Ammonites are almost ubiquitous fossils in Mesozoic Marine deposits, and as such have been used extensively in interpreting and dating these deposits. They were free-swimming Cephalopods, related to modern Octopus, Squid, Cuttlefish and Nautiloids, and like other Molluscs are thought to have preserved the isotope ratios elements such as oxygen found in the surrounding water within their shells, which, since the ration of oxygen isotopes relates directly to temperature, can be used to determine the temperature of the water in which the animals lived. However in order to establish whether such studies are accurate, it is necessary to be able determine precisely where the animals lived when alive. Ammonites had chambered shells which are thought to have housed gases to aid buoyancy, and their closest living analogues, the Nautiloids (which also have chambered shells) are known to be capable of floating for very long distances after death, leading to the possibility that fossil Ammonites found in deposits and used to interpret their environment may have drifted there from distant locations.
In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 16 November 2015, a team of scientists led by Jocelyn Sessa of the Division of Paleontology at the American Museum of Natural History describe the results of a study of Ammonites from the Late Cretaceous Owl Creek Formation of northern Mississippi which attempted to determine the original environment of the living animals.
Three groups of Ammonites are present in the Owl Creek Formation, Baculites, Scaphites, and Sphenodiscids, as well as numerous Bivalves, Gastropods and both planktonic and benthic Foraminifera. The deposits are interpreted as having been laid down on a coastal plain at a depth of between 70 m and 150 m, and most probably at about 100 m.
Representative mollusks from the Owl Creek Formation. (A) Left lateral and ventral views of a Discoscaphites iris macroconch; (B) right lateral and ventral views of a Discoscaphites iris microconch; (C) right lateral view of Discoscaphites iris, showing a healed injury; (D) ventral and right lateral views of Eubaculites latecarinatus; (E) ventral and right lateral views of Eubaculites carinatus; (F) left lateral view of Sphenodiscus pleurisepta; sutures are visible because most of the shell is missing; (G) Gyrodes crenata; (H) Nucula percrassa. Sessa et al. (2015).
Scaphite Ammonites have distinctive, sex-dependent adult morphologies, making it possible to easily differentiate adult male, adult female and juvenile specimens. The population in the Owl Creek Formation was found to comprise 52% females, 44% males and 4 % juveniles, proportions thought to be indicative of a living community, rather than (for example) a site visited periodically by females to lay eggs. About 5% of the Scaphites show healed injuries associated with failed predation attempts, a proportion which has been seen in other populations thought to have been preserved in situ. Baculite Ammonites are less easy to assign to sexes, however of the specimens from Owl Creek for which the sex could be determined, the proportions were roughly equal. Sphenodiscids were far less common than the other two groups, and all of those found were too fragmentary for their sexes to be determined, though all were thought to be adults.
Analysis of oxygen isotope ratios in the planktonic Foraminiferans Planoheterohelix globulosa and Rugoglobigerina rugosa (which are thought to have lived close to the surface) from the Owl Creek Formation yielded temperatures of about 26°C, while the benthic Foraminiferans Lingulogavelinella sp. and Gavelinella sp. (which are thought to have lived on the seafloor) produced temperatures of about 20°C, as do Oysters from the same strata (also thought to have lived on the seafloor). A wider assemblage of Bivalves and Gastropods (which will include species which lived in sediments beneath the seafloor) gave an average temperature of 19°C, giving a total temperature gradient of 7°C; as planktonic Foraminiferans are prone to producing extensive blooms in warmer months, this is thought likely to be a warm season temperature gradient, with the difference lower in cooler months (the water temperature at about 100 m would be expected to fluctuate little with the season.
Temperatures estimated from oxygen isotope ratios from Scaphite and Baculite Ammonites aligned closely with those for benthic Foraminiferans and Molluscs, strongly suggesting that they spent most of their lives close to the seafloor in the area where they were preserved. Sphenodiscids on the other hand produced warmer temperature estimates though with a wide range of results that range from those found in Planktonic Foraminiferans to the warmer end of the ranges seen in benthic foraminiferans and Molluscs, suggesting that these Ammonites spent at least part of their lives in a warmer environment than that enjoyed by the Scaphites and Baculites.
Carbon isotope ratios can be used as a proxy for depth in palaeoenvironmental studies, though this is more complex than the ratio between temperature and oxygen isotopes. An 0.8‰ variation in carbon isotope ratios was found between the planktonic and benthic Foraminiferans, which is roughly consistent with what we would expect between planktonic Foraminiferans and benthic species living at a depth of about 100 m today. However the situation is more complex in Molluscs, as the carbon isotope ratio is established during photosynthesis by the Foraminiferans, whereas the Molluscs are secondary or even tertiary consumers (i.e. animals which eat the photosynthetic Foraminiferans, or animals which eat those animals), so these ratios can be overwritten by metabolic processes. Typically in modern benthic Molluscs the proportion of the isotope carbon 13 is about 1‰ higher than in Foraminiferans from the same environment, and this is seen in the Owl Creek Bivalves and Gastropods.
The situation is more complex still with Ammonites. which lived in the water column, and are believed to have been strict carnivores. Previous studies have found that they have lower proportions of carbon 13 in their shells than benthic Molluscs in the same environment, and this was found to be the case in the Owl Creek Ammonites, with the highest proportions found in the Scaphites, the Baculites having intermediate levels and the Sphenodiscids the lowest levels, which Sessa et al. suggest may indicate that the Scaphites lived lowest in the water column, the Baculites slightly higher and the Sphenodiscids highest of all.
Previous studies have suggested that Baculites lived in lower to middle waters and Scaphites lived close to the seafloor at intermediate depths, both of which appear to be consistent with the Owl Creek findings. Sphenodiscids have previously been suggested to be inshore species, living in shallow waters close to the coast. Sessa et al. feel that this is also likely in the Owl Creek specimens, which are likely to have floated further from the shore after death, eventually losing their buoyancy and sinking in deeper water.
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