I had never seen these flowers before. Erica noticed them in the field that "bumps out" toward the creek on the east side of the park. According to Texas Wildflowers, they are Pink Sensitive Briar, or Shame Vine (Schrankia uncinata). The plant is so named because its leaves are so sensitive that the slightest touch will cause them to fold up. The plant also curls up at night and on overcast days--just like the dogs! I did not get down on the ground to smell for myself, but my field guide says that the flowers are "fragrant." I'll have to ask Muzzy.
Also nearby is this specimen of lantana. According to my field guide, this one is on the lam from someone's garden. It's not the wild Texas lantana (Lantana horrida), which has deep orange and yellow flowers. Its cream and pink petals mean that it is L. camara, "mainly a cultivated plant, but some have escaped." Send for the dogs!