Epitornoceras baldisi ( Leanza, 1968 )

Allaire, Ninon, Rustán, Juan J., Korn, Dieter, Vaccari, Emilio, Ezpeleta, Miguel & Balseiro, Diego, 2024, The late Givetian (Middle Devonian) ammonoid Epitornoceras Frech, 1902 from Argentina (southwestern Gondwana), Geodiversitas 46 (7), pp. 327-341 : 334-337

Epitornoceras baldisi ( Leanza, 1968)

( Figs 6 ; 7 )

Tornoceras baldisi Leanza, 1968 : figs 1-4.

non Tornoceras (Tornoceras) cf. baldisi House, 1978 : pl. 6, figs 9, 10.

TYPE MATERIAL. — Holotype. Argentina • Specimen CPUBA 8088 designed by Leanza (1968), stored at the University of Buenos Aires (re-illustrated here, see Fig. 6 ).

DIAGNOSIS. — Epitornoceras reaching at least 50 mm conch diameter. Extremely discoidal and involute adult stage (CWI c. 0.25; umbilicus closed at 30 mm dm); whorl profile strongly compressed (WWI c. 0.40 at 30 mm dm). Flanks weakly convex, convergent, umbilical margin continuously rounded. Without any ventrolateral grooves. Suture line with broadly rounded, nearly symmetric adventive lobe with convergent flanks; adventive lobe wider than deep.

TYPE LOCALITY AND HORIZON. — Uppermost part of the Chavela Member, uppermost part of the Chigua Formation, Chinguillos Group, north of Angualasto town, San Juan province, Argentina.

OCCURRENCE. — The species is known only from the type locality (Chavela Member, Chigua Formation, San Juan province, Argentina), late Givetian.

MATERIAL. — Ten specimens, INGEO-PI-2002 to 2011.

DESCRIPTION

The conch is extremely discoidal (CWI c. 0.25) with a strongly compressed whorl section (WWI c. 0.40) in the subadult stage (30-50 mm dm); its umbilicus is completely closed (specimens INGEO-PI-2011, INGEO-PI-2007 and INGEO-PI-2004; Fig. 7 A-C, I). The flanks are weakly convex and convergent, and the venter is narrowly rounded; the umbilical margin is continuously rounded. At a smaller diameter (20-30 mm dm), the conch is also discoidal, but has a slightly opened umbilicus (specimens INGEO-PI-2005 and INGEO-PI-2002; Fig. 7 F-H). At about 20 mm dm, the coiling rate is very high (WER c. 2.35). Based on specimen INGEO-PI-2003 ( Fig. 7Q, L ), the morphology of the earlier whorls can be investigated by the use of latex casts: around 10-15 mm dm, the conch is thinly discoidal (CWI c. 0.35-0.40) with a weakly compressed whorl section (WWI c. 0.50-0.60) and a high whorl expansion rate (WER c. 2.10). Surprisingly, the umbilicus appears to be entirely closed.

In most specimens, partially preserved suture lines or septal surfaces are visible. The entire suture line can be reconstructed for specimen INGEO-PI-2007 ( Fig. 7C, E ); at wh c. 18 mm, it consists of a narrow and deep external lobe, a narrow and subacute E/A saddle, a broadly rounded, wide and deep nearly symmetric adventive lobe with convergent flanks, followed by a similarly broadly rounded and deep but wider saddle. Suture lines are also relatively well preserved in specimen INGEO- PI-2004 ( Fig. 7D, I ); the shape of the adventive lobe and the following saddle is similar to specimen INGEO-PI-2007 ( Fig. 7E ); the external lobe and the ventrolateral saddle are not visible. Interestingly, the larger specimens show a septal thickening when comparing with smaller ones (i.e., increase in thickness of the septal wall when it meets the shell; compare the wideness of the suture lines of specimens INGEO- PI-2007 and INGEO-PI-2004, Fig. 7C, I , with specimen INGEO-PI-2002, Fig. 7G ). This trend of increasing septal thickness toward adulthood (seeKlug et al. 2015b) was already documented in several ammonoids ( Westermann 1971) as in modern nautilids ( Collins & Ward 1987).

Although cracked, the shell is partially preserved in one specimen (INGEO-PI-2011, Fig. 7A ); it exhibits iridescent colors. Growth lines are not visible.

REMARKS

This material fits well with the description of Korn & Bockwinkel (2022) defining the genus Epitornoceras Frech, 1902 with a compressed and involute conch, a completely closed umbilicus in the pre-adult stage but slightly opened in the juvenile stage, sutural formula E A L I with broadly rounded adventive lobe, narrow and subacute ventrolateral saddle. Epitornoceras differs from the other genera of the subfamily Tornoceratinae by its very narrow and subacute ventrolateral saddle ( Korn & Bockwinkel 2022).

Four other species of Epitornoceras have been described: Epitornoceras mithracoides (Frech, 1888) from the Rhenish Mountains of Germany ( Bockwinkel et al. 2013; Bockwinkel & Korn 2015; Korn & Bockwinkel 2022), Epitornoceras transmediterraneum Korn & Bockwinkel, 2022 from the Anti-Atlas of Morocco ( Bockwinkel et al. 2009, 2013, 2017; Bockwinkel & Korn 2015; Korn & Bockwinkel 2022), Epitornoceras peracutum (Hall, 1876; holotype described by Miller 1938) and Epitornoceras dennisoni Kirchgasser, Baird & Brett, 2020 reported from North America (seeTable 1 for a comparison of the morphological characters of the different species). House (1965, 1978)also identified forms showing affinities with Epitornoceras peracutum (refigured in House & Kirchgasser 2008).

