heart of any paleontologist, but not the very best. And currently, David Schwimmer (Columbus State University, Georgia) is working on extraordinary preservation in the Middle Cambrian Conasauga Shale of the Coosa Valley in NE Alabama and NW Georgia (USA). He presented a talk on the subject at the SE GSA meeting in Biloxi a few days ago. Cheers, Andrew Andrew K. Rindsberg Geological Survey of Alabama -----Original Message----- From: paleonet-owner@nhm.ac.uk [mailto:paleonet-owner@nhm.ac.uk] On Behalf Of joseph00@esc.cam.ac.uk Sent: Thursday, April 07, 2005 5:06 AM To: paleonet@nhm.ac.uk Subject: paleonet 'Exceptional' preservation? Something different. Having just come back from a few days' fieldwork in Wales, and returned with yet another site with 'exceptional preservation' that has been unnoticed in previous decades of study, a few questions arise. In this case, the fossils include fragile hexactinellid sponges, palaeoscolecid material, and a holothurian, from the Middle Ordovician. None of these are definitely preserving truly soft tissues, but they do rank as something very unusual. Over the past few years, we (Lucy Muir & I) have found five sites of this type, in different lithologies, and in a very small area, two of them with extremely labile tissues preserved. In the latest site, and the most spectacular of the others, the fossils are from a classic locality which has been heavily studied, and where fieldtrips have been going repeatedly for decades. The fossils aren't obvious, true - most of them are small, and could easily be overlooked - but because we're working on ecosystems rather than a specific group, we still try to collect anything that looks interesting. It can always be discarded back at the office, after all. The fossils are also rather uncommon at the new place - we got five or six really interesting specimens in a day, plus loads of trilobites, brachiopods etc. But we were working on a small, weathered exposure in non-ideal conditions, and went through a very small amount of rock. Compare the abundance with somewhere like the Soom, or even Chengjiang, and it's very rich indeed. So, the question is, just how exceptional is 'exceptional' preservation? From what we've seen (particularly with the sponges, which need to be effectively buried alive to be preserved), these deposits are actually almost the norm - we just need to have patience, and look at the fine details, the small fragments, and out they pop. Part of the problem seems to be the 'search image.' In general we look for one group, trilobites, graptolites or whatever, and tend to ignore the small, 'difficult' fossils in the field. A lot of our finds are not even visible in the field - it's only bulk sampling or chance that reveals them under a microscope. Sponges in particular can look like stains on the surface, until the spicules leap out at higher magnification. I don't doubt that the Burgess Shales of the world are truly exceptional, but how many fall into the gap between those and 'normal' deposits? There are a number of papers on single extraordinary fossils (an ostracode with soft tissue, and the favositid zooids from Canada spring to mind), with apparently no sign of any other soft tissue. Is it the case that most deposits contain the odd freak of preservation, but most are almost barren of such things, or are a large number of deposits actually rich in inconspicuous, perhaps small or easily weathered, soft tissue fossils? There's a major emphasis on the celebrated sites, with 'complete' ecosystems, which is all very well. But each of these deposits is only one environment, and for some times, we have very few such sites. If there are many more smaller occurrences of exceptional fossils, it could open a major new angle for looking at the evolution of ecosystems. So, does anyone else have these experiences of finding unusually preserved things on a regular basis, or is it just that we happen to work in a remarkable area? Hoping for some interesting responses, Joe Botting --------------------------------------------- Department of Earth Sciences University of Cambridge Downing Street Cambridge CB2 3EQ Phone: ( +44 ) 1223 333400 Fax: ( +44 ) 1223 333450 -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.308 / Virus Database: 266.9.4 - Release Date: 4/6/2005
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