Protocyptendoceras fuenzalidae Cecioni, 1965

Cichowolski, Marcela, 2009, A review of the endocerid cephalopod Protocyptendoceras from the Floian (Lower Ordovician) of the Eastern Cordillera, Argentina, Acta Palaeontologica Polonica 54 (1), pp. 99-109 : 103-106

Protocyptendoceras fuenzalidae Cecioni, 1965

Fig. 4.

1965 Protocyptendoceras fuenzalidae sp. nov.; Cecioni, 1965: 9, pl. 2: 1–3.

1965 Protocyptendoceras corvalani sp. nov.; Cecioni, 1965: 11, pl. 2: 11–13.

1965 Protocyptendoceras teicherti sp. nov.; Cecioni, 1965: 12, pl. 3: 1–5.

Material examined.—Types: Holotype of Protocyptendoceras fuenzalidae ( JUY−P 29 by original designation) (Fig. 4F 1 –F 3). Other material examined: Paratypes JUY−P 30 (Fig. 4G) and JUY−P 31 (only a saggital cut) of P. fuenzalidae . Furthermore, the specimen ( JUY−P 28) designed as the holotype for P. corvalani by Cecioni 1965 —here considered to be a junior synonym of P. fuenzalidae —was analyzed (Fig. 4H). Forty four specimens from the Sepulturas Formation of La Ciénaga: CEGH−UNC 23117, 23118b, 23120a, 23121c, 23122, 23123a, b, 23125, 23126a–c, 23127, 23131, 23132a–j, 23133, 23134a–d, 23136, 23137a–c, 23138a, b, 23139, 23140, 23141, 23143, 23145, 23146, 23147, 23148, and 23182a, b.

Diagnosis.—As for the genus.

Description.—Orthoconic longiconic conch. Cross section is circular to subcircular, slightly depressed, in particular in later ontogenetic stages, with a compression ratio ranging from 0.67 to 1.1 (Fig. 4A 2, F 1, J). Maximum diameter at the living chamber: 18 mm. Maximum diameter at the phragmocone: 13.2 mm. Apical angle variable, mean approximately 5.35 °, with a maximum of 8.6 ° and a minimum of 1.7 ° (this was measured based both on dorsoventral expansion rate and lateral expansion rate, obtaining almost identical values, see Table 2). Shell surface only with growth lines (Fig. 4I). Mean cameral depth (expressed as a percentage of the dorsoventral diameter): 22.88 (max: 31, min: 15, see Table 2). In specimen CEGH−UNC 23132a, a general cameral length of 2–2.5 mm was measured, descending adorally to 1.8 mm, septal approximation indicates the maturity of the individual (Fig. 4A 1). Suture almost straight, with a shallow, narrow ventral lobe (with a depth approximately 10% of the dorsoventral conch diameter) being well preserved in specimen CEGH−UNC 23137c and in the holotype (Fig. 4C, F 3). Septal depth was calculated only in specimen CEGH−UNC 23132d (which was longitudinally cut and polished), being 16% of the dorsoventral diameter. Moderate to large siphuncle, diameter varying from 27 to 54% of the dorsoventral diameter (mean: 41.7%). This parameter does not seem to change proportionally with ontogeny but it probably varies intraspecifically (see Table 2). The position of the siphuncle is marginal, being in contact with the ventral shell wall (Fig. 4A 1, F 1, J). Septal necks subholochoanitic and connecting rings thick (Fig. 4K 1, K 2). External shape of the siphuncle shows oblique lateral ridges and grooves that form an adoral ventral projection and an adapical dorsal sinus reflecting the positions and orientation of the septal foraminae (Fig. 4L 1, M). Endocones weakly asymmetrical, with the endosiphotube being slightly closer to the ventral side. Cross section generally droplet−shaped (Fig. 4L 2, N), but sometimes rounded (Fig. 4A 2). Endosiphuncular deposits often recrystallised (Fig. 4D, E), although in specimen CEGH−UNC 23148 they are very well preserved (Fig. 4N). Spiculum 3 mm in maximum diameter and between 5 and 7 mm in length (Fig. 4D, E). A single endosiphotube is preserved, apparently without internal structures (Fig. 4D). Apical parts are rarely preserved, however one example of a probable external mould is present ( CEGH−UNC 23132b). The first 5 chambers estimated as being 2.8 mm in length, while the width of the probable first chamber is estimated as being 1.1 mm.