As a general observation, the conch of E. baldisi appears to be more slender (i.e., extremely discoidal, see Table 1 ) at the adult stage than in all other Epitornoceras species (by excluding E. peracutum from the comparison, for which the conch shape is not well known; the description of this species is almost only based on its suture line, see Miller 1938).

Epitornoceras baldisi differs from E. transmediterraneum in the shape of the umbilicus: the umbilical wall is obliquely flattened caused by an umbilical shell thickening in E. transmediterraneum (i.e., funnel-shaped umbilicus), while in E. baldisi the umbilical margin is continuously rounded (as in all others species, see Table 1 ). The two species also differ when comparing the suture line. The suture line of E. baldisi possesses a narrower external lobe, in E. transmediterraneum the E lobe is V-shaped and wider. Furthermore, the A lobe is considerably deeper in E. transmediterraneum , and the flanks are nearly parallel ( Korn & Bockwinkel 2022). In the latter, the A lobe is deeper than wide ( Korn & Bockwinkel 2022), while in E. baldisi the A lobe is slightly wider than deep ( Fig. 7D, E ).

Epitornoceras baldisi and E. mithracoides are close in terms of conch shape (seeTable 1). However, differences can be seen in the suture line: in E. mithracoides , the A lobe is asymmetric and as wide as it is deep ( Korn & Bockwinkel 2022), while in E. baldisi , the A lobe is nearly symmetric (and wider than deep), furthermore the L saddle is deeper and considerably wider ( Fig. 7D, E ).

Epitornoceras baldisi mainly differs from the North American species ( E. peracutum and E. dennisoni ) in its suture line (see Table 1 ). Epitornoceras peracutum possesses an E lobe much wider (with concave flanks), and an A lobe much deeper and more asymmetric, furthermore, the dorsolateral saddle is comparatively less wide ( Miller 1938). Epitornoceras dennisoni also possesses an E lobe much wider (with convex flanks), and a flat-topped dorsolateral saddle ( Kirchgasser et al. 2020).

The type material of E. baldisi was originally attributed to the genus Tornoceras . Two specimens were described: the holotype CPUBA 8088 and CPUBA 8089, re-illustrated here inFig. 6. The conch of the holotype is involute with an entirely closed umbilicus ( Fig. 6A, B ); suture lines are visible (see Leanza 1968: fig. 4) and their shape is characteristic of the genus Epitornoceras Frech, 1902 . The counterpart of the specimen shows the external mold with imprints of the growth lines ( Fig. 6B ). As indicated by Leanza (1968), they have a biconvex course (characteristic of the genus Epitornoceras ; see Korn & Bockwinkel 2022). The holotype and the new specimens described herein share very similar characteristics and came from the same levels and localities, so they most likely correspond to the same species. The additional specimen of Leanza (1968), CPUBA 8089, consists of a poorly preserved internal mold ( Fig. 6C ) of an involute conch; the internal whorl shows a slightly opened umbilicus (pre-adult stage).

The specimen from Bolivia identified as Tornoceras (Tornoceras) cf. baldisi by House (1978: pl. 6, figs 9, 10) has a much stouter conch than E. baldisi . Thus, it can be neither identified as E. baldisi nor as E. cf. baldisi .

In Morocco, the genus Epitornoceras occurs from the topmost middle Givetian ( Pharciceras aff. amplexum Zone ) to the topmost late Givetian ( Petteroceras errans Zone ) deposits ( Becker & House 2000; Bockwinkel et al. 2009, 2013, 2015, 2017; Aboussalam & Becker 2011; Becker et al. 2013; Aboussalam et al. 2018). In Germany, Epitornoceras was reported in one late Givetian biozone ( Pseudoprobeloceras pernai Zone ; Korn & Bockwinkel 2022). In North America, Epitornoceras was recognized in the lower part of the late Givetian (i.e., E. aff. peracutum ; upper part of the Geneseo shale, House 1965, 1978) and in early Frasnian rocks (i.e., E. dennisoni ; Kirchgasser et al. 2020). Epitornoceras was also reported in early Famennian deposits from Australia ( Petersen 1975). House (1978), however, stated that the specimens identified there as Epitornoceras peracutum by Petersen (1975) must be assigned to a different genus. Therefore, the Argentinian Epitornoceras most likely indicates a late Givetian age (see also Discussion section below for more details about biostratigraphic correlations).

The specimen whose shell shows iridescent colors (INGEO- PI-2011; Fig. 7A ) is the subject of further investigations. Chemical analyses and SEM observations will be performed to understand the origin of these colors, and the results will be presented in a future study.

Figures

FIG. 6. — Historical material of Epitornoceras baldisi (Leanza, 1968), identified as Tornoceras baldisi Leanza, 1968 by Leanza (1968), from the Chavela Creek (“Quebrada de la Chavela”),Chavela Member, Chigua Formation (San Juan Province, Argentina): A, B, holotype CPUBA 8088 (part and counterpart showing the external mold of the conch); C, additional specimen CPUBA 8089. Scale bars: 2 mm.

FIG. 7. — New specimens of Epitornoceras baldisi (Leanza, 1968) from the type area of the Chigua Formation (San Juan Province, Argentina): A, B, specimen INGEO-PI-2011; C, specimen INGEO-PI-2007; D, suture line of specimen INGEO-PI- 2004 at wh c. 33 mm; E, reconstructed suture line based on specimen INGEO-PI-2007 at wh c. 18 mm; F, specimen INGEO-PI-2005 (counterpart showing the external mold of the conch); G, H, specimen INGEO-PI- 2002 (part and counterpart showing the external mold of the conch); I, specimen INGEO-PI-2004;J, specimen INGEO-PI-2006; K, L, specimen INGEO-PI-2003 (L corresponds to latex cast of internal whorl). Scale bars: 2 mm. The pictures of some specimens have been flipped (A, B, and K).