HEAVY LIQUID SEPARATION TECHNIQUES FOR MICROVERTEBRATE FOSSILS FROM A NEWARK SUPERGROUP DEPOSIT AND IMPLICATIONS FOR OTHER SITES

Extracting, or picking, fossils from screenwash concentrate is by far the most time-consuming aspect of microvertebrate studies. We report here on a successful use of heavy liquids (sodium polytungstate or ‘SPT') to separate fossils of diverse microvertebrates (fish and amniotes) from concentrates of the Upper Triassic Moncure microvertebrate site, Sanford subbasin, North Carolina.

The basics of heavy-liquid separation have been reviewed in the literature, we report the following innovations, recommendations and findings:

1. We have, through water-density tests and SPT trial and error, attempted to quantify the ideal densities for various types of fossils (fish scales, reptile teeth, bone fragments, etc.)

2. We have, through random sampling of the “floated bodies,” run simple counting tests of the effectiveness of the SPT separation and found a fossil occurrence of less than 0.4%, with the residual fossils all being imbedded in another, less-dense substrate.

3. We use an immersed screen to facilitate recovery of the fossils and other “heavies” without decanting SPT. We also keep all containers holding SPT solution inside other, larger containers, thus avoiding SPT loss from drips or spills during ladling/general working.

Our procedure differs from the reviewed accounts on several other points, and a basic outline of how to use SPT in the manner we did would be as follows: weigh the SPT then mix it in with water until the desired density is achieved (2.73-2.77 g/mL for the Moncure site), pour resulting solution into a deep (>7cm) container with a mesh net with a long (nonmetal) handle or drawstring at the bottom, and then pour in the sample that requires separation. Leave the sample in the solution for 5-10 minutes, stirring regularly to avoid rafting.

Once density differentiation is achieved simply take the ladle and, entering the solution obliquely (gently submerse only the edge of the ladle), scoop out the float and pour the ladle's contents into a funnel lined with a layer of mesh netting and an unbleached coffee filter and allow the SPT solution to drip into a graduated cylinder for reuse. We strongly recommend frequently rechecking the SPT solution's density at this stage. Once the entire sample has been processed, simply extract the immersed net and remove the fossils and other heavies from the net/screen by immersion in DI water.