Semirostrum ceruttii

Overview

Semirostrum ceruttii is an extinct porpoise known from Californian fossils. Its lower jaw extends conspicuously far beyond the upper snout. A 2014 study described this projection beyond the snout as the greatest known among mammals at the time.^[1]

Key facts

The inside of a fossil can also be examined. Researchers used CT to identify long channels within the lower jaw. They discussed the possibility that these structures were associated with sensory function.^[1]

Time and place

The 2014 description presents material from the San Diego and Purisima formations in present-day California, United States. The paper connects these occurrences with Pliocene seas.^[1]

The time range describes source context, not the complete duration of this species.

Size, form and skeleton

The front of the lower jaw is fused over a long distance and projects forward. This elongated region has almost no teeth, while small conical teeth occur farther back. The front and rear of the jaw thus differ in form.^[1]

The total length of Semirostrum is given as approximately 3 m. This estimate was used in a 2024 comparative study of marine life and represents the entire body with its elongated lower jaw.^[2]

Habitat and reproduction

Did it seek food near the seafloor? Based on oblique tooth wear and internal jaw structures, researchers proposed that it may have probed the bottom with its lower jaw.^[1]

Fossil discovery

The study describing the new species used CT to examine the well-preserved type specimen. It explained the unusual fossil by reading the external jaw outline alongside its internal channels.^[1]

References

1. Racicot et al. (2014) — A porpoise with an unusual lower jaw from Pliocene California
2. Pimiento et al. (2024) — Extinct large marine organisms of the Phanerozoic
3. Bisconti (2003) — The evolutionary history of Balaenidae and Balaenula

Research questions

Its lifestyle was not interpreted from the jaw alone. The study also discusses the scapula, sternum, and unfused neck vertebrae. It connected several features to develop a hypothesis about food searching.^[1]

Observation

First locate the tips of the upper snout and lower jaw. Compare their positions, then move your gaze toward the rear region where teeth remain to make the unusual jaw clearer.^[1]