The enigmatic palynomorph Tortotubus Protuberans from the latest Ordovician-Early Silurian: A possible land-derived plant fragment in nearshore depositional environments

Tortotubus protuberans was first described by Johnson (1985) from the Silurian (Llandovery) near Mill Hall in Pennsylvania, U.S.A. Since then it has been described from a number of northern Gondwana localities (Chad, Libya, Oman, Saudi Arabia) and Laurentia ranging in age from latest Ordovician to Silurian. T. protuberans occurs as tubular organic structures believed to derive from nematophytes – a group of probable land plants of uncertain affinities (Wellman 1995). Tubular organic structures comparable to T. protuberans reported by Johnson (1985) were found in drill cores from shallow boreholes in the eastern Kufra Basin of southeastern Libya and in the southwestern Kufra Basin in northern Chad in uppermost Hirnantian or lowermost Rhuddanian sedimentary strata (Le Hérissé et al. 2013; Thusu et al. 2013). T. protuberans fragments up to 1 mm long were recovered for SEM analysis by moderate acid treatment with HCl and HF without bleaching and centrifugation. The resulting organic residue was sieved through a 50-micron sieve, and T. protuberans was then hand-picked with a glass micropipette from a watch glass filled with distilled water. In Libya, T. protuberans has a wide geographical distribution. In addition to the reported presence in the Kufra Basin, it is reported in well A1-46 in the Cyrenaica Platform and in wells A1-81 and I1C-81 in the Quattara Graben and well E1-81 in the Dalma High in the southern Cyrenaica Platform, eastern Libya (Richardson 1998; B. Thusu, personal observations). In these localities, T. protuberans is associated with cryptospores, Tasmanites sp., scolecodonts, acritarchs, chitinozoans and remains of eurypterids. In the Llandovery of central Pennsylvania, it is associated with an abundant and predominantly land-derived palynoflora (Johnson 1985). The cryptospore-Tortotubus association observed in several, geographically apart basins may suggest that both palynomorphs may be outwash, land-derived plant debris deposited in nearshore sedimentary environments. In conclusion, T. protuberans shows a wide geographical distribution but seems to be restricted to marginal marine (nearshore) environments in close proximity to the shoreline. Therefore, it may be useful as an environmental indicator. Its presence in Lower Palaeozoic nearshore shales in North Africa may be accompanied by diminished source rock potential as recorded in the examined cores from the Kufra Basin. It is interesting to note that the occurrence of T. protuberans and cryptospores in the stratigraphic record coincides with the evolutionary period of early land plants in the latest Ordovician to earliest Silurian. These observations encourage future studies.