by Constance Van Beek,
Preparator, Fossil Vertebrates
The Field Museum
Chicago, IL 60605
A 52 million year old fish fossil, a large predatory fish, Phareodus encaustis from the Green River Formation, was excavated in a Wyoming quarry and brought back to the Field Museum for fossil preparation in 2015. 'Fossil preparation' means removing rock or other materials (collectively known as 'matrix') from the fossil remains. There are various types of preparation, and removing the matrix from this ancient fish required 'mechanical preparation'.
Mechanical preparation was utilized in three stages with the appropriate tools.
1. 'Micro-jack'- a range of miniature jack-hammers that run on compressed air removed the top layers of matrix, getting close to the bones and scales without touching them.
2. 'Pin vise'- a hand-held pencil sized tool that holds a thin carbide rod, which can be sharpened to a super sharp point,was useful for delicate detail work.
3. 'Air abrasion- a miniature sand-blaster that shoots out very fine abrasive powder from a small nozzle. The air pressure and powder flow was fine-tuned to remove matrix effectively without damage to the fossil.
There were several large Phareodus candidates for this preparation project. All of them were digitally x-rayed to reveal features still hidden under limestone, and the best one was chosen for prep.
The work began with micro-jack work. The prep started on the bigger bones of the skull, then moved on to the spine vertebrae. As more bones would be revealed, pin vise was used to define delicate details of bones and thin ribs and fin rays.
The work proceeded from the skull to the torso of the fish ('length'), from its dorsal surface (top) to its ventral (bottom) ('width').
Finally, it was time for the super-fine detail work of prepping the fins and tail. There was also a light layer of matrix all over the fish in general. This detail work was done with air-abrasion, with both air pressure and powder flow set to 'low', so as not to damage the exposed bones and scales.
A final light blowing of compressed air removed all traces of abrasive powder still clinging in spots.
Total prep time exceeded 90+ hours.
Voilá! Phareodus encaustis was now ready for its close-up, on view in The Secrets Of Fossil Lake exhibit.