Sunday 18 February 2018

Early Whales of the North Pacific

The majority of living cetacean species belong to a group called the odontocetes, or "toothed whales". Most of them aren't really what we think of as "whales" at all, since the group includes all of the dolphins and porpoises. The group does, however, include a number of much larger species - some of them just really big dolphins, such as the killer whales, but others being less closely related, such as sperm whales, beaked whales, and narwhals.

The odontocetes as a whole stretch back far into the fossil record, with the oldest known examples dating back to around the dawn of the Oligocene epoch 34 million years ago. They rapidly spread across the globe, unhindered by the geographical barriers that often affect more land-based mammals. (The continents were all separate at the time, which would have helped, since it was possible to swim right around the globe without leaving the mid-latitudes).

The two oldest fossil species of odontocete that have so far been formally described are Mirocetus, from Azerbaijan, and Simocetus, from Oregon. The last common ancestor of all living odontocetes probably lived a few million years later than they did, but quite how they would have related to it is rather less clear. While both have a number of primitive features, as one would expect for animals living so far back, Simocetus is generally thought to be more closely related to living forms than its Asian counterpart.

In fact, the general part of the world that Simocetus hails from - the north-eastern Pacific - has been a good hunting ground for all sorts of fossil aquatic mammals of similar age. These include seal-like animals and primitive mysticetes (that is, relatives of today's "toothless" or "baleen whales"), as well as other animals with no close living relatives. But, while there are apparently relevant fossils sitting in museum cupboards still waiting to be scientifically described, until recently, Simocetus was the only named odontocete.

That changed in 2016, with the description of Arktocara, a 25-million year old porpoise-like animal from Alaska, and the oldest known odontocete that can be ascribed to a family that still exists. While it probably lived in coastal waters, its closest living relative is the endangered South Asian river dolphin (Platanista gangetica), although it has a surprisingly large number of fossil relatives from across the globe. But now, a third species has been added to the list.

Olympicetus avitus is known from two partial skulls dating from the mid-Oligocene of Washington state, around 28 million years ago. While not a record-breaker for age, this still places it as having lived before the last common ancestor of all living species probably would have done, alongside a number of other primitive odontocetes from across the world.

The skulls in question consist of bones that had not yet fully knitted together, and have teeth that are not fully formed. This indicates that neither belonged to an adult, and the smaller one has natural gaps in it that suggest it had likely only just been born when it died. Given that this is so, and that the fossils only consist of partial skulls and the first three vertebrae of the neck, it's obviously difficult to say how large these animals would have been in life.

Nonetheless, using a mathematical formula devised for the purpose, the researchers estimate that it was somewhere between 130 and 178 cm (4'3" to 5'10") in length. So about porpoise-sized, then; this is a time when even pilot whales were a long way off in the future.

The teeth of modern whales, dolphins, and porpoises tend to all be identical in any given animal, and they're usually all simple in shape. (The only exceptions are whales with tusks, such as narwhals). Olympicetus however, had teeth that still varied along the length of the jaw, as they do in most other mammals, and most of them were serrated, with multiple sharp points. They'd all fallen out of the skull, making it hard to know for sure what order they go in, but the one that the researchers suspect was at the back of the jaw has a distinctly molar-like shape, something not seen even in other fossil odontocetes.

This alone is enough to place it outside any living family of odontocetes, something supported by the fossil's age. It's surely a member of some early and "primitive" branch of toothed whales that arose before any of the modern sorts did, but exactly where does it fit in the larger family tree?

The answer to this question is complicated by the fact that we don't really know what the family tree of such early fossil whales really looks like, although there have been no shortage of suggestions. The author of the paper describing the new species performs his own analysis, expanding on existing methods, and concludes that all existing fossils fall into one of two groups, one of which includes (among others) Simocetus and all of the living species.

The other group maps closely to the previously named xenorophid family, which includes a number of fossil species known from the Oligocene of what is now the southern US Atlantic seaboard, but which would now (if the analysis is correct) also include the early Asian species Mirocetus. These were dolphin-like animals, with at least two known species apparently being capable of echolocation.

Olympicetus, unfortunately, shares features with both of these groups, and so is either a very "primitive" member of one or the other group, or a member of some third lineage that died out early on. Either way, it can plausibly be described as one of, if not the, most primitive odontocetes known.

One of the reasons that this is particularly significant comes from an unusual feature of the hind part of the animal's upper jaw. This is expanded in a way that suggests it must have rooted an unusually large facial muscle. And the reason that that is significant is that, in living species, this particular muscle is part of the echolocation apparatus that they use to modulate sonar pings.

Now, the reason that we know the xenorophids could use echolocation is that the relevant parts of their ear have been preserved. These don't exist in the partial skulls we have for Olympicetus, so we can't say that it could do the same, let alone how good such sonar might have been if it existed. The muscle might, after all, have done something else before it got co-opted for echolocation.

But it's an intriguing possibility that, such a short time after their first appearance, and separation from the earliest primitive mysticetes, odontocetes may already have been capable of using underwater biosonar, something so key to the lifestyles of their modern counterparts.

[Photo by Jessica Redfern of the NOAA. In the public domain.]

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