Saturday, February 18, 2023

Updates from the CCNHM prep lab 2: more progress on our basilosaurid whale

For a quick recap of the excavation of "Elizabeth" our basilosaurid whale we collected during Covid winter, check out these posts: Part 1  Part 2  Part 3  Part 4


The beautiful right mandible and molars of the small basilosaurid whale, with Sarah Boessenecker's (admittedly tiny) hand for scale.

The end of the excavation - a large jacket being carried to the truck by a comically oversized front-loaded.

Back in November 2020 we managed to go on a brief three hour collecting trip at Giant Quarry in Harleyville, South Carolina, that resulted in the discovery of a partial basilosaurid whale skeleton - which we collected over an additional 7-8 days of excavation spread out over the following two months. For me personally it was a pretty stressful excavation given the timing (middle of the semester), short days (winter), cold temperatures (winter again), volunteer-based crew (couldn't always find good help, and sometimes folks disappeared to go find shark teeth instead of digging), and the fact that it was my first ever "big" excavation (constant worry about the jackets being too heavy to remove or breaking during flipping).


 Ann-Frances and I cutting the jacket open back in October behind the building.

It took a while but after we finished preparing the skull and mandible blocks of a partial skeleton of the Oligocene dolphin Xenorophus (which I'll cover in a different blog post), we were able to start opening up the smaller blocks from the Harleyville whale in September. By October our two preparators, Ann-Frances and Rebecca, had zipped through most of the small blocks and were chomping at the bit for more. We opened up the block that I initially suspected contained the right mandible - and within a few hours of work Ann-Frances had turned up several unexpected teeth and a jumble of bone fragments. After a bit of exploratory prep, the condyle of the suspected mandible turned up - but did not clearly connect to anything looking like an in situ mandible. While in the field, we saw that there were some filled troughs with loose limestone that had clearly been cut by the large hook-shaped 'rippers' on quarry bulldozers* that cross-cut all of the quarry surfaces. I knew that several large cuts, spaced out about three feet apart, had removed parts of the skeleton. In this case, it seems as though the limestone had become re-cemented. I recalled limestone matrix that had once been soft becoming rock-hard on my boots within a day or so of drying out, which confused me at the time - but I now realize was a bit of foreshadowing. Once having this realization I feared that this mandible jacket might only have fragments inside.

*Quarry bulldozers deploy these rippers and drag them across the limestone surface, breaking up the limestone into disaggregated blocks so that they can be pushed along into piles, and then scooped up by front loader tractors into dump trucks. The limestone is too hard to directly excavate into by a single dedicated machine.

The mandible fragment with in situ p1 (1st premolar, left) and p2 (second premolar, right), exposed when a large piece of limestone popped off the top of the block.

 And after some additional preparation...


The reassembled fragment now fully prepared - I would like to photograph and perhaps photogrammetize this tooth before we glue the mandible back together here.

 My fears were substantiated by further preparation, however - some of the fragments are quite good. Individual teeth, no matter how fragile, somehow survived the process, and Ann-Frances quickly located a section of jaw - what I originally thought was the maxilla, but after further inspection, is in fact part of the left mandible including the in situ p1 and p2. This jaw fragment was discovered when a large slab of limestone from the top of the jacket split horizontally right through the fragment - giving a nice view of the tooth roots of the p2.

 The upper right P3 (?) and perhaps the upper left P4 or M1, and some maxilla fragments, which were sitting below the section of mandible photographed above.

 One lower molar completely prepared and another emerging from the limestone. The lower molars of basilosaurids are distinctive in that they only have accessory cusps on the posterior (distal) side of the crown, and a smooth ridge on the anterior (mesial) edge.

Both of the lower molars removed. A fracture running through the tooth on the left resulted in collapse of that missing cusp, which Ann-Frances is confident she can reconstruct - these teeth are hollow and incompletely formed since this is a juvenile, making them quite fragile.

After removal of this fragment, additional mandible fragments were uncovered. Last weekend, Ann-Frances was on a roll and located five additional teeth - the tip of one crown, two complete lower molars, an upper molar, and what I think is an upper premolar (P3 or P4) still in a chunk of maxilla. Several additional maxilla fragments are present. If identified correctly, this is major - we've got both maxilla and mandible fragments.


The reassembled parts of the left mandible as of yesterday (Feb 17). It's looking quite good!

Though I was initially quite disappointed at the scatter of bone fragments in the supposed left mandible jacket, I know that we have at least one very well preserved right mandible. And regardless - many mandible fragments were able to be reassembled today, and many mandible fragments were collected from the surface. Nearly the entire coronoid process was reassembled today, as was much of the ventral margin of the left mandible.

The many possible identities of our specimen - Zygorhiza, Dorudon, Chrysocetus - more likely one of the latter two. Images from Kellogg (1936: Review of the Archaeoceti) and Uhen (2013: Review of North American Basilosauridae).

As for the identity - we're no closer at this point, though now that some teeth are out of the matrix, some direct measurement-based comparisons can be made. The problem is that there are no fewer than three named small-bodied basilosaurid whales in the uppermost Eocene (Priabonian stage) rocks of the southeastern USA: Dorudon serratus, Chrysocetus healyorum, and Zygorhiza kochii. Zygorhiza is asserted to be absent from rocks on the Atlantic coastal plain in the Carolinas, but an occurrence beyond the gulf coast would not surprise me. Problems arise with Dorudon serratus, which is only known from the holotype juvenile specimen which only preserves deciduous cheek teeth and little informative cranial morphology (which, mind you, does not vary much among basilosaurids in the first place). To frustrate matters somewhat, Chrysocetus healyorum is also from the uppermost Eocene of South Carolina, but direct comparisons with Dorudon serratus were not made by Uhen and Gingerich (2001), only indirect comparisons with adult Dorudon atrox - which I understand, since there are no overlapping tooth morphology between Dorudon serratus and Chrysocetus healyorum. Chrysocetus is interpreted as being monophyodont - but maybe it wasn't, and it and Dorudon serratus could be synonymous, as there isn't much morphology to separate them. As for our specimen - such a fossil could help weigh in on possible synonymy. At present, the teeth are nearly completely smooth and lack any enamel striations seen in Zygorhiza, and so Dorudon and Chrysocetus seem possible identities for now.

This specimen is a juvenile, and it has many permanent teeth including lower molars - which do not have milk (deciduous) precursors, and several very well-developed premolars that may be milk teeth. This specimen is critical because one way or the other, it 1) might help clarify whether Chrysocetus actually is monophyodont (one set of teeth) or 2) help clarify what the permanent teeth of Dorudon serratus look like. It's a huge win-win for us!

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