It is also the first known bird to have lost the long tail of Archaeopteryx and develop fused tail vertebrae ( a pygostyle ) . Recent cladistic analyses suggest that Confuciusornis is the most primitive pygostylian bird ; it has a more primitive skull than Archaeopteryx , but it is the first known bird to have lost the long tail of Archaeopteryx and develop fused tail vertebrae ( a pygostyle ) .