ST. LOUIS — The Cowboys and Rams have swapped underachieving former first-round picks, with offensive tackle Alex Barron going to Dallas and linebacker Bobby Carpenter headed for St. Louis.

Carpenter made only two starts in four seasons for the Cowboys, one in the regular season and one in the playoffs, and was a backup inside linebacker in a 3-4 defense last season. The Rams have a 4-3 defense and the 250-pound Carpenter could start alongside fellow Ohio State products James Laurinaitis, a second-round pick last year, and Na’il Diggs. A fourth St. Louis linebacker, Larry Grant, also played for the Buckeyes.

“That’s kind of an anomaly there,” Carpenter said. “It’s going to be great to come in and play with those guys.”

Carpenter was the 18th pick of the 2006 draft and Barron was the 19th choice in 2005. The trade was finalized Monday and Carpenter welcomed a fresh start.

“We’ve had a good team for a lot of years and it’s been tough to get on the field,” Carpenter said of his years in Dallas. “I think it’s going to be a great opportunity.

“They’ve got some great defensive minds and really a young team, so I think it’s going to be really enthusiastic, a lot of energy.”

The 6-foot-7, 315-pound Barron played in 76 games for St. Louis, all but two of them starts, and has been at least a mild disappointment after moving full-time to left tackle last year as the replacement for Orlando Pace.

Barron had been a restricted free agent before agreeing to a one-year tender and has been one of the most penalized offensive linemen in the NFL during his career, leading the league with five holding calls last season.

Barron was benched for disciplinary reasons the second half of a loss to the 49ers last October.