The Origin of Filter Feeding in Whales

Abstract

As the largest known vertebrates of all time, mysticetes depend on keratinous sieves called baleen to capture enough small prey to sustain their enormous size [1]. The origins of baleen are controversial: one hypothesis suggests that teeth were lost during a suction-feeding stage of mysticete evolution and that baleen evolved thereafter [2-4], whereas another suggests that baleen evolved before teeth were lost [5]. Here we report a new species of toothed mysticete, Coronodon havensteini, from the Oligocene of South Carolina that is transitional between raptorial archaeocete whales and modern mysticetes. Although the morphology and wear on its anterior teeth indicate that it captured large prey, its broad, imbricated, multi-cusped lower molars frame narrow slots that were likely used for filter feeding. Coronodon havensteini is a basal, if not the most basal, mysticete, and our analysis suggests that it is representative of an initial stage of mysticete evolution in which teeth were functional analogs to baleen. In later lineages, the diastema between teeth increased-in some cases, markedly so [6]-and may mark a stage at which the balance of the oral fissure shifted from mostly teeth to mostly baleen. When placed in a phylogenetic context, our new taxon indicates that filter feeding was preceded by raptorial feeding and that suction feeding evolved separately within a clade removed from modern baleen whales.

Tooth Loss Precedes the Origin of Baleen in Whales

“…The functional transition from teeth to baleen in mysticetes has remained enigmatic because baleen decays rapidly and leaves osteological correlates with unclear homology; the oldest direct evidence for fossil baleen is 25 million years younger [3] than the oldest stem mysticetes (36 Ma). Previous hypotheses for the origin of baleen [4,5] are inconsistent with the morphology and phylogenetic position of Maiabalaena. The absence of both teeth and baleen in Maiabalaena is consistent with recent evidence that the evolutionary loss of teeth and origin of baleen are decoupled evolutionary transformations, each with a separate morphological and genetic basis [2,6].…”

Evolution of cranial telescoping in echolocating whales (Cetacea: Odontoceti)

“…), and Coronodon (Geisler et al. )), to the baleen whales of today (Marx and Fordyce ). In contrast, the oldest known odontocete whales display clear morphological features of the suborder (Uhen ), and possess many of the traits we associate with toothed whales today, including ultrasonic hearing (Churchill et al.…”

“…), benthic suction feeders ( Mammalodon; Fitzgerald ), and filter feeders that relied on teeth ( Coronodon; Geisler et al. , although see Hocking et al. ) or baleen (Eomysticetidae; Boessenecker and Fordyce ) or possibly a combination of both (Aetiocetidae; Deméré et al.…”

“…Most work on disparity in whales during the Oligocene has focused on mysticetes, which attained their greatest disparity during this epoch . At no other time period do we see the presence of such a wide diversity of feeding morphologies within Mysticeti, including raptorial predators (Janjucetus; Fitzgerald 2006, and Fucaia;, benthic suction feeders (Mammalodon; Fitzgerald 2010), and filter feeders that relied on teeth (Coronodon; Geisler et al 2017, although see Hocking et al 2017) or baleen (Eomysticetidae; Boessenecker and Fordyce 2015) or possibly a combination of both (Aetiocetidae; Deméré et al 2008).…”

Cetacean Skull Telescoping Brings Evolution of Cranial Sutures into Focus

“…Several advances in cetacean biology provide new context for understanding the evolution of telescoped morphologies that was unavailable to Miller (1923) and Kellogg (1928aKellogg ( , 1928b, including recognition that cetaceans are nested within terrestrial artiodactyls (Gingerich, 2001;. A robust and TELESCOPING AND CRANIAL SUTURE EVOLUTION growing fossil record of forms that preceded the telescoped skulls of modern cetaceans (and that may be intermediate between them and their un-telescoped ancestors) permits evolutionary processes to be inferred and hypotheses to be tested (e.g., Hampe and Baszio, 2010;Velez-Juarbe et al, 2015;Churchill et al, 2016;Marx and Fordyce, 2016;Geisler et al, 2017;Peredo et al, 2017;Pyenson, 2017;Churchill et al, 2018). New technology, especially noninvasive 3D imaging, is making rare and fragile specimens more accessible for research and allows telescoped morphologies-especially those of fetal specimens-to be studied for the first time "…a pattern of skull bone position referred to as 'telescoping'…" (Armfield et al, 2011) "…cetacean skulls became more telescoped as the nasal openings migrated dorsally (Miller, 1923)."…”