High Encephalization in a Fossil Rorqual Illuminates Baleen Whale Brain Evolution

Abstract

Baleen whales are considered underencephalized mammals due to their reduced brain size with respect to their body size (encephalization quotient [EQ] << 1). Despite their low EQ, mysticetes exhibit complex behavioral patterns in terms of motor abilities, vocal repertoire, and cultural learning. Very scarce information is available about the morphological evolution of the brain in this group; this makes it difficult to investigate the historical changes in brain shape and size in order to relate the origin of the complex mysticete behavioral repertoire to the evolution of specific neural substrates. Here, the first description of the virtual endocast of a fossil balaenopterid species, <i>Marzanoptera tersillae</i> from the Italian Pliocene, reveals an EQ of around 3, which is exceptional for baleen whales. The endocast showed a morphologically different organization of the brain in this fossil whale as the cerebral hemispheres are anteroposteriorly shortened, the cerebellum lacks the posteromedial expansion of the cerebellar hemispheres, and the cerebellar vermis is unusually reduced. The comparative reductions of the cerebral and cerebellar hemispheres suggest that the motor behavior of <i>M. tersillae</i> probably was less sophisticated than that exhibited by the extant rorqual and humpback species. The presence of an EQ value in this fossil species that is around 10 times higher than that of extant mysticetes opens new questions about brain evolution and provides new, invaluable information about the evolutionary path of morphological and size change in the brain of baleen whales.

Neuroanatomy of the Cetacean Sensory Systems

“…In dolphins, it is mainly innervated by the mandibular branch of the trigeminal nerve (V3), through the auriculotemporal nerve and several of its branches, although it likely follows the terrestrial mammals Bauplan with secondary contribution through the facial (VII), glossopharyngeal (IX) and vagal nerves (X), and possibly also the accessory nerve (XI) and nerves connected to the most cranial cervical dorsal root ganglia [ 188 ]. In delphinoids, V3 exists in the basicranium separately, through the foramen ovale along the posteromedial margin of the alisphenoid bone [ 189 , 190 , 191 , 192 ]. It gives off the small auriculotemporal nerve, which turns ventro-laterally in the direction of the tympanic bulla and dorsally near the mandibular joint, where it gives off several thin branches that project into the area of the external ear canal [ 193 ].…”

Cranial Material of Long-Snouted Dolphins (Cetacea, Odontoceti, Eurhinodelphinidae) from the Early Miocene of Rosignano Monferrato, Piedmont (NW Italy): Anatomy, Paleoneurology, Phylogenetic Relationships and Paleobiogeography

“…The sedimentary sequence of eastern Monferrato includes several units among which the Pietra da Cantoni (PdC) is located at the top. The PdC consists of clastic rocks, typical of a relatively shallow marine depositional environment and influenced by waves; the sediments are indicative of a paleobiotope (developed in a warm tropical sea) located in an outer subsiding carbonate platform and slope, showing a transgressive trend and a deepening of the basin from east to west [2,3,24] and literature therein).…”

“…Unfortunately, the exact stratigraphic location of the fossil within the PdC is unknown; therefore, since it cannot be referenced to any of the biozones mentioned above, it is generically assigned to the late Burdigalian (approximately 16-19 million years ago) [3] (Figure 1).…”

“…In Piedmont, the local geological history was characterized by a series of Miocene and Pliocene paleoenvironments with remarkable biodiversity, especially of cetaceans [2]. The fossil record of cetaceans in Piedmont, which is in large part preserved in the collection of the Museo Paleontologico Territoriale dell'Astigiano (hereinafter MPTA) [3], contributes to the reconstruction of the main evolutionary trends of these marine mammals in the Mediterranean Miocene, providing crucial information for the reconstruction of the taxonomic changes and turnover patterns that affected the cetacean communities of the central Mediterranean area during the Neogene [4]. In this basin, the odontocete diversity has consistently increased from the latest Aquitanian-early Burdigalian to reach a peak in diversity during the Burdigalian-Langhian [4].…”

“…These include several units, among which the Pietra da Cantoni Formation (hereinafter PdC) occupies the uppermost portion. A number of fossil cetaceans were found in the PdC (which is Burdigalian-upper Langhian in age) over the last three centuries [3]. The PdC accumulated in an outer subsiding carbonate platform and slope, being characterized by a transgressive trend and a deepening of the basin from east to west.…”