Abstract
Archaic toothed mysticetes represent the evolutionary transition from raptorial to bulk filter feeding in baleen whales. Aetiocetids, in particular, preserve an intermediate morphological stage in which teeth functioned alongside a precursor of baleen, the hallmark of all modern mysticetes. To date, however, aetiocetids are almost exclusively Late Oligocene and coeval with both other toothed mysticetes and fully fledged filter feeders. By contrast, reports of cetaceans from the Early Oligocene remain rare, leaving the origins of aetiocetids, and thus of baleen, largely in the dark. Here, we report a new aetiocetid, Fucaia buelli, from the earliest Oligocene (ca 33–31 Ma) of western North America. The new material narrows the temporal gap between aetiocetids and the oldest known mysticete, Llanocetus (ca 34 Ma). The specimen preserves abundant morphological detail relating to the phylogenetically informative ear bones (otherwise poorly documented in this family), the hyoid apparatus and much of the (heterodont) dentition. Fucaia comprises some of the smallest known mysticetes, comparable in size with the smallest odontocetes. Based on their phylogenetic relationships and dental and mandibular morphology, including tooth wear patterns, we propose that aetiocetids were suction-assisted raptorial feeders and interpret this strategy as a crucial, intermediary step, enabling the transition from raptorial to filter feeding. Following this line of argument, a combination of raptorial and suction feeding would have been ancestral to all toothed mysticetes, and possibly even baleen whales as a whole.
Highlights
Aetiocetids are a clade of archaic toothed mysticetes known exclusively from the Oligocene [1], recent, as yet unpublished, reports may hint at a possible survival until the late Early Miocene [2]
Apart from Willungacetus and a referred Early Oligocene specimen of Aetiocetus cotylalveus (USNM 256593 [6]), all reported aetiocetids date from the Late Oligocene. Considering their intermediate morphology, this temporal distribution is rather striking, given that (i) the earliest toothed mysticete, Llanocetus denticrenatus, is known from the latest Eocene; and (ii) even chaeomysticetes are attested from the late Early Oligocene onwards [13]
Our results differ from those of Marx & Fordyce [13] in two important respects: (i) OCPC 1178, an as yet undescribed Early Miocene specimen from California, is located inside Aetiocetus as sister to A. polydentatus; little can as yet be said about this result, except that it is unsurprising in light of the young age of both OCPC 1178 and A. polydentatus relative to other members of Aetiocetus; and (ii) Fucaia is sister to Aetiocetus + OCPC 1178, to the exclusion of Morawanocetus and Chonecetus
Summary
Aetiocetids are a clade of archaic toothed mysticetes known exclusively from the Oligocene [1], recent, as yet unpublished, reports may hint at a possible survival until the late Early Miocene [2]. Apart from Willungacetus and a referred Early Oligocene specimen of Aetiocetus cotylalveus (USNM 256593 [6]), all reported aetiocetids date from the Late Oligocene Considering their intermediate morphology, this temporal distribution is rather striking, given that (i) the earliest toothed mysticete, Llanocetus denticrenatus, is known from the latest Eocene (ca 34 Ma [12]); and (ii) even chaeomysticetes are attested from the late Early Oligocene onwards [13]. It is likely that the apparent lack of Early Oligocene aetiocetids reflects a patchy fossil record, as shown by the mere handful of cetacean specimens from this period, rather than a genuine biological phenomenon We reduce this temporal gap by describing a new species and genus from the Early Oligocene Makah Formation of the Olympic Peninsula, WA, USA. The new species represents one of the smallest mysticetes yet described and highlights the rather humble origins of the giants that plough the modern oceans
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