In very rare situations, a jockey may become a dominant figure at a track. He will influence the outcome of races almost regardless of the ability of the horses he is riding.

This happened during Steve Cauthen's heyday in New York. It happens occasionally when a great jockey like Laffit Pincay or Angel Cordero experiences a sensational hot streak. And it happened this fall at Calder Race Course, when local horseplayers found themselves asking almost automatically before every race, "Who's Mary riding here?"

Mary L. Russ made converts of many sexist bettors who previously may have viewed all female jockeys with skepticism or contempt. And when she beat out Jacinto Vasquez to rank as the leading rider at Calder, she was only the second member of her sex to accomplish such a feat at a major race meeting. (Denise Boudrot did it at Suffolk Downs.)

Only a year ago, when she was starting her career, Russ would have confirmed the prejudices of any misogynist horseplayer. She had the typical credentials of the typical female jockey. At the age of 27 she was fulfilling a long-deferred dream. She had fallen in love with horses growing up in Tampa and, she said, "I wanted to work with them. So I went to Ocala and broke yearlings and lived on a farm, and I was in heaven. But when I came here and watched races I fell in love with the track."

Russ started exercising horses at the track in 1978, and in 1981 trainer Manny Tortora finally gave her a chance to ride in competition. The careers of most female jockeys who start this way would have been very short and very undistinguished. But a lifelong friend of Russ said, "With Mary this wasn't a phase. She was not going to give this up. She never thought like that for a minute."

Russ exercised diligently to increase her stamina and get her weight down. She went to the track every morning at dawn to gallop horses and develop the all-important contacts with trainers. And she approached the sport cerebrally, making a conscious effort to learn from all her early riding experiences. "Every time you ride," she said, "you see something different, you go through different predicaments. Every race was like a new page in a book."

If intelligence was Russ' chief asset, Calder was the optimal place to employ it. The Miami track had a powerful bias this year: the inside part of the strip was so advantageous that almost every race was won by a horse on the rail. Even superior animals couldn't win by circling the field.

Russ observed this, heard Tortora preach the importance of staying on the rail, and heeded the lesson. Because the rail was strong enough to carry even weak riders or bad horses to victory, Russ started winning races, and reinforced her determination to get to the inside whenever she could.

While other jockeys might be reluctant to stay on the rail for fear of being blocked, Russ learned--from encountering enough "predicaments"--that a hole would eventually open for her. It almost always did; her ability to get through on the rail became a daily source of amazement at Calder. And so even her longshot mounts always seemed to get into contention.

As she pursued and overhauled Vasquez, a two-time Kentucky Derby winner, to capture the Calder riding title, Russ encountered none of the resentment from males that greeted most of the pioneer female riders. The local racetrackers know how hard she worked for this, and veteran racing columnist Art Grace remarked, "As long as I've been around the track, I've never seen anyone with such backing. Everyone wants her to succeed."

Still, succeeding at Calder was a far cry from succeeding at Hialeah, where an influx of big-name riders like Jorge Velasquez, Eddie Maple, Jeffrey Fell and Jean-Luc Samyn would make it harder to get mounts or win races. Besides, there was going to be no Calder rail to give Russ a shortcut to victory.

But the first week of racing here indicated that Russ doesn't need shortcuts any more. Her accomplishments at Calder have clearly given her a sense of self-confidence that shows in every phase of her riding.

She is adaptable enough to circle the field on a track like Hialeah, which permits it. She is strong enough to duel though the stretch. She almost never gets her mounts into trouble. She currently ranks second to the great Velasquez in the Hialeah riders' standings, and even at this great track horseplayers may soon find themselves asking, "Who's Mary riding here?"