Náttúrufræðingurinn - 1993, Blaðsíða 67

árekstrar meiri háttar loftsteina eða
halastjama virðast fylgja (Sepkoski 1990).
Það er engu líkara en lífríki jarðar sé
stjórnað af geimklukku sem slær á 26
milljón ára fresti. Enn er hart deild um
gildi og reyndar raunveruleika þessarar
tíðni, en það er til dæmis athyglisvert að
hún er svipuð og tíðnin í sveiflu sól-
kerfisins um miðás Vetrarbrautarinnar
(Rampino og Caldeira 1992).
HEIMILDIR
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