Today two blogposts are published
because they relate strongly to one another. Shortly there will be a post on Monodon, the narwhal, which introduced me to whale skull asymmetry, which I then researched and found the following study from 2011.
A paper by Fahlke et al. 2011 reported,
“Eocene archaeocete whales gave rise to all modern toothed and baleen whales (Odontoceti and Mysticeti) during or near the Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, with asymmetry linked to high-frequency sound production and echolocation.”
This is not true
when more taxa are added to a phylogenetic analysis looking at whales. Archaeocetes are not basal to baleen whales (Mysticeti) in the LRT.
Fahlke et al. did not look at tenrecs,
which nest basal to archaeocetes and pakicetids in the large reptile tree (LRT, 1187 taxa). Hemicentetes (Fig. 2) echolocates (Gould 1965) and travels in pods — and it has a torsioned skull (Fig. 2). Baleen whales (mysticetes) had a separate ancestry with desmostylians, apart from archaeocetes.
A torsioned skull further cements tenrecs to archaeocetes and odontocetes. Fahlke et al. did not look at desmostylians either.
Taxa basal to tenrecs
in the LRT, like the elephant shrew, Rhynchocyon, do not have a torsioned skull. So that trait originated with a sister to Hemicentetes.
In an interview, Fahlke reported,
“This means that the initial asymmetry in whales is not related to echolocation,” said Fahlke, who is working with Philip Gingerich, an internationally recognized authority on whale evolution, at the U-M Museum of Paleontology.
Oh, yes, asymmetry is related to echolocation!
Expand that taxon list to tenrecs, read Gould (1965) and everything will fall into place. The origin of echolocation in the ancestors of whales goes back to the mid-Cretaceous, based on the separation of Madagascar (where tenrecs live) from Pakistan and India (where whale ancestors like the tenrec, Indohyus) are found.
Fahlke’s backstory from the U. of Michigan webpage:
“The actual skull on which the model was based was noticeably asymmetrical, but Fahlke and colleagues at first dismissed the irregularity.
“We thought, like everybody else before us, that this might have happened during burial and fossilization,” Fahlke said. “Under pressure from sediments, fossils oftentimes deform.” To correct for the deformation, coauthor Aaron Wood, a former U-M postdoctoral researcher who is now at the University of Florida, straightened out the skull in the digital model. But when Fahlke began working with the “corrected” model, the jaws just didn’t fit together right. Frustrated, she stared at a cast of the actual skull, puzzling over the problem.
“Finally it dawned on me: Maybe archaeocete skulls really were asymmetrical,” Fahlke said. She didn’t have to go far to explore that idea; the U-M Museum of Paleontology houses one of the world’s largest and most complete archaeocete fossil collections. Fahlke began examining archaeocete skulls, and to her astonishment, “they all showed the same kind of asymmetry?a leftward bend when you look at them from the top down,” she said.”
Fahlke JM, Gingerich PD, Welsh RC and Wood AR. 2011. Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water. PNAS 108 (35) 14545-14548; https://doi.org/10.1073/pnas.1108927108
Gould E 1965. Evidence for Echolocation in the Tenrecidae of Madagascar
Proceedings of the American Philosophical Society 109 (6): 352-360. online here.
U of Michigan story on the Fahlke team’s discovery here