Mustonen opened the scoring at 9:43, following his own rebound and poking it past American goalie Anthony Stolarz.
McCoshen tied the game when his shot from just inside the blue line went off the stick of a Finnish player and bounced past goalie Joonas Korpisalo at 14:04.
Play turned nasty a few times in the first, and the center of attention was U.S. forward J.T. Compher. The Buffalo Sabres draft pick centered a line with Tyler Motte and Hudson Fasching, and twice in the period got the puck to the front of the net, with the result being all 10 skaters piled in the blue paint.
The second such occurrence saw Compher take a few swipes at Korpisalo after the goalie thought he had control of the puck, and Korpisalo responded by throwing a punch at Compher. Compher was sent off for slashing, while Finland's Henri Ikonen was called for slashing.
Back with more after the second period.