Whale experts consider the origin of the cetacean brain without a valid phylogenetic context

Orliac and Thewissen 2021
looked at the brain endocast of the tenrec-to-pakicetid transitional taxon, Indohyus (Figs. 1, 2), but omitted comparisons to extant tenrecs. Instead they held on to the myth of an artiodactyl ancestry for all whales. Sadly, these whale experts did not realize that: 1) Indohyus is a tenrec (Fig. 4) and: 2) baleen whales (mysticetes) had an entirely different ancestry arising from hippos (= mesonychids, not artiodactyls), anthracobunids and desmostylians (Figs. 3–5). This has been online for five years and at ResearchGate for three years.

The authors also overlooked
the cranial torsion (= assymmetry, Fig. 1) found here in tenrecs that echolocate, a trait retained by living odontocetes like Orcinus (Fig. 2), and not found in mysticetes.

Indohyus is not an artiodactyl. It is a tenrec in the large reptile tree (LRT, 1838+ taxa; subsets Figs. 4, 5), which minimizes taxon exclusion by its wide gamut of included taxa.

“Raoellidae are sister group to Cetacea and the new morphological observations allow for outlining some of the early steps of the evolutionary history of the cetacean brain.”

Cetacea is not a monophyletic clade as discussed in a paper here in 2018 and first hypothesized here in 2016.

“The combination of primitive artiodactyl features and typical cetacean characters is unique about the Indohyus endocast. The fact that it presents the symplesiomorphic brain pattern observed in earliest Artiodactyla indicates that the cetacean brain derives from a very simple, plesiomorphic brain, with simple neocortical folding pattern, widely exposed midbrain, and concurrent small neocortex expansion.”

Taxon exclusion is present here. Note how whale authors never find a sister for Indohyus within the Artiodactyla. That’s because it isn’t there. In like fashion authors never find a sister for pterosaurs within Dinosauria, but keep insisting the two clades are somehow related.

“On the other hand, the Indohyus endocast shows characters that also occur in early cetaceans. These include modifications of the olfactory tract with narrow, elongated olfactory bulbs and peduncles, accompanied by a posterior location of the braincase in the cranium.”

Which other overlooked taxa have an elongate olfactory bulbs? Tenrecs (Fig. 2).

“The derived endocranial cast features of Indohyus mainly reflect changes in cranial architecture and these are most probably associated with modifications of the masticatory apparatus and a shift in diet.”

The authors overlook the echolocation abilities of tenrecs and Indohyus, their ability to send out signals through the nasal area, as in odontocetes, along with a torsioned skull for receiving signals slightly out of phase for more accurate 3D imagery.

“Indohyus meninges (= the three membranes that envelop the brain and spinal cord) were very thin like in most terrestrial artiodactyls and it had no extensive rostral or lateral retia mirabilia (= a complex of arteries and veins lying very close to each other). It however shows a branching pattern of ramification of intraosseous blood sinuses above the cerebellum that might represent the initial development of the caudal venous rete mirabile that would have colonized the endocranial cavity later on, in early archaeocetes.

Good to know, but take a look at tenrecs.

Some definitions according to Wikipedia:
“Raoellidae are an extinct family of semiaquatic digitigrade artiodactyls in the clade Whippomorpha. An exceptionally complete skeleton of Indohyus from Kashmir suggests that raoellids are the “missing link” sister group to whales (Cetacea).”

In the LRT Indohyus is basal only to pakicetids, archaecetids and odontocetes, not to mysticetes. ‘Cetacea’ is not a monophyletic clade (Figs. 4, 5).

“Whippomorpha is a group of animals that contains all living cetaceans (whales, dolphins, etc.) and hippopotamuses, as well as their extinct relatives. All Whippomorphs are descendants of the last common ancestor of Hippopotamus amphibius and Tursiops truncatus. This makes it a crown group.

In the LRT this definition includes edentates (=xenarthrans), uintatheres, rhinos, macrauchenids, chalicotheres, elephants and horses, taxa not intended for inclusion when originally defined.

Choosing to exclude taxa
is a common trait among established PhDs in paleontology. That’s why it is the number one problem in paleontology. PhDs got into paleontology to make discoveries, but when others make discoveries that change or challenge their hypotheses, a less welcoming attitude ensues.

References
Orliac MJ and Thewissen JGM 2021. The Endocranial Cast of Indohyus (Artiodactyla, Raoellidae): The Origin of the Cetacean Brain. Journal of Mammalian Evolution (advance online publication) DOI: https://doi.org/10.1007/s10914-021-09552-x
https://link.springer.com/article/10.1007/s10914-021-09552-x

wiki/Raoellidae
wiki/Whippomorpha