Thomas was drafted by the Dolphins in the sixth round in 2008. He spent the 2008 season on injured reserve for the most part but bounced back nicely in 2009, starting 12 out of 16 games. This year, the Dolphins released him during the final round of roster cuts, and this month he worked out with a number of teams before signing with Detroit.
It looks like the Lions decided to sign Thomas in order to add some depth to the guard position. Stephen Peterman and Dylan Gandy are both a bit banged up, so the Lions apparently wanted to make sure someone is there to step in if Peterman and Gandy go down.
To make room on the roster for Thomas, the Lions released Caleb Campbell, who was just signed off the practice squad last weekend. He was active for the Cowboys game, and now he is going to be re-signed to the practice squad assuming he clears waivers.