Weirdest skull of all: Eubalaena australis (Southern right whale)

There are few mammal skulls
that depart from the basic placental pattern (bauplan) like that of Eubalaena australis (Desmoulins 1822; 15-18 m in length; extant), the Southern right whale (Fig. 1). All the parts are there. They just have weird shapes and are shifted around a bit.

The giant mouth
is much deeper than in Caperea (Bisconti 2012, Fordyce and Marx 2013 ) or Balaenoptera (Fig. 2; Linneaus 1758), with much longer baleen. The skull is relatively larger and the postcrania shorter and deeper with relatively larger forelimbs than Balaeonoptera. The mandible moves relatively little. The coronoid process is absent. The lower lip rises to meet the upper jaw, as in Caperea (Fig. 2), but much more so due to the great height of the giant skull.

Note the hammerhead cranium in dorsal view
creating a T-shape with the addition of that long triangular rostrum (Fig. 1). The mandible is much wider than the rostrum. That resulting space between them is where the water exits after entering from the front and after being filtered by the tall baleen strips.

Surprisingly,
five digits are present on the manus following a long lineage of thumb-less ancestors. So the reappearance of the thumb here is a minor atavism.

The cervical series
is tightly interwoven, like a stack of bent playing cards. The caudal transverse processes have broad tips. The chevrons are longer than deep.

The pygmy right whale (Caperea marginata)
(Fig. 2; Gray 1846; 6m; extant) looks like a small, slender right whale with tiny flippers. While it nests with Eubalaena in the LRT (Fig. 9), it retains something of a gray whale (Eschrichtius, Fig. 4) appearance. Caperea is the smallest of all baleen whales, by far.

The cervicals are all fused to one another. The dorsals have paddle-shaped ribs, but don’t be fooled by the museum mounts (Fig. 2, above). Such ribs cannot fit into a living Caperea without rotating posteriorly about 45º (Fig. 2, below). The lumbars are reduced to one. The chevrons are shorter than long, as in right whales and cetotheres.

The dorsal fin is above the sacral vertebrae, distinct from odontocetes, in which the dorsal fin is above the dorsal ribs.

By comparison
Balaenoptera (Fig. 2) the blue whale, is longer, with a relatively smaller, shallower skull. It feeds by expanding a giant throat sack with water inflow then expelling it with the tongue and by throat constrictions.

The California Gray Whale
(Eschrichtius robustus; 15m; extant; Gray 1864; Fig. 7) nests at the base of all tested mysticetes (Fig. 9) and provides the best clues to envision the post-crania of Behemotops, and other derived desmostylians.

The maxilla of the gray whale retains a tooth alveolus
for the canine which is aligned along the jawline in Desmostylus (Fig. 8), a trait not found elsewhere among mammals. The dentary tip also retains a tooth alveolus, similar to that found in Desmostylus.

Note the maxilla of this Desmostylus
(Fig. 8) shares many traits with basal mysticetes, including a general toothlessness (molars still are present below the orbit), a concave maxilla ventral margin, and a dorsal naris. The dentary has anteriorly oriented, tusk-like incisors, growing from alveoli similar to those in the gray whale (Fig. 7).  I wonder if we’ll someday find teeth in a dissection of the Gray Whale. Someone should look for them. I wonder if someday someone will find teeth in a dissection of the Gray Whale. Someone should look for them.

The evidence keeps mounting
that mysticetes and odontocetes had separate origins among desmostylians and tenrecs, respectively.

References
Desmoulins C 1822. Baleine. Dictionnaire Classique d’Histoire Naturelle 155-165
Gray JE 1864. “Eschrichtius“. Annals of the Magaztine Natural History. 3 (14): 350.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

wiki/Balaenoptera
wiki/Eubalaena
wiki/Caperea
wiki/Eschrichtius