Monday, March 1, 2010

Prep update: Purisima Formation mysticete skull 1

In 2005 I received a permit to collect vertebrate fossils from a locality in the Purisima Formation. This project lasted two summers, and resulted in the collection of dozens of shark teeth and vertebrae, calcified cartilage skate jaws, pinniped bones and teeth, several bird bones, odontocete and mysticete bones, including a handful of tympanics, a complete lower jaw of Herpetocetus, a porpoise cranium, and the big kahuna: a 300+ lb plaster jacket with some sort of mysticete skull inside. I wasn't really sure what it was in the field, although I suspected it was a balaenopterid (i.e. rorqual whale, like a Minke, Humpback, Fin, or Blue whale) on gut feeling alone.
The cranium prior to excavation, June 2005.

I first spotted the skull over thanksgiving break in November, 2004, with a flashlight just after sunset. I thought it was a skull at the time, but hadn't seriously thought about getting a permit yet. After I started the permit application process, I actually forgot entirely about the skull, and hadn't thought seriously about excavating it. When I finally stumbled across it again, I thought "why the hell not?" and up until the last few weeks, I've regretted the decision. Bottom line - I spent five months preparing the soft matrix from the ventral side, and in Spring 2006 I moved it to Museum of the Rockies, where I've been (intermittently, given my school schedule) preparing it with pneumatic airchisels since.

Day 1 of the excavation - we started at 5 in the afternoon, and stopped at about 10pm, digging by lantern, headlamp, and heavy metal.

Four years of preparing it with pneumatic tools - this skull was encased in a ridiculously hard concretion which only gets harder toward the center; concretions from elsewhere in the Purisima Formation are "nice" in that they 1) are of constant hardness throughout, and 2) the rock splits off the bone in these other cases. For this specimen, rock never splits off the bone, and in most cases I've had to prep all but 1-2mm of rock away and then grind it down with the airchisel, so some surfaces have definitely gotten a little scored. No matter, because all the sutures are visible, and it looks pretty nice anyway (unless you look at it with a hand lens). So shoot me; if I'd done it 'better', I'd have another year or two of prepwork to do. As it is, I'm leaving some matrix in the left temporal fossa, and elsewhere.

Vicki Jacklich and Liz Johnson (North Carolina State University) assist with the excavation, on Day 2.

A closeup of the skull, which is upside down. The rostrum is missing, and the vomer is the elongate bone pointing to the left. The frontal is the roughly triangular bone in the middle, the arcuate bone is the lateral crest of the supraoccipital shield, which slightly overhangs the temporal fossae (filled with matrix); the base of the squamosal is preserved at left.

I love surprises; this is a broken, partial basking shark (Cetorhinus maximus) gill raker found associated with the skull. I've found two more during preparation.

Here Vicki and I are putting the finishing touches on a preliminary jacket to protect some exposed bone.

Liz Johnson (NCSU) assisting with plaster jacketing.

The end of day 3: the skull is now trenched and tunneled under (see rock hammer) and ready for the rest of the plaster jacket to be applied.

Another view of the skull bearing pedestal after trenching and tunneling.

The completed top jacket prior to flipping on Day 4; I didn't have much time to take photos after this.

After the top jacket dried (which took over a day, due to the extremely humid air and my lack of jacketing experience), my temporary field assistant (and friend since 1st grade) Matt Berrini and I used what resources we had, and played 'egyptian' for a while: the ancient egyptians used earth for monument construction. For building pyramids, they built an earthen ramp; for raising obelisks, they had a chamber filled with sand; after the obelisk was raised by ropes, the obelisk was propped up by the sand in the chamber, which was slowly removed, allowing the obelisk to become upright with comparatively little further struggle. So - we built a fairly large 'ramp' of beach sand up to the jacket (as the skull was about 1 meter above the beach) so that the jacket did not break apart and collapse during flipping. In retrospect, the incredible density of the concretion probably indicated we could have probably let it fall anyway (although parts of the bones protruded from the concretion's boundaries, and may have broken).

Anyway, that's all for now, but soon I'll have another post about the prep process, and let you in on the last 5 years of my life.


Jose Ramon Santana Vazquez said...
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Tony Edger said...

Thoroughly enjoyed this account of your field work and look forward to more as you prep the skull. When did it become clear that only the skull was there?

Robert Boessenecker said...

I'll have a post fairly soon about the initial prep process - I'm currently in the final stages of prep (i.e. mechanical preparation will be done by tuesday, the rest of the skull pieces glued on by wednesday, and the cradles constructed by friday, and saturday will be in my car on its way to UCMP).

I figured there might not be much associated with the skull at first due to the fact that no other bones were exposed. We didn't find any bones during trenching, and there were no cervical vertebrae articulated at the back of the skull, so I figured there wasn't an associated skeleton. Additional erosion in the intervening years hasn't exposed anything new.