What is "gruae"

Gruae is a clade of birds that contains the order Opisthocomiformes (hoatzin) and Gruimorphae (shorebirds and rails) identified in 2014 by genome analysis. Despite its unusual and primitive morphology, genetic studies have shown the hoatzin is not as primitive or as ancient as once thought, and that it could be very derived bird that reverted or retain some plesiomorphic traits.