[PaleoMammalogy • 2018] Maiabalaena nesbittae • Tooth Loss Precedes the Origin of Baleen in Whales
 |
| Maiabalaena nesbittae
Peredo, Pyenson, Marshall & Uhen, 2018
|
Highlights
• Maiabalaena nesbittae is 33 million year old fossil baleen whale from Oregon
• Maiabalaena has neither teeth, nor baleen
• Early whales lost teeth entirely before the evolutionary origin of baleen
• Despite no teeth or baleen, these whales were effective suction feeders
Summary
Whales use baleen, a novel integumentary structure, to filter feed;
filter feeding itself evolved at least five times in tetrapod history
but demonstrably only once in mammals. Living baleen whales (mysticetes)
are born without teeth, but paleontological and embryological evidence
demonstrate that they evolved from toothed ancestors that lacked baleen
entirely. The mechanisms driving the origin of filter feeding in
tetrapods remain obscure. Here we report Maiabalaena nesbittae gen. et sp. nov., a new fossil whale from
early Oligocene rocks of Washington State, USA, lacking evidence of
both teeth and baleen.
The holotype possesses a nearly complete skull
with ear bones, both mandibles, and associated postcrania. Phylogenetic
analysis shows Maiabalaena as crownward of all toothed mysticetes,
demonstrating that tooth loss preceded the evolution of baleen. 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 than the oldest stem mysticetes (∼36
Ma). Previous hypotheses for the origin of baleen 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. Understanding these
macroevolutionary patterns in baleen whales is akin to other
macroevolutionary transformations in tetrapods such as scales to
feathers in birds.
Keywords: baleen, cetacea, filter-feeding, mysticeti, suction feeding
 |
| Figure 1. Cranial Elements of the Holotype of Maiabalaena nesbittae, USNM 314627. |
Systematics
Cetacea; Pelagiceti;
Neoceti; Mysticeti;
Maiabalaena nesbittae gen. et sp. nov.
Etymology: Maiabalaena combines Maia-, meaning mother, and -balaena, meaning whale. Named for its phylogenetic position as basal to baleen-bearing mysticetes. The specific epithet nesbittae honors Dr. Elizabeth A. Nesbitt for
her lifetime of contribution to paleontology of the Pacific Northwest
and her mentorship and collegiality at the Burke Museum of Natural
History and Culture in Seattle, Washington, USA.
 |
| 3D models of select specimens in lateral view with artistic reconstructions of their feeding modes: (B) Basilosaurus isis; (C) Coronodon havensteini; (D) Maiabalaena nesbittae; and (E) Balaenoptera musculus.
These panels illustrate the loss of a
functional dentition, the intermediate phase with neither teeth nor
baleen, and the subsequent origin of baleen. Illustrations are original
artwork by Alex Boersma (www.alexboersma.com).
|
 | |
| Figure 2. Phylogenetic
Relationships of Stem Mysticetes Illustrating the Evolutionary Loss of
Teeth and Subsequent Origin of Baleen Figure illustrates a composite
phylogeny including results from this analysis (Figure S4) and recently
published analyses. (A) Time calibrated simplified phylogeny, with collapsed clade resolution for Mammalodontidae, Aetiocetidae and Eomysticetidae, and crown Mysticeti. | (B–E) Colored bars indicate groups figured; gray bars indicate groups not figured. Panels (b–e) represent 3D models of select specimens in lateral view with artistic reconstructions of their feeding modes: (B) Basilosaurus isis; (C) Coronodon havensteini; (D) Maiabalaena nesbittae; and (E) Balaenoptera musculus. These panels illustrate the loss of a functional dentition, the intermediate phase with neither teeth nor baleen, and the subsequent origin of baleen. Illustrations are original artwork by Alex Boersma (www.alexboersma.com). |
 |
| This is an artistic reconstruction of a mother and calf of Maiabalaena nesbittae nursing offshore of Oregon during the Oligocene, about 33 million years ago. While Maiabalaena would
not have been able to chew or filter feed, muscle attachments on the
bones of its throat indicate it likely had strong cheeks and a
retractable tongue. These traits would have enabled it to suck water
into its mouth, taking up fish and small squid in the process. The
ability to suction feed would have rendered teeth, whose development
requires a lot of energy to grow, unnecessary. The loss of teeth, then,
appears to have set the evolutionary stage for the baleen, which the
scientists estimate arose about 5 to 7 million years later. |
Carlos Mauricio Peredo, Nicholas D. Pyenson, Christopher D. Marshall
and Mark D. Uhen. 2018. Tooth Loss Precedes the Origin of Baleen in
Whales. Current Biology. DOI: 10.1016/j.cub.2018.10.047
Whales Lost Their Teeth Before Evolving Hair-like Baleen in Their Mouths si.edu/newsdesk/releases/whales-lost-their-teeth-evolving-hair-baleen-their-mouths via @Smithsonian
Toothless, 33-Million-Year-Old Whale Could Be an Evolutionary ‘Missing Link’ gizmodo.com/toothless-33-million-year-old-whale-could-be-an-evolut-1830739126 via @gizmodo
Nenhum comentário:
Postar um comentário
Observação: somente um membro deste blog pode postar um comentário.