Longipteryx enters the LRT with gastralia AND a large sternum

Zhang et al. 2001 originally described
Early Cretaceous Longipteryx as an ‘opposite bird’ twenty years after the first enantiornithine was described in 1981.

According to Wikipedia, “The affiliations of Longipteryx are not resolved.”

The clade of Pygostylia was named by Chatterjee 1997
to include all avialans with short, fused caudals. Even though this definition is an early example of “Pulling a Larry Martin” other bird workers (e.g. O’Connor et al. 2016) have embraced it. Chiappe 2001 added three other unique characters, perhaps forgetting that many extant birds lack a pygostyle. These include the kiwi (Apteryx), ostrich (Struthio), rail (Rallus) and members of the extinct Hesperornis clade.

This allows for the possibility ofreversals.
That allows for Rapaxavis (Fig. 3), a sister to Longipteryx, to have a robust pygostyle. This allows Longipteryx (Fig. 1) to have a shorter tibia than femur, a trait convergent with several giant theropods and Triassic herrerasaurids. This allows for the possibility of several different types of pygostyle appearing, which is in evidence.

Don’t make clades based on traits
then expect nature to follow them. Instead, let nature and your software tell you how to lump and split clades and where trait exceptions (= reversals) and convergence occur.

References
Chiappe Luis 2001. Chiappe L and Witmer L (eds). Basal bird phylogeny in Mesozoic Birds: above the heads of dinosaurs. University of California Press.
O’Connor JK, et al. 2016. An Enantiornithine with a Fan-Shaped Tail, and the Evolution of the Rectricial Complex in Early Birds. Current Biology, 26(1): 114-119.
Zhang F, Zhou Z, Hou L and Gu, G 2001. Early diversification of birds: Evidence from a new opposite bird”. Chinese Science Bulletin. 46 (11): 945–949.

wiki/Longipteryx
wiki/Enantiornithes
wiki/Pygostylia