A restoration of Mammalodon by Brian Choo (published in Fitzgerald, 2009).
In the introduction to his 1883 lecture on whales, the English anatomist William Henry Flower said;
Few natural groups present so many remarkable, very obvious, and easily appreciated illustrations of several of the most important general laws which appear to have determined the structure of animal bodies, as that selected for my lecture this evening. We shall find the effects of the two opposing forces–that of heredity or conformation to ancestral characters, and that of adaptation to changed environment, whether brought about by the method of natural selection or otherwise–distinctly written in almost every part of their structure. Scarcely anywhere in the animal kingdom do we see so many cases of the persistence of rudimentary and apparently useless organs, those marvellous and suggestive phenomena which at one time seemed hopeless enigmas, causing despair to those who tried to unravel their meaning, looked upon as mere will-of-the-wisps, but now eagerly welcomed as beacons of true light, casting illuminating beams upon the dark and otherwise impenetrable paths through which the organism has travelled on its way to reach the goal of its present condition of existence.
As presented by Flower, whales were excellent examples of evolutionary change. They were mammals well-adapted to life at sea and yet they still retained anatomical quirks which testified to their origin from terrestrial creatures. (And, interestingly, Flower was one of the first naturalists to suggest that whales had evolved from artiodactyls.) Frustratingly, however, only a handful of early fossil whales were known at the time, and while there was no doubt that whales had evolved the fossil proofs of their evolution were largely missing. Naturalists could only speculate on how early whales transitioned into an aquatic lifestyle, and just as mysterious was the origin of the largest animals on earth, the baleen whales.
Numerous discoveries made during the past three decades have greatly filled in the gaps in our understanding of whale origins, however, and have turned one of the greatest evolutionary mysteries into a literal textbook example of evolution. Thanks to intertwining lines of fossil and genetic evidence we now know that whales evolved from artiodactyls (even-toed hoofed mammals) a little more than 55 million years ago, but these early whales were quite unlike their living relatives. Whales of a modern sort, ones belonging to the groups containing modern toothed whales on one hand and baleen whales on the other, evolved much later, around 35 million years ago. The origin of these groups has not received as much public attention, but recent research has given scientists a new look at what some of the earliest relatives of today’s blue and humpback whales might have been like.
Based upon anatomical, fossil, developmental, and genetic evidence scientists know that baleen whales (technically called mysticetes) evolved from ancestors with teeth, especially since some fossil baleen whales had both teeth and baleen at the same timeboth teeth and baleen at the same time. Clearly baleen whales evolved from a member of the toothed, archaic whales called archaeocetes, of which Basilosaurus is perhaps the most famous member. But which archaeocete are the earliest baleen whales closest to, and what were these whales like? Were the first baleen whales like their modern counterparts, or were they something different? These are some of the questions considered by Erich Fitzgerald in his new, exquisitely-detailed study of the early baleen whale Mammalodon colliveri, just published in the Zoological Journal of the Linnean Society.
Up until now the fossil baleen whales that received the most attention were the several species of Aetiocetus (~34-24 million years ago) and their relatives, known primarily from fossils found along the west coast of the United States. As incongruous as it might sound these were baleen whales with teeth, or whales that bridge the anatomical gap between early whales with teeth and modern mysticetes. Yet these were not the first members of the group. Even older mysticetes have been found in southeastern Australia, among which were the frightening Janjucetus and Mammalodon.
A collection of fossil whale skulls (from Fitzgerald, 2009).
Though they were most certainly mysticetes Janjucetus and Mammalodon did not look very much like their living relatives, nor were they quite like the earlier archaeocetes such as Basilosaurus. When viewed from the top they had skulls shaped like squat triangles which were rounded at the front, and their teeth were more prominent than in later mysticetes such as Aetiocetus. The ~24 million year old Mammalodon, especially, was short-snouted compared to its close relatives, and based upon its skull size it was one of the smallest mysticete whales to have ever lived (even smaller than the pygmy right whale, which is between 4 and 6 meters long). These anatomical facts have important implications for how Mammalodon may have fed.
Lateral view of the lower jaw of Mammalodon (from Fitzgerald, 2009).
The preserved teeth of Mammalodon show something strange. They are heavily worn, so much so that the distinctive parts of the tooth crown were obliterated to give way to a flat inclined surface. Part of the wear on these teeth can be attributed to the way its teeth interlocked. Its teeth did not form a side-to-side cutting shear as in Basilosaurus, but rather they were arranged right on top of each other. This means that as Mammalodon opened and closed its mouth its teeth would have lightly rubbed against each other on the front and back of the teeth, but this alone cannot explain the extreme amount of wear seen in the teeth Fitzgerald described.
