Unusual concentrations of foraminiferal tests interpreted to be the remnants of worm tubes 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 at Mission Viejo. The foraminifera indicate deposition occurred at bathyal depths. Surrounding strata are fine-grained, thin bedded and lack heavy bioturbation, features typical of the siliceous facies of the upper Monterey Formation and indicative of disaerobic conditions.
The foraminiferal concentrations are arranged in tube-like shapes similar to those constructed by some modern marine worms. Because agglutinated worm tubes readily disaggregate after the worm’s death, very few tubes have been found in the fossil record. In addition, although some modern worms construct their tubes using foraminiferal tests, the fossil record of this phenomenon to date is represented by a single specimen from the Lias (Lower Jurassic). The fossil tubes from the Miocene of California have closest affinity with the polychaete worm genus Pectinaria, which includes Recent species living at littoral to abyssal depths. Foraminiferal tests were likely the predominant sand-sized particles available to worms in the Miocene environments, thus the worms likely did not purposefully select foraminiferal tests. Low-oxygen conditions might have enhanced the potential for tube preservation, but subsequent leaching at the Carmel localities removed most calcareous test material from the sediment.