oc eras ▁is ▁a ▁genus ▁of ▁ce phal op od ▁belonging ▁to ▁the ▁Gast ri oc er at idae ▁family . ▁They ▁are ▁an ▁ext inct ▁group ▁of ▁am m ono id , ▁which ▁are ▁sh elled ▁ce phal op ods ▁related ▁to ▁squ ids , ▁be lem n ites , ▁oct op uses , ▁and ▁c utt le fish , ▁and ▁more ▁dist antly ▁to ▁the ▁na util oid s . ▁ ▁References ▁▁ ▁The ▁Pale obi ology ▁Database ▁accessed ▁on ▁ 1 0 / 0 1 / 0 7 ▁ ▁Category : G oni at it ida ▁gener a ▁Category : G ast ri oc er at idae <0x0A> </s> ▁In ▁the ▁theory ▁of ▁general ▁relativ ity , ▁the ▁Gib b ons – H aw king ▁effect ▁is ▁the ▁statement ▁that ▁a ▁temperature ▁can ▁be ▁associated ▁to ▁each ▁solution ▁of ▁the ▁Ein stein ▁field ▁equations ▁that ▁contains ▁a ▁caus al ▁horizon . ▁It ▁is ▁named ▁after ▁Gary ▁Gib b ons ▁and ▁Stephen ▁Haw king . ▁ ▁The ▁term ▁" ca us al ▁horizon " ▁does ▁not ▁necessarily ▁refer ▁to ▁event ▁hor iz ons ▁only , ▁but ▁could ▁also ▁stand ▁for ▁the ▁horizon ▁of ▁the ▁visible ▁universe , ▁for ▁instance . ▁ ▁For ▁example , ▁Schwar z sch ild ▁spac etime ▁contains ▁an ▁event ▁horizon ▁and ▁so ▁can ▁be ▁associated ▁a ▁temperature . ▁In ▁the ▁case ▁of ▁Schwar z sch ild ▁spac etime ▁this ▁is ▁the ▁temperature ▁ ▁of ▁a ▁black ▁hole ▁of ▁mass ▁, ▁satisfying ▁ ▁( see ▁also ▁Haw king