But as the leaves got bigger they were rounded. This came up near a bunch of self-seeded tomatoes, Mexican mint and morning glories. It's about 6 inches tall now. Any ideas?
It is buffalobur, Solanum rostratum. In the nightshade family.
Looks a lot like watermelon, or cucumber.
Yikes, that's a new sticker plant, haven't seen those before. It's gone. Thanks hortster!