High Encephalization in a Fossil Rorqual Illuminates Baleen Whale Brain Evolution

Area of Science:

• Paleontology
• Neuroscience
• Evolutionary Biology

Background:

• Baleen whales (mysticetes) are considered underencephalized, with low brain-to-body size ratios (encephalization quotient [EQ] << 1).
• Despite low EQ, mysticetes display complex behaviors, but their brain evolution remains poorly understood.
• Investigating fossil whale brains is crucial for understanding the historical changes in brain morphology and the evolution of complex behaviors.

Purpose of the Study:

• To describe the virtual endocast of a fossil balaenopterid, Marzanoptera tersillae, from the Pliocene.
• To investigate the brain size and morphology of M. tersillae and compare it to extant mysticetes.
• To explore the implications of M. tersillae's brain evolution on understanding baleen whale brain evolution.

Main Methods:

• Virtual endocast reconstruction of the fossil balaenopterid Marzanoptera tersillae.
• Analysis of brain morphology, including cerebral hemispheres and cerebellum.
• Comparison of encephalization quotient (EQ) and brain organization with extant baleen whales.

Main Results:

• Marzanoptera tersillae exhibited an exceptionally high EQ (around 3) compared to extant baleen whales.
• The fossil whale's brain showed a distinct morphology: anteroposteriorly shortened cerebral hemispheres, reduced cerebellar vermis, and lack of posteromedial expansion in cerebellar hemispheres.
• These features suggest M. tersillae likely had less sophisticated motor behaviors than extant rorquals and humpbacks.

Conclusions:

• The high EQ in M. tersillae challenges the notion of consistently low encephalization in baleen whales.
• The unique brain morphology indicates a different evolutionary trajectory for brain organization in this fossil species.
• This study provides critical data on the evolutionary path of brain size and morphology in baleen whales, opening new avenues for research.