FOSSIL BALEEN WHALES (MYSTICETI) SHOW THAT CEREBRUM SIZE ACHIEVED MODERN SIZE BEFORE BALEEN EVOLVED

The lineage of baleen whales (Mysticeti) first appeared in the latest Eocene, although these early baleen whales had teeth and lacked baleen for filter-feeding. The earliest baleen whales also possessed organs for olfaction, which they inherited from their terrestrial ancestors and continue to possess today. By contrast, olfactory organs are absent in living lineage of toothed whales (Odontoceti), the sister group to mysticetes, implying that olfaction was lost in odontocetes (all crown cetaceans have also lost the ability to taste, as well). As with other mammals, olfaction in cetaceans is functionally regionalizd in the olfactory bulb, which can be inferred directly from natural endocasts of the braincase and computed tomography (CT) scanning.

To understand the evolution of olfactory bulbs (OB) and its potential connection to brain size evolution, we used CT scanning to examine 12 mysticetes, including both stem (e.g., Mammalodontidae, Aetiocetidae) and crown Mysticeti, along with two species of stem cetaceans, four (extant and extinct) species of Odontoceti, and four species of semi-aquatic and terrestrial artiodactyls. These datasets were obtained using multiple CT scanning techniques including helical CT scanning and micro-focus CT scanning. We then created 3D digital endocranial casts to examine the morphological differences, and the relative evolutionary changes in brain morphology and size along the lineage of mysticetes. We determined that the OB is present in all the extinct and extant Mysticeti. However, the size and the length of the olfactory tract varied among the groups. In the stem mysticetes, generally, OB size is greater and the olfactory tract is longer than that all toothed baleen whales or crown baleen whales.

Interestingly, cerebrum size in early mysticetes was as large as modern mysticetes, although it was less complex. While previous research indicated that the dorsal OB in cetaceans is absent because the D domain region was lost in the early cetacean evolution, we found that fossil mysticetes retained the dorsal side of OB. Our work shows that baleen whale brain and sensory organs evolved in strongly regionalized ways, which were likely decoupled from innovations such as filter-feeding and extreme gigantism.