Fitzgerald suggests that a very different sort of living marine mammal might provide a clue as to how the teeth of Mammalodon became so worn; the walrus. When walrus hunt for invertebrates in the muddy sediment on the seafloor they quickly and powerfully retract their tongues to create suction, thereby drawing food into their mouths. A lot of sediment goes along with this, though, even some small stones, and as a walrus does this over and over its teeth are abraded.
Previously it had been thought that Mammalodon might filter out small prey with its teeth, or maybe even some archaic form of baleen, but there is no solid evidence that it did either. Instead it seems more probable that Mammalodon was a suction-feeder in the same manner as living walrus and some whales, and this hypothesis is supported by its short snout. From a mechanical perspective Mammalodon could more efficiently create heavy suction forces than a whale with a long snout, and it is no surprise, then, that there is some resemblance between the skulls of Mammalodon and living whales that employ suction feeding, such as belugas. Likewise, Mammalodon appears to have had eyes positioned to give it binocular vision, and it therefore could keep an eye on what it was stirring up in the mud. Inferring behavior from anatomy can be difficult, but Fitzgerald makes a pretty solid case that Mammalodon was a suction-feeder.
A phylogeny of fossil whales with corresponding drawings of skulls (from Fitzgerald, 2009).
But how does Mammalodon relate to other whales? This is where the discussion has to get a little more technical. The group containing the earliest whales, or everything from Pakicetus to Basilosaurus, is called the Archaeoceti. So many new species and genera have been found so quickly that the relationships of the archaeocetes to each other are a bit fuzzy, but the Archaeoceti undoubtedly contains the ancestors of the other major group of whales, the Neoceti. The Neoceti can be split into two subgroups, the odontocetes (toothed whales, such as orcas and porpoises) and the mysticetes (baleen whales), with Mammalodon sitting fairly close to the base of the early baleen whales. This means that it might provide some clue as to what the common ancestor of the odontocetes and mysticetes was like, which in turn might contain some hint as to what subgroup of archaeocetes the first members of the Neoceti evolved from.
To start to untangle the relationships of these whales Fitzgerald compared the traits of Mammalodon to not only other fossil mysticete whales, but to archaeocetes such as Georgiacetus and the basilosaurids (Basilosaurus, Dorudon, and Zygorhiza). These archaeocetes were among the most aquatically-adapted of the entire group, and if Mammalodon and other early mysticetes corresponded closely to one of them this connection could be useful in examining the details of how the first members of the Neoceti evolved. The results of the analysis recovered the basilosaurids as the closest whales to the Neoceti, confirming the connection to these fully aquatic archaeocetes. This does not mean that Fitzgerald identified the ancestor of the Neoceti and Mammalodon, but rather that the earliest members of the Neoceti probably evolved from a basilosaurid whale.
(Interestingly, though, Fitzgerald notes that there is a collection of 34 million year old fossil whales from South Carolina that have been said to be the earliest mysticetes, yet are quite different from the basilosaurids and other mysticete whales. Where do they fit into this picture? No one knows, and we will have to wait for them to be described and named before comparing them to the hypothesis put forward by Fitzgerald.)
Two views of the skull of Mammalodon (from Fitzgerald, 2009).
Even so, Mammalodon was a fairly specialized mysticete that lived about ten million years after the hypothesized time of origin for its group. It contains a suite of archaic traits that place it near the base of the baleen whale family tree but it itself could not have been the ancestor of other mysticetes such as Aetiocetus or living forms. This also means that it cannot necessarily be taken as a model for how the earliest baleen whales evolved. Although closely related to Janjucetus, Mammalodon was quite different, and it could very well be a specialized mud-grubber that tells us more about the diversity of extinct baleen whales than the origins of modern ones.
Then again, it is tempting to think about how baleen might have been an advantage for early suction-feeding mysticetes. Baleen would allow them to filter out more food and keep some of the sediment from going down their throat. As it stands now, though, this speculative hypothesis is only a just-so story that requires more evidence to back up. New discoveries will have to be made and old discoveries will have to be reexamined, but regardless of all that I applaud Fitzgerald’s excellent contribution to our understanding of early baleen whales.
FITZGERALD, E. (2010). The morphology and systematics of (Cetacea: Mysticeti), a toothed mysticete from the Oligocene of Australia
Zoological Journal of the Linnean Society DOI: 10.1111/j.1096-3642.2009.00572.x