It's about time, right?

The 49ers' defensive line has been so good, for so long, that it deserves its own nickname.

So, make room, "Purple People Eaters" and "Fearsome Foursome."

Niners defensive line coach Jim Tomsula has a moniker that neatly captures the spirit of a starting unit featuring defensive tackles Justin Smith and Ray McDonald and noseguard Isaac Sopoaga, 905 pounds of snarling malevolence.

Ready for it? The NFL's next great nickname is ... the Muppets.

Huh?

Tomsula will attempt to explain: "If you ever see those kids in kindergarten, some of the kids take the colors and they put nice colors up and they try to draw squares? I think all the guys in my room were the ones with a big mess, a blob of stuff, you know? That's kind of the way it all is, because it's nuts in there! It's crazy! Stuff's flying around and you're like: 'Whoa, man! Boom!' "

Tomsula was describing a typical scene in the defensive line's meeting room, where attendees routinely get their chops busted for violations ranging from hideous haircuts to $200 sneakers to, in the case of the doughy Tomsula, an unfortunate amount of chest hair flowing from his partially unzipped windbreaker.

Said McDonald: "We're like, 'You've got to shave that taco meat off your chest, Jim.' "

There's clearly plenty of fun and games - and we haven't even touched on backup noseguard Ricky Jean Francois' "Peanut Butter and Jelly Dance."

Paul Sakuma/ASSOCIATED PRESS

On the field, however, the Muppets are seriously no joke.

Entering Monday's game against the visiting Steelers, San Francisco's 3-4 defense ranks first in the NFL in rushing yards allowed per game (70.5) and yards per carry (3.2) and is the first unit since the 1920 Decatur Staleys not to allow a rushing touchdown in the season's first 13 games.

The Niners are poised to break the NFL's all-time record for fewest rushing touchdowns allowed in a season (two), a mark held by three teams that played at least two fewer games than the 16 San Francisco will play this season. The 49ers are almost certain to break the league's record for fewest rushing touchdowns allowed in a 16-game season (four).

The touchdown-less streak has raised plenty of eyebrows around the league. But the topic hasn't been broached in that meeting room, where seemingly anything goes - except for football talk not related to X's and O's or wins and losses.

"We don't like talking about records and stats and stuff like that," McDonald said. "We just like go out there on Sunday and play hard."

The no-frills mentality has been hammered home by Tomsula, who began instilling a rugged approach long before head coach Jim Harbaugh issued blue-collar work shirts this season. In his fifth season in San Francisco, Tomsula, 43, is the Niners' longest-tenured coach, the only assistant retained by Mike Singletary and Harbaugh.

A Pittsburgh native, he delivered newspapers, chopped firewood and sold entry rugs to support his family during his years as an assistant coach at Charleston Southern and Catawba College. Just 14 years ago, he was working as an unpaid assistant at Catawba and living in a red Cadillac with a black Lab and a calico cat.

So about those gas-station-style shirts from Harbaugh ...

"I've got those shirts for real," Tomsula said. "That's not the first time I've had one of those shirts. My whole family's got those shirts."

His charges have fallen in line with their homespun, blue-collar leader, whom they refer to as "Super Mario" for his resemblance to a distinctly Italian video-game character. On the flip side, Jean Francois refers to him as both a big brother and a father figure. Smith, an 11-year veteran, has termed him one of the best coaches he's ever been around.

The 49ers' run defense was a stellar spot on a series of nonwinning teams during the first four years of Tomsula's tenure. San Francisco ranks second in the NFL in rushing yards allowed per game (84.4) since 2007.

"There's a comfort level between all of us in that room, how we communicate and get the point across," Smith said. "And it's worked well. The statistics don't lie. ... We all get along together, but none of it would matter if he wasn't a knowledgeable guy."

Smith, a two-time Pro Bowl selection who is being mentioned as a Defensive Player of the Year candidate this season, is routinely ridiculed by his linemates for his perceived lack of natural ability. The media often refer to Smith as a "high-motor guy," suggesting the No. 4 overall pick in the 2001 draft is all heart and hustle.

Smith, of course, is easily the most accomplished member of the defensive line.

Among the Muppets, though, no one is immune.

"We all just kind crack jokes on each other all day," Smith said. "Really nothing we can get into in the media, but we bust on each other pretty good."