Similicaudipteryx yixianensis

Three growth stages of Similicaudipteryx yixianensis. The largest is based on the holotype specimen apparently collected (ironically) from the Jiufotang formation. That the Yixian form is actually Similicaudipteryx is doubtful, IMO, but the Jiufotang specimen is largely incomplete anyway, lacking a skull and feather impressions. It does, however, have a pygostyle, probably in order to support the large fan of tail feathers found in the older of the immature referred specimens. As you can see in my other caudipterid restorations, this tail is an order of magnitude larger than any so far known for Caudipteryx.

Interestingly, the youngest juvenile has much more obviously Caudipteryx-like proportions of the wing and tail, but famously, its remiges and rectrices are of a more "primitive" form, the feather stage where the shafts are composed of a solid ribbon only barbed at the end. These are also found in the display feathers of Epidexipteryx, Confuciusornis and Protopteryx. Note that Richard Prum, though (a well known feather development expert) has speculated that the apparent ribbon-feathers of the Similicaudipteryx juvenile could simply be feathers in mid-molt, when new primaries sprout from basal "tubes". It would be strange for the tubes to be so long, though.

Feathers aside, the main difference between Similicaudipteryx and Caudipteryx are the former's well-developed third digit, and the pygostyle (possibly indicating an even larger tail in the adult, which I restored here). It also seems to have proportionally larger feet, especially the adult.

References:
[link] and photos/description in:
* Xu, X., Zheng, X. and You, H. (2010). "Exceptional dinosaur fossils show ontogenetic development of early feathers." Nature, 464: 1338-1341.