Problems and solutions in the ancestry of baleen whales

Earlier we looked at
the newly recovered desmostylian ancestors to baleen whales here. In the large reptile tree (LRT) the desmostylians Behemotops, Paleoparadoxia and the anthracobunids Janucetus, and Anthracobune nest basal to the baleen whales, Balaenoptera and Eubalaena.

By contrast toothed whales
nest in the LRT with long-tailed aquatic tenrecs.

cetaceans are paraphyletic. Mysticeti nest with desmostylians, several clades apart from Odontoceti + Tenrecidae. That’s testable, of course, so feel free to do so yourself. Anyone can.

Unfortunately, until yesterday
there were two problems with this hypothesis of relationships.

Problem #1:
In the LRT, odontocetes and mysticetes nested together when tested together, but separately when tested separately (deleting one whale clade, then the other in successive tests). Likely that was due to the fact that toothed and baleen whales are so convergent that, until now, everyone assumed they had a common ancestor with flippers and flukes.

A second cause may be due to the mysticete-like desmostylian, Behemotops, which is known from a fragment of a skeleton that did not link it strongly enough to its four-legged sisters. Rather it always nested with baleen whales, no matter where they nested.

Figure 2. Two specimens attributed to Desmostylus, but are not congeneric, compared to scale with sister taxa including Paleoparadoxia and Balaenoptera, the blue whale.

Figure 1. Two specimens attributed to Desmostylus, but are not congeneric, compared to scale with sister taxa including Paleoparadoxia and Balaenoptera, the blue whale. Note the gradual increase in the relative size of the skull, but no giant swimming tail. Even so, this is the best line up of mysticete whales recovered so far in the LRT (Fig. 2). Note the phylogenetic miniaturization in the transitional taxon, Behemotops!

Problem #2: All known desmostylians
had a vestigial tail (Fig. 1) — nothing like the massive organ of propulsion found in mysticete whales. This has ALWAYS been a problem with the hippo ancestor hypothesis for whales in general.

Figure 1. Subset of the large reptile tree focusing on the Odontoceti and Mysticeti and their closest relatives, which are not each other.

Figure 2. Subset of the large reptile tree focusing on the Odontoceti and Mysticeti and their closest relatives, which are not each other.

Solution #1: When the desmostylian, Desmostylus hesperus (Marsh 1888), and the odontocete, Zygorhiza kochii (True 1908), are added to the LRT (Fig. 2) the Odontoceti and the Mysticeti finally nest apart when tested together. Both of these transitional taxa are known from complete skeletons that link them to both ancestors and descendants. The addition of these two taxa makes the transition more seamless.

Solution #2: The RBCM specimen referred to Behemotops includes a partial skull, 12 thoracic vertebrae, 17 rib fragments, a very partial scapula and most of a humerus. Authors Beatty and Cockburn 2015 also report, “No other skeletal elements of desmostylians are found anywhere nearby, and the only other bone found in the area was a caudal vertebrae from a Tursiops-sized odontocete.” 

Now, I’m no whale expert,
so I don’t know if odontocete caudal vertebrae can be distinguished from mysticete caudal vertebrae on shape alone. However, in today’s world, all mysticetes are much larger than bottle nose dolphins (the above-mentioned Tursiops truncatus) — and so are their tail vertebrae.

Beatty and Cockburn had no idea
that Behemotops was a transitional taxon linking desmostylians to mysticetes, so if they found an associated caudal vertebra, they would expect it to be pencil-thin based on other desmostylians, not dolphin-sized. We should explore the possibility that the one caudal vertebra found with Behemotops does indeed belong to Behemotops given that this taxon is transitional to Mysticeti.

Keep an eye on Behemotops.
At present it is the ‘Archaeopteryx’ or ur-mysticete of baleen whales, just as Leptictidium is the ‘Archaeopteryx‘ or ur-odontocete of toothed whales.

Convergence with tenrecs
Some living and extinct tenrecs have a long muscular tail. Others have vestigial tails. Perhaps tenrec tail variety is convergent with desmostylian tail variety.

Phylogenetic miniaturization in transtional mysticetes
Earlier we looked at phylogenetic miniaturization at the genesis of several new clades within the Reptilia and at the origin of the Reptilia. Given the large size of desmostylians and the giant size of mysticete whales, I note only a slight reduction in the size of the Behemotops skull at the transition (Fig. 1), but even that counts. And it should encourage us to look further in Behemotops bone beds.

The reduction in limb size
in Desmostylus #2 (Fig. 1) together with a longer fore limb than hind limb — plus the relatively large size of the skull vs. torso — are all transitional traits leading to Mysticeti. Also note the reduction in the anterior teeth and the narrowing of the anterior jaws. No other taxon in the LRT are closer to Mysticeti than the desmostylians. These observations further cement the desmostylian / mysticete relationship. Sometimes you just have to see them all together in one place (Fig. 1) to get the picture that echoes evolutionary events that took place over millions of years.

And now we may have an inkling
as to why desmostylians were such weird mammals. They were becoming ever weirder still as baleen whales.

Beatty and Cockburn 2015
tentatively agreed with tradition when they considered desmostylians to be afrotheres and tethytheres, more closely related to elephants and sirenians (and anthracobunids) than to hippos, mesonychids (and anthracobunids). To their credit, however, they wrote. “The possibility that desmostylians and anthracobunids may not be Afrotherians should be considered.”  The LRT has solved many such problems with logical and testable solutions that are not tentative.

Beatty BL and Cockburn TC 2015. New insights on the most primitive desmostylian from a partial skeleton of Behemotops (Desmostylia, Mammalia) from Vancouver Island, British Columbia. Journal of Vertebrate Paleontologoy 35(5):e979939: 15 pp.
Marsh OC 1888.
Notice of a new fossil sirenian, from California. American Journal of Science 25(8):94–96.
True FW 1908. The fossil cetacean, Dorudon serratus Gibbes. Bulletin of the Museum of Comparative Zoology. 52 (4): 5–78.


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