Buttercup

@ AnnaWhipkey: I agree on that, I think you´ve got it right

ranunculus are taller than potentilla and don't have a red fruit. See: http://osuext.intermountaintech.org/download/creeping%20buttercup.pdf This looks like your other photo, which does have red fruit, Potentilla indica (mock strawberry).

I've just posted a buttercup from my yard that is definitely ranunculus; perhaps you could compare the two. Evening primrose in this part of the world is yellow but a much larger and different flower, growing waist-high at times, and definitely not having any resemblance to our buttercup or to what you show here. Our ranunculus buttercup trails along the ground.

Oenothera has 4 petals and a large X-shaped pistil

http://en.wikipedia.org/wiki/Potentilla well... I don´t know, the leafs do not look the same...

Pink Evening Primrose (Showy Primrose) Oenothera speciosa Evening primroses are often called buttercups. There are evening primroses that are yellow with black centers, and then there is the pink evening primoses. The pink evening or "showy primrose" has broad petals that are may be pink, light pink or almost white, with pink or red veins and yellow centers. They open at dusk in northern parts of the state, but wither the next day. In the rest of the state, the blooms stay open all day. I stand corrected.

I don´t think it is a buttercup /ranunculus, the leafs and flowers look more like Potentilla to me

i am in the same town..and my buttercups are pink.

i am in the same town..and my buttercups are pink.

Looks different from ours, but I suspect this is a buttercup, Ranunculus sp.