Friday, March 3, 2023

Updates from the CCNHM Lab 4: further reassembly of our basilosaurid whale mandible

Since I last wrote about preparation of "Elizabeth", the basilosaurid whale we collected during winter 2020-2021, our preparators have been banging away on it - and Ann-Frances has really exposed quite a bit of new mandibular material. Nearly all the large bones that were obvious in the limestone plaster jacket have now been removed, and it's pretty clear the entire left mandible was busted up by the mine equipment and was lightly re-cemented prior to discovery. There is still some hope that there might be some intact bone further down in the jacket.


The lower left P3 (third premolar) of Elizabeth, our as-yet-unidentified basilosaurid whale, still embedded in a chunk of the mandible.

 The fragment of jaw with a beautiful in situ posterior premolar has turned out to be the the left lower third premolar (P3) - mildly disappointing that it's not part of the maxilla, but this is good for two reasons: 1) it means that we now have a complete series of premolars in the left mandible, along with two molars; the right mandible has at least two molars, either all three of them or possibly just the P4 through M2. 2) It also means that there is still hope that the maxilla is in one piece somewhere in one of our jackets: if a fragment of maxilla was in this mandible jacket, we'd have an even worse puzzle to put back together.

 The exploded-view of the left mandible and premolars - we're waiting on some smaller fragments and won't glue any of these back in place until after we've photographed (and photogrammetized!) everything.

We also have evidence of at least two sets of teeth - there are some sub-millimeter thin partially formed enamel crowns that are hollow. These would almost certainly have to be permanent premolars, as there are already lower molars that have roots and thicker enamel - and might suggest that the premolars stuck in the jaw are milk teeth.

The fragments and teeth jigged up into position - I was focused on the teeth so I did not stick the coronoid process in place.

And here's a closeup of the premolars - what a beauty!

Now I would be remiss if I didn't speculate more on the identification. These teeth to me are looking to me so far to most closely resemble Dorudon serratus. This is great news because the holotype looks like someone blew it apart with dynamite (and also doesn't preserve any mandibular teeth). This would considerably expand the hypodigm (sample) of the species. However, I haven't yet done any serious comparisons with the type material and it's possible the comparative specimens we have at CCNHM are not identified properly. I'll have to double check Mark Uhen's (2013) basilosaurid review again!

Sunday, February 26, 2023

Geology by paddleboard and other geological (and paleontological) snapshots from the tropics

This Friday Sarah and I are heading to Playa del Carmen with my family for spring break - since 2018 we've been going to the Caribbean/tropical Atlantic for spring break, since the tropics are so damn close to Charleston and it's pretty affordable* to get to. We've done the Bahamas, Turks and Caicos, and Grand Cayman (twice) and seen some neat stuff. This time, we get to combine the tropical Caribbean with the terrestrial biodiversity of the Yucatan peninsula and the cultural/archaeological heritage of the Mayan civilization - I'm 1/4 Mexican, and while my Spanish is admittedly rudimentary, I love the food, history, and culture and this will be my third time to Mexico since I first went to Baja for Shark Week in 2019 and Mexico City a few months later. The best part about going to Playa del Carmen is the fact that we won't be anywhere near Cancun during spring break and away from all of those college students I have to be around during the school year!

 Regardless, I've been meaning to share these photos for quite some time, and since I'm getting excited for our upcoming trip to Mexico, here are a number of snapshots with no particular theme other than showcasing some neat outcrops I paddleboarded by and some various tropical fossils from Turks and Caicos in 2020 and Grand Cayman in 2022. The 2020 photos are taken with a GoPro and aren't great, but the 2022 shots I believe are all with an Olympus TG6 and the photo quality is quite good.

*For our very, very delayed honeymoon (delayed because of my Ph.D.; we wanted to go to Rarotonga in the Cook Islands, just a few hours flight from NZ, but couldn't afford it; after moving to South Carolina it took a few years to scrape anything together owing to how expensive rent is) - we thought about going to the Bahamas, but was uneasy about plane tickets. One day while walking past my colleague's office door I saw a map of Florida and saw that Grand Bahama island is actually on the same map and only like 90 miles offshore - we could probably take a boat! There's a three hour ferry ride to Grand Bahama from Fort Lauderdale and it only costs like 100-150$  per person, round trip. Once there hotels cost ~90-200$ per night. We were able to do five nights and all travel for about 1200$.

A nice afternoon paddle and workout on the canals on the south side of Providenciales in Turks and Caicos - this is taken March 18, 2020 - we were supposed to be enjoying vacation in the tropics but lockdowns started happening across the USA after we left. We flew on a plane weeks before masks were even recommended or the virus thought to be spread through respiratory droplets, unlike SARS-CoV-1. Needless to say our 2020 trip was not the most relaxing. The day we arrived was the first day of panic-buying in grocery stores on the island.


 Shortly after venturing out down the canals I noticed these beautiful exposures of cross-bedded oolites - oolites are a type of carbonate sand (oolite) made out of ooids, which are tiny limestone grains that grow through time. Many beautiful white sand beaches in the Bahamas and Turks and Caicos are made of ooids.

