Praeactinocamax aff. plenus Blainville, 1825

Košťák, Martin & Wiese, Frank, 2008, Lower Turonian record of belemnite Praeactinocamax from NW Siberia and its palaeogeographic significance, Acta Palaeontologica Polonica 53 (4), pp. 669-678 : 671-674

Praeactinocamax aff. plenus Blainville, 1825

Figs. 2 , 3 , 4A–I.

Material.—Three uniquely well preserved alveolar fractures with phragmocones and several belemnite fragments of Praeactinocamax aff. plenus (MSU 3025−3). They were collected together with undetermined gastropods, small bivalves ( Cucullaea sp. ), Inoceramus agapensis Khomentovsky, 1992 (upper Cenomanian–?lower Turonian) and Mytiloides cf. hattini (see Dhondt and Naidin 2004), suggesting an early Turonian age. Zakharov et al. (1989) showed also the First Appearance Data (FAD)—of M. labiatus in Bed VII ( Fig. 1B ).

Description.—The material studied represents two distinct morphologies: (i) Morphotype A ( Figs. 2A, C , 3A , 4A–C, E, H) characterized by a low cone−shaped alveolar fracture, which shows in this respect the greatest similarities with those of P. primus ( Arkhangelsky, 1912) and P. plenus , (ii) Morphotype B ( Figs. 2B , 3B , 4D, G, I) characterized by a very shallow pseudoalveolus (not exceeding 1.5– 2 mm). The calcitic rostrum is fragmentarily preserved ( Fig. 4A). It is medium sized (not exceeding ca. 65–70 mm) and is ventrally markedly flattened. The flanks are slightly depressed and form a dorsolateral depression. No dorsolateral double furrows or vascular imprints have been recognized. The alveolar angle is ca. 21°. The diameter of the protoconch is about 1 mm. The calcitic posterior part of the rostrum terminates in a low cone−shaped alveolar fracture and/or an extremely shallow pseudoalveolus or depression with a pit in the centre ( Figs. 2 , 3 ). This part juts into a white, anterior part of the rostrum, which is usually not preserved. The boundary between the white and the dark calcitic parts is very sharp ( Fig. 4), without any transitional zones albeit the laminar growth lamellae continue from the calcitic into the white anterior part part without any visible interruptions. A shallow pit in the centre of the alveolar fracture marks the position of the protoconch ( Fig. 4C–E). The area between the walls of the aragonitic anterior part of the rostrum and the phragmocone is formed by aragonitic matter ( Barskov et al. 1997; Dauphin et al. 2007). However, in the specimen figured on Fig. 4D, G, this space was more altered than the walls.

Discussion.—Of interest is the fact that the anterior parts of the rostra, including the phragmocone, are preserved. The preservation of the anterior parts of belemnite rostra is exceptionally rare and only few records of this can be found in the literature ( Saemann 1861 –1862; Schlüter 1876; Moberg 1885; Jeletzky 1948). The original composition of the anterior part, separated from the calcitic posterior part by the so−called “alveolar fracture” (see Figs. 2 , 3 ) was considered to have been primarily horny or formed by some another undetermined organic material without, or containing only a very low, calcium carbonate content (Saeman 1861–1862; Crick 1904), whereas the calcitic posterior part is generally regarded as being composed of low−Mg calcite. Naidin (1969) supposed that the alveolar fracture originated as a result of postmortal diagenetic processes, while Barskov et al. (1997) and Dauphin et al. (2007) suggested, also based on material from the Taimyr Region, that the original material of the anterior part of the rostrum was aragonite.

