Ammonites were spiral - shelled maritime shellfish that stray throughout the Mesozoic seas until the Cretaceous – Paleogene mass extinction event . Despite how abundant they are in the dodo record , we do n’t fuck that much about their environmental science . Now , researchers trying to reconstruct their home ground let on where in thewater columnthese omnipresent fossils preferred to live . The findings , publish inProceedings of the National Academy of Sciencesthis workweek , might help researcher redo ancient mood .
Because ammonites are diverse and their shells preserve so well , these ocean creatures are an important index number fossil for dating rock layer . For example , the 35 - million - year - long record of Upper Cretaceous deposits in the U.S. Western Interior Seaway has been divided into 66 ammonite zones . At the upper Maastrichtian Owl Creek Formation in northern Mississippi , three common ammonite families – Baculitidae , Scaphitidae , and Sphenodiscidae – co - occur with other well - preserved organism living at various depths , from planktonic ( surface dwelling ) critter to benthic ( bottom dwelling ) ones . This area offer a rarefied chance to constrain depth habitats of ammonites by compare isotopic compositions of all the co - occurring groups of organisms : Shell chemistry , specifically oxygen isotope , register the temperature of the surrounding water .
To recreate the weewee pillar profile and determine where ammonites lived , a team led byJocelyn Sessafrom the American Museum of Natural History performed isotopic analyses on ammonites as well as planktonic and benthic protists call order Foraminifera . Of the 553 Owl Creek Formation mollusc specimens they studied , the squad ended up with 234 isotopic measure .
Baculites and scaphites , they found , have exchangeable chemical compositions as benthal organism – which mean they lived near to the ocean floor . The third house of ammonites , the sphenodiscids , was more close associated with planktonic foraminifera and those from the warmer half of the benthic environments . That mean they lived near the sea surface . proportional to baculites and scaphites , sphenodiscids were rarified and poorly preserve : Their shells likely reached Owl Creek by float seaward after death .
Being able to mold where different types of ammonites survive – using the oxygen isotope of their shells – may help oneself researcher construct preceding climate . " Ammonites have n’t been used much in temperature reconstructions because we did n’t know whether they were control surface dwellers , bottom dwellers , or perhaps even both if they swam through unlike weewee masses , " Sessa explains to IFLScience . " Now that the profoundness habitats of these three groups have been set up , they could be used to furnish temperature estimates of picky water masses in the future . "
