It is shown that a mechanism involving only random motion of the sun with respect to the surrounding star field can account for the ~1 per 25 Myr characteristic frequency of large cometary impacts on Earth. In the proposed mechanism, the sun travels through the Oort Cloud of an encounter star, most typically a Type M dwarf, while the dwarf flies through the Oort cloud of our Sun. As a result, Oort Cloud objects from our Solar System are precipitated in large numbers to impact planets in the dwarf star system, while the dwarf's Oort Cloud objects are destabilized to impact planets in our Solar System. It is shown that it is this exchange of Oort cloud object between stellar systems, rather than the precipitation of Oort Cloud objects within a stellar system, that can account for the apparent periodicity of mass extinctions. Because the sun is more massive than ~90% of stars, its Oort cloud extends further, resulting in it delivering about a factor of three more bombardments on other solar systems than our Solar System receives. About 60% of the bombardments on our Solar System are found to be delivered by Type M dwarfs, about 20% by type K dwarfs, with the remaining 20% being delivered by stars of type G or larger. Foreign star Oort cloud objects can be captured by our Sun at typical ranges of 10 AU, resulting in a cometary approach to perihelion within a few years. It is found that assuming an effective Oort Cloud radius of 40 000 AU for a star of solar mass, increasing in size with the square root of the mass, accounts for the observed characteristic frequency of mass extinction events on Earth, given the local stellar number density of 0.003 stars per cubic light year. The frequency of mass extinction events in other solar systems would increase or decrease in linear proportion to the local stellar number density. It is shown that this exchange of materials between solar systems during close stellar encounters could be an important mechanism for spreading life throughout the galaxy. Implications for the evolution of life on Earth and in other solar systems are discussed.

Chitinozoans are considered as reproductive bodies of marine invertebrates, called chitinozoophorans. These chitinozoophorans were most likely to have been small, pelagic or necto-pelagic, soft-bodied, probably wormlike animals, and judging from the size of chitinozoans, they probably measured from a few millimetres to a few centimetres in length. The chitinozoophorans most likely survived by grazing on phytoplankton. There is no evidence of a large colonization of the pelagic niche in the Cambrian, but from the Early Ordovician onward, this niche was exploited chiefly by graptolites and chitinozoophorans. Both groups inhabited nearshore and offshore habitats, but in contrast to the graptolites, the chitinozoans displayed their highest diversity at high latitude, in less distal (that is, upper and lower offshore) environments. The chitinozoan group evolved rapidly during the Ordovician and reached its maximum Ordovician diversity in the late Darriwilian. From the first occurrence of chitinozoans in early Tremadocian times, to the biodiversity crisis in latest Ordovician times, nearly 80 % of the morphological innovations took place. Until their extinction in the latest Devonian, chitinozoans survived through several biodiversity crises: in the early Late Ordovician, late Hirnantian, late Wenlock, earliest Emsian, and in the latest Frasnian (Kellwasser event). During the melting of the Hirnantian ice sheet, most Ordovician genera and species became extinct, but some genera extended beyond the boundary (e.g. Spinachitina, Belonechitina, Cyathochitina, Ancyrochitina). The Hirnantian glaciation was not directly responsible for the dramatic extinction of organic-walled microfossils, but it certainly accelerated the extinction of lineages that had already been weakened since the early to mid-Katian. The late Wenlock and earliest Emsian graptolite crises affected the chitinozoophorans to a lesser degree, and the latest Frasnian Kellwasser event did not greatly affect chitinozoophorans. The disappearance of the chitinozoan group at the end of the Famennian resulted from a combination of factors, for example, the chitinozoophorans probably no longer had the genetic potential for successful adaptations to successive drastic environmental changes (only one species is known from the latest Famennian), their usual niche was invaded by more efficient groups, and their usual food supply disappeared or was no longer sufficient. The latter factor is supported by the contemporaneous decline in phytoplankton.