Hi everyone. Thanks to Jerry Harris (who also sent me the "Proornis" paper), I was able to get a hold of the description of Lingyuanornis parvus. It turns out this bird is just the counterpart of Liaoxiornis delicatus. I'm not sure which was published first, so I don't know the proper name for this species. However, I do believe it is a juvenile enantiornithine, which would explain the small size. I plan to get a Details segment completed soon, but it would help to know which name is correct.
Ji Qiang & Ji Shu'an. 1999. A new genus of the Mesozoic birds from Lingyuan, Liaoning, China. CHINESE GEOLOGY 1999 (3) No 262, 1999: 45-48, illustr.
Lianhai Hou & Peiji Chen, 1999. Liaoxiornis delicatus gen. et sp. nov., the Smallest Mesozoic Bird Chinese Science Bulletin 44: 834-838