  Chondrites are one of the most primitive objects in the solar system, and
keep the record of the degree of thermal metamorphism experienced in their
parent bodies. This thermal history can be classified by the petrologic type.
We investigate the thermal evolution of planetesimals to account for the
current abundances (known as the fall statistics) of petrologic types 3 - 6
ordinary chondrites. We carry out a number of numerical calculations in which
formation times and sizes of planetesimals are taken as parameters. We find
that planetesimals that form within 2.0 Myr after the formation of Ca-Al-rich
inclusions (CAIs) can contain all petrologic types of ordinary chondrites. Our
results also indicate that plausible scenarios of planetesimal formation, which
are consistent with the fall statistics, are that planetesimals with radii
larger than 60 km start to form around 2.0 Myr after CAIs and/or that ones with
radii less than 50 km should be formed within 1.5 Myr after CAIs. Thus, thermal
modelling of planetesimals is important for revealing the occurrence and amount
of metamorphosed chondrites, and for providing invaluable insights into
planetesimal formation.
