A San Francisco art dealer plead guilty to selling two rhino horns to an undercover agent for $55,000.

The man, Lumsden Quan, was working with Edward Levine of Mill Valley to sell the horns to a federal agent who posed as a wildlife collector. According to SFGate, the two men said they acted as brokers. Quan plead guilty on Friday.

The two horns they sold were of the black rhinoceros, a critically endangered species. The SF Zoo's Twitter superstar, Boone, is also a black rhinoceros. While Boone hasn't commented specifically on this case, he's been a big proponent of rhino conservation on Twitter:

Levine, a previously convicted drug dealer, is awaiting trial and Quan is scheduled to be sentenced in December.