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Tracking the Evolution of Giant Cephalopods

DWhatley

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#1
Spineless Giants Track Oceanic Revolutions
by Brian Switek

Today’s giant and colossal squids were hardly the first invertebrate giants to inhabit the seas. Squishy and shelly critters with sizes over a foot and a half long have evolved multiple times during the last 500 million years and are well-known among paleontologists who specialize in spineless species. Endoceras giganteum, a 451 million year old cephalopod that lived inside an elonged cone of a shell, could get to be about 15 feet long, and there are rumors of lost specimens 30 feet in length. The 404 million year old sea scorpion Jaekelopterus rhenaniae has been estimated to be over eight feet long, and the 465 million year old trilobite Hungioides stretched nearly three feet long. And that’s just a few of prehistory’s immense invertebrates.
 

DWhatley

Cthulhu
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#2
Stable isotope data (δ18O, δ13C) of the ammonite genus Simbirskites – implications for habitat reconstructions of extinct cephalopods
Kevin Stevens, Jörg Mutterlose, Kurt Wiedenroth 2014 (subscription)

Abstract
The accurate habitats of the extinct ammonites and belemnites are largely unknown. Most ammonites are thought to have had a pelagic lifestyle, while belemnites are often considered to have dwelled in deeper, colder waters. Stable isotope analysis (δ18O, δ13C) provides a useful method to reconstruct the habitats of these two groups of extinct cephalopods, but is at the same time limited by various unknowns, including the isotopic composition of the past seawater, ‘vital’-effects and diagenesis.
In this paper, stable isotope data from 12 ammonite shells (Simbirskites spp.) from the upper Hauterivian of the Lower Saxony Basin (northwest Germany) are presented. Ontogenetic isotope profiles have been obtained from six of these shells in order to reconstruct habitat changes during their lifetime. These new ammonite data are combined with published stable isotope data from belemnites (Hibolithes jaculoides) and TEX86 palaeotemperature-estimates from the same section.
The combination of isochronous ammonite and belemnite stable isotope data shows that the resulting values of both groups go along with each other. Calculated palaeotemperatures of the two groups differ by 5 – 10 °C, with belemnites always showing the colder temperatures. All juvenile shells show an increase of the δ13Camm to more positive values throughout their ontogeny; they are stable in an adult shell-fragment. The incorporation of carbon isotopes in the ammonite shell therefore correlates with ontogeny and is influenced by metabolic effects, probably linked to the shell growth rate. The isotope profiles further suggest vertical habitat changes during the ontogeny of the ammonites, indicating that the δ13Camm values might record a combination of environmental and metabolic signals. Taking into account studies on oxygen isotope fractionation of recent aragonitic and calcitic mollusk-shells, a similar pelagic habitat is proposed here for both, ammonites and belemnites.
 

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