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Hi all, I thought this specimen was worth posting! This placoderm from the Taemas/Wee Jasper area of southern NSW has beautifully intricate internal preservation. Not certain what the hollow structures are but perhaps they're blood vessels? It was acid prepped it quite a few years ago when I was far less experienced (not that I'm that experienced now), so I'm surprised it came out this well. Edit: John Long said as follows, "Looks like a buchanosteid left rear part of braincase with spinal nerves enclosed in perichondral bone tubes" This paper provides some context on the fossils from the area. The fish fossils from here are renowned for their intricate preservation! LINK Excuse the patchy ammonium chloride application, these photos took a long time to process so I dread redoing them.
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A few months ago I started to prepare this specimen which we collected five or so years ago near Wee Jasper, NSW. Specimens from this site must be acid prepared since they are encased in a hard limestone, making mechanical preparation impossible. Included will be my method for preparing this specimen, as well as my mistakes! Placoderm fossils from this site are typically found as black cross sections, just like the pictured specimen. Bone can be distinguished from other fossils in most cases due to the bumpy texture present on the exterior surface. To consolidate the specimen before working on it, I applied very dilute paraloid-b72 to the bone. At this stage, a dilute solution is best to ensure the paraloid can penetrate as deep into the bone as possible. If the ratio of paraloid to acetone is too high, the solution will be too viscous and the paraloid will only coat the surface of the bone. Next, I air scribed the bulk of the matrix I wanted to remove in order to greatly speed up the process. Here is the specimen after a couple of baths in 8% acetic acid (double strength store bought vinegar): Between acid baths, I let the specimen soak in water for a couple of days to remove any residual vinegar and prevent the build up of crystals which may form inside the bone and damage it once it dries out. Once dry, I applied more of the dilute paraloid to the freshly exposed bone with an eye dropper, making sure the bone was soaked and well consolidated. A photo showing matrix carefully removed with an air scribe between acid baths: I think it was around this point I made a very annoying and easily preventable mistake - I broke the fossil while moving it. It was easy to glue the pieces back on with a stronger, more viscous solution of 20% paraloid but now there are some ugly cracks through the specimen in places. I opted for paraloid instead of superglue because it is easily removable, if I'm not satisfied with my reattachment I just have to apply some acetone and the glue will dissolve again. I broke a couple of other pieces off too which is incredibly annoying, fragile fossils and I don't tend to mix well! Here are some photos of the finished specimen: Note the cracks on the right side of the specimen in this photo. I can't remember how I broke so much off at one time but it is incredibly annoying and so easily avoidable! The fracture towards the left side of the specimen here is natural, it may have been filled in by a band of silica which makes up much of the remaining rock. The crack in this photo was a natural fracture, but it looks like a larger chunk of bone fell off when I was gluing it back together before I started the acid preparation. This photo was taken at almost same angle as the "before" photo below it for reference! Here it is easy to see where the blueish limestone dissolved away, leaving only the bone and the bands of silica which make a convenient stand and support for the fossil. Thank you for reading:)
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G'day, We recently found this partial placoderm skull at a property near Wee Jasper, NSW and I'm wondering what part of the skull it is. We found a lot of other stuff on that trip too, and I'll do a trip report on that eventually. Thanks, Izak
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