The unusually straight, somewhat flattened mandibles of the gray whale Eschrichtius robustus - skeleton on display at the Charleston Marine Life Center in Charleston, Oregon (not THAT Charleston!). Photo by the author.
I've been studying fossil baleen whales from the Purisima Formation of northern California for about twenty years now, and I am always learning new things. So far, baleen whales (mysticetes) from the Purisima Formation tend to belong to three families: the Balaenopteridae (rorquals), Cetotheriidae (dwarf whales, mostly my old friend Herpetocetus), and the Balaenidae (right whales). There are about a dozen species from the Purisima, over half of which are balaenopterid whales. Balaenopterids, or 'rorquals', include the blue, humpback, fin, minke, and sei whales - all the species with many elongate throat pleats. including archaic forms like Norrisanima miocaena, "Balaenoptera" cortesi var. portisi, and Parabalaenoptera (originally reported from the Santa Cruz Mudstone of Marin County a bit further north). There are also some more derived forms, like the minke whale like species Balaenoptera bertae, which I named when I was a Ph.D. student. The Purisima Formation is latest Miocene to late Pliocene in age - roughly 7-2 million years old - and most other marine rocks elsewhere dating to the same time period are also dominated by balaenopterid whales.
The ~five meter long, strongly arched mandibles of "Hope" the blue whale, Balaenoptera musculus, at the Natural History Museum in London. Photo by author.
I do plan on doing an entire blog post on the fossil record and evolution of gray whales, but a short summary is warranted here. Gray whales are in the family Eschrichtiidae, and today, there is only a single species - Eschrichtius robustus - which lives only along the Pacific coast of North America. In the past, Eschrichtius robustus also inhabited the western North Pacific as well as the North Atlantic. There are a handful of fossil gray whales. The oldest is Archaeschrichtius ruggerioi, from the upper Miocene of Italy, dating to about 7.5-11 Ma. This specimen is based on an isolated mandible with an unusual double coronoid process. I'll talk about the difference between balaenopterid and eschrichtiid mandibles later in this short article. A relatively small gray whale, originally named Cetotherium gastaldii in the 1880s from Italy, was renamed Eschrichtioides gastaldii in 2005 by Michelangelo Bisconti. This specimen has a skull that is slightly more 'primitive' than the modern gray whale - and it dates to the early Pliocene (~4-5 million years old). The only other extinct genus of gray whale, Glaucobalaena, is quite fragmentary - consisting only of paired left and right chunks of the basicranium - is from the late Pliocene of, you guessed it - Italy. The only named extinct eschrichtiid outside of Italy is Megapteropsis robusta, a chimaeric assemblage of fossils named by P.J. Van Beneden in 1872. One of the specimens is a rather well-preserved mandible, which Van Beneden figured in excellent detail. There is a well-represented but unnamed species of gray whale from the late Pliocene (2-4 million year old) San Diego Formation, known from multiple skulls and skeletons. This taxon was misidentified as a balaenopterid whale for a long time - and since gray whales and balaenopterids are sister taxa and forma clade, Balaenopteroidea, it's not surprising that some fossils would more closely resemble balaenopterids. As it happens, the genus Eschrichtius is quite derived in many ways.
Another putative gray whale, Gricetoides aurorae, was named from the Pliocene Yorktown Formation at the Lee Creek Mine. The holotype is a skull that very much looks like a regular balaenopterid, but the bulla is clearly from a gray whale. However, they are not associated. The skull, I think, is from an extinct balaenopterid, and I won't discuss it further here - other than to say that the bulla does record a gray whale in the Pliocene rocks of the western North Atlantic.
The remaining fossils of gray whales are all from the genus Eschrichtius. There's a late Pliocene mandible fragment of Eschrichtius from Belgium, a late Pliocene skeleton of Eschrichtius sp. from Japan, and an extinct species in the modern genus, Eschrichtius akishimaensis, from the early Pleistocene of Japan. Lastly, there are a few fossils from the Pleistocene of California - my colleague Cheng-Hsiu Tsai wrote a paper that I coauthored where we reported a tympanic bulla of Eschrichtius from the early Pleistocene Rio Dell Formation of California, which still is the oldest record of the modern genus from the eastern North Pacific. There is also a complete skull and partial skeleton of a gray whale, likely Eschrichtius robustus or a closely related extinct species yet to be named, from the late Pleistocene San Pedro Sand of Los Angeles. I'll summarize the eschrichtiid fossil record in more detail in a later post.
