Details of Thomas' new deal were not made public.
Thomas is heading into his seventh season and has started 25 games since 2010, with 19 of those coming with the San Diego Chargers and Pittsburgh Steelers from 2013-14. Thomas was signed to a $760,000 base salary on June 7 and will serve as a backup on a star-studded defensive line that includes Aaron Donald, Robert Quinn, Michael Brockers and Will Hayes as starters.
Arkin was a fourth-round draft pick in 2011 who spent 2012-14 with the Dallas Cowboys, Miami Dolphins and Indianapolis Colts, respectively, receiving minimal playing time. The 28-year-old was signed to the Rams' practice squad in January.
The moves gives Los Angeles nine offensive and defensive linemen.