Semigothograptus meganassa ( RICKARDS et PALMER, 2002)

Štorch, Petr & Manda, Štěpán, 2019, Little Known Homerian (Lower Silurian) Graptolites From Kosov Quarry Near Beroun, The Czech Republic, Fossil Imprint 75 (1), pp. 44-58 : 48-49

Semigothograptus meganassa ( RICKARDS et PALMER, 2002)

Text-figs 3c , 4g

2001 Gothograptus nassa HOLM (wide form); Kozłowska- Dawidziuk et al., pp. 151–152, text-fig. 4, figs 8–12.

2002 Gothograptus ? meganassa sp. nov.; Rickards and Palmer, pp. 228–231, text-fig. 4.

2016 Semigothograptus meganassa (RICKARDS et PALMER) ; Kozłowska, pp. 536–540, figs 1d, 2b, 3, 4b, 7a–d.

2019 Semigothograptus aff. meganassa (RICKARDS et PALMER) ; Manda et al., figs 8h, 10d.

H o l o t y p e. Specimen no SX.28550a-b (part and counterpart) from the upper Homerian at Aston Piggot, Long Mountain, Shropshire, UK; figured by Rickards and Palmer (2002: fig. 4a, b).

M a t e r i a l. Single complete flattened rhabdosome from the uppermost lundgreni Biozone of Kosov 2 section (sample K2/6; see Manda et al. 2019: fig. 2) and several poorly preserved fragments from the same level, questionably assigned to this form.

D i a g n o s i s. As for the monospecific genus, see

Kozłowska (2016).

D e s c r i p t i o n. Rhabdosome with 9 distinct pairs of thecae is 10.5 mm long. It widens from 1.36 mm at the first thecal pair, through 1.87 mm at the 3 rd thecal pair to the maximum width 2.1 mm attained at the 5 th thecal pair. Reticulum is dense proximally, becoming much less developed in the distal part. Proximal end rounded, no details of ancora can be recognized in the present material. Rhabdosome development is non-finite, without appendix. The free, continuous nema runs inside the rhabdosome, and further distally out of the thecate part. Genicular hoods are only slightly developed in the two most proximal pairs of thecae. Subsequent thecal apertures are furnished with solid, nassa - type genicular hoods. Rather long lateral apertural rods, convex to the centre of the lateral wall are seen on the right distal thecae of the present specimen ( Text-figs 3c , 4g ), which also exhibits prominent oblique lateral lists of the ancora sleeve. 2TRD increases from 1.5 mm at th2 to 1.9 mm at th5.

R e m a r k s. This form can be readily distinguished from its presumed gothograptid ancestors by its unfinished rhabdosome (lacking an appendix) with free nema, and maximum width of about 2.1 mm, which is attained as late as at the 5 th thecal pair. It resembles Gothograptus nassa ( HOLM, 1890) in having solid, rather prominent apertural hoods, but markedly differs in having a non-finite rhabdosome widening from 1.3–1.4 mm to the maximum 2.1 mm attained at about the fifth thecal pair. The present material from the Kosov 2 section moves the FAD of S. meganassa from the parvusnassa Biozone down to the uppermost lundgreni Biozone. The present FAD of S. meganassa predates the so far known FAD of the widely recorded and very common G. nassa, and therefore calls into question its derivation from G. nassa as presumed by Kozłowska (2016).

Semigothograptus meganassa is known from Avalonia ( England, Rickards and Palmer 2002), Baltica ( Poland, Kozłowska 2016) and peri-Gondwanan Europe ( Germany, Thuringia, G. meganassa, sensu Jaeger (1991) ; the Czech Republic, G. nassa wide form, sensu Kozłowska et al. (2001)). In the Czech Republic, this form occurs in Všeradice (Kozłowska et al. 2001), Nesvačily (PŠ, unpublished personal observation) and Kosov (this paper) sections. Semigothograptus meganassa has been reported from the post-extinction parvus, nassa and praedeubeli-deubeli biozones ( Kozłowska 2016), but the present occurrence in the pre-extinction uppermost lundgreni Biozone qualifies S. meganassa for second retiolitine survivor of the middle Homerian lundgreni mass extinction event.Palaeogeographic distribution of S. meganassa coincided with that of G. nassa and other species of the genus Gothograptus (see Kozłowska 2016, Kozłowska et al. 2019).

Figures

Text-fig. 3. Retiolitid graptolites from the upper lundgreni Biozone of the Kosov section. a, b – Gothograptus kozlowskii KOZŁOWSKA-DAWIDZIUK, 1990: a – PŠ 3870, finite rhabdosome with curved apertural spines from sample K2/13; b – PŠ 3878, sample K2/11. c – Semigothograptus meganassa (RICKARDS et PALMER, 2002): PŠ 3863, non-finite rhabdosome with free nema, sample K2/6. All specimens ×10, scale bar 1 mm.

Text-fig. 4. Retiolitid graptolites from the upper lundgreni Biozone of the Kosov section. a–c – Gothograptus aff. domeyki KOZŁOWSKA et al., 2019: a – PŠ 3869, finite rhabdosome in profile with small nassa-type genicular hoods on 1st and 2nd pair of thecae, sample K2/18; b – PŠ 3905, specimen in latero-ventral view showing narrow orifices, thickened thecal lips and very small genicular hood on th 1, sample K2/22; c – PŠ 3877b, specimen in obverse view with small genicular hoods on th11 and th12, and small lateral spines in the next two thecae, sample K2/15. d–f – Gothograptus kozlowskii KOZŁOWSKA-DAWIDZIUK, 1990: d – PŠ 3878, specimen showing broadened bases of the apertural spines, sample K2/11; e – PŠ 3870, finite rhabdosome in obverse profile view, sample K2/13; f – PŠ 3868, latero-ventral (subscalariform) view, sample K2/21. g – Semigothograptus meganassa (RICKARDS et PALMER, 2002): PŠ 3863, sample K2/6. All specimens × 10, scale bar 1 mm.