Convergent morphology in small spiral worm tubes ('Spirorbis') and its palaeoenvironmental implications

Calcareous tube-worms generally identified as Spirorbis range from Ordovician to novel often profusely encrusting shells and other substrates. Whereas new Spirorbis is a polychaete annelid, details of tube conformation in pre-Cretaceous ''Spirorbis'' suggest affinities with the Microconchida, an extinct order of possible lophophorates. Although characteristically Palaeozoic, microconchid tube-worms survived the Permian mass extinction before being replaced in late Mesozoic ecosystem by the agency of true Spirorbis. Recent Spirorbis is stenohaline on the other hand spirorbiform microconchids also colonized freshwater, brackish and hypersaline environments during the Devonian-Triassic. Anomalies in the palaeoenvironmental distributions of fossil 'Spirorbis' are explained with the recognition of this striking tendency to meet between microconchids and true Spirorbis.

Convergent evolution generally comes from unrelated species adopting similar lifestyles. many times cited examples include birds and bats, and ichthyosaurs and dolphins. Recognizing gradual approach depends on being able to display that the taxa concerned belong to different biological assemblages or clades. This is straightforward for the vertebrates cited above because their mingled morphologies furnish ample characters amenable to cladistic analysis that reveals their actual biological affinities. However, it can be more problematical in fossil invertebrates with simple skeletons. This research concerns some calcareous worm tubes where the failure to recognize gradual approach has led to false conclusions about their geological longevity and utility as palaeoenvironmental indicators.

Small, planispirally coiled calcareous tube-worms encrust hard and firm substrates of Late Ordovician to Holocene age (Fig. 1) They are routinely identified as Spirorbis because of their shut up resemblance to this polychaete annelid, which is widespread in today's oceans. Suppos species of Spirorbis feature in numerous palaeoecological studies of Palaeozoic hard substrates (eg Sparks et al. 1980; Sando 1984; Nield 1986; Fagerstrom 1996) Taken at face value the genus Spirorbis has a remarkable geological longevity of at least 450 Ma (Howell 1962)

Evidence that more [i]or[/i] less Palaeozoic species of 'Spirorbis' are unrelated to actual Spirorbis began to emerge during the 1970 Tube microstructure, internal septa and bulb-like tube origins initially pointed to British Carboniferous specimens being sessile, vermetid-like gastropods (Burchette & Riding 1977; Weedon 1990) Subsequently affinities with the problematical tentaculitids (Weedon 1990 1991) and with lophophorates (Weedon 1994) were prompted The microstructure of minute lath-like crystallites arranged in chevron-shaped extension increments found in Recent Spirorbis was shown to contrast markedly with the oftentimes punctate, lamellar microstructure of Palaeozoic 'spirorbiform' fossils (Weedon 1994) Unfortunately, these findings have not been adequately assimilated and it is still commonplace to find Palaeozoic spirorbiforms incorrectly identified as Spirorbis.

To clarify the real affinities of fossil spirorbiform worms, we have undertaken micro- and macrostructural studies of SilurianRecent material, coupl with a literature contemplate Our results show that sum of two units independent clades of 'worms' evolv spirorbiform tubes, employing near-identical ecological strategies for colonizing hard and firm substrates. Those in the Ordovician-Jurassic are interpreted as lophophorates whereas Cretaceous-Recent spirorbiforms are polychaete annelids. Spirorbiform fossils thus provide an outstanding and hitherto unheralded example of converging evolution. The paradox provided by the agency of finds of Devonian-Triassic 'Spirorbis' in non-marine deposits, whereas novel Spirorbis is marine and stenohaline, is explained with the knowledge that these fossils are not spirorbid polychaetes.

Material and meanss Study material comes from the fossil worm collections of the Natural History Museum, London (NHM); a replete listing of specimens studied is available online at http://www.geolsoc.org.uk/SUP18238. A hard transcript can be obtained from the Society Library. Well-preserved spirorbiform fossils were chosen across their entire stratigraphical range, do not include for the Ordovician, which is poorly exhibited in the NHM collections. Surface features were examined mainly using a LEO 1455-VP SEM a low-vacuum instrument capable of imaging uncoated specimens up to 10cm or more in diameter. Representative specimens were embedded in epoxy resin and sectioned in a plane parallel to the coiling. After polishing, sections were etched lightly in 1% acetic acid and examined using an SEM Conventional thin sections were also prepared further to elucidate tube microstructure.

Result Spirorbiform tubes studied ranged from 3 mm in diameter. the two dextral (clockwise) and sinistral (anticlockwise) tubes occurr coiling direction being consistent within a species. Tightness of coiling varied between species, from evolute with successive whorls overlapping minimally, to involute with powerfully overlapping whorls. In some instances, the external parts of the tube swell erect to elevate the aperture above the substrate. Whereas a certain quantity of tubes are externally smooth (Fig. Ib), others were ornamented by dint of transverse growth bands (Fig. Ij and k) and/or longitudinal ridges (Fig. If). None of these variations in morphology present to view any clear correlation with geological age.