424
Journal of Paleontology 92(3):412–431
Figure 10. Boxes (minimum, maximum, median, and first and second quartile) showing WER, WWI, HW/HH ratio and WH/CL ratio in Silurian Discoceras from Gotland; g: D. graftonense,s: D. stridsbergi,l: D.
lindstroemi.Graycross indicates value in holotype of G. graftonense (data from Foerste, 1925).
Ordovician tarphycerid genera, is restricted to the proximal areas of low-latitude platforms and occasionally more distal areas of black shale sedimentation between 20°N and 30°S (Table 4, Fig. 11). The occurrence of Discoceras in Llandovery off-shore
black graptolitic shales of the Prague Basin (Perunica micro- plate, peri-Gondwana), which were deposited under anoxic near-bottom conditions (Štorch, 2006) well removed from a carbonate platform, resembles Ordovician occurrences in black graptolite shales of Baltoscandia (Kröger et al., 2009; Rasmussen and Surlyk, 2012). The earlier Silurian anoxic episode restricted almost all fauna except graptolites in peri- Gondwanan basins. Nautiloid immigration from low latitudes into peri-Gondwanan basins with isolated elevated submarine regions is linked with a decline in early Silurian anoxia, and activation of currents beginning in the middle Llandovery to Wenlock (Stridsberg, 1988b; Manda, 2008b; Histon, 2012; Evans et al., 2015; Fig. 11). Pioneer nautiloid immigrants were forms with a coiled
shell. Aeronian Discoceras is the earliest known stray immi- grant into the Prague Basin; later, in the latest Llandovery, the discosorid Phragmoceras Broderip, 1839 in Murchison (1839) appeared. Permanent nautiloid populations and continuous faunal exchange in the Prague Basin appeared in the late Wenlock (Manda, 2008b). The occurrence of Discoceras in black off-shore shales in association with graptolites indicates its migration potential and swimming ability in surface currents across an open sea. Dispersion potential also could have been enhanced for possible planktotrophic juveniles transported by ocean currents. This also explains the dispersion of
Figure 11. Paleogeographic map (after Torsvik and Cocks, 2013) showing distribution of tarphycerids and current system (after Wilde et al., 1991) in the late Wenlock. Alternative paleographic position of Central Bohemia (2), microcontinent Perunica (1), respectively, after Cocks and Torsvik (2002); this position corresponds better with the dispersion pattern of Discoceras, which never occurs in cool water areas influenced by subpolar current (Silurian tarphycerids are unknown in such areas of peri-Gondwana including Austria, France, Germany, Spain, etc.). A indicates Australia.
D. graftonense in distant continents (e.g., Laurentia and the Northeast China Plate). Empty shells could have been trans- ported together with immigrants via current (Hamada et al., 1980). However, even the fine details of the shell sculpture are well preserved, suggesting that drift time and distance of empty shells were probably relatively short. The lack of bioerosion exclude their long-running drift between continents, which is consistent with rather limited post-mortem transport suggested for early Paleozoic nautiloid assemblages (Flower, 1957; Hewitt and Watkins, 1980; Frey, 1989).
Embryonic development and hatching time in Tarphycerida
The tarphycerid early shell is planispiral and tightly coiled with a very small umbilical perforation (Furnish and Glenister, 1964; Dzik, 1984). Its apex is blunt, the first chamber is curved and cup-like, and its length varies between 0.8–3.5mm (Stumbur, 1959; Shimansky and Zhuravleva, 1961). The nepionic con- striction characterizing the end of the embryonic phase in later nautilids is not present, but sculpture usually shows some change in growth line spacing. However, the sculpture develop- ment in embryonic and juvenile growth stages of tarphycerid shells is still poorly known. Consequently, the internal struc- tures of the phragmocone have been used for determination of hatching time. This is a methodological approach inferred from extant Nautilus (Schindewolf, 1934; Stumbur, 1959). A change in septal spacing between the seventh and eight
chambers in Nautilus coincides with its emergence from an egg and formation of the nepionic constriction (Naef, 1921–1923;
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