Small, spirally coiled tubeworms from the Middle Jurassic, previously assigned to the polychaete spirorbid genera Spi− rorbis and Spirorbula, are the youngest known examples of the problematical Microconchida, a group that is more com− mon in Palaeozoic and Triassic deposits. Three species are recognized, all assigned to the new genus Puntaconchus. In overall morphology, they represent remarkable homeomorphs of spirorbid polychaetes. However, their porous tubes and skeletal microstructure imply a very different mode of growth from that of true spirorbid polychaetes which are first recorded unequivocally from the Cretaceous (Taylor and Vinn 2006). Whereas the tubes of spirorbids are formed from a mucus paste containing calcium carbonate crystallites that is applied episodically around the tube aperture, those of microconchids were apparently formed as continuous secretions from an epi− thelium that lined the interior of the tube.
The Jurassic microconchid Punctachonus differs from its Palaeozoic–Triassic predecessors in having walls with very large pores, contrasting with the imperforate or minutely po− rous tubes found in older taxa. At least some species of Punctachonus have a fibrous tube ultrastructure, a feature not yet recorded among Palaeozoic or Triassic microcon− chids. Microconchid tubes, by analogy with brachiopods, bryozoans and molluscs, are inferred to have possessed an outer organic layer (periostracum or cuticle). Maintenance of this layer may have been aided by the pores in the calcareous skeleton which would have permitted a direct link between the outer organic layer and the soft tissues of the animal within the tube. The fibrous tube ultrastructure and the large pores probably indicate a high organic content in the skeleton compared to Palaeozoic microconchids that lack pores and possess tabular crystallites with lower surface areas than the fibres of Punctaconchus. Thus, the tubes of Punctaconchus may have been more flexible and easier to repair in the event of damage.
Punctaconchus is not only the youngest known micro− conchid, but also the youngest tentaculitoid, a class that in− cludes tentaculitids, cornulitids, and trypanoporids. These vermiform, calcareous tube−building animals flourished in the Palaeozoic and were a distinctive, if minor, component of Palaeozoic marine faunas. Uniquely among tentaculitoids, some microconchids were able to inhabit reduced salinity en− vironments (Taylor and Vinn 2006). The youngest non−ma− rine microconchids are recorded from the Triassic (e.g., Ball 1980). However, the Jurassic genus Punctaconchus is known only from marine strata.
More research is needed on Mesozoic tubeworms to deter− mine the timing of the switchover from spirorbiform micro− conchids to spirorbid polychaetes. The youngest known microconchids, described in this paper, date from the Late Bathonian. None of the supposed spirorbid polychaetes re− corded from the pre−Cretaceous seem actually to belong to this group: some are microconchids and others are likely to be spi− rally coiled serpulids. True spirorbids are, however, present in the Cretaceous, at least as early as the Cenomanian. Therefore, current evidence suggests that there was no overlap in the ranges of spirorbiform microconchids and spirorbid poly− chaetes, implying extinction of the former group before origi− nation of the latter. It may be notable that, according to the data of Parsch (1956), there is a fivefold increase in the number of spirally coiled serpulid species between theMiddle and Upper Jurassic. This suggests a major diversification of annelid tube− worms immediately after the presumed extinction of spirorbi− form microconchids at the end of Middle Jurassic. Ecospace vacated by spirally coiled spirorbiform microconchids could thus have been reoccupied by spirally coiled serpulid poly− chaetes and eventually by true spirorbids.
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