Karen Richardson told NBC Montana that she was chaperoning a fifth-grade trip to the park when two tourists arrived at the ranger station with the animal in the back of their SUV. She said the pair had taken the calf because they were worried it was cold.
Rob Heusevelet, a father of a student on the class trip, told East Idaho News the tourists "sincerely thought they were doing a service and helping that calf."
The unidentified pair were ticketed, and the animal was returned to where it had been found, the website reported.
A bison expert said the baby animal probably wasn't healthy and may have been abandoned by its mother, or else it wouldn't have been caught so easily.
"From about 3 days old, they can run up to 30 miles per hour," bison rancher Troy Westre told NBC Montana. "If the mother is around, she's going to kill you any chance she gets."
Although President Barack Obama recently signed the National Bison Legacy Act into law, officially making the American bison the national mammal of the United States, park officials do not want tourists approaching the animals.
Yellowstone visitors should remain at least 25 yards from bison, and signs around the park warn people to keep their distance:
There have been a growing number of incidents between people and animals in Yellowstone, including at least five injuries last summer when tourists got too close to bison while trying to take selfies.
UPDATE: The National Park Service said in a statement on Monday that they euthanized the calf after it was abandoned by its herd:
"Last week in Yellowstone National Park, visitors were cited for placing a newborn bison calf in their vehicle and transporting it to a park facility because of their misplaced concern for the animal's welfare. In terms of human safety, this was a dangerous activity because adult animals are very protective of their young and will act aggressively to defend them. In addition, interference by people can cause mothers to reject their offspring. In this case, park rangers tried repeatedly to reunite the newborn bison calf with the herd. These efforts failed. The bison calf was later euthanized because it was abandoned and causing a dangerous situation by continually approaching people and cars along the roadway."