Mammalodon and Janjucetus Move to the Odontocete Clade

Traditional ‘toothed’ mysticetes,
Mammalodon and Janjucetus (Fig 1), were always a little different than the other mysticete ancestors in the large reptile tree (LRT, 2319 taxa). As of today those differences are no longer a factor. Scoring changes move both close to Aetiocetus (Fig 1), a transitional archaeocete – odontocete without a melon, but with single-rooted premolars and molars.

Corrections like this are part of the scientific method.

The long low snout pf Aetiocetus is reduced
in Mammalodon and Janjucetus (Fig 1). Earlier the LRT nested these two with a similar short-snouted taxon, Anthracobune (Fig 2), a taxon that continues to link hippos with desmostylians + mysticetes in the LRT.

The problem that raised a curious eyebrow
was the retroarticular process. It was extremely long in Anthracobune (Fig 2) – and missing in Janjucetus and Mammalodon (Fig 1).

I should have tested this earlier.
We looked at convergent mysticete-odontocete taxa a few days ago.

My guess is
a much smaller phylogenetically miniaturized transitional taxon will someday be discovered that more closely links long-snouted Aetiocetus (Fig 1) with its short-snouted (neotonous), but not smaller relatives, Mammalodon and Janjucetus. Presently these two taxa are terminal. They stand alone morphologically. Presently long-snouted taxa gave rise to long-snouted odontocetes like Simocetus and Physeter (the extant sperm whale). By ccontrast, dolphins have a shorter snout – by convergence with Janjucetus and Mammalodon – also by phylogenetic miniaturization and neotony according to the LRT.

Whale workers will tell you
that Aetiocetus, Janjucetus and Mammalodon are mysticete ancestors (Fordyce and Marx 2016, Geisler et al 2017). This works – but only in the absence of pertinent taxa found in the LRT that completely separate odontocete ancestors from mysticete ancestors.

Add taxa to your own LRT to find out for yourself. Apologies for earlier errors.

Janjucetus hunderi
(Fitzgerald 2006; late Oligocene; est. 3.5m in length) was originally considered the basalmost Mysticeti (baleen whale), but it had large serration teeth, the whale-like post-crania imnagined by Fitzgerald is supported by phylogenetic bracketing, but this is a relative of Aetiocetus, basal to Odontoceti (toothed whale). The skull was more than twice as wide as tall. The orbit was at mid skull, not in the posterior half, as in related taxa. This is a result of phylogenetic miniaturization and neotony.

Mammalodon colliveri
(Pritchard 1939; Late Oligocene, est 3 m in length) was previously considered a basal mysticete with teeth, closely resembling Janjucetus. Here it nests with Janjucetus and archaeocetes leading to odontocetes. The snout is shorter than the cranium.

Mammalodon hakataramea
Fordyce and Marx 2016)

References
Emlong D 1966. A new archaic cetacean from the Oligocene of Northwest Oregon. Bulletin of the Museum of Natural History, University of Oregon. 3: 1–51.
Fitzgerald EMG 2006. A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proceedings of the Royal Society B 273:2955-2963.
Fordyce RE and Marx FG 2016. Mysticetes baring their teeth: a new fossil whale, Mammalodon hakataramea, from the Southwest Pacific. Memoirs of Museum Victoria 74: 107–116.
Geisler JH; Boessenecker RW; Brown M; Beatty BL 2017. The Origin of Filter Feeding in Whales. Current Biology. 27 (13): 2036–2042.e2. doi:10.1016/j.cub.2017.06.003
Pritchard G B 1939. On the discovery of a fossil whale in the older tertiaries of Torquay, Victoria. The Victorian Naturalist 55:151-159.

wiki/Aetiocetus
wiki/Janjucetus
wiki/Mammalodon