卵子发育模型阅读真题.pdfVIP

Passage1442015-09-19-2ModelsofEggDevelopment

1.Severaldifferenttheorieshavebeenputforwardtoexinhowthehard-sedeggsof

land-dwellingreptiles(e.g.lizards)evolvedfromthesofteggsthatamphibians(e.g.

andtoads)layinwater.TheRomermodelofeggdevelopmentisnamedafterthelateAlfred

Romer,apaleontologistwhoalsobecamedirectoroftheHarvardUniversityMuseumof

ComparativeZoology.Hisspecialtywasearlyreptilesbecause,hefelt,theywerethekeyto

understandingthegreatreptilediversificationseenintheLatePaleozoicandMesozoicEras

(around230millionyearsago).Romershypothesiswasthatsomeaquaticamphibians¨Cthat

is,amphibianslivinginwater¨Ccalledanthracosaursbegantolaytheireggsonlandatabout

thetimethattheywereevolvingreptile-likeskeletalfeatures.Indeed,someoftheseearly

amphibiansandearliestreptilesaresosimilarintheirskeletonsthattheexact

transitionpointfromotheotherisstilldifficulttodetermine.Eventually,though,

thetransitionwasmade,buttheseearlyreptilesremainedaquatic.Theadvantageforlaying

eggsonlandwasprimarilytoavoidtheaquaticlarval(pre-adult)stageduringwhich

immatureamphibiansliveexclusivelyinwaterwithitsinherentriskofpredatorsanddrying

ofponds.However,thelandhasitsownsetofdangers,notleastofwhichisthedryingeffect

oftheatmosphere.Tocopewiththeseproblems,aseriesofprotectivemembranesdeveloped

aroundtheegg,ludingahards.Onlylaterdidthereptilescompleyabandonan

aquaticlifestyle.

2.AnotherhypothesiswasproposedbyGermanpaleontologistRolfKohring,whosespecialtyis

fossileggs.InKohrinodel,amphibiansduringtheMississippianepoch(360¨C320

millionyearsago)spreadintonutrient-p