All studied specimens belong to the genus Eutrephoceras, which is the first record of this genus in the Koppeh Dagh Basin, as well as in Iran. Even though the specimens are rather poorly preserved, we identified the following species (see systematic paleontology below): E. clementianum (d’Orbigny, 1840), E. sublaevigatum (d’Orbigny, 1850) and E. bouchardianum (d’Orbigny, 1840).
Previous studies documented several ammonite-rich beds and proposed the basic age framework for the Taherabad section of the Aitamir Formation and for adjacent areas (Lehmann et al., 2019; Mosavinia, 2008; Mosavinia & Wilmsen, 2011, 2017; Mosavinia et al., 2014). We used the ammonite occurrences and zones to provide the age information for the stratigraphic levels in which we collected our specimens. Figure 2 shows the stratigraphic distribution of the collected nautilids accompanied by ammonite records, and the suggested substage boundaries. Albian ammonite biostratigraphic studies for the Taherabad section were carried out by Mosavinia et al. (2014) and Lehmann et al. (2019), but no zonations were given for the interval in which we collected our specimens. Nevertheless, some of the reported ammonites helped us to constrain the substage boundaries within the Albian. By contrast, Cenomanian ammonites from the Taherabad section documented by Mosavinia and Wilmsen (2011, 2017) indicate a sequence of important bioevents that indicate a position in the Mantelliceras mantelli to Acanthoceras rhotomagense zones.
The base of the section lies 100 m below the Sanganeh/Aitamir formation boundary, which is referred to as 0 m. The nautilid occurrences in the Taherabad section begin with E. clementianum (NAT-1) at the second glauconitic sandstone bed of the Aitamir Formation (= 129 m level). E. clementianum occurs between the latest Aptian index ammonite Hypacanthoplites uhligi (Fig. 3a–b) (at the 45 m level) and Hoplites (Hoplites) baylei (at the 290 m level), i.e., the middle to late Albian.
E. sublaevigatum (NAT-2) occurred at the 490 m level of the Taherabad section, just above the ammonite taxa Mariella (Mariella) bergeri (Fig. 3d–e), Semenoviceras michalskii (Fig. 3f–i) and Placenticeras mediasiaticum. S. michalskii and M. (M.) bergeri are the main classical components of the late Albian Dipoloceras cristatum and Stoliczkaia dispar zones. The upper part of the Taherabad section yielded two specimens of E. bouchardianum. At 612 m, the first specimen (NAT-3) of this species was found within the M. mantelli Zone, indicating an early Cenomanian age. The highest recorded nautilids at Taherabad are E. bouchardianum (NAT-5) and Eutrephoceras sp. at the 660 m level. They fall within the Mantelliceras dixonii Zone. This zone is marked by the middle Albian Turrilites costatus (Fig. 3j), Turrilites scheuchzerianus, Forbesiceras baylissi, Cunningtoniceras cunningtoni and Acanthoceras rhotomagense (Fig. 3k–m) at the top, suggesting an early Cenomanian age for the stratigraphic interval between the 625 m–700 m levels.
Order Nautilida Agassiz, 1847
Superfamily Nautilaceae de Blainville, 1825
Family Nautilidae de Blainville, 1825
Genus Eutrephoceras Hyatt, 1894
Eutrephoceras bouchardianum (d’Orbigny, 1840)
*1840 Nautilus bouchardianum d’Orbigny: 75, pl. 13, Figs. 1–3.
2015 Eutrephoceras bouchardianum (d’Orbigny, 1840); Machalski and Wilmsen; p. 497, text-Figs. 3A–D, 4A–B (with synonymy).
Material: Two specimens (NAT-3 and NAT-5 from the early Cenomanian).
Description: NAT-5 (Fig. 4d–g) measures 134 mm in conch diameter. The umbilicus is closed. The whorl section is very involute and depressed (WWI = 0.81–0.92; Table 1). The whorl width is the greatest at the umbilicus. The venter is widely rounded. The shell is only partially preserved and smooth. The suture line is straight to slightly sinuous. There are nine suture lines in a half whorl. The position of the siphuncle was not observable. NAT-3 (Fig. 4a–c) measures 98 mm in conch diameter. The umbilicus appears nearly closed. The whorl section is widely rounded. This specimen is slightly less inflated than NAT-5 (WWI = 0.78–0.80; Table 1). The suture line appears to be nearly straight, although it is only partially exposed. There are 11 septa per half whorl. The siphuncle is not visible.
Discussion: E. bouchardianum is characterized by its depressed whorl section (lectotype: ww/dm = 0.82; Wiedmann, 1960), which we tentatively consider a diagnostic character of the species. E. sublaevigatum is similar, but has a less inflated whorl section (lectotype: ww/dm = 0.70 Wiedmann, 1960). According to Wiedmann (1960), E. bouchardianum has a triangular whorl section, whereas E. sublaevigatum is rounded. However, the lectotype figured by Tintant and Gauthier (2006a) shows a somewhat widely rounded venter. Also, the hypotype of E. bouchardianum (Wiedmann, 1960, Table 19, Fig. I) exhibits a whorl section that looks similar to the lectotype of E. sublaevigatum. Thus, we cannot confirm that whorl section shape is a character that separates the two species. Wiedmann (1960) also mentioned that E. sublaevigatum differs from the present species in having a more peripherally located siphuncular position. However, this needs to be comprehensively reinvestigated. For example, Landman et al. (2018) and Tajika et al. (2020) found a significant change in the position of the siphuncle through ontogeny in Eutrephoceras dekayi from the lower Maastrichtian in Montana, USA, and Tajika and Klug (2020) also discovered a decreasing ontogenetic trend of siphuncular position in the post-hatching ontogeny of Nautilus. The lectotypes of some Albian-to-Cenomanian Eutrephoceras species, E. montmollini and E. bouchardianum are broken phragmocones, whereas E. sublaevigatum is a specimen with body chamber preserved. The ontogenetic changes of siphuncular position across/within species have not been investigated. The question arises whether the siphuncle position was always compared between the same growth stages or not. In addition to the above-mentioned characters, E. sublaevigatum is more evolute than E. bouchardianum according to Machalski and Wilmsen (2015), although we do not see such differences in the lectotypes of each species (i.e., both species seem to have a closed umbilicus). E. montmollini appears similar to E. bouchardianum in having an inflated whorl section (lectotype: ww/dm = 0.74), but differs in having a wider umbilicus and a central siphuncle. Detailed examinations of the morphological changes occurring during the ontogeny of each species are urgently needed to improve Eutrephoceras taxonomy and phylogeny.
