The ray-finned fish blackbox: unprecedented morphological diversity and the interplay between brain and endocast

Vertebrates are unique among animals as they bear a neurocranium, which almost completely encapsulates the brain. Thus, within vertebrates, the evolution of neuroanatomical features might be tightly correlated with the evolution of their surrounding neurocranial ossifications, as these pose spatial constraints to brain size and shape. The endocranial cast, or surrounding neurocranial ossifications, poses spatial constraints to brain size and shape. The endocranial cast, or ‘endocast’, represents an important structure for the study of vertebrate neuroanatomical evolution as it has been considered to be closely associated with brain morphology. This assumption has been tested for many extant vertebrate lineages, including archosaurs, mammals and non-tetrapod sarcopterygian fishes, but brain–endocast correlation has never been studied for ray-finned fishes. Given that ray-finned fishes represent roughly half of extant vertebrate diversity and show remarkable morphological disparity, understanding the relationship between brain and endocast in this major vertebrate lineage becomes paramount for understanding vertebrate neurocranial evolution. Here, we describe previously unknown morphological diversity of endocasts in extant ray-finned fishes, compare it to endocasts from the fossil record and explore the relationship between endocast and brain across a large sample of extant taxa. Our results indicate that endocasts are a poor predictor of brain morphology for ray-finned fishes but can provide valuable information regarding neurocranial diversification and its ties to ecological features.