Abstract
Baleen is a comb‐like structure that enables mysticete whales to bulk feed on vast quantities of small prey, and ultimately allowed them to become the largest animals on Earth. Because baleen rarely fossilises, extremely little is known about its evolution, structure and function outside the living families. Here we describe, for the first time, the exceptionally preserved baleen apparatus of an entirely extinct mysticete morphotype: the Late Miocene cetotheriid, Piscobalaena nana, from the Pisco Formation of Peru. The baleen plates of P. nana are closely spaced and built around relatively dense, fine tubules, as in the enigmatic pygmy right whale, Caperea marginata. Phosphatisation of the intertubular horn, but not the tubules themselves, suggests in vivo intertubular calcification. The size of the rack matches the distribution of nutrient foramina on the palate, and implies the presence of an unusually large subrostral gap. Overall, the baleen morphology of Piscobalaena likely reflects the interacting effects of size, function and phylogeny, and reveals a previously unknown degree of complexity in modern mysticete feeding evolution.
Highlights
Baleen is the key adaptation that allows mysticetes to filter small prey directly from seawater, and is central to understanding their ecology and evolution (Pivorunas, 1979)
MUSM 3292 is a cetotheriid based on the presence of: (i) a distally expanded compound posterior process of the tympanoperiotic; (ii) an anteriorly expanded paroccipital concavity flooring the facial sulcus via a distinct posteroventral flange; (iii) a low and broadly triangular coronoid process of the mandible; and (iv) a posteriorly extended angular process of the mandible (Whitmore & Barnes, 2008; El Adli et al 2014; Marx & Fordyce, 2016); and belongs to P. nana based on the broad exposure of the vomer on the palate, and the wide and transversely rounded vomerine crest (Bouetel & de Muizon, 2006; Fig. 2)
Similar calcification of the intertubular horn occurs in B. acutorostrata, but detailed comparative data are currently lacking for most extant species (Szewciw et al 2010)
Summary
Baleen is the key adaptation that allows mysticetes to filter small prey directly from seawater, and is central to understanding their ecology and evolution (Pivorunas, 1979). Because of its keratinous nature, baleen generally decays along with the remainder of the soft tissue. Descriptions of fossilised baleen are rare, and currently restricted to specimens (e.g. fossil rorquals) that closely resemble extant whales in their morphology and, presumably, lifestyle (Esperante et al 2008; Bisconti, 2012; Gioncada et al 2016; Marx & Kohno, 2016).
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