I'm getting ready to do some remodeling in my home office, which calls for mounting shelves that can ideally hold quite a bit of weight. I started by trying to map out the studs in my drywall, but the stud finder was very inconsistent with it's readings. knocking didn't seem to produce any measurable difference either. Finally I decided to use a small hole punch and try to identify the studs. However
every time I punctured the wall, I hit something hard.

Perplexed, I decided to cut a small hole in my drywall to see what was going on. It looks like instead of studs, the support is wood running horizontally all over.

Is it safe to drill right into this type of support or should I be using some type of anchor?

Note that it's a 100+ year old house with an extension that was added in the past few decades.