No specimens with a complete living chamber are preserved. A 35 mm long fragment is illustrated, however this shows neither muscle attachment scars nor the shape of the aperture (Fig. 4I).

Discussion.—Based on a revision of Cecioni’s (1965) original material, and a detailed study of the recently collected specimens from La Ciénaga, it may be concluded that only one species belonging to the genus Protocyptendoceras occurs in the Purmamarca region. The senior synonym of the species is P. fuenzalidae Cecioni, 1965 .

Cecioni (1965) stated that his material (with the exception of specimens assigned to P. teicherti Cecioni, 1965 ) was found in early Tremadocian strata, on the right margin of the Purmamarca River, 4 km upstream of Purmamarca. However, he also stated that these fossils came from the “Chañarcito Limestones” of Harrington and Leanza (1957). There is some discrepancy between the geographical location and the stratigraphical age and formation to which Cecioni’s (1965) material has been assigned. The Chañarcito Formation (Tremadocian) crops out near Purmamarca railway station ( Harrington and Leanza 1957), east of the Río Grande (Quebrada de Humahuaca) (see Fig. 2 ), and far from the Purmamarca River locality. Furthermore, no nautiloid cephalopods have been found in these horizons, which are not limestones but essentially black shales ( Harrington and Leanza 1957). It is here concluded that the specimens assigned by Cecioni (1965) to P. fuenzalidae and P. corvalani were collected from La Ciénaga, and on the evidence of the associated fauna are Floian in age.

P. teicherti was described based on erratic material (apparently a single specimen), without a precise geographical or stratigraphical provenance. Nowadays, the use of specimens for taxonomic descriptions that lack accurate provenance data is discouraged. In this paper, and based on Cecioni’s (1965) description, P. teicherti is considered as a junior synonym of P. fuenzalidae . The original material should be revised in or−

Fig. 4. Endocerid cephalopod Protocyptendoceras fuenzalidae Cecioni, 1965 , from the Floian of La Ciénega, Purmamarca, Eastern Cordillera of Argentina. H A. CEGH−UNC 23132a in ventral view [arrow points to the septal approximation (A 1)], and in posterior view showing the most apical part preserved, with the outline of the siphuncle and endocones retouched (A 2). B. CEGH−UNC 23134b in dorsal view. C. CEGH−UNC 23237c in ventral view, with the suture line retouched in part. Arrow points to the ventral lobe. D. CEGH−UNC 23127, polished section of isolated siphuncule, showing the endosiphotube, spiculum and recrystallised endocones. E. CEGH−UNC 23136, polished section of isolated siphuncle showing the spiculum and recrystallised endocones.

F. JUY−P 29 (holotype of P. fuenzalidae Cecioni, 1965 ), transverse section of the most apical portion preserved (F 1), and in ventral view (F 2 and F 3), with the detail of the suture line on the ventral surface retouched to indicate the ventral lobe (F 3). G . JUY−P 30 (paratype of P. fuenzalidae Cecioni, 1965 ) in lateral view. H . JUY−P 28 (holotype of P. corvalani Cecioni, 1965 ) in ventral view. I . CEGH−UNC 23145 , broken living chamber in lateral view .

J. CEGH−UNC 23132, cross section of the rock sample. Note the siphuncle position in the specimens and the relation between the siphuncle and shell diameter. K. Detail of the septal necks. K 1, CEGH−UNC 23132a, detail of the siphuncular structure from a ventral view of the entire conch (lacking external shell wall). Note subholochoanitic septal necks and thick connecting rings; K 2, CEGH−UNC 23132d, detail of the siphuncular structure from a longitudinal view of a cut and polished specimen, showing subholochoanitic septal necks, in both cases with a graphical scheme to the right (Sn, septal neck; Cr, connecting ring). L. CEGH−UNC 23146, isolated siphuncle in lateral view (L 1), and in polished cross section, showing the droplet shape of the endocones to the right (L 2) (ventral side downward). M. CEGH−UNC 23147, isolated siphuncle in dorsal view. N. CEGH−UNC 23148, detail of the siphuncle in polished cross section, showing the preserved structure of the endocones (not recrystallised). Ventral side downward.

der to provide a more definitive conclusion. However, to date this has not been found in the Cecioni's Collection at the JUY−P.

