In the Middle–Upper Jurassic boundary of the External Subbetic, there are abundant discontinuities with neptunian dikes and sills composed of Callovian–lower Tithonian deposits. In cavities developed on a slope with escarpments, cryptobiontic communities were preserved in life position. These cavities were excavated beneath a hardground covering the upper surface of Bathonian oolitic limestones deposited in very shallow pelagic carbonate platforms. The biogenic crusts are composed mainly of serpulids and Frutexites—laminated dendrolitic microstructures—and, secondarily, by sessile foraminifera. The serpulids were pioneer organisms during colonization of the walls of small cavities in stressed shadow-cryptic environments. Serpulid aggregates then grew downward from the top walls of the cavities. The colonization of serpulid tubes was mainly after the death of organisms, first by microborers, secondly by Frutexites, and later by sessile foraminifera. The preferential colonization of these cryptic environments by serpulids can be interpreted as due to possible photophobic behavior and or the possibility of avoiding space competition or predation.

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