Suction feeding preceded filtering in baleen whale evolution

Abstract

Marx F.G., Hocking D.P., Park T., Ziegler T., Evans A.R. and Fitzgerald, E.M.G. 2016. Suction feeding preceded filtering in baleen whale evolution. Memoirs of Museum Victoria 75: 71-82.The origin of baleen, the key adaptation of modern whales (Mysticeti), marks a profound yet poorly understood transition in vertebrate evolution, triggering the rise of the largest animals on Earth. Baleen is thought to have appeared in archaic tooth-bearing mysticetes during a transitional phase that combined raptorial feeding with incipient bulk filtering. Here we show that tooth wear in a new Late Oligocene mysticete belonging to the putatively transitional family Aetiocetidae is inconsistent with the presence of baleen, and instead indicative of suction feeding. Our findings suggest that baleen arose much closer to the origin of toothless mysticete whales than previously thought. In addition, they suggest an entirely new evolutionary scenario in which the transition from raptorial to baleen-assisted filter feeding was mediated by suction, thereby avoiding the problem of functional interference between teeth and the baleen rack.

A behavioural framework for the evolution of feeding in predatory aquatic mammals

“…These predictions are borne out by the fossil record e.g. in the form of raptorial sperm and beaked whales preceding their suction feeding extant relatives [57,58], toothless suction feeders arising from within largely raptorial clades [59], and suction feeding archaic mysticetes [18,54]. Each of these occurrences represents an independent evolutionary event that supports the directionality of our model, and together they suggest that our framework applies irrespective of where, or how often, specializations for aquatic feeding have taken place.…”

“…As the water is expelled, food is prevented from leaving the mouth by the semi-closed jaws, teeth or a specialized filter. Where prey items are relatively large, it is sufficient for the jaws, lips, gums and/or teeth to act as a simple sieve, or 'cage jaw' [18,42]. By contrast, small prey, such as krill or copepods, require the use of a specialized filter, such as the elaborate, interlocking teeth of leopard and crabeater seals, or the racks of comb-like, keratinous baleen plates found in mysticetes [5,7,43].…”

Decoupling Tooth Loss from the Evolution of Baleen in Whales

“…The suction feeding hypothesis was articulated recently by Marx et al (2015Marx et al ( , 2016b, who noted the presence of tooth wear indicative of suction feeding in an undescribed aetiocetid. On this basis, they suggested that suction feeding may have been the ancestral condition for the earliest mysticetes, and that stem mysticetes underwent first a transition from raptorial feeding to suction feeding and then subsequently transition again to bulk filter feeding.…”

“…Considering the tenuous link between palatal foramina and baleen, this hypothesis better reflects the aetiocetid feeding condition given the available morphological data. However, Marx et al (2016b)'s hypothesis did not perform ancestral state reconstructions with dense taxonomic sampling, limiting the power of phylogenetic inference for this feeding mode. As only one of several feeding morphologies within Aetiocetidae, suction feeding may be plesiomorphic as the basic mammalian condition for suckling (Werth, 2000).…”

“…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).…”