Semirostrum fossil map
Where fossils of Semirostrum have been found, shown on the Earth as it looked about 5 million years ago and on today’s map.
Dots are fossil collections recorded in the Paleobiology Database. The “then” view moves today’s fossil sites back to their positions 5 million years ago (the time slice nearest the median age of the fossils) using the Merdith et al. (2021) plate model; positions are approximate. Open on the timeline map →
| Group | Early mammals |
|---|---|
| Lived | 7.246–2.58 million years ago (Messinian to Piacenzian) |
| Periods | Neogene |
| Family | Phocoenidae |
| Named by | Racicot et al. 2014 |
| Fossil records | 8 occurrences at 4 locations (PBDB) |
| Found in | United States |
| Rock formations | Purisima, San Diego |
About Semirostrum
Semirostrum ceruttii is an extinct porpoise that lived between 5 and 1.5 million years ago (Ma), during the Pliocene epoch. The species is highly distinctive due to the extremely long symphysis on the lower jaw, reaching lengths of 85 centimeters (33.46 in), while that of a modern porpoise is 1–2 centimeters (0.39–0.79 in) long. The main hypothesis regarding its use is that it probed along the sediment in the murky estuaries and shores of what is now California in search of food, which would easily be scooped up the symphysis and into the jaws. It is the only known species in the genus Semirostrum. The etymology of the name means "half beak", referring to the upper jaw being half the length of the lower.
Fossils of Semirostrum have an unusually long, fused symphysis, longer than any known mammal. CT scans revealed elongate mental and accessory canals within the symphysis, which are similar to those found in birds of the genus Rynchops, meaning that the elongated symphysis was likely involved in sensory function.
Text in “About” is quoted from the Wikipedia article Semirostrum (revision 1320230140), licensed under CC BY-SA 4.0; introduction excerpted verbatim, formatting removed. Ages, family, authority and fossil records: Paleobiology Database, accessed 23 October 2026. Ages are those of the rock layers the fossils were found in. Wikidata: Q17126060.
