Info

After MacLeod 1998.

After MacLeod 1998.

that Phanerozoic extinction patterns show a periodicity with an interval length of 26 Myr (Raup and Sepkoski 1984) has spurred intensive debates about extinction mechanisms but has received, with increasing accuracy of the geological time scale, little support from other researchers. Currently, few paleontologists would subscribe to the idea that the mass extinctions show any periodicity, despite continued search for possible (mainly extraterrestrial) mechanisms (e.g., invisible companion of the sun, "nemesis," inducing a comet shower when passing through Oort Cloud; an eccentrical "planet X'' doing the same job; the passing of our solar system through the spiral arms of our galaxy, again perturbing bolides in the Oort cloud; movement of the solar system through the galactic plane; Sepkoski 1990). It comes as a major surprise that a new study indicates again that the diversity of life on earth followed some cyclic pattern, this time with a 62-Ma periodicity (Rohde and Muller 2005). Yet this pattern has yet to be scrutinized, and it might simply be the inevitable outcome of the applied methods.

The phase after a mass extinction usually shows a peculiar fauna/flora that change gradually during the time following the extinction. Some terminology has proven to be helpful for the characterization of these intervals (Kauffman and Erwin 1995; O Figure 16.5). The extinction phase is the time where most of the affected taxa had their last appearance. This is followed by the survival phase where some groups not severely affected by the extinction (holdover taxa) already began to diversify. During the survival phase, there is usually a small number of opportunistic taxa that could temporarily spread at the expense of the other fauna.

O Figure 16.5

Faunal patterns during extinctions and subsequent recovery. After Hallam & Wignall 1997

O Figure 16.5

Faunal patterns during extinctions and subsequent recovery. After Hallam & Wignall 1997

Lazarus taxa (Jablonski 1986) are those that disappear, probably by becoming very rare (and not retreating to refugia; Wignall and Hallam 1999) during the survival phase only to later reappear during the recovery phase when new ecosystems are being established, while Elvis taxa (Erwin and Droser 1993) were those that entered newly the fossil record but mimicked the shape of extinct taxa. The new communities were largely shaped by progenitor taxa, the most prolific during the recovery phase (Kauffman and Erwin 1995; Hallam and Wignall 1997).

It was long recognized that reefs showed, after mass extinctions, a prolonged recovery interval. This "reef gap'' (Hallam and Wignall 1997; Stanley 2001; Jablonski 2003), which is evident after all of the "big five'' and also after the extinction event after the end of the Lower Cambrian (extinction of the Arche-ocytha; Copper 1988), probably reflects the longer time it needs to reassemble complex ecosystems (Jablonski 2003).

Was this article helpful?

0 0

Post a comment