The main differences between the three species described by Cecioni (1965) lie in chamber spacing, and rather dubiously, the form of the suture line. In his description of the three species, Cecioni (1965) made reference to the number of chambers present in an interval equivalent to the lateral diameter of the conch, but omitted mention of the ontogenetic stage at which the shell diameter was measured. Given the preservational differences between the specimens, it may be that Cecioni did not consider giving comparative measurements. In any case, the supposed variability of the material under study may be considered as intraspecific (see Table 2). With regard to the suture, Cecioni (1965) did not make specific mention of the differences between P. fuenzalidae and P. corvalani , but indicated that a difference could exist. Subsequently, he mentioned that in P. teicherti the suture presented a more prominent ventral saddle. Again, this degree of variability is common within a single species. Considering these observations, it is concluded that the recognition of three separate taxa is not justified.

The differences in the suture between the newly described material and the specimens described by Cecioni (1965) may be found in the ventral part, where a lobe should be present instead of a saddle. However, study of specimen JUY−P 29 (holotype of P. fuenzalidae ) shows that a shallow ventral lobe is present (Fig. 4F 3), with a very similar aspect to that present in specimen CEGH−UNC 32137 c (Fig. 4C) .

In the newly described material the layering of the connecting rings mentioned by Cecioni (1965) for P. fuenzalidae is not evident (Fig. 4N). Mutvei (1997), when studying the siphuncular structure of some endocerid genera using SEM, noted that the connecting ring is a continuation of the spherulitic−prismatic layer, one of three structural layers of the septal neck present in this order. He mentioned that the structural differentiation of the connecting ring reported by previous authors ( Flower 1941, 1964; Teichert and Glenister 1954) probably results from diagenesis. Furthermore, the siphuncular structure in endocerids agrees in detail with that of living Spirula and Nautilus ( Mutvei 1997) .

The conical endosiphuncular deposits (endocones) were secreted by the siphuncular epithelium ( Mutvei 1964). Regarding the shape of these deposits, some variability exists in the studied species (Fig. 4A 2, L 2, N). Most specimens show a droplet shape in cross section, but some of them have a rounded contour. However, Mutvei (1997) stressed the intraspecific variability of these structures, advising caution with regard to their use in the definition of species.

With respect to the length of the septal necks, Cecioni (1965: 8), in the diagnosis of the genus, wrote: “long septal necks, of approximately one third of the chambers length”. But in the description of the species P. fuenzalidae , he mentioned that septal necks were of approximately three−quarters of chamber length. Although the diagrams shown in Cecioni’s plates are not clear, it appears that the septal necks are rather long (e.g., Cecioni 1965: pl. 2: 7; pl. 3: 5). In the material studied here it can be seen that the septal necks are subholochoanitic (Fig. 4K 1, K 2). The differences observed in this character could be due to ontogenetic variability, as in other genera of the same family (e.g., Proendoceras Flower, 1955 ).

Aceñolaza et al. (1977) reported the presence of Cyptendoceras sp. in the Lower Ordovician of the Precordillera (San Juan Province, Argentina). Based on the description of the single studied specimen (considered by Aceñolaza et al. 1977 to be a proterocameroceratid), it is similar to the material described here in most aspects, although the presence of endocones is not mentioned. However, the calcite infilling of the siphuncle suggests that they may be present in a recrystallised state.

Finally, it is worth noting the similarities between this genus and forms such as Anthoceras Teichert and Glenister, 1954 , in view of the equivalent age assigned to cephalopod assemblages from Western Australia.

Stratigraphical and geographical distribution.—Sepulturas Formation, Floian Stage. La Ciénaga, about 5 km west of Purmamarca, Jujuy Province, Eastern Cordillera of Argentina.

Figures

Fig. 2. A. Map of Argentina with the Jujuy region in black. B. Jujuy region with the study are indicated. C. Location map of the fossil locality (La Ciénaga), 5 km northwest of Purmamarca.