Sunday 20 November 2011

When Whales Walked the Land

Protocetus, a close relative of the new species
The Eocene, the second epoch of the Age of Mammals, was a time of many strange creatures. The mammals were well established by this point, but few of the modern groups of mammal we are familiar with had yet evolved, and those that had did not necessarily look the same as they do today. Take the whales, for example:

Today, there are two basic types of whale. The odontocetes, or toothed whales, are the largest group, and include the porpoises and dolphins, as well as several larger species, including the mighty sperm whale. The other group are the mysticetes, or baleen whales, which have no teeth, and are instead filter feeders. This latter group includes the right whales and, perhaps most famously, the blue whale, which, so far as we know, is the largest animal ever to have lived.

Both of these groups first appeared at the end of the Eocene, but in those days, they shared the seas with a third, older, type of whale, that would die out during the following, Oligocene, epoch. These were the archaeocetes, and they include the original whales from which all the others later evolved.

Like most mammals, the archaeocetes did, of course, have teeth. But their teeth were very different from those found in living toothed whales. One of the distinctive features of mammals, compared with most reptiles, is that they are heterodont, that is to say, that they have more than one kind of tooth in their jaws. They have cutting incisors at the front of the mouth, followed by stabbing canines, and then grinding premolars and molars for chewing up food. Of course, not all mammals have all four kinds of teeth - herbivores, in particular, have often lost the canines over the course of their evolution - but the general principle remains the same.

But toothed whales, dining as they do, on fish and soft-bodied squid, are unusual (though not unique) among mammals in having lost this pattern altogether. The teeth of whales and dolphins are usually all the same, simple, conical shape, and, in the few exceptions, they're so peculiar and highly specialised that they can't possibly be fitted into the usual categories of incisors, canines, and cheek teeth. But this wasn't the case among the archaeocetes.

That alone would have made the archaeocetes look quite different to modern whales. When they opened their mouths, they would have revealed teeth looking more like those of a land mammal than those we are used to in, for example dolphins - perhaps more like the teeth of a long-snouted dog than anything else. Nor is this the only way in they were different from the whales that followed, as is illustrated by a new fossil reported earlier this month by Giovanni Bianucci and Philip Gingerich.

This was not the result of any fossil-hunting expedition. Instead, it was found in a block of marble dug out of a stone quarry in Egypt. When the stone arrived in Italy, the architects who had planned to use it discovered the presence of the fossil, which, being made of a softer material than the surrounding stone, made it useless for its intended purpose. Fortunately for science, rather than throwing it away, they donated it to the University of Pisa.

The fossil turned out to consist of the head, backbone and ribs of a previously unknown species of whale, now officially named as Aegyptocetus tarfa, after its place of origin. From the geology at the quarry, they determined that it lived around 40 million years ago, towards the end of the Eocene. When it was alive, it would have swum in the shallow waters of the Tethys Sea, a broad body of water that, in those days, separated Africa and India in the south from Europe and Asia in the north - parts of it remain today as the Mediterranean Sea and Persian Gulf.

The skull measures about 68 cm long, which probably means that the living animal would have been very small by modern standards - much smaller than, say, a killer whale, although still considerably larger than a typical dolphin. It had a full complement of teeth, and, together with other features, this enabled the researchers to identify it not only as an archaeocete, but as a relative of another Egyptian fossil, Protocetus. In the paper, they make much of the unusual sloping shape of its head, something that sets it apart from its relatives, although they concede that they have no idea what the significance of this might be.

The skull has a slight asymmetry, a feature seen much more strongly in living whales, and the result of the movement of the nostrils up onto the forehead to form a "blow-hole". The placement of nostrils further back on the head is a common feature of aquatic mammals and reptiles, presumably making it easier for them to breathe while swimming. Seals, for example, may not have a blow-hole, but they do have nostrils placed much further back than those of a dog or horse, and this animal was probably much the same.

The way that the original block of marble had been cut into slabs also meant that it was possible to see cross-sections of the nasal cavities, and these turned out to contain large sinuses and well-formed turbinate bones. These latter bones form baffles within the nasal cavity that are covered with cells responsible for our sense of smell, which this animal presumably had to a fairly high degree. That's significant, because living whales are quite different, having little, if any sense of smell, since, being below water for so much of the time, they have no use for one.

Furthermore, the backbone has strong spines that, in life, would have anchored some of the muscles mammals use when walking. Although its legs weren't preserved, this implies that the animal was, at the very least, capable of moving about on land, and perhaps finding food there. On the other hand, the structure of its ears suggests that it would have been far better at hearing underwater than it would have been in air.

Taking this mixture of features together - smell adapted for land, hearing adapted for water, and so on - we can say that, unlike any living whale, Aegyptocetus was only semi-aquatic. Although there's no way to know for sure, it may have lived somewhat like a seal, able to move about on land, but doing much of its hunting in the sea.

We know that it died at sea, because of some other interesting features of the skeleton. There are bite marks on some of the ribs, matching the teeth of a shark, so the animal was clearly attacked at some point. The attack may not have been fatal, and the fact that the ribs are still there certainly suggests that the shark can't have eaten too much of it even if it was, but it does tell us something of the animal's life. More significantly, perhaps, the skeleton shows marks from barnacles that must have encrusted it after it settled to the bottom - a clear indicator it was underwater at the time. The barnacles are mostly found on one side of the skeleton, so we can even tell which way up the body was lying in the soft sediment of the sea floor.

Aegyptocetus lived too late to be the direct ancestor of modern whales, but its existence shows that, even as its cousins were just beginning to take to the sea full-time, some whales continued to live along the boundary, taking the best from both the dry and wet worlds.

[Image from Wikimedia Commons]

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