The early Miocene balaenid Morenocetus parvus from Patagonia (Argentina) and the evolution of right whales

BUONO, MÓNICA R.; FERNÁNDEZ, MARTA S.; COZZUOL, MARIO A.; CUITIÑO, JOSÉ I.; FITZGERALD, ERICH M.G.

PeerJ

PeerJ

Año: 2017 vol. 5

Balaenidae (right and bowhead whales) are a key group in understanding baleenwhale evolution, because they are the oldest surviving lineage of crown Mysticeti,with a fossil record that dates back ∼20 million years. However, this record is mostlyPliocene and younger, with most of the Miocene history of the clade remainingpractically unknown. The earliest recognized balaenid is the early MioceneMorenocetus parvus Cabrera, 1926 from Argentina. M. parvus was originally brieflydescribed from two incomplete crania, a mandible and some cervical vertebraecollected from the lower Miocene Gaiman Formation of Patagonia. Since then it hasnot been revised, thus remaining a frequently cited yet enigmatic fossil cetacean withgreat potential for shedding light on the early history of crown Mysticeti. Herewe provide a detailed morphological description of this taxon and revisit itsphylogenetic position. The phylogenetic analysis recovered the middle MiocenePeripolocetus as the earliest diverging balaenid, and Morenocetus as the sister taxonof all other balaenids. The analysis of cranial and periotic morphology ofMorenocetus suggest that some of the specialized morphological traits of modernbalaenids were acquired by the early Miocene and have remained essentiallyunchanged up to the present. Throughout balaenid evolution, morphologicalchanges in skull arching and ventral displacement of the orbits appear to be coupledand functionally linked to mitigating a reduction of the field of vision. The bodylength of Morenocetus and other extinct balaenids was estimated and the evolution ofbody size in Balaenidae was reconstructed. Optimization of body length on ourphylogeny of Balaenidae suggests that the primitive condition was a relatively smallbody length represented by Morenocetus, and that gigantism has been acquiredindependently at least twice (in Balaena mysticetus and Eubalaena spp.), with theearliest occurrence of this trait in the late Miocene?early Pliocene as represented byEubalaena shinshuensis.