The timing of the transition is intriguing since an asteroid impact 65 million years ago is believed to be the proximal cause of the extinction of the dinosaurs and many other components of Earth¹s biota at the Cretaceous-Tertiary boundary.

Instabilities in the asteroid belt (below) can be due to both orbital and secular resonances. The former are responsible both for gaps (the 3:1 and 2:1 cases) and groups, also called families (the Hildas in the 3:2 and the Trojans in the 1:1 resonance). Secular resonances are regions in which the precession of an asteroid's orbit has the same frequency as that of a major planet. The secular resonance ₆ is associated with Saturn, while ₃ and ₄ are associated with Earth and Mars, respectively. The latter are believed to have played major roles in delivering the martian meteorite ALH84001 to the Earth.

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Mercury's perihelion (first page)

Earth and Mars (page 2)

Mercury's eccentricity (page 3)

The chaotic jumps int he secular frequencies g₃ and g₄ cause shifts in the location of secular instabilities in the asteroid belt and might trigger increases in the flux of Earth-impacting bodies. The asteroids most affected by these shifts belong to the Hungaria family, a group having semi-major axes of about 1.8-2.0 AU, low eccentricities, and inclinations of 18-20 degrees.

It might have been a member of this family, (a "Hungarian") that lost its orbital stability due to a sudden shift in the secular frequencies and slammed into Earth 65 million years ago.