Monday, December 14, 2009

Partial mysticete jaw (more fossil preparation)

About two years ago I came across a partial lower jaw of a small baleen whale in a cliff. The outwardly bowed part of the jaw was facing the outside of the cliff, and the middle had eroded away, leaving the anterior and posterior ends. I immediately collected the posterior end, which was (and currently still is) in a small nodule, and awaits further preparation. However, what is preserved indicates it belongs to Herpetocetus - my favorite cetacean. The posterior mandible of Herpetocetus is very distinctive; the three main features of this part of the jaw (the coronoid process, mandibular condyle, and angular process) are all anteroposteriorly elongate relative to other mysticetes, with the angular process projecting posteriorly as a flat flange. Anyway, as the specimen isn't prepared yet, I'm not going to show you pictures of the interesting part.

After I collected the posterior part, I tried to excavate the anterior portion; I successfully removed two segments which broke along natural cracks, and a third portion (which I figured at the time was the anterior tip) which was stuck in a nodule, and was rather stubborn. I decided to leave that part in the cliff, and return later under more favorable conditions. The next few visits it remained, and I checked up on it; I figured noone else would disturb it (or even spot it). Well, I got lazy, and over thanksgiving, I returned with the intention of collecting it, and some fresh pick marks were around it, and whoever it was had chipped away a little bit of bone, much to my chagrin. Anyway, I excavated the rest of it immediately; fortuitously the other person had started a trench around the concretion, so it took a mere 5 minutes to carve around it and pop it out - and it did make a 'pop' sound and land with a resounding thud on the beach sand, to which I said to myself "if I had known it would have been that friggin easy, I woulda done it two years ago!".The anterior dentary before and after chiseling.
The problem was, I had no idea whether or not the two pieces would even connect, given the damage done by the other collector, and two years of erosion. And I had to fly with the fossil in my duffel bag all the way back here to Montana to find out. I "gingerly" chipped away some pieces of the concretion with a rock hammer and chisel, which split the rock from the bone perfectly.

The anterior 1/3 of the Herpetocetus mandible.

Then, when I got back to Bozeman, I took the fossil to campus and nervously matched up the two sides - a lot of bone was definitely missing, but (thank god) the two pieces matched up along the ventral side of the bone, preserving the natural length (which is kind of moot anyway, given that I'm missing the middle - kind of, because I measured the missing distance when I was in the field). Some major acid preparation is in order, as well as some serios airscribing and microblasting on all three pieces. These specimens are very similar to mandibles of Piscobalaena, a cetotheriid from the Pliocene of Peru, and the probable sister taxon to Herpetocetus (Bouetel and Muizon, 2006). To be totally honest, the piece of the mandible I've shown you looks pretty damn similar across much of Chaomysticeti (baleen bearing mysticetes), with the possible exception of balaenids.

The mandible of Piscobalaena nana from the Early Pliocene of Peru (from Bouetel and Muizon, 2006)

This is one of five partial Herpetocetus dentaries I've collected from the Purisima Formation (none are currently in museum collections at UCMP or SCMNH), and is the second most complete specimen; one is a complete, ~4' long, very large and robust dentary, and the others are all posterior dentaries (i.e. posterior 1/3). Two of these are extremely small (i.e. one is a fragment where the shaft of the dentary was only 2.5cm high), and likely represent neonates or extremely young individuals. These specimens, along with a nearly complete skull, a partial skull, half a dozen petrosals, and several tympanics will be the subject of a study by Jonathan Geisler and myself. In addition, there are two more possible (one definite) Herpetocetus crania in-situ, which will (hopefully) be excavated over winter break.

For more information on cetotheriids, see Alton Dooley's recent post on cetotheriids at Updates from the Vertebrate Paleontology Lab.

Bouetel, V., and C. de Muizon. 2006. The anatomy and relationships of Piscobalaena nana (Cetacea, Mysticeti) a Cetotheriidae s.s. from the early Pliocene of Peru. Geodiversitas 28:319-395.


Doug said...

Why is Herpetocetus your favorite?

Also, what becomes of all the fossils you find?

Robert Boessenecker said...

Why is Herpetocetus my favorite... well, for one, its strange; its also very tiny, roughly the size of a large pilot whale. Many of its bones are easily identified (skull, jaw, petrosal, tympanic, and parts thereof); in the Messinian (latest Miocene) and Pliocene, it is the sole surviving cetotheriid genus in the northern hemisphere - all other mysticetes are balaenids, balaenopterids, and eschrichtiids. That, and I have a LOT of material to describe, and there is still a lot to figure out about Herpetocetus (relation to Nannocetus and Piscobalaena, feeding function, etc.).

All of my fossils are destined to museum collections (I've just been slow at doing so). I've already donated several dozen specimens to the Santa Cruz Museum of Natural History, and much of my remaining collection (along with material I collected under permits) is going to UCMP over the next year (in increments, so I don't go crazy curating and cataloging stuff for a month straight).

Doug said...

Interesting. I had seen a fossil whale at the California Academy of Sciences (very long time ago, like, years before it was closed for renovation) that was very small, like, no more then ten feet long. I think the plaque said it was a new species.

That's good to hear. For some reason whenever i go north i have an urge to stop by that little museum in Santa Cruz (oddly, it's so small and yet, it's the second oldest museum in CA). When i start finding stuff, they will most likely head for museums (though i may hang on to them to see if i can't get this overly ambitious idea of mine off the ground).

Anonymous said...

Bobby, I tried to send you an email, but was unsuccessful. I saw a paper on early whale evolution that might pique your interest.

Send me an email:
solius_symbiosus at yahoo dot com for details.