A typical mysticete from the late Miocene and Pliocene of California: the extinct balaenopterid whale Parabalaenoptera baulinensis, from the upper Miocene (~8 Ma) Santa Cruz Mudstone of Bolinas in Marin County, California. Photo by the author.
Complete skulls of fossil gray whales are quite rare, being known pretty much from Italy and San Diego. I've been paying attention to more fragmentary remains for a long time - the earbones are quite distinctive. Unfortunately, the fossils of some early balaenopterids share some features in common with gray whales, so it can be difficult to distinguish between the two. One such feature is the fenestra rotunda on the periotic bone - in Eschrichtius robustus, this foramen is open dorsally, which until recently, was thought to be unique to Eschrichtius. In all other cetaceans, this foramen is circular and surrounded by bone. Recently, a paper by Bisconti reported the periotic anatomy of the blue whale, Balaenoptera musculus, and reported that an earlier specimen was probably a misidentified fin whale. Turns out, the blue whale also has an open fissure-like fenestra rotunda. And, so do many early balaenopterids. Are some of these early balaenopterids actually outside the balaenopteridae? Are some actually gray whales, or do they diverge earlier? Indeed, there are a couple of Eschrichtius-like periotics from the Purisima Formation - but these happen to match the periotics of a small balaenopterid known from more complete skulls in the San Mateo Formation of southern California, and may possibly represent Kennedycetus pericorum from Baja California (which I saw in person at UABCS in La Paz this January). The tympanic bulla is a little more reliable, and while gray whale bullae are not rare in Pliocene rocks of the Atlantic coast - uncommon, I'd say - I've not found any obvious eschrichtiid bullae in the Purisima Formation.
The mandible of the minke whale, Balaenoptera acutorostrata, with cross sections in approximate location relative to the above diagram. Modified from Demere (1986).
The mandible of baleen whales is occasionally overlooked by paleocetologists, but generally the anatomy mandible really conforms to family-level clades. Right whales, for example, have strongly bowed mandibles with massive elevated condyles that look like basketballs at the end of the mandible, lack a coronoid process, and are twisted dramatically at the anterior tip; these mandibles are also quite thick and have a nearly circular cross-section for most of their length. Right whale mandibles can be quite large - 1.5 meters, up to about 5 meters in length. Cetotheriids are generally quite small (1-2 meters), but have a slightly bowed mandible and often an elongate, low, and laterally hooked coronoid process; the cross-section is usually oval. Balaenopterids look quite similar to cetotheriid mandibles, but differ in a couple ways - the mandibles may be as small as 1.5 meters, and up to 6 meters in the blue whale - but in the fossil record, usually smaller than three meters. The cross-section is D-shaped, or planoconvex, and often with a sharp ventral ridge; the coronoid process is finger-like and laterally hooked. The angular process is thick and separated from the condyle by a deep groove. Lastly, balaenopterid mandibles are often laterally bowed, like cetotheriids, but then medially bowed near the condyle, resulting in a sinuous shape.
The mandible of gray whales, on the other hand, is quite distinctive. The mandible is nearly straight, lacking the extreme bowing of balaenopterids, and absolutely lacking the sinuous profile. Most mandibles would be 1.5-3 meters long. The mandible is dorsoventrally deep with a flattened shape, and the mandible can also be somewhat dorsally arched. The posterior end is the most diagnostic, however. Like balaenids, the coronoid process is highly reduced to a small tubercle, and the mandibular condyle is very large and elevated - elevated far above the dorsal edge of the mandible. However, unlike a balaenid, the condyle is narrow, a bit more like a balaenopterid.
