The swordfish dolphins no one told you about

Here's a group I wish was better known: these are “𝘌𝘶𝘳𝘩𝘪𝘯𝘰𝘥𝘦𝘭𝘱𝘩𝘪𝘴” 𝘴𝘢𝘭𝘦𝘯𝘵𝘪𝘯𝘶𝘴 (skull) and 𝘡𝘪𝘱𝘩𝘪𝘰𝘥𝘦𝘭𝘱𝘩𝘪𝘴 𝘢𝘣𝘦𝘭𝘪 (rostrum), members of the longirostrine “swordfish” dolphins (Eurhinodelphinidae) from the Miocene of Lecce, Italy, displayed at the Museum of Paleontology of Naples (the label says "Rhabdosteidae", but this name is now considered problematic, see Fordyce and de Muizon, 2001).

Many different odontocete lineages converged on a longirostrine, swordfish-like morphology, especially during the Miocene, but eurhinodelphinids were particularly swordfish-like, having a mandible that was much shorter than the rostrum. Approximately one-third of this long rostrum was formed by the premaxilla, which in at least some genera (including 𝘡𝘪𝘱𝘩𝘪𝘰𝘥𝘦𝘭𝘱𝘩𝘪𝘴) had an elongated edentulous termination (McCurry and Pyenson 2019; Tosetto et al. 2023).

Two competing ideas have been formulated to explain the function of this hyperelongate rostrum, the first postulating that it may have been used to stab and stun prey, just like a swordfish. As said above, hyper-elongation of the rostrum evolved many times independently during the Miocene, a fact that may suggest there were some environmental factors favouring this adaptation, such as warmer waters with small and fast prey, and sea-level highs allowing niche partitioning. Furthermore, the circular cross-section of the rostrum of at least some eurhinodelphinids means that they could be swept from different directions, not just do lateral sweeping (McCurry and Pyenson 2019).

Alternatively, they may have been similar to living river dolphins in using their rostra to dig prey out of the sediment, as suggested by deep lateral grooves on the rostrum, the edentulous termination of which may have been a sensory organ (Racicot 2018). This also agrees with the non-­orth­og­onal semicircular canals observed in at least some taxa, suggesting likely slow head movements (Lambert et al. 2022). If so, they may have been slow-moving shallow water denizens, as also suggested by unfused and relatively long cervical vertebrae (Racicot 2018). Cochlear measurements also indicate that at least some taxa produced narrow-band high-frequency echolocation clicks, which in modern odontocetes are correlated with reduced vocal repertoire and sociality, and also to avoidance of predators, which in the case of eurhinodelphinids may have included squalodontids and macroraptorial sperm whales (Lambert et al. 2022).

Eurhinodelphinids were a mostly Miocene group, widespread across the North Atlantic, the Mediterranean, the western South Pacific, and the Paratethys, a vast epic­ontinental sea that once extended fr­om Central Eur­pe t­o Central Asia (Lambert et al. 2022). 

Their position within Odontoceti is still quite unstable, with different analyses variously finding them as close to Ziphiidae (or to a clade comprising Physeteroidea, Ziphiidae, and Delphinida), or close to Platanistidae (Tosetto et al. 2023).

References 

• Fordyce, E., de Muizon, C. (2001). Evolutionary history of cetaceans: a review. In: Mazin, J.M., de Buffrenil, V. (Eds.), Secondary Adaptations of Tetrapods to Life in Water. Verlag Dr. Friedrich Pfeil, Munchen, München, pp. 169–234.
• McCurry, M.R, Pyenson N.D. (2019). Hyper-longirostry and kinematic disparity in extinct toothed whales. Paleobiology. 45(1):21–29. doi:10.1017/pab.2018.33.
• Tosetto, V., Damarco, P., Daniello, R., Pavia, M., Carnevale, G., Bisconti, M. (2023). 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. Diversity 2023, 15, 227. https://doi.org/10.3390/d15020227
• Racicot, R.A. (2018). Dolphins, porpoises, and monodontids, evolution. In Encyclopedia of Marine Mammals, B. Wu¨rsig, J.G.M. Thewissen and K. Kovacs, eds. (Academic Press), pp. 271–274.
• Lambert, O., Wanzenböck, G., Pfaff, C., Louwye, S., Kriwet, J., & Marx, F. G. (2023). First eurhinodelphinid dolphin from the Paratethys reveals a new family of specialised echolocators. Historical Biology, 35, 1074–1091.