Preparation of a Devonian Acanthodian Jaw

[izak_]

I recently decided to try a new (for me) method of preparation which involves dissolving away bone in hydrochloric acid. This method was used on the specimens published in various papers on the Hunter Siltstone, an important Devonian fish locality in NSW. Fossils are dissolved in hydrochloric acid, leaving an impression of the bone which is then cast in latex and/or resin. This method is used because the rocks tend to split right through the bone itself, so you miss a lot of the features present on the exterior. After a cast of the specimen is made, it can be coated in black ink and photographed with a coating of ammonium chloride so all the details are clearer.

 

Here is the specimen I decided to try - a partial acanthodian jaw. This is what it looked like as split, just a cross section of the bone.

 

 

The first step is dissolving away the bone in hydrochloric acid. I don't have any photos of this step but I just used a store bought bottle I had laying around, not watered down or anything. I think it took 2-3 days for all of the bone to dissolve away on this specimen. Keep in mind hydrochloric is nasty stuff, definitely best to wear gloves, avoid splashing it and use it in a well ventilated space!

 

After leaving the rock to soak in water for a few days after the acid bath, I was ready to make a latex cast of the impression. The first coat of latex should be super thin to ensure as much detail as possible is captured. After the second or third thin layer, I started to apply thicker layers. The latex I used ended up being from 1999, it was only bought a few years ago so mustn't be in very high demand! 

 

Here is a photo of the latex cast and the finished specimen. In the second photo I applied a coating of indian ink for good contrast when I eventually applied ammonium chloride for photography.

 

 

This was my first time using ammonium chloride so it took a lot of attempts before I was happy with my results. I'm still a bit unsatisfied, not sure if it's because of the ammonium chloride itself or if it's just showing up imperfections in the cast more. 

 

This ended up being an ischnacanthid acanthodian jaw, either Grenfellacanthus or something new. Have linked the Grenfellacanthus paper here for those interested: LINK

 

[rocket]

well done!

[Fossildude19]

Wow. I think you got amazing results, Izak!
Congratulations on this! 

[FossilDAWG]

Really informative post.  Thanks!

 

For the general audience I will note that this method only works when the matrix is some clastic (sandstone/siltstone/shale) that is not affected by the acid.   If the matrix is carbonate such as limestone the matrix as well as the bone will dissolve and the fossil will be lost.  In cases where the matrix is limestone, sometimes the bone can be removed by scribing instead of acid before the cast is made.

 

Don

[Tidgy's Dad]

Super. 

[izak_]

Another specimen from here I finished preparing today. This rock is full of interesting things, mostly placoderm armour but also some sarcopterygian material. I think the jaw is a sarcopterygian, perhaps Eusthenodon gavini? See this paper for more info.