Supayacetus muizoni

Supayacetus muizoni was an archaeocete cetacean belonging to the Basilosauridae, a group of fully aquatic primitive whales that lived during the middle-late Eocene, approximately 40–37 million years ago, in the coastal waters of present-day Peru. It represents a very important transitional form in the early evolution of whales, showing a combination of primitive traits inherited from older archaeocetes and characteristics already clearly adapted to full marine life. Its fossils come from the Paracas Formation, in the Pisco basin, a region exceptionally rich in fossil marine vertebrates from the South American Pacific.

The genus was described in 2011 from the holotype specimen MUSM 1465, a partial skeleton composed of cranial fragments, vertebrae, ribs, teeth, and forelimb and sternum elements. The name Supayacetus combines “Supay”, Inca deity of the underworld and death, with the Greek term ketos (“whale”), while the epithet muizoni honors French paleontologist Christian de Muizon for his contributions to the study of Peruvian paleontology.

In biometric terms, it was a relatively small to medium-sized whale for a basilosaurid, probably with a length close to 5–6 meters, clearly larger than early archaeocetes like Protocetus but smaller than later giants like Basilosaurus. Its body must have been elongated and streamlined, with forelimbs transformed into functional flippers and powerful axial musculature for undulatory swimming. Although no complete caudal remains have been preserved, it is reasonable to infer the presence of a developed caudal fin similar to that of other basilosaurids.

Anatomically, Supayacetus is particularly interesting because it preserves a mosaic of evolutionary characters. Its sternum featured a T-shaped manubrium, a condition considered relatively primitive and closer to semi-aquatic archaeocetes such as Rodhocetus or Georgiacetus than to more derived basilosaurids. This morphology suggests that it still retained certain ancestral anatomical configurations despite being an already fully marine whale. The shoulder was composed of a broad and robust scapula and a hemispherical humerus, well developed to support stabilizing movements of the pectoral fins during swimming.

Its dentition reveals an active marine predator. Its posterior teeth possessed a large central cusp accompanied by accessory denticles, a typical characteristic of basilosaurids adapted for gripping and tearing slippery prey. This indicates a diet probably based on bony fish, small marine vertebrates, and cephalopods, which it would have caught in relatively productive coastal environments. It would not have practiced filter feeding like modern baleen whales, but rather direct predation by biting.

The paleoenvironment where it lived was very different from present-day Peru. During the Eocene, the region was covered by a relatively shallow and surprisingly cold coastal sea for the time, probably influenced by ocean currents somewhat comparable to the current Humboldt system. The presence of cold-water foraminifera and remains of pelagic fish suggests highly productive marine ecosystems capable of sustaining large early marine predators. In this environment, it coexisted with other archaic cetaceans such as Ocucajea, as well as even more primitive protocetids.

From an evolutionary point of view, Supayacetus muizoni holds a relevant position because it helps to understand the transition between early archaeocetes and fully modern whales. For years it was interpreted as one of the most basal basilosaurids, due to its archaic anatomical features, although recent phylogenetic studies have suggested a possible closer relationship with derived forms close to the ancestors of modern cetaceans (Neoceti). This makes Supayacetus a key piece in understanding how marine locomotion, thoracic anatomy, and predatory adaptations evolved in the first fully oceanic cetaceans.