There once was a widespread and successful clade of birds that became extinct at the end of the Cretaceous. These are the Enanitornithes, or opposite birds, because their scapula/coracoid joint tabbed the opposite way of living birds.
Protopteryx fengningensis (Zhang and Zhou 2000, Barremian/Aptian, 131 mya) the most primitive enantiornithine, was starling-sized (10cm) had no pygostyle, had two elongate tail feathers that lacked barbs and ram. Preserved with flight and down feathers.
Cathayornris yandica (Zhou, Jin and Zhang. 1992, IVPP V9769, Aptian, Early Cretaceous) toothy jaws, large naris, domed skull, large claws on first two wing fingers, coracoids large and equal to the scapula length, long gracile curved pubis, pygostyle shorter than remaining caudals.
Iberomesornis romerali (Sanz and Bonaparte 1992, LH-22, Barremian, Early Cretaceous, 125 mya) tiny bird (8.7 cm axial column) with short wings (20 cm wingspan), pygostyle.
Sinornis santensis (Sereno and Rao 1992, Early Cretaceous) sparrow-sized, excellent flyer with large perching hallux despite having unfused hand and wrist bones. Only moderately curved manual unguals.
Sulcavis geeorum (O’Connor et al., 2013, Early Cretaceous, BMNH Ph-000805) has short gastralia in place of a sternum, or perhaps a sternum is really the result of fused gastralia. The teeth are robust and have tiny grooves radiating from the tip.
Neuquenornis volans (Chiappe and Calvo 1994; Late Cretaceous, 85 mya, MUCPv-142) larger than the others (30cm), had a fused occiput, a large hallux.
Gobipteryx minuta (Elzanowski 1974, Campanian, Late Cretaceous) Adult skull: 4.5 cm, the size of a dove. Toothless beak. Nares smaller than antorbital fenestra. Parietals fused. Only 19 presacrals, with 6 dorsals. Ready to fly upon hatching.
Concornis lacustris (Sanz and Buscalioni 1992, Barremian, Early Cretaceous) 13 cm skeletal length, smallish, generalized bird with large hallux, sternum with keel and deep notches.