Written in stone: history of serpulid polychaetes through time

Although the fossil record of annelids in general is poor, calcareous tube-building Serpulidae are a notable exception. The "stumbling block" of understanding the serpulid fossil record is obtaining reliable taxonomic interpretations of fossil tubes based on morphology. Luckily, serpulid tubes demonstrate high variety of ultrastructures and nonuniform mineralogical composition, which can be used as new tools for decrypting the fossil record. Ancient Late Ediacaran (580- 541 Ma) and Paleozoic (541-252 Ma) rocks contain diverse tubicolous fossils that have often been erroneously interpreted as annelids, and serpulids, in particular. Palaeozoic to Middle Jurassic coiled spirorbiform tubes, often referred to as Spirorbis, had been shown to be microconchids, a group of probable lophophorate affinity. The most ancient records of unequivocal serpulids date back to the Middle Triassic (~244 Ma) of the Mesozoic, and from the Earliest Jurassic (~200 Ma) fossil serpulids become common. From the latest Jurassic (~146 Ma) serpulids colonised hydrocarbon seep environments and possibly also penetrated the deep sea. Concerted efforts of paleontologists and zoologists are needed for further understanding of serpulid evolutionary history. The serpulid fossil record can become a valuable instrument for calibration of "molecular clocks" in polychaetes, which would allow dating not only divergence events in serpulids, but also in annelid groups that lack a representative fossil record.