Jeholornis and Oviraptors

I suggest that secondarily flightless oviraptors descended (in both senses) from a flying creature like Jeholornis. Notice the similarities in morphology, time and location.

https://en.wikipedia.org/wiki/Jeholornis

Jeholornis (meaning "Jehol bird") is a genus of avialans that lived between approximately 122 and 120 million years ago during the early Cretaceous Period in China. Fossil Jeholornis were first discovered in the Jiufotang Formation in Hebei Province, China (in what was previously Rehe Province, also known as Jehol—hence the name) and additional specimens have been found in the older Yixian Formation.[1] Jeholornis had long tails and few small teeth, and were approximately the size of turkeys,[2] making them among the largest avialans known until the Late Cretaceous. Their diet included seeds of cycads, Ginkgo or similar plants. Jeholornis were relatively large, primitive avialans, with a maximum adult length of up to 80 cm (2.6 ft).[2] Their skulls were short and high, similar to other primitive avialans like Epidexipteryx and to early oviraptorosaurs like Incisivosaurus.

https://en.wikipedia.org/wiki/Avialae

Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.[8][9]

https://en.wikipedia.org/wiki/Oviraptorosauria

Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, with or without bony crests atop the head. They ranged in size from Caudipteryx, which was the size of a turkey, to the 8 metre long, 1.4 ton Gigantoraptor.[4] The group (along with all maniraptoran dinosaurs) is close to the ancestry of birds. Analyses like those of Maryanska et al (2002) and Osmólska et al. (2004) suggest that they may represent primitive flightless birds.[5][6]

https://en.wikipedia.org/wiki/Caudipteridae

Caudipteridae is a family of oviraptorosaurian dinosaurs known from the Early Cretaceous of China. Found in the Yixian and Jiufotang Formations, the group existed between 125-120 million years ago.

https://en.wikipedia.org/wiki/Protarchaeopteryx

Protarchaeopteryx (meaning "before Archaeopteryx") is a genus