Life span and growth rate of Middle Jurassic mesohibolitid belemnites deduced from rostrum microincrements

The microincrements of nine Middle Jurassic mesohibolitid belemnite rostra (Hibolithes jurensis, Pachybelemnopsis fusiformis and Pachybelemnopsis “subhastatus”) were analysed to measure the ontogenic age and growth rate of the belemnites. The microincrements are interpreted to have been formed daily based on analogy with extant coleoid cephalopods and the frequency of circadian rhythms in animal activity. The mesohibolitid belemnites are found to have had a short life span of ca. 1 year. The short life span of the belemnites studied is similar to the life span of modern coleoids (squids, cuttlefishes and common octopuses). It restricts the utility of belemnite rostra as a proxy for average secular seawater temperatures and chemistry. Short-lived cycles (2–8 days) in the belemnite growth rate are linked to variations in the metabolic activity of these animals. Minor diagenetic alteration revealed by the dull cathodoluminescence of some rostra is found to have no effect on the preservation of the growth rings. 1 Institute of Geological Sciences, Polish Academy of Sciences, ul. Twarda 51/55, PL 00-818 Warszawa, Poland; Present address: Polish Geological Institute – National Research Institute, Rakowiecka 4, PL 00-975 Warszawa, Poland; e-mail: hubert.wierzbowski@pgi.gov. pl Doyle, 1990; Dunca et al., 2006; Fig. 1). As well-preserved epirostra are rarely found, most studies focus on the orthorostra. The calcite of the belemnite rostra shows radiaxial-fibrous fabrics and an undulose extinction pattern under crossed nicols due to the curvature of crystal c-axis (Richter et al., 2011). Well-preserved rostra contain a significant amount of intracrystalline and intercrystalline organic matter as indicated by the excess amount of carbon in the belemnite calcite and by direct organic carbon analyses (TOC up to 0.8 wt%; Sælen, 1989; Florek et al., 2004; Dunca et al., 2006; Wierzbowski, Joachimski, 2009). Pristine rostra show alternating dark (laminae ob­ scurae) and light (laminae pellucidae) growth lines visible in transmitted light (Müller-Stoll, 1936). Microscopic growth rings consisting of topographic highs and lows are additionally visible on polished and etched surfaces of the rostra (Barskov, IntroductIon Belemnite rostra are composed of radially arranged low-Mg calcite crystals accreted periodically in concentric growth rings (Veizer, 1974; Sælen, 1989). The rostra are dense and relatively resistant to diagenetic alteration and therefore are frequently found in Jurassic and Cretaceous marine rocks. The belemnite rostrum formed as a heavy counterweight to the light phragmocone and is composed of the primordial rostrum – a small juvenile rostrum, which covers the protoconch and the juvenile part of the phragmocone, as well as the solid orthorostrum (Müller-Stoll, 1936; Spaeth et al., 1971; Bandel et al., 1984; Hewitt et al., 1999; Doguzhaeva et al., 2003; Fuchs, 2012). An additional epirostrum that grew from the apex of the orthorostrum may sometimes occur (Müller-Stoll, 1936; Spaeth et al., 1971; 2 Hubert Wierzbowski 1970; Sælen, 1989; O’Neill et al., 2003; Dunca et al., 2006; Wierzbowski, Joachimski, 2009). The occurrence of the growth rings in the rostra is interpreted as being the result of periodical variations in organic matter content or changes in the calcite crystal size and density (Müller-Stoll, 1936; Sælen, 1989; Dunca et al., 2006). The rostral growth rings (microincrements) are similar to growth lines found on the surfaces or transverse sections of the beaks, stylets, statoliths, gladii, and cuttlebones of modern octopuses, squids and cuttlefishes. The growth increments of the extant cephalopods are regarded or demonstrated to be daily and therefore are used as a proxy for their life-span and growth rate (Choe, 1963; Kristensen, 1980; Nakamura, Sakurai, 1991; Perez et al., 1996; Arkhipkin, Perez, 1998; Rocha, Guerra, 1999; Hernández-López et al., 2001; Durholtz et al., 2002; Šifner, 2008; Hermosilla et al. 2010; Aguiar et al., 2012). The cephalopod growth lines may also provide information on periodical changes in the growth rate, variations in environmental conditions, food availability and sex of specimens (Arkhipkin, Perez, 1998). The usage of cephalopod growth increments may however be limited owing to their indistinctiveness in some areas of the skeletons (Choe, 1963; Perez et al., 1996; Arkhipkin, Perez, 1998). This may result in the underestimation of the individual age of a cephalopod (Perez et al., 2006). A bias associated with different methods of the determination of growth line chronology is additionally reported (Lipinski, Durholtz, 1994; González et al., 2000). Belemnite microincrements, by analogy with modern cephalopods and fortnight cyclicity, are interpreted as being formed during daily periods (Dunca et al., 2006; Wierzbowski, Joachimski, 2009). The sclerochronology of the belemnite rostra, comprising entire ontogenic stages, was studied in a Jurassic species – Megateuthis giganteus (Dunca et al., 2006). Dunca et al. (2006) counted from 370 to 570 growth rings and estimated the life span of the specimens of Mega­ teuthis giganteus studied to 1–2 years. A similar life span was postulated for Jurassic Hibolithes belemnites by Wierzbowski and Joachimski (2009) based on fragmentary growth ring analysis. It is worth noting that belemnite life span has also been estimated using quasi-cyclic variations in the δ18O values of the rostra. These data give discrepant results ranging from 2 to 5 years (cf. Urey et al., 1951; Stevens, Clayton, 1971; Zakharov et al., 2011). The oxygen and carbon isotope compositions of wellpreserved bulk belemnite rostra or their fragments are widely used in palaeoenvironmental reconstructions (e.g. Anderson et al., 1994; Podlaha et al., 1998; McArthur et al., 2000, 2004, 2007; Bailey et al., 2003; Wierzbowski, 2004; Rosales et al., 2004; Wierzbowski, Joachimski, 2007; Gómez et al., 2008; Price, Rogov, 2009; Wierzbowski et al., 2009, 2013; Nunn, Price, 2010; Mutterlose et al., 2010, 2012; Gómez, Goy, 2011; Korte, Hesselbo, 2011; Armendáriz et al., 2012). Mg/Ca and Sr/Ca ratios of belemnite rostra are sometimes employed as an independent temperature proxy (McArthur et al., 2000, 2004, 2007; Bailey et al., 2003; Rosales et al., 2004; Nunn, Price, 2010; Li et al., 2012; Wierzbowski, Joachimski, 2009; Wierzbowski et al., 2013). Knowledge of belemnite habits, life span and ontogeny is needed for the proper insight into their geochemical record. This is also a key question for deciphering Mesozoic seawater temperatures and chemistry. In view of conflicting estimates of belemnite life span listen above the aim of the present contribution is to conduct a sclerochronology study of new specimens of Jurassic belemnites and draw conclusions about their longevity and growth periodicity. The present survey is based on several well-preserved Middle Jurassic Hibolithes and Pachybelemnopsis rostra of the family Mesohibolitidae Nerodenko, 1983 (a former name of the family was “Belemnopseidae Naef, 1922”). These genera were common during the Middle–Late Jurassic and are often analysed for their stable isotope composition. Rostrum cavum Rostrum solidum Epirostrum