Are pecan shells poisonous

No

The shell itself is made of cellulose and lignin, a common seed coat which is there to provide a hard barrier to protect the inner material (in this case - nut meat). These seed coats are typically made of cells called sclereids. Sclereids also occur in other foods that are commonly ingested such as Pears, Apples, cinnamon, and cloves.

Pecan shell fiber is actually a beneficial ingredient which contains more antioxidants than raw berries such as cranberries, blueberries, and raspberries.