It may be difficult to make out here, but this photo shows that the lower part of the section has smaller scale cross-bedding than that above - trough cross-bedding, with individual beds terminating over a distance of 1-2 meters instead of being dominated by laterally extensive, seaward-dipping planar laminations above. Trough cross-bedding suggests subtidal migration of ooidal sand dunes to me, and the planar laminated oolite above is likely instead a beach deposit - the swash zone on the beach itself where the waves go up and down in the intertidal zone. If this is correct then the 3-4 meter sequence here represents shoaling from subtidal to relatively shallower intertidal deposits. I'm not certain whether or not these are early Holocene or Pleistocene sediments. There isn't much published on the sedimentology of this island, which I find surprising.

What a geologist on vacation sees! This is a large cross-bed in the upper unit, showing a bit more complex bedding than normal - there was likely some sort of channel here perhaps. Click on the lower photo to see my annotations and the pattern of bedding. According to some very, very coarse geological maps there are Pleistocene beach deposits along the south side of the island in this area, so perhaps the swash zone of the beach is a reasonable interpretation.

 A couple of queen conch shells (Lobatus/Strombus gigas) in ?Pleistocene beach deposits on the north side of Providenciales.

 And modern queen conch shells in modern ooidal sand on the south side of Providenciales. Here's a tip for those who want a nice conch shell to bring home (provided the country allows it - which Turks and Caicos does not) - find a seafood restaurant that serves conch fritters (an amazing appetizer in the Caribbean) and go exploring nearby. Frequently, you'll find some piles of shells (and trust me - you'll smell them first, so walk upwind - if you dare!) and will be able to pick out a couple of nice trophies.

A queen conch (Strombus/Lobatus gigas) shell from the Ironshore Formation near Smith's Barcadere in Grand Cayman. I have no idea if collecting such a specimen is legal or not so I left it.

Several queen conch shells in situ within the Ironshore Formation at Smith's Barcadere on Grand Cayman.


A large brain coral - perhaps
Pseudodiploria (symmetrical brain coral), one of the more commonly seen large brain corals in the area today. However, there are certain corals that are completely extinct today in the Caribbean that occur in the Ironshore Formation.

 Many transported brain coral fragments mixed together with queen conch shells - under the Dunham classification scheme for limestones, since the large bioclasts are touching, I think this would be classified as a rudstone. None of these corals looked to be in situ.

A relatively large top snail - Lithopoma most likely, species uncertain owing to the age. It seems to be ornamented with larger, smoother ribs than extant Lithopoma americana. The modern snails below are beaded periwinkles, Cenchritis muricatus, which were perhaps 1 cm in length, and this fossil shell was about 2-3 cm in diameter.

 At the other end of the island we chanced upon another exposure of the Ironshore Formation at Barefoot Beach after our last round of snorkeling on our 2022 trip. I found one of the most interesting fossils of the trip - this fossil
Pocillopora, a cauliflower coral! Pocillopora is perhaps one of the most characteristic corals found today in Indo-Pacific reefs - but preserved in the Caribbean during the Pleistocene. Fossils like this one indicate that it became regionally extinct sometime after 125,000 years, which is the age of the Ironshore Formation. Similar specimens dating to the Sangamonian interglacial ~125 Ka have also been reported from rocks of the Dry Tortugas at the southwest end of the Florida Keys.

An excellent but small brain coral in the Ironshore Formation, Barefoot Beach, Grand Cayman.
These last two are admittedly more geomorphological than geological, but I never get tired of these extreme wave cut overhangs (coastal notches) in tropical limestone. Note the solution pitting on the roof of the notch -this indicates that on some level dissolution of limestone is occurring. As it happens, this is actually a bit of a controversy in tropical geology: seawater in the tropics is saturated with respect to calcium carbonate and quite basic, so how is there any dissolution occurring, which requires a low pH? pH changes, perhaps dissolution occurs during the winter when the water is undersaturated - perhaps this dissolution is mediated by algae. Most of the notch formation is probably from sandblasting, but there is *some* degree of dissolution happening, and in other tropical settings estimated to account for 10% of notch formation (presumably by volume). I wondered how the hell these notches formed because the waves are not large, and the notch is very shallow and very deeply incised (laterally anyway). Along temperate coastlines notches tend to be much shallower, and without much solution pitting.


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.

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!

Friday, February 3, 2023

Updates from the CCNHM prep lab: mystery bone (Pliocene Purisima Formation, California)

Frequently I am able to identify vertebrate fossils in the field based on their cross-section or if a little bit of the bone is exposed: when dealing with fieldwork in the Miocene-Pliocene Purisima Formation in Northern California, perhaps 80-90% of vertebrate fossils are ribs and vertebrae from cetaceans (whales and dolphins). If I can tell something is a whale rib or vertebra, I move on - these bones are not particularly useful or informative. And, to be clear, I'm talking about whether or not such a specimen could be published - does it represent a new record for the fossil assemblage? Is it diagnostic enough to name a new species from? If I can tell it's something else - a flipper bone, particularly a humerus, radius, or ulna from a whale, dolphin, or pinniped - that's something I can work with. Some vertebrae are even useful - particularly the atlas or axis from a cetacean. Skull bones, earbones, partial skulls, and complete skulls are desirable (and in that order). Skeletons are most exciting - but very, very rare in the Purisima Formation owing to the rapid skeletonization and disarticulation that occurs in shallow well-oxygenated marine shelf settings. Usually, I can narrow down a marine mammal bone to one of these categories - 'not interesting', 'possibly/probably worth collecting', or 'holy shit!' A vertebra, possible skull bone, and a nice skull would fall into these categories (respectively). 

