Gothograptus, Kozłowska & Lenz & Melchin, 2009

Kozłowska, Anna, Lenz, Alfred & Melchin, Michael, 2009, Evolution of the retiolitid Neogothograptus (Graptolithina) and its new species from the upper Wenlock of Poland, Baltica, Acta Palaeontologica Polonica 54 (3), pp. 423-434 : 425-426

Evolution of Neogothograptus

Morphological and stratigraphic data suggest that the genus Neogothograptus may have evolved from either Gothograptus ( Kozłowska−Dawidziuk, 2004) or Baculograptus (Lenz

Baltica, Laurentia, Baltica erratic Baltica, Holy South China, Australia, Bartoszyce Germany, Kyrgyzstan, Arctic, boulders, Cross Mts., southern New South borehole, Baltic boulders Central Asia Canada Poland Poland Yunnan Wales Poland N. reticulatus N. thorsteinssoni N. reticulatus N. balticus Gothograptus ? Gothograptus N. purus labiatus N. thorsteinssoni N. eximinassa N. romani N. reticulatus N. ornatus ? probably = N. balticus ? N. melchini N. romani N. ornatus N. purus reticulatus Gothograptus sp. N. alatiformis sp. nov. N. purus N. alatiformis and Kozłowska−Dawidziuk, 2002). The main similarity among these genera is the construction of a finite rhabdosome terminating in an appendix. Gothograptus and several species of Neogothograptus possess similarly developed genicular processes ( Kozłowska−Dawidziuk 1995), whereas the main difference is the position of the nema ( Fig. 2). In all species of Gothograptus , the nema is attached throughout, whereas in every species of Neogothograptus , as in all other upper Homerian and Ludlow retiolitids, the nema is free ( Bates et al. 2005). Species of Baculograptus lack genicular processes but have an internal structure that may be an intermediate state in which the nema is free proximally, but attached distally, as in Eisenackograptus .

Gothograptus is the only retiolitid genus known to have survived the Cyrtograptus lundgreni extinction event (Porębska et al. 2004) and a single species, Gothograptus nassa , first appearing in the earliest post−extinction Pristiograptus dubius – Gothograptus nassa Biozone and ranging into the middle upper Homerian Colonograptus praedeubeli Biozone (Kozłowska et al. 2001) , is the only known species that spans this interval. By comparison, the genus Neogothograptus was, until very recently, known only from the Ludlow, leaving a considerable biostratigraphic gap between the highest occurrence of Gothograptus and the lowest occurrence of Neogothograptus . The new evidence, presented in this paper, shows that Neogothograptus ( N. reticulatus sp. nov.) first appeared in the C. praedeubeli Biozone. This evidence is further enhanced by the very recent discovery of another species of Neogothograptus ( N. eximinassa ) from the uppermost upper Homerian, Colonograptus ludensis Biozone of Germany (Maletz 2008; and Fig. 3 herein). These gap−closing occurrences suggest that based on biostratigraphic evidence we can suggest that: (i) Gothograptus nassa was a possible ancestor to either or both species of the upper Homerian species of Neogothograptus and/or Baculograptus ; and (ii) either of the upper Homerian species of Neogothograptus could have been ancestral to the Ludlow species of Neogothograptus . Since the first occurrence of species of Baculograptus are known from the C. praedeubeli Biozone , as in Neogothograptus , it is possible that these two genera may show an ancestor−descendant relationship, or be sister taxa, derived from Gothograptus .

It is important, however, that too much reliance is not based on our current understanding of the known ranges of species occurrences for phylogenetic interpretations. New studies of isolated faunas (such as this study and Maletz 2008) are still regularly yielding new taxa, so it is clear that our sampling of Homerian retiolitids is still far from complete. It is also clear that the best−known assemblages both in terms morphology and taxonomic diversity have come from collections of isolated material extracted from carbonate strata. Such collections are uncommon in the Gothograptus nassa / Pristiograptus parvus Biozone in comparison with under− and overlying strata. Therefore, it is likely that despite intensive collection of flattened graptolite assemblages in this interval in many parts of the world, the scarcity of isolated faunas has resulted in undersampling of the diversity of retiolitids in this interval. It is therefore possible that other species of Gothograptus could have survived the Cyrtograptus lundgreni Event to become ancestral to Neogothograptus / Baculograptus . For example, there are significant similarities between G. storchi Lenz and Kozłowska, 2006 from the Cyrtograptus lundgreni Biozone and several species of Neogothograptus and Baculograptus . The finite rhabdosomes ending with an appendix, dense reticulation, similar thecal profile, and well−developed geniculum are very similar in these taxa. A similar possibility is the survival of the G. kozlowskii . This species has genicular hoods similar to those in G. nassa in the proximal end, and long reticulated hoods in the mesial and distal thecae. G. kozlowskii , may then, be ancestral to both G. nassa , and later species of Neogothograptus .

It is likely then, that further studies will yield more previously unknown species that may help to fill in our understanding of the ranges of morphologies among these genera and their stratigraphic distribution.

Figures

Fig. 3. SEM micrographs showing comparison of the genicular hoods of mature rhabdosomes of Neogothograptus Kozłowska−Dawidziuk, 1995 species (A–D) and Gothograptus nassa Holm, 1890 (E). A, B. Neogothograptus eximinassa Maletz, 2008, Baltic erratic boulder from Wetterhammer, Thuringia, Germany, Colonograptus ludensis–Monograptus gerhardi Biozone. C, D. Neogothograptus reticulatus sp. nov. C. ZPAL G. 41/7, Bartoszyce borehole 1630.7 m, Colonograptus praedeubeli Biozone. D. ZPAL G 41/2, Baltic erratic boulder 59, Jarosławiec, Poland. E. Gothograptus nassa, Bartoszyce borehole 1655.8 m, Pristiograptus dubius– Gothograptus nassa Biozone, whole hood (E1) and enlargement showing bandages covering hood (E2).