In our specimens, there is a very sharp and distinct boundary between the aragonitic anterior, and the calcitic posterior part of the rostrum ( Fig. 4). This contact likewise marks the morphological feature known as the alveolar fracture. Judging from a wealth of literature data ( Naidin 1964, 1969; Christensen 1974, 1997a; Košťák and Pavliš 1997) and our own studies based on thousands of specimens (MK) of Praeactinocamax primus , P. plenus , and related species from Tadzhikistan in the east to England in the west in several different sediments (i.e., sandstone, marl, claystone, limestone etc.) with totally different diagenetic, histories, the shape of the alveolar fracture shows great morphological stability. In the case of P. primus , virtually no differences in this feature have been observed, while P. plenus shows slight differences in alveolar fracture morphology (see also morphometric analysis in Christensen 1997a: 67). The observed stability of the alveolar fracture morphology cannot be explained by diagenetic processes. Instead, the clear demarcation line between the calcitic and aragonitic part of the endoskeleton is suggestive of complex biomineralization process, which must have been genetically controlled. As can be seen from younger representatives of Praeactinocamax , the progressive calcification of the anterior part of the rostrum and the resultant progressive deepening of the pseoudoalveolus are important features in the evolution of this genus (Košťák 2004) as well as other Late Cretaceous belemnite lineages i.e., Goniocamax – Belemnitella (see Ernst 1964; Christensen and Schulz 1997; Christensen 1995, 1997a, 2000). The morphology of the alveolar fracture can thus be considered a reliable taxonomic feature of the Belemnitellidae . A conical alveolar fracture as described above is exclusively known from the genera Actinocamax Miller, 1823 and Praeactinocamax .

All species of Actinocamax are characterized by a high−conical alveolar fracture and only slight differences are observable between the species. The mean value of the length of the rostra is between 30–35 mm and it usually does not exceed 55 mm ( Christensen and Schulz 1997). The earliest species of this genus ( A. verus antefragilis Naidin, 1964 ) occurred in the early Turonian and the genus became extinct about the Early/Late Campanian boundary ( Christensen 1997a). No high−conical alveolar fracture (as the one of the most important taxonomic feature) is present in the material studied and hence these specimens can be safely excluded from Actinocamax . The size of the guard remains supports

KOŠŤÁK AND WIESE—TURONIAN BELEMNITID IN NW SIBERIA 673

p A B phr ar p C pr H F phr p p D pr I p E G

this idea. The guard diameter at protoconch area is about 10 mm in specimens studied, while in Actinocamax it is less than 1–2 mm (both, the top of high conical alveolar fracture and protoconch area are identical in any species of this genus). The maximum lateral diameter in Actinocamax rarely exceeds 5 mm (in few Coniacian and Santonian species) whereas lateral diameter of the studied Siberian specimens under question is two times larger.

Praeactinocamax is characterized by medium−sized guards not exceeding 115 mm in length ( Christensen 1997a). The alveolar fracture is usually low cone−shaped, however, a relative deep (up to 6 mm) pseudoalveolus can be developed in some species ( Naidin 1964; Košťák 2004). The depth of pseudoalveolus and the shape of the alveolar fracture are definitely not a result from preservation, and own data from material from various facies (i.e., sandstones, marlstones, limestones etc.) clearly show a morphological stability of this feature in all Praeactinocamax species ( Naidin 1964; Košťák 2004). In Praeactinocamax , a trend towards a deepening of the pseudoalveolus can be observed. However, the morphologic lineage [ P. primus ; early to middle Cenomanian— P. plenus ; late Cenomanian— P. planus ( Makhlin, 1965) ; late Turonian— P. cobbani ( Christensen, 1993) ; middle Coniacian– P. groenlandicus ( Birkelund, 1956) ; early Santonian] kept a stable morphology of the alveolar fracture. The development of the pseudoalveolus is based on the deepening of the central pit and the calcification of its margin. A first slight deepening occurs in the late Cenomanian P. plenus ( Christensen 1990, 1997a; Košťák and Pavliš 1997) and it is well developed in P. triangulus Naidin, 1964 (early Turonian), a possible derivative of P. plenus ( Naidin 1964; Christensen 1974; Košťák 2004; Košťák et al. 2004). Within this context, our material can be attributed to Praeactinocamax .

This feature has been reported from several P. plenus populations (see Christensen 1974; Košťák and Pavliš 1997; Christensen 1997a) that have been partly referred to P. triangulus , a species formerly regarded as a subspecies of P. plenus ( Naidin 1964; Košťák 2004). A very shallow to deeper pseudoalveolus (2–5 mm) is also known in the early Turonian species P. sozhensis ( Makhlin, 1973) , P. contractus ( Naidin, 1964) , and P. sp. 1 sensu Košťák (2004). However, due to the extremely limited material available from the section under discussion more precise taxonomic comments at species level are not possible, but we safely can re−assign the material to Praeactinocamax ( P. aff. plenus ).