With these features in mind, it's easy to identify mandibles, often even when represented by partial specimens. This was the case with the mandible fragment reported by Tsai et al. xxx from Belgium. In my case, no specimen yet collected from the Purisima Formation clearly represents an eschrichtiid - though I would expect them to be there. Especially the unnamed species represented in the San Diego Formation - the Purisima shares many other marine mammal species, including Valenictus chulavistensis, Callorhinus gilmorei, Parapontoporia sternbergi, Semirostrum ceruttii, and others.
The partially poached mandible in the cliff. An inexperienced collector has removed much of the anterior 2/3 of the mandible but left the posterior end, and the tip in place.
But what about specimens I've seen out in the field? Here is a mandible that I saw poking out of the cliff when I was doing some fieldwork under a California State Parks permit back in about 2010 or so. While I was away at college at MSU Bozeman, I got an email from one of the rangers, alerting me to this specimen. Someone had dug out the middle part of the mandible. That was unfortunate - I had thought about excavating it, but it would have been a pain, as its location in the intertidal zone at the base of a cliff would require it to be collected in several segments to be reassembled rather than as a complete plaster jacket. Given the damage to the specimen, its risky location, and my impending move to New Zealand, I decided against collecting the specimen. But, maybe the specimen can still be informative?
The anterior end of the mandible, left in the cliff. It's difficult to see but the anterior end is dorsoventrally deep and transversely narrow - more like a gray whale (Eschrichtiidae) than a rorqual (Balaenopteridae).
The posterior end of the poached mandible in lateral view. The coronoid process is hidden, but the mandibular condyle looks like it is quite elevated - perhaps above the level of the coronoid, and certainly much higher than the base of the coronoid.
And an oblique view of the posterior end of the mandible. Here you can see that the mandible was quite straight - the excavated trough corresponding to the inner edge of the mandible curves only about 10-15 degrees or so.
A few features tell us very quickly what this mandible does NOT belong to. First, it has a relatively small mandibular condyle, and a slightly sinuous (medially bent) posterior part, and, evidently, at least a low coronoid process that is nearly raised to the level of the condyle - all of these indicate that it is not from a right whale (Balaenidae). Second, it is much too large to be Herpetocetus - the only Pliocene cetotheriid in the North Pacific - and the mandibular condyle is rounded, not set off to the side and flattened. The angular process is rounded, not like the posteriorly directed shelf seen in Herpetocetus; Herpetocetus also doesn't have a 'reflexed' posterior end. The reflexed or sinusoidal outline of the posterior end indicates that the mandible is a balaenopteroid - either a gray whale in the Eschrichtiidae, or a balaenopterid. The mandibular condyle seems quite high - much higher than what appears to be the base of the coronoid. The mandible is not strongly arched, unlike many, if not most, extinct balaenopterids. The anterior end seems quite tall and narrow. All of these features suggest that this could belong to an early gray whale with some balaenopterid-like features. However, we don't know what the middle of the mandible looks like, and the posterior end and the coronoid, at the time of my photos, were still embedded in rock. So, it is entirely possible that these features might be misinterpreted owing to an 'optical illusion' - or what I call an 'erosional mirage'. Sometimes, the way a specimen erodes out makes it look like something else, something more exotic. To borrow a phrase from medicine, "when you hear hoof beats, look for horses, not zebras." In field paleontology, we're always trying to find "zebras", and an odd plane of erosion in combination with wishful thinking has led many astray before (including myself!). In this case, gray whales are the "zebra" and balaenopterids are the "horses" (if it wasn't obvious). One important caveat here: this is by no means a fossil example of the modern gray whale Eschrichtius. This is admittedly one of a few early eschrichtiids with balaenopterid like features. We should expect to find the "San Diego gray whale" in the Purisima Formation at some point, given that they are the same age and baleen whales tend to have very large areas of distribution. Unfortunately, we'll never know - the specimen completely eroded away between the time that I left for my Ph.D. in New Zealand and before I applied for a new permit in 2015. However, this is in part OK, because some other enticing discoveries at other Purisima Formation localities have been made - and once they are prepared, I am certain will demonstrate the presence of gray whales in the same unit.
On a final note: if the person who dug this out of the cliff happens to read this, please get in touch with me. I'd love to see the specimen if it still exists.
More on gray whales later this year! Stay tuned...
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