I was wondering what this marking means, it looks like an upside-down square fermata or an upside-down down bow marking with a dot. I found it in a piece for cello by Nadia Boulanger.
By convention, a square fermata has a longer duration than a rounded fermata.
It's not "upside down". Traditional notation convention usually tries to put the fermata over the note head, rather than the note stem. If the note is stemmed-down, them the fermata goes over the notehead, and the fermata dot will be below the fermata line.
If the note is stemmed-up, the fermata will be below the notehead, and the fermata dot will be above the fermata line.
Fermatas may also be oriented so that it clear to which voice they apply in multi-part writing. For example, in four part vocal music you will find fermatas for the soprano and tenor voices are typically above the staff, and those for the alto and bass voices below the staff, and inverted.
The example you give has only one voice, but the fermata has been placed below the staff so as not to interfere with the trill notation above the note.