Extinct alien species that inhabited New Montana (YTS-4885002-3825-IV; MPA). The Thyresta were endoskeletal four-limbed creatures equipped with two horn-like constructions that could act as manipulators, weapons and social signals. Apparently they lived in complex familial nests, developing a fairly sophisticated civilisation before destroying themselves in a nuclear war 3.6 million years ago.
They are sometimes called "mankind's contenders", as cultural analysis suggests that if they had not destroyed themselves they might have developed the same kind of dynamic expansionist civilisation as the terragens did. This species is the most recent example of an intelligent race which failed to avoid the existential risks associated with the early stages of advanced technology; several older examples have been found on various worlds, but the remains are generally less well preserved. If a species develops, and uses, high tech weapons before it achieves interplanetary travel it is quite likely to share the fate of the Thyresta.
A related species, Thyresta Minor, possibly a devolved form or subspecies of the original Thyresta remain and is being provolved by MPA organisations on orbital habitats surrounding New Montana.