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Gigantism Precedes Filter Feeding in Baleen Whale Evolution

2018; Elsevier BV; Volume: 28; Issue: 10 Linguagem: Inglês

10.1016/j.cub.2018.04.027

1879-0445

R. Ewan Fordyce, Felix G. Marx,

Paleontology and Evolutionary Biology

Baleen whales (Mysticeti) are the largest animals on Earth, thanks to their ability to filter huge volumes of small prey from seawater. Mysticetes appeared during the Late Eocene, but evidence of their early evolution remains both sparse and controversial [1Mitchell E.D. A new cetacean from the Late Eocene La Meseta Formation, Seymour Island, Antarctic Peninsula.Can. J. Fish. Aquat. Sci. 1989; 46: 2219-2235Crossref Scopus (128) Google Scholar, 2Lambert O. Martínez-Cáceres M. Bianucci G. Di Celma C. Salas-Gismondi R. Steurbaut E. Urbina M. de Muizon C. Earliest mysticete from the Late Eocene of Peru sheds new light on the origin of baleen whales.Curr. Biol. 2017; 27: 1535-1541.e2Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar], with several models competing to explain the origin of baleen-based bulk feeding [3Deméré T.A. McGowen M.R. Berta A. Gatesy J. Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales.Syst. Biol. 2008; 57: 15-37Crossref PubMed Scopus (207) Google Scholar, 4Marx F.G. Hocking D.P. Park T. Ziegler T. Evans A.R. Fitzgerald E.M.G. Suction feeding preceded filtering in baleen whale evolution.Mem. Mus. Vic. 2016; 75: 71-82Crossref Scopus (46) Google Scholar, 5Hocking D.P. Marx F.G. Fitzgerald E.M.G. Evans A.R. Ancient whales did not filter feed with their teeth.Biol. Lett. 2017; 13: 20170348Crossref PubMed Scopus (29) Google Scholar, 6Geisler J.H. Boessenecker R.W. Brown M. Beatty B.L. The origin of filter feeding in whales.Curr. Biol. 2017; 27: 2036-2042Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar]. Here, we describe a virtually complete skull of Llanocetus denticrenatus, the second-oldest (ca. 34 Ma) mysticete known. The new material represents the same individual as the type and only specimen, a fragmentary mandible. Phylogenetic analysis groups Llanocetus with the oldest mysticete, Mystacodon selenensis [2Lambert O. Martínez-Cáceres M. Bianucci G. Di Celma C. Salas-Gismondi R. Steurbaut E. Urbina M. de Muizon C. Earliest mysticete from the Late Eocene of Peru sheds new light on the origin of baleen whales.Curr. Biol. 2017; 27: 1535-1541.e2Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar], into the basal family Llanocetidae. Llanocetus is gigantic (body length ∼8 m) compared to other early mysticetes [7Tsai C.-H. Ando T. Niche partitioning in Oligocene toothed mysticetes (Mysticeti: Aetiocetidae).J. Mamm. Evol. 2015; 23: 33-41Crossref Scopus (25) Google Scholar, 8Fitzgerald E.M.G. The morphology and systematics of Mammalodon colliveri (Cetacea: Mysticeti), a toothed mysticete from the Oligocene of Australia.Zool. J. Linn. Soc. Lond. 2010; 158: 367-476Crossref Scopus (102) Google Scholar, 9Slater G.J. Goldbogen J.A. Pyenson N.D. Independent evolution of baleen whale gigantism linked to Plio-Pleistocene ocean dynamics.Proc. Biol. Sci. 2017; 284: 20170546Crossref PubMed Scopus (114) Google Scholar]. The broad rostrum has sharp, widely spaced teeth with marked dental abrasion and attrition, suggesting biting and occlusal shearing. As in extant mysticetes, the palate bears many sulci, commonly interpreted as osteological correlates of baleen [3Deméré T.A. McGowen M.R. Berta A. Gatesy J. Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales.Syst. Biol. 2008; 57: 15-37Crossref PubMed Scopus (207) Google Scholar]. Unexpectedly, these sulci converge on the upper alveoli, suggesting a peri-dental blood supply to well-developed gums, rather than to inter-alveolar racks of baleen. We interpret Llanocetus as a raptorial or suction feeder, revealing that whales evolved gigantism well before the emergence of filter feeding. Rather than driving the origin of mysticetes, baleen and filtering most likely only arose after an initial phase of suction-assisted raptorial feeding [2Lambert O. Martínez-Cáceres M. Bianucci G. Di Celma C. Salas-Gismondi R. Steurbaut E. Urbina M. de Muizon C. Earliest mysticete from the Late Eocene of Peru sheds new light on the origin of baleen whales.Curr. Biol. 2017; 27: 1535-1541.e2Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar, 4Marx F.G. Hocking D.P. Park T. Ziegler T. Evans A.R. Fitzgerald E.M.G. Suction feeding preceded filtering in baleen whale evolution.Mem. Mus. Vic. 2016; 75: 71-82Crossref Scopus (46) Google Scholar, 5Hocking D.P. Marx F.G. Fitzgerald E.M.G. Evans A.R. Ancient whales did not filter feed with their teeth.Biol. Lett. 2017; 13: 20170348Crossref PubMed Scopus (29) Google Scholar]. This scenario differs strikingly from that proposed for odontocetes, whose defining adaptation—echolocation—was present even in their earliest representatives [10Geisler J.H. Colbert M.W. Carew J.L. A new fossil species supports an early origin for toothed whale echolocation.Nature. 2014; 508: 383-386Crossref PubMed Scopus (75) Google Scholar].