[SunkenCiv]

:') I think there are some things which may have skewed previous data; the Middle Kingdom only lasted 400 years, built with mud brick and very soft stone (and pilfered Old Kingdom work in larger, harder stone), doesn't appear to have been a wealthy era for Egypt, and ended in a pretty grim manner.

[Berosus]

It looks like I spoke too soon. A British website called Digital Egypt asserts that anthropoid coffins were introduced during the Middle Kingdom. These two were found in a tomb belonging to two men. Apparently with each of them, the mummy was placed in a newfangled anthropoid coffin, and that went into one of the more typical rectangular boxes.

Source: http://www.digitalegypt.ucl.ac.uk/rifeh/mk/tombbrother.html