All of this is a long-winded way of saying when I find something in the middle category - probably interesting - I usually leave the field with the fossil, but maybe not a clear idea of what it is.That usually changes after a little bit of preparation. Rarely, do I ever continue to prepare something and have no clue what the hell it is after much of the bone is exposed - this has happened to me with Purisima Formation fossils only once before - a weird bone fragment I collected in 2007, which is still not identified. Well, it happened again! I am once again stumped. And I LOVE getting stumped. When I get stumped, it generally means that whatever I'm trying to identify is either 1) a tiny uninformative piece of something really big or 2) something really unusual and exciting. 


The mystery bone in situ in the Purisima Formation.

How I initially interpreted the specimen in the field - as a tuskless, isolated left maxilla of a small specimen of the walrus Valenictus (specimen at San Diego Natural History Museum shown upside down for comparison). I thought the part labeled in blue was the edge of a tusk socket and the foramen in red as the infraorbital foramen. However, there was quite a bit more, and the other side was somewhat cylindrical.

In December I collected a relatively large bone fragment from near the top of the Purisima Formation under a state parks permit - it's a locality I've collected and studied extensively, but is still providing new surprises and challenges. When I first spotted the bone, I thought that maybe - just maybe - it was the tusk socket from a partial walrus skull. When I pulled the whole thing out, it was certainly not that - the back side was smooth, finished bone, almost resembling a rib - though it was clearly not a rib. I shrugged and then wrapped it up in paper towel, and when I got back to my folks' house, wrapped it in a bit more newspaper and in an oversized ziplock bag for the flights back to the east coast. On Friday of last week I opened it up, and thought if I started prepping it I would have an ID before I went home for the weekend - but I was wrong.

 The mystery bone in three different views, as I left it Friday night.

  The mystery bone after a little bit more preparation earlier this week.


So far, there are a few possibilities - but let's discuss what it is NOT first: it's pretty clearly not a vertebra fragment or a rib, unless it's a very messed up (e.g. pathologic) rib. I also do not think it's part of a baleen whale mandible, though there are some vague similarities. I've also compared it with limb bones from the sea cow Hydrodamalis and walruses, for which it is no match, and can confirm immediately that it's not a part of a cetacean forelimb bone. So, some possibilities in no particular order:

1) Partial walrus pelvis. Several candidate walruses are present in the California Pliocene, and they have big, chunky pelves. This would be neat, but it's not quite a good match - there isn't anything that looks like an acetabulum (hip socket).

2) Unusual baleen whale mandible, posterior end. The expanded feature at one end looks like it could be a broken mandibular condyle, and there is a bony flange below that somewhat resembles an angular process - however, there is no mandibular canal.


3) Sea cow premaxilla. There are some similarities between this and the left premaxilla of Hydrodamalis, which is already known from the Pliocene of California - Hydrodamalis cuestae, a larger extinct relative of the recently extinct Hydrodamalis gigas. The match isn't perfect, and some parts bother me. One problem is that the texture seems to be somewhat too porous.

The squamosal of baleen whales is quite diagnostic, and there are some similarities with the squamosal of a dwarf right whale, but it is not a great match.

Comparison of mystery bone with the left squamosal of Balaenula astensis, a dwarf right whale from the Pliocene of Italy (photos by Felix Marx). The mystery bone is quite flattened from front to back. However, the small size of the mystery bone would necessitate this specimen being very small, likely a juvenile, and perhaps some of the differences in morphology could be explained by growth.

4) Squamosal from a very tiny dwarf right whale. In this scenario the expanded process I initially mistook for a tusk alveolus would be the glenoid fossa (jaw joint) of the squamosal, and the small flange opposite would be the zygomatic process. If this is accurate, it would be a very, very small right whale, smaller than the Pliocene taxon Balaenula, and perhaps as tiny as Balaenella brachyrhynus from the Pliocene of Belgium. However, there is one major problem here: the bone is just not wide or deep enough, and there's no hint of the occipital articulation - the occipital shield is the triangular part at the back of the skull, and there should be radially oriented ridges for the occipital bone along with a platform raised up from the rest of the squamosal.

5) And here we're starting to get desperate: baleen whale pelvis. There's a slim resemblance, but admittedly it is less than fantastic.

6) Some unusual sort of land mammal bone. This part of the Purisima Formation is late Pliocene, so there are already some large ground sloths and a slew of elephants present in North America. However, nothing immediately comes to mind that looks similar.

7) One last possibility, and I need to humble myself here: perhaps this bone falls into the first category from further above in the post - a smallish piece of something really big, now too small to recognize immediately.

There is more preparation to tackle, so hopefully after some more cleaning the identity will be clarified!