Morenocetus: a small Early Miocene right whale ancestor

Another taxon to consider
in our search for the ancestors of right whales. This one is small enough to have a skull similar in size to that of the earlier (Oligocene) desmostylian, Desmostylus (Fig. 1).

Figure 1. Morenocetus and related right whale skulls, Eubaelana and xx to scale along with the Oligocene ancestor in the LRT, Desmostylus.

Figure 1. Morenocetus and related right whale skulls, Eubaelana and Balaenella, to scale along with the Oligocene ancestor in the LRT, Desmostylus. Note the size of the Morenocetus skull is quite similar to that of the earlier Desmostylus, which already has reduced hind limbs. Only the cranial portion of the Morenocetus skull is known and was shown in Buono et al. 2012. All skull drawings are from Buono et al. 2012. They were set to the same scale here.

Buono et al. 2012 report,
“The earliest recognized balaenid is the early Miocene Morenocetus parvus Cabrera, 1926 from Argentina. M. parvus was originally briefly described from two incomplete crania, a mandible and some cervical vertebrae collected from the lower Miocene Gaiman Formation of Patagonia. Since then it has not been revised, thus remaining a frequently cited yet enigmatic fossil cetacean with great potential for shedding light on the early history of crown Mysticeti. Here we provide a detailed morphological description of this taxon and revisit its phylogenetic position. The phylogenetic analysis recovered the middle Miocene Peripolocetus as the earliest diverging balaenid, and Morenocetus as the sister taxon of all other balaenids.The analysis of cranial and periotic morphology of Morenocetus suggest that some of the specialized morphological traits of modern balaenids were acquired by the early Miocene and have remained essentially unchanged up to the present.”

Figure 1. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Figure 2. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Wikipedia reports, from Buono et al. 2012:
“Morenocetus is distinguished from more derived balaenids in the narrow exposure of the squamosal lateral to the exoccipital, a short supraorbital process of the frontal, straight lateral edges of the supraoccipital, and a postorbital process of the frontal oriented posteriorly. It can be distinguished from the only other Miocene balaenid, Peripolocetus in having a dorsoventrally expanded zygomatic process of the squamosal. The body length of Morenocetus is estimated at 17 to 18 feet (5.2 to 5.5 m), and the rostrum is moderately arched dorsoventrally in contrast to crown Balaenidae.”

Buono et al. 2012
did not include nearly toothless desmostylians in their taxon list when they analyzed ‘cetacean’ relationships, but continued the myth of the monophyletic clade ‘Cetacea’ due this taxon exclusion issue.

A paper describing the triple origin of whales
can be accessed here and here.

References
Buono MR, Fernández MS, Cozzuol MA, Cuitiño JI and Fitzgerald EMG 2017. The early Miocene balaenid Morenocetus parvus from Patagonia (Argentina) and the evolution of right whales. PeerJ 5:e4148; DOI 10.7717/peerj.4148
Demere T and Pyenson N 2015. Filling the Miocene ‘Balaenid Gap’ – the previously engimatic Peripolocetus vexillifer Kellogg, 1931 is a stem balaenid (Cetacea: Mysticeti) from the Middle Miocene (Langhian) of California, USA. Journal of Vertebrate Paleontology 35 (Supplement): 115A.
El Adli JJ, Deméré TA and Boessenecker RW 2014. Herpetocetus morrowi (Cetacea: Mysticeti), a new species of diminutive baleen whale from the Upper Pliocene (Piacenzian) of California, USA, with observations on the evolution and relationships of the Cetotheriidae. Zoological Journal of the Linnean Society 170 (2): 400–466. doi:10.1111/zoj.12108.

wiki/Peripolocetus
wiki/Morenocetus

 

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