Naidin et al. (1978), Teys et al. (1978), Barskov et al. (1997), and Dauphin et al. (2007) identified the belemnites from the Lower Agapa River section as Goniocamax . This generic assignment cannot be followed here due to the unequivocal Praeactinocamax character of the alveolar end in these belemnites. Goniocamax is characterized by the so−called “bottom of ventral fissure”, a line connecting the pseudoalveolus wall (near the protoconch area) with the ventral rostrum surface at the ventral notch ( Christensen and Schulz 1997). Christensen and Schulz (1997) established the stratigraphic range of Goniocamax to be from the base of the Coniacian through lower Santonian. Košťák (2005) described the late Turonian species G. christenseni from Central Russia. The origin of Goniocamax is currently under examination by one of the authors (MK): no forms related to Goniocamax appeared until the late middle Turonian Inoceramus lamarcki Zone ( Košťák et al. 2004; Košťák 2005).

Dauphin et al. (2007), based on material from the same section, showed that the anterior white part consists of aragonite. They introduced a hypothesis concerning the original mineralogy of the entire rostrum. Microstructural and geochemical composition studies showed that the aragonite in all the shells from the Bed VII nodules (including the belemnites) had been affected to a greater or lesser extent by diagenetic alteration. The coexistence of both calcitic and aragonitic components in the same rostrum favoured their hypothesis that the primary mineralogy of the entire rostrum was aragonite rather than low Mg−calcite), and that the originally aragonitic posterior part of the rostrum had become secondarily calcitized in the course of diagenesis. Dauphin et al. (2007: text−fig. 1) showed an irregular and diffuse boundary between the calcitic—in their view diagenetically altered—posterior and the primary aragonitic anterior part of the rostrum. In fact, this may well be an expression of a geochemical/diagenetic boundary and we agree that there is geochemical evidence for diagenetic alteration. However, on both morphological and geochemical grounds, we do not agree with their final conclusion concerning the primary mineralogy of the posterior part of the rostrum, which we consider to have been composed of low−Mg calcite. Further evidence that this part of the rostrum of Praeactinocamax was primarily composed of low−Mg calcite rather than aragonite derives from combined cathodoluminescence, geochemical and microstructural investigations of well preserved posterior rostra of P. plenus composed of low Mg−calcite from Upper Cenomanian chalks of southern England (see Voigt et al. 2003: fig. 2). Those authors demonstrated that the parts of the rostra composed of completely fresh (i.e., unaltered) calcite were non−luminescent, while any diagenetically altered parts showed reddish−orange luminescence due to the early diagenetic incorporation of Mn in the calcite lattice ( Machel et al. 1991). The good preservation of the unaltered calcite was supported not only by the low concentrations of Mn and Fe, and high concentrations of Sr, but also by carbon stable isotope data.

Stratigraphic and geographic range.—Early Turonian of NW Siberia.

Figures

Fig. 2. Two morphotypes Praeactinocamax aff. plenus. Schematic sketches of morphotype A (A) in direct comparison with morphotype B (B) from Kazakhstan (Upper Cenomanian). Lateral sections of rostra showing alveolar end of morphotype A (C, specimen IGP KK8/15) and morphotype B (D, specimen IGP 170/11).

Fig. 3. The alveolar fracture (alveolar end) reconstruction of Praeactinocamax aff. plenus with the position of phragmocone. A. Morphotype A, specimen MSU 3025−3/1 with low cone−shaped fracture, in ventral (A1) and lateral (A2) views. B. Morphotype B, specimen MSU 3025−3/2 with a very shallow pseudoalveolus with a pit in the centre, in ventral (B1) and lateral (B2) views.

Fig. 1. A. The geographic position of the North Siberian Upper Cretaceous belemnite locality at the Agapa River (Taimyr Peninsula). B. Schematic sketch of the exposed section at the find locality. Belemnite levels are indicated, for more details see text.