The Origin and Initial Rise of Pelagic Cephalopods in the Ordovician

Abstract

BackgroundDuring the Ordovician the global diversity increased dramatically at family, genus and species levels. Partially the diversification is explained by an increased nutrient, and phytoplankton availability in the open water. Cephalopods are among the top predators of todays open oceans. Their Ordovician occurrences, diversity evolution and abundance pattern potentially provides information on the evolution of the pelagic food chain.Methodology/Principal FindingsWe reconstructed the cephalopod departure from originally exclusively neritic habitats into the pelagic zone by the compilation of occurrence data in offshore paleoenvironments from the Paleobiology Database, and from own data, by evidence of the functional morphology, and the taphonomy of selected cephalopod faunas. The occurrence data show, that cephalopod associations in offshore depositional settings and black shales are characterized by a specific composition, often dominated by orthocerids and lituitids. The siphuncle and conch form of these cephalopods indicate a dominant lifestyle as pelagic, vertical migrants. The frequency distribution of conch sizes and the pattern of epibionts indicate an autochthonous origin of the majority of orthocerid and lituitid shells. The consistent concentration of these cephalopods in deep subtidal sediments, starting from the middle Tremadocian indicates the occupation of the pelagic zone early in the Early Ordovician and a subsequent diversification which peaked during the Darriwilian.Conclusions/SignificanceThe exploitation of the pelagic realm started synchronously in several independent invertebrate clades during the latest Cambrian to Middle Ordovician. The initial rise and diversification of pelagic cephalopods during the Early and Middle Ordovician indicates the establishment of a pelagic food chain sustainable enough for the development of a diverse fauna of large predators. The earliest pelagic cephalopods were slowly swimming vertical migrants. The appearance and early diversification of pelagic cephalopods is interpreted as a consequence of the increased food availability in the open water since the latest Cambrian.

The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden

“…A total of 34 longitudinal color bands are preserved around the entire circumference of the specimen PMU 26835. This occurrence of longitudinal color bands is known from other species of Isorthoceras (see Kröger et al 2009; Fig. 28), but their significance for taxonomy is not known, because of the rarity of this type of preservation.…”

“…Crustaceans, which have a poor preservation potential (Plotnick 1986), are the main component in the diet of modern cephalopods (Glynn & Enochs 2011). The most common and the most diverse Boda cephalopods are orthocerids (Table 3), which are interpreted as vertical migrants in the water column (Westermann 1998;Kröger et al 2009). Therefore, a cephalopod feeding within the water column is likely (Kröger et al 2009).…”

“…The most common and the most diverse Boda cephalopods are orthocerids (Table 3), which are interpreted as vertical migrants in the water column (Westermann 1998;Kröger et al 2009). Therefore, a cephalopod feeding within the water column is likely (Kröger et al 2009). This could be interpreted as indirect evidence for a high abundance and diversity of nektonic and planktonic crustaceans in the Boda reef waters.…”

“…Actinocerids are abundant in contemporaneous cephalopod assemblages of low palaeolatitudes and in Late Ordovician reef settings and are present in contemporaneous assemblages of Norway and Estonia (Teichert 1930;Strand 1934;Kröger et al 2009). This absence or extraordinary rarity is provisionally interpreted as evidence for a depositional environment in relatively deep waters or in relatively cold waters.…”

A probable case of heterochrony in the solutan Dendrocystites Barrande, 1887 (Echinodermata: Blastozoa) from the Upper Ordovician of the Prague Basin (Czech Republic) and a revision of the family Dendrocystitidae Bassler, 1938

“…In modern seas, asteroids, teleost fish and decapod crustaceans are the main predators feeding on benthic, free-living echinoderms (Fujita 1992, Aronson & Blake 1997). In Late Ordovician times, both asteroids and cephalopods were already diverse and probably among the top predators feeding on benthic organisms (Blake & Guensburg 1994, Lefebvre 2007, Kröger et al 2009, Blake 2013. However, asteroid remains and cephalopod conchs are both rare or absent in Dendrocystites-dominated horizons, thus suggesting a low intensity of predation pressure.…”