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!