Manda and Turek—Silurian Nautiloidea, Tarphycerida
Arnold et al., 1987; Ward, 1987; Tajika et al., 2015). Both reflect stress following the emergence of the hatched animal from the egg capsule (Arnold et al., 1987, 2010). Although the change in septal spacing is often linked with hatching in Nau- tilus, the intraspecific variability in the chamber length pattern is high (Stenzel, 1964). Except in Ophioceras (Turek and Manda, 2016), the variability of septal spacing in early ontogeny has not been studied in early Paleozoic nautiloids. A change in septal spacing in the first whorl in Ordovician tarphycerids was linked with hatching by Schindewolf (1934). Stumbur (1959, 1960) studied the early ontogeny in four genera representing the main evolutionary lineages of Ordovician tarphycerids. According to him, the hatching in tarphycerids is manifested by a sudden change in septal spacing—the first five to seven phragmocone chambers are longer than those following, in which length decreases gradually or abruptly. Following this hypothesis, the shell of early-hatched tarphycerids would have reached 1.25– 1.5 whorls (WH=4–8mm, SD=10–20mm) and the phragmo- cone would consist of 5–7 chambers. Consequently, the hatch- ing size in Tarphycerida would be close to that in post-Triassic nautilids (Wani et al., 2011). Stumbur (1959) supported his conclusion with two species of Ordovician Discoceras from Norway, illustrated by Sweet (1958). However, in these species, the change in septal spacing occurs between the eight and ninth chambers. Sweet (1958) illustrated a specimen of Estonioceras proteus demissa Holm, 1885 in which the first two chambers are long and the third chamber markedly shortened; length of the following chambers gradually decreased, and after the ninth chamber increased again. Similarly, in Eurystomites amplectens Ruedemann, 1906 (pl. 18, fig. 5) and Tarphyceras multicameratum Ruedemann, 1906 (pl. 19, fig. 3), the first two chambers are longer than the following ones, which have almost equal lengths. In Estonioceras imperfectum (Quenstedt, 1845), the first chamber is long, the second chamber is shorter, the third to eighth chambers are almost the same length as the second; the ninth and following chambers are, again, markedly shorter (Stumbur, 1959; fig. 1a). Stumbur (1959) considered this development atypical, and assumed that the change in length between the seventh and eighth chambers indicates hatching. However, Shimansky and Zhuravleva (1961, p. 78) described another tarphycerid, Trocholites sp., with a high first chamber, a much shorter second chamber, followed by chambers with gradually increasing lengths. Differing interpretations of the significance of changes in
septal spacing during early development were addressed by Shimansky and Zhuravleva (1961, p. 84). They suggested two early ontogenetic paths in tarphycerids: (1) the shell in the egg capsule possessed one or two chambers and a body chamber (e.g., Estonioceras imperfectum and Trocholites sp.), in which case, the hatched animal differed from an adult in having a small curved shell; and (2) the embryonic shell consisted of five to six chambers and a body chamber (all other tarphycerids), and early-hatched animals possessing coiled shells that did not differ from adults. Nevertheless, such differing hatching size reflect- ing remarkable differences in the early development of hatchl- ings seems unlikely in such closely related taxa. Turek and Manda (2016) demonstrated that septal spacing
in Silurian Ophioceras is not coupledwith hatching; hatching in this case undoubtedly preceded the change in septal spacing,
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and juveniles had small curved shells with only one phragmo- cone chamber. However, Ophioceras is evolutionarily the youngest tarphycerid with some evolutionary novelties (e.g., a single ventral retractor muscle scar), and it is uncertain whether this type of embryonic development is applicable to older tarphycerid taxa.
Hatching indication in the Silurian Discoceras
The apex of D. graftonense is very similar in shape to that of Ordovician species of Discoceras. The earliest part of the shell is smooth; growth lines have been observed starting at about three-eighths of the first whorl and at whorl height 7–9mm
(SD=16–22mm) growth lines become raised. Length of the first chamber is ~2.7mm, and the second to sixth phragmocone chambers are about one-third longer than following chambers (Fig. 13.4). Change in septal spacing occurs at a whorl height of 3mm, when the shell attained one whorl (SD=8.8mm). Following Schindewolf´s (1934) and Stumbur´s (1959) approach to linking hatching with change in septal spacing, the shell of hatched D. graftonense would have reached one and one-half of a whorl and consisted of six phragmocone chambers plus a body chamber about half a whorl in length (WH=5.2 mm, SD=14 mm).Achange in sculpture appeared immediately after the supposed indication of hatching. However, in specimen RM Mo 59803 (Fig 13.1), the difference in chamber length is not obvious, because chamber lengths increase gradually. The early shell of D. lindstroemi n. sp. is similar in shape
three-quarters of the first whorl (WH=5mm)—preceded the change in septal spacing (Fig. 4.9). The character of the groove and its adapical widening on the ventral side of the shell indicate serious mechanical damage caused by a predator. The injury originated after hatching and thus preceded the change in septal spacing (i.e., the shell of the hatched individual did not reach three-quarters of a whorl). Close to three-eighths of the first whorl, a change in shell coiling occurs, and growth lines are enhanced. It is highly probable that the change in septal spacing
and caecum position to D. graftonense, but differs in having much less pronounced growth lines, and a change in septal spacing occurs between the fifth and sixth chambers (WH= 2.6mm, shell reached first whorl; Figs. 12.2, 13.5). If this change really indicates hatching time (Stumbur, 1959), then the whorl height would have reached 5mm and shell diameter 12mm. Aunique morphological feature—a deep groove situated at
did not coincide with hatching in Discoceras, as well as in Tar- phycerida in general (see Turek and Manda, 2016). According to Stumbur (1959), the growth sculpture appeared in tarphycerids after hatching. However, in Ophioceras, fine growth lines have been observed on the first chamber. In the Ordovician species Discoceras vasalemmense Balashov, 1953, growth lines are dis- cernible in about a half of the first whorl; they are regularly arranged, and in the adjacent juvenile part of the shell no striking change in their spacing has been observed. In SilurianDiscoceras,
growth lines appear at about three-eighths to one-half of the first whorl. Also at close to three-eighths of the first whorl, a slightly changed course of the shell spire and slight shell expansion has been observed. Taken together, there is as yet no
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