Last Modified: Wednesday, October 26, 2011 3:29 PM

JAIME ARON // American Press AP Pro Football Writer

ARLINGTON, Texas (AP) — So the first time the Dallas Cowboys asked DeMarco Murray to be an important part of their offense, all the rookie did was run for more yards than Hall of Famer Tony Dorsett and NFL rushing king Emmitt Smith ever gained in any game.

His 253 yards were the ninth-most by anyone in NFL history and the second-most ever by a rookie. He broke Jim Brown’s record for the most yards ever against the Rams, a mark that had been around since 1957.

This breakout performance begs one question: What can the kid do if he actually gets to start?

A third-round pick from Oklahoma, Murray came into this season as Dallas’ third-stringer. He moved up to second for Sunday’s game because starter Felix Jones had a high ankle sprain, then made himself right at home by running 91 yards for a touchdown on his first carry, putting the Cowboys on their way to a 34-7 victory.

“I never thought in a million years that I’d ever have a day like this,” Murray said. “This is what I’ve been working hard for since my Pop Warner days.”

His long touchdown was the second-longest in franchise history, behind only Dorsett’s NFL-record 99-yarder in January 1983.

It was a heck of a way for him to get into the end zone for the first time, and it more than doubled his career rushing total of 71 yards coming into the game.

It also showed that St. Louis’ run defense was as bad as statistics indicated.

The Rams (0-6) came in last in the league, allowing 163 yards per game, more than two first downs worse than anyone else. Not only were they shredded by a novice, but they couldn’t even take advantage of a left guard, Montrae Holland, who’d been unemployed all season until signing with Dallas on Tuesday.

Murray ran through huge holes all game, often not getting hit until he reached the secondary.

When he ran out of gas during a 43-yard run in the fourth quarter, fourth-stringer Phillip Tanner came in and gained the final 35 yards of that drive, capping it with his first career touchdown, further proof of how much room there was for Dallas runners. The Cowboys finished with 294 yards rushing, fourth-most in team history.

“Until they decide they are going to tackle the way it’s coached and the way we ask them to do it, it’s probably not going to change,” said St. Louis coach Steve Spagnuolo, whose background is defense.

For Dallas (3-3), the real satisfaction was ending a two-game losing skid and emphatically breaking a stretch of 11 straight games decided by four points or fewer.

The Cowboys never trailed and left no reason for team owner Jerry Jones to criticize coach Jason Garrett’s play-calling — except maybe to wonder why Murray didn’t have a bigger role in the offense until this game.

Murray had 187 yards through three quarters, and the Cowboys were up 20-7. Because they wanted to run to protect the lead, it was clear that Murray would get a shot at Smith’s club record of 237 yards set Oct. 31, 1993, at Philadelphia.

Murray followed with a 43-yard run that could’ve gone for a 73-yard touchdown had he not run out of gas along the way. He finished it 2 yards shy of Smith’s mark.

“I just wanted to get down and protect the ball,” he said.

The record fell on his next try, an 8-yard run. He finished with 25 carries and an average of 10.1 yards. Dallas ran for 294 yards overall, spiking a season average of 84.8 that had been among the league’s worst.

For the local fans, it was a terrific start to a baseball-football doubleheader between teams from Dallas-Fort Worth and St. Louis. Game 4 of the World Series began just down the street less than an hour after this game ended. Josh Hamilton of the Rangers and Lance Berkman of the Cardinals showed up in uniform as honorary captains for the pregame coin toss.

The Rams were the perfect foe for the Cowboys to cure all that ailed them. In addition to their trouble stopping the run, they were without quarterback Sam Bradford (high ankle sprain) and were averaging the fewest points in the league even with him.

A.J. Feeley made his first start since 2007 and was 20 of 33 for 196 yards with one interception and one sack. But the offense gained only 4 yards in the third quarter, and had only two good drives. The first ended in a 6-yard touchdown run by Steven Jackson that got St. Louis within 14-7. The other ended with a fourth-and-goal from the 1 that was stuffed in the final minutes.

“We couldn’t get any rhythm and left the defense on the field entirely too long,” Jackson said. “For whatever reason, this keeps being the same old story.”

Jackson finished with 70 yards, 46 coming on the touchdown drive.

St. Louis also saw right tackle Jason Smith and backup defensive tackle Darell Scott carted off. Both were taken to a hospital for evaluation and were expected to fly home with the team. Smith, a Dallas native, had a strained neck; Scott’s injury was called a blow to the head.

Dallas’ Tony Romo was 14 of 24 for 166 yards, with two touchdowns. He didn’t have to throw much because the running game was doing so well. However, he hit Dez Bryant for four passes and a touchdown in the second half, which was significant because they’d hooked up for only two catches after halftime all season. Romo’s other TD throw went to Jason Witten.

The Cowboys’ rookie kicker Dan Bailey made two field goals, including a career-best 51-yarder, giving him 16 in a row.

NOTES: St. Louis’ Brandon Lloyd caught six passes for 74 yards in his St. Louis debut. He was acquired from Denver earlier this week. ... One record Murray didn’t set was most yards by a former Oklahoma Sooner. His predecessor Adrian Peterson went for 296 in 2007, which is both the NFL single-game record and the rookie record. ... Murray had the longest first-career TD since Detroit’s Bob Hoernschemeyer went 96 yards against the New York Yanks in 1950.