Timarcha goettingensis/tenebricosa
The obvious difference between these two species is size. Although there is some overlap T. tenebricosa is by far larger (11-18mm) with T. goettingensis smaller (8-13mm).

goettingensis: Barely constricted at base.
tenebricosa: More strongly constricted at base.

goettingensis: More shining, puncturation coarser.
tenebricosa: Dull, puncturation finer.

T. goettingensis

T. tenebricosa

Just in case you cant use a ruler,an idea of the size difference.