Sunday, February 26, 2023

Updates from the CCNHM lab 3: Oligocene dolphin (Olympicetus) partial skull with earbones - acid preparation


 A specimen of the early toothed baleen whale Fucaia bubbling away in an acid bath in 2018 - specimen collected and donated by Terry Iversen.

Since 2016 I've been continuously preparing fossils donated by super collector James (Jim) Goedert from the Pacific Northwest - Jim has written nearly as many papers as I have but considers himself an amateur collector, though he's anything but; his incredible collecting earned him the Strimple Award from the Paleontological Society a few years ago (2019), which recognizes outstanding achievements in amateur paleontology. Jim and the late Gail Goedert (who passed away a few years ago) collected extensively in Oregon and Washington in the 1980s and since the 1990s focused their attention on the Olympic Peninsula. Jim today collects all over, but the units he has targeted for fossils earmarked for my studies at CCNHM (Mace Brown Museum of Natural History) are chiefly from three rock units, in descending order of importance: Pysht Formation ("middle" Oligocene), Makah Formation (early Oligocene), and the Lincoln Creek Formation (late Oligocene). Jim knows that we aren't set up at CCNHM for a lot of mechanical preparation of ultra-hard matrix, so he's thoughtfully sent us hundreds of pounds of specimens that occur in highly soluble siltstone concretions that can instead be acid prepared. Perhaps half to two thirds of specimens he sends us are from the Murdock Creek locality, where the holotype of the simocetid-grade dolphin Olympicetus avitus was collected (also by the Goederts).

Here's the holotype (left) and paratype (right) skulls of Olympicetus avitus from the Pysht Formation. These fossils are about 26-28 million years old. From Velez-Juarbe (2017).

A skull of Olympicetus sp. from the same locality, the first one I ever finished acid-prepping. My colleague Rachel Racicot took the periotic on loan and micro-CT scanned it, and found that it didn't seem to have any cochlear adaptations for echolocation! We published a paper in 2019 that this specimen suggests that echolocation likely evolved twice: once within the Xenorophidae, and once within 'main line' odontocetes, but after the 'simocetids' (Racicot et al., 2019). This skull is very small, maybe 2/3 the size of the Olympicetus holotype, and represents a perinatal individual (young calf, likely under a year old).


One of these small concretions, field number JLG 387, was sent to us in 2016 and I didn't start acid prepping it until last fall. It looks like a nondescript gray siltstone boulder at first glance - but, Jim inspects every surface of these concretions before moving on...

...and behold! A tympanic bulla. This is a cross-section through the outer lip of a tympanic bulla - so at least there's one earbone. I did not know at the time if the thick part (involucrum) or just a bit of the edge was preserved. Fortunately, about 2/3 of it was in there.

 Concretion JLG 387 after about three weeks of acid prep. A second tympanic bulla has emerged! It's the bulbous thing on the upper left with a dark spot on it on the right photo.

 A few weeks later - much of the new bulla has emerged on the left, and it is in articulation with the squamosal! This suggests a periotic is sandwiched in between. There are also at least four teeth preserved. the original tympanic bulla exposed in cross-section is on the lower right side of the block in this image.


JLG 387 after a few more weeks of acid preparation - the periotic is now nicely exposed, and the first bulla has been freed completely from the concretion.

A closer look at the exposure of the articulated periotic and tympanic bulla of JLG 387, about 2-3 months into preparation.

And what's left in acid as of this past week: S, squamosal; P, periotic; B, bulla; H, hyoid (stylohyal? thryohyal?); F, frontal(?). I'm pretty excited about this specimen because the ossicular chain, accessory ossicle of the bulla, periotic, tympanic bulla, and squamosal are all still in articulation. I think I will leave them in articulation so we can put the whole thing into microCT. The specimen should be fully prepared within a week or so!

And to bring it back to the beginning, here's the main part showing the right squamosal outlined in red and in comparison with the Olympicetus avitus holotype skull. The bulla would be exposed on the ventral side and completely obscure the periotic from view - the side we're looking at faces the endocranial space where the brain is, and we can only see it here because the squamosal has disarticulated from the rest of the braincase.

So, what do we stand to learn from this individual? We have many other skulls with periotics that represent Olympicetus avitus - probably the most common species in concretions at Murdock Creek. This one in particular is significant as it is the first where the bulla, periotic, squamosal, all three ossicles (malleus, incus, stapes) and the accessory ossicle are preserved in situ and in articulation, so I'll do my best to keep them preserved that way. There's some interesting stuff going on wiht the teeth, and this is only the second or third specimen I know of with any part of the hyoid apparatus - critical for interpreting the early evolution of suction feeding adaptations (or, lack thereof for that matter) in dolphins.

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