A new Early Oligocene toothed ‘baleen’ whale (Mysticeti: Aetiocetidae) from western North America: one of the oldest and the smallest

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.

Decoupling Tooth Loss from the Evolution of Baleen in Whales

“…In contrast, other stem Mysticeti from the mid-late Oligocene (latest Rupelian to Chattian marine stages) are much better known from described material, illustrating vastly different feeding morphologies for geographically and phylogenetically separate groups: The relatively small Mammalodontidae (∼3 m in total length), including Janjucetus hunderi (Fitzgerald, 2006), Mammalodon colliveri (Fitzgerald, 2010), and Mammalodon hakataramea (Fordyce and Marx, 2016) from Australia and New Zealand; the equally small Aetiocetidae (∼2-4 m in length), from the North Pacific Ocean (Emlong, 1966;Marx et al, 2015); and the minke-sized Eomysticetidae (∼6-8 m in length), found in both Northern and Southern hemispheres, represented by several genera (Boessenecker and Fordyce, 2015a,b). Both Mammalodontidae and Aetiocetidae unequivocally possessed functional upper and lower dentitions, while Eomysticetidae either possessed severely reduced or entirely lacked dentition.…”

“…Pending studies that more broadly sample these traits across mysticete phylogeny, we argue that patterns of dental evolution and loss in cetaceans should not be a priori linked, neither in step-wise nor correlative models. Lastly, stem Mysticeti show at least four different lineages with markedly disparate feeding morphologies (Mitchell, 1989;Fitzgerald, 2006Fitzgerald, , 2010Boessenecker and Fordyce, 2015a;Marx et al, 2015). The functional and ecological interpretations of these extinct morphologies are unclear and, we argue, potentially over interpreted.…”

“…Similarly, remains of comparatively sized, toothed cetaceans from the Oligocene of South Carolina (published informally as the "Charleston mysticetes" but otherwise unnamed and undescribed) have been attributed to stem Mysticeti, despite preserving many stem cetacean characteristics (Fitzgerald, 2010). Both Llanocetus and the "Charleston mysticetes" have been coded for phylogenetic analyses (Marx et al, 2015;Boessenecker and Fordyce, 2016), resulting in equivocal phylogenetic placements among stem Mysticeti. However, the lack of any published data on their feeding morphology limits our understanding of their role in the evolution of tooth loss.…”

“…Aetiocetid teeth are notably smaller, simpler, and more widely spaced than mammalodontids, suggesting a range of feeding modes. Among aetiocetids, only Fucaia buelli preserves a dentition exhibiting occlusion, interpreted for mastication, rather than filtration, based on wear patterns (Marx et al, 2015).…”

“…The third (posterior baleen hypothesis) suggests that functional baleen evolved posterior to vestigial adult teeth retained at the distal tip of the rostrum and dentaries, with the dentition and baleen aligned in the rostrum (Boessenecker and Fordyce, 2015a,b). A fourth (suction feeding hypothesis) proposes suction feeding as the ancestral condition, suggesting a transition first from raptorial feeding to suction feeding and then subsequently to bulk filter feeding (Marx et al, 2015(Marx et al, , 2016b. We elaborate on this idea to raise this latter possibility to suggest that a transitional step in stem mysticetes included taxa bearing neither teeth nor baleen (Figure 1).…”