一种罕见的化石记录(A Rare Fossil Record)

thepreservationofembryosandjuvenilesisarateoccurrenceinthefossilrecord.thetiny,delicateskeletonsareusuallyscatteredbyscavengersordestroyedbyweatheringbeforetheycanbefossilized.ichthyosaurshadahigherchanceofbeingpreservedthandidterrestrialcreaturesbecause,asmarineanimals,theytendedtoliveinenvironmentslesssubjecttoerosion.

still,theirfossilizationrequiredasuiteoffactors:aslowrateofdecayofsofttissues,littlescavengingbyotheranimals,alackofswiftcurrentsandwavestojumbleandcarryawaysmallbones,andfairlyrapidburial.giventhesefactors,someareashavebecomeatreasuryofwell-preservedichthyosaurfossils.

thedepositsatholzmaden,germany,presentaninterestingcaseforanalysis.theichthyosaurremainsarefoundinblack,bituminousmarineshalesdepositedabout190millionyearsago.overtheyears,thousandsofspecimensofmarinereptiles,fishandinvertebrateshavebeenrecoveredfromtheserocks.

thequalityofpreservationisoutstanding,butwhatisevenmoreimpressiveisthenumberofichthyosaurfossilscontainingpreservedembryos.ichthyosaurswithembryoshavebeenreportedfrom6differentlevelsoftheshaleinasmallareaaroundholzmaden,suggestingthataspecificsitewasusedbylargenumbersofichthyosaursrepeatedlyovertime.

theembryosarequiteadvancedintheirphysicaldevelopment;theirpaddles,forexample,arealreadywellformed.onespecimenisevenpreservedinthebirthcanal.inaddition,theshalecontainstheremainsofmanynewbornsthatarebetween20and30incheslong.

whyaretheresomanypregnantfemalesandyoungatholzmadenwhentheyaresorareelsewhere?thequalityofpreservationisalmostunmatchedandquarryoperationshavebeencarriedoutcarefullywithanawarenessofthevalueofthefossils.butthesefactorsdonotaccountfortheinterestingquestionofhowtherecametobesuchaconcentrationofpregnantichthyosaursinaparticularplaceveryclosetotheirtimeofgivingbirth.