518 mya in the Cambrian period, a worm like creature named Facivermis ("torch worm") adapted to a tube dwelling lifestyle that lost it's need for it's lower limbs. This article describes how Facivermis is the earliest known animal to lose body parts ('secondary loss'). They also place Facivermis as a Lobopodian.
https://www.sciencedaily.com/releases/2020/02/200227114446.htm
The journal article can be found here for anyone interested (open access) - https://www.cell.com/current-biology/fulltext/S0960-9822(20)30119-6?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982220301196%3Fshowall%3Dtrue