Eutrephoceras clementianum (d’Orbigny, 1840)
*1840 Nautilus Clementianus d’Orbigny: 77, pl. 13, Figs. 1–6.
1960 Eutrephoceras clementianum (d’Orbigny) 1840; Wiedmann: pl. 168, pl. 18, Fig. J (with synonymy).
2006b Eutrephoceras clementianum (d’Orbigny, 1840); Tintant and Gauthier: pl. 2, Figs. 5a–b, 6).
2008 Eutrephoceras clementianum (d’Orbigny, 1840); Kennedy et al.; pl. 8, Figs. 8, 9.
2018 Eutrephoceras cf. clementianum (d’Orbigny, 1840); Ayoub-Hannaa et al.; Figs. 4A–E, 5A, B.
Material: One specimen (NAT-1 from the latest Aptian–early Albian).
Description: NAT-1 measures 254 mm in conch diameter with eroded body chamber preserved. The umbilicus is nearly closed. The whorl section is somewhat inflated (WWI = 0.58–0.75) and trapezoidal/rectangular. This specimen has the highest whorl expansion rate among the specimens documented in this paper (WER = 3.43; Table 1). The suture line is slightly sinuous. Septal crowding in the last 13 septa indicates that it is a mature/submature specimen. There are 10 septa in a half whorl. The siphuncle is not visible.
Discussion: According to Wiedmann (1960), this species has a trapezoidal whorl section and is inflated (lectotype: WWI = 0.72). Wiedmann (1960) also mentioned that the siphuncle is dorsally located in this species. However, the lectotype figured by Tintant and Gauthier (2006b; Fig. 5a, b) does not show the siphuncle position since the body chamber of the specimen covers the phragmocone. Presumably, the position of the siphuncle was assumed based on a poorly preserved syntype (a single chamber; Tintant & Gauthier, 2006b; pl 2, Fig. 6). Also, the siphuncular position shifts towards the dorsal margin in at least some taxa (Tajika et al., 2020). The above-mentioned chamber was likely formed slightly before the sexual maturity based on the size. These suggest that the dorsal location of siphuncle may be the result of its ontogenetic change. It is also worth mentioning that our specimen is much larger (254 mm) than the lectotype (180 mm). Assuming that the lectotype is an adult specimen, it is possible that Eutrephoceras in Koppeh Dagh Basin grew and reached a much larger adult size than the one in France. In modern Nautilus, it is known that different geographic populations exhibit different adult sizes (Saunders, 1987; Tajika et al., 2018). Our discovery may imply that different geographic populations in fossil nautilids have a similar trend.
Eutrephoceras sublaevigatum (d’Orbigny, 1850)
*1850 Nautilus Sublaevigatus d’Orbigny: Prodrome II, S. 189.
1960 Eutrephoceras sublaevigatum (d’Orbigny) 1960; Wiedmann; p. 165, pl. 19, fig. O, pl. 20, fig. A, pl. 23, fig. L. (with synonymy).
2006c Eutrephoceras sublaevigatum (d’Orbigny, 1840); Tintant and Gauthier: pl. 4, fig, 3a, b; pl. 5, Fig. 1a, b, 2a–c.
2017 Eutrephoceras sublaevigatum (d’Orbigny, 1850); Tajika et al.; Fig. 5C, C, K, L.
2021 Eutrephoceras sublaevigatum (d’Orbigny, 1850); Jattiot et al.; Fig. 27D–P.
Material: One specimen (NAT-2 from the late Albian).
Description: NAT-2 measures 167 mm in conch diameter. The umbilicus is barely visible but appears nearly closed. The whorl section is inflated (WWI = 0.68) and rather narrowly rounded. This specimen bears the smallest whorl expansion rate (WER = 3.06; Table 1) among the documented specimens. The suture is slightly sinuous. There are 13 septa in a half whorl.
Discussion: As discussed above, E. sublaevigatum appears similar to E. bouchardianum but differs in whorl width index (see the discussion for E. bouchardianum). As in E. clementianum discussed above, our specimen of E. sublaevigatum is larger (167 mm) than the lectotype (110 mm). Provided that the lectotype is an adult specimen, our specimen has a larger adult size, which is congruent with our hypothesis that Eutrephoceras attained a larger adult size in Koppeh Dagh Basin.
Material: One incomplete specimen (NAT-4 from the early Cenomanian).
Description: NAT-4 is an incomplete broken phragmocone that measures 91 mm in whorl width. The whorl section is broadly rounded. The suture line is slightly sinuous.
Discussion: The specimen is assigned to Eutrephoceras based on the suture line and broadly rounded whorl section. However, the preservation does not allow for species identification.