Coombs et al. 2020 re-study odontocete skull asymmetry

Coombs et al. 2020 described odontocete (toothed whale) skull asymmetry
but did not trace it back to its origins in tenrecs (Fig. 1), as we did here two years ago. Without a valid phylogenetic context, the answers they sought evaded the Coombs team.

Nine years ago
whale skull asymmetry was studied by Fahlke et al. 2011, likewise without including tenrecs.

Figure 1. Skull asymmetry in odontocete whales from Fahlke et al. 2011.

Figure 2. Hemicentetes an extant echolocating tenrec, also has a twisted skull, like its descendants, the odontocete whales.

From the Coombs et al. abstract:
“Unlike most mammals, toothed whale (Odontoceti) skulls lack symmetry in the nasal and facial (nasofacial) region. This asymmetry is hypothesised to relate to echolocation, which may have evolved in the earliest diverging odontocetes.”

Earlier. See figure 1.

“Early cetaceans (whales, dolphins, and porpoises) such as archaeocetes, namely the protocetids and basilosaurids, have asymmetric rostra, but it is unclear when nasofacial asymmetry evolved during the transition from archaeocetes to modern whales.”

Earlier. See figure 1.

“Early ancestors of living whales had little cranial asymmetry and likely were not able to echolocate.”

Incorrect conclusion. Add taxa. See figure 1. And see Gould 1965, who described echolocation in tenrecs.

Figure 1. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.

Figure 2. Odontoceti (toothed whale) origin and evolution from tree shrews to killer whales. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.

Illustrations like these
(Fig 2) can be extremely helpful for ‘seeing’ evolution take place in a series of micro-evolutionary events. Typical of evolution, several lineages go extinct, while one or a few continue to the present day. Here we are lucky enough to have a few flesh and blood tenrecs at the genesis and several odontocetes to compare. This would make a great PhD project.

Coombs et al. 2020
are still not aware that the traditional clade ‘Cetacea’ is no longer valid because odontocete ‘whales’ arise apart from mysticete ‘whales’ in the large reptile tree. Click the links in this paragraph and in the citations below to get more backstory.

Coombs EJ, Clavel J, Park T, Churchill M and Goswami A 2020. BMC Biology 18:86
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;
Gould E 1965. Evidence for Echolocation in the Tenrecidae of Madagascar
Proceedings of the American Philosophical Society 109 (6): 352-360. online here.


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