Foraminifera Used In the Construction of Miocene Polychaete Tubes, Monterey Formation, California, USA

Unusual concentrations of foraminiferal tests occur in the Miocene Monterey Formation near Carmel and Mission Viejo, California. Lithostratigraphic and biostratigraphic correlations firmly establish their ages as late middle Miocene near Carmel and early late Miocene in Mission Viejo. The foraminifera indicate benthic environments were likely bathyal depths and disaerobic, as the strata are fine-grained, thin beds that are not heavily bioturbated – typical features of the siliceous facies of the upper Monterey Formation.

The foraminifera make up tubes similar to those constructed by some modern marine worms. Because agglutinated worm tubes readily disaggregate after the worm's death, very few of them have been found in the fossil record. And although some modern worms construct their tubes with foraminiferal tests, the fossil record of this phenomenon to date is represented by a single specimen from the Lower Jurassic. The fossil tubes from California have closest affinity with the genus Pectinaria, which includes Recent species living at littoral to abyssal depths. Foraminiferal tests were likely the most predominant sand-sized particles available to the worm in these particular environments; the worm did not purposefully select foraminiferal tests. Disaerobia may then have enhanced the chances of tube preservation, but subsequent leaching at the Carmel localities removed most calcareous test material from the sediment.