Gothograptus nassa

Kozłowska, Anna, 2016, A new generic name, Semigothograptus, for Gothograptus? meganassa Rickards & Palmer, 2002, from the Silurian post- lundgreni Biozone recovery phase, and comparative morphology of retiolitids from the lowermost upper Homerian (upper Wenlock), Zootaxa 4208 (6), pp. 534-546 : 542-543

The Gothograptus nassa type of genicular hoods

Genicular processes are typical of many retiolitids, mostly in post- lundgreni Biozone species. One of them is the genicular hood, typical of Gothograptus nassa Holm, 1890 , named here the nassa type. In the evolutionary history of the retiolitines, nassa type genicular hoods are present in four species, all of them within the Gothograptus lineage, which started during the lundgreni Biozone, and ranges into to the lower Ludfordian leintwardinensis Biozone ( Kozłowska 2015). The species are Gothograptus kozlowskii Kozłowska-Dawidziuk, 1990 from the lundgreni Biozone, Gothograptus nassa from the dubius / nassa Biozone and lower part of the praedeubeli Biozone, Semigothograptus meganassa ( Rickards & Palmer, 2002) from the uppermost part of the dubius / nassa Biozone and ludensis Biozone, and Neogothograptus eximinassa Maletz, 2008 from the ludensis/gerhardi Biozone.

The nassa type of genicular hoods are developed only on the first thecal pair in the oldest species, Gothograptus kozlowskii , whereas the rest of the genicular processes are largely of reticulated construction. In mature tubaria of Gothograptus nassa , Semigothograptus meganassa and Neogothograptus eximinassa genicular hoods of the nassa type are well-developed on every theca ( Kozłowska et al. 2009).

The detailed structure of the hood is shown herein in two well-preserved specimens of Semigothograptus meganassa and G. nassa ( Fig. 7 ). The hood is built from many, very thin and densely packed parallel subjacent bandages as shown in detail on Fig. 7 B, F, G. The bandages grow proximally from the genicular list forming a structure of high density. Irregular bandages, growing also proximally and started above the geniculum, are secreted on the outer surface of the hood, strengthening it during astogeny ( Fig. 7 A, E–H). The hoods of thecae within mature tubaria may occlude the whole thecal aperture leaving very little space for the zooids.

There are some differences in the density of the parallel bandages forming the nassa type of hoods. The irregular bandages forming the outer layer of the hood may be weakly developed. The most dense and solid hoods are those of Gothograptus kozlowskii and G. nassa , whereas the hoods of Semigothograptus meganassa ( Fig. 7 A– D), and hoods of Neogothograptus eximinassa ( Maletz 2008, fig. 9H, K) have some space between the parallel bandages. However, in the mature specimens of Semigothograptus meganassa described by Rickards & Palmer (2002), the hoods form solid structures as in the G. nassa hoods ( Rickards & Palmer 2002, text-fig. 4A–F).

In the diagnosis of the Gothograptus ? meganassa, Rickards & Palmer (2002, p. 229) note that the hoods are “heavily pigmented but with clear microfusellar tissue”. SEM examination of the present material reveals that these hoods have distinctive pustules on the parallel bandages as well as on the typical irregular bandages covering the hoods ( Fig. 7 ). The pustules are a unique structure known only on the bandage surfaces of some retiolitids ( Bates et al. 2005). Thus the pustules identified under SEM, clearly indicate the bandages.

The morphology of the Neogothograptus eximinassa hoods is shown by Maletz (2008, fig. 9H, K) on SEM pictures. They are similar to the Semigothograptus meganassa hoods but the enlargement is too small to see the details. The pustules are very weakly visible on the parallel structures building the genicular hoods (Maletz, personal communication). Maletz (2008) regarded the hoods as composed of microfusellar structure.

Figures

FIGURE 7. SEM pictures of nassa type genicular hoods of Gothograptus nassa Holm, 1890 and Semigothograptus meganassa (Rickards & Palmer, 2002). A – D. Semigothograptus meganassa, ZPAL G. 54 / 1, 1649.2 m, Bartoszyce IG- 1 drill core, Poland, uppermost part of dubius / nassa Biozone, A. hood of th 2 2 with marked surface bandages (arrow). B. enlargement showing pustules on parallel bandages (arrow), C. hood of th 1 2 with marked space between bandages (arrow), D. hood of th 3 2, surface bandages (arrow). E – H. Gothograptus nassa, Bartoszyce IG- 1, 1655.8 m, dubius / nassa Biozone, E. whole hood of th 3 2, irregular bandages arrowed, specimen ZPAL G. 54 / 3. F – G. enlargement of hood, ZPAL G. 54 / 9, F. showing pustules on parallel bandages (arrows), G. presenting edges of subjacent bandages (arrows). H. fragment of hood of mature specimen with thick layer of irregular bandages with distinctive pustules and parallel bandages (arrow), ZPAL G. 54 / 6.