Abstract
Die oorsake van massauitsterwings in die geologiese verlede bly nog 'n fassinerende onderwerp vir bespreking. Die moontlikheid van katastrofiese ekstraterrestriële oorsake word tans nog druk bespreek en wissel van supernova tot die wyd gepubliseerde asteroiedimpakteorie van Alvarez en medewerkers wat op die aanwesigheid van groot hoeveelhede iridium op die grens tussen Kryt en Tersiêr gebaseer is. So 'n impak kon moontlik 'n wêreldwye vuur en gevolglike as tot gevolg gehad het wat die son se strale gekeer en 'n sogenaamde kernw inter veroorsaak het; die moontlikheid van suurreën is ook nie uitgesluit nie. Baie outeurs verw erp egter die ekstraterrestriële teorie en meen dat m assauitsterw ings aan verskillende terrestriële oorsake toegeskryf kan word. Moontlikhede is die verlaging van watertem peratuur in oseane, die teru gtrekkin g van die see sodat tussengetydiere aan droë toestande blootgestel word, die oorstroming van die land deur seewater en geofisiese aktiwiteite van die aarde. Die idee dat baie diergroepe, soos die d inosouriërs, nie skielik uitgesterf het nie, maar geleidelik, het ook heelwat aanhangers. Biotiese faktore soos siektes of die akkum ulering van letale gene kon w aarskynlik slegs 'n rol in agtergronduitsterw ing gespeel het, maar kon nie m assauitsterwings veroorsaak het nie. Groot veranderinge in ekostelsels kon egter wel tot m assauitsterw ings gelei het. Soos belangstelling in m assauitsterwing gedurende die laaste aantal jare toegeneem het, is daar baie gegewens vir die meeste uitsterw ingsgebeure versam el. Een belangrike onbeantwoorde vraag is of die sogenaamde katastrofiese gebeure - soos di teen die einde van dieKrytperiode - sowel kwalitatief as kwantitatief van die talle ander kleinskaalse uitsterw ingsgebeure verskil. 'n Aspek wat almal nog dronkslaan is die selektiwiteit ten opsigte van groepe wat tydens sekere periodes uitsterf. Die oorsake van massauitsterwings is w aarskynlik 'n sameloop van Koers 53(2)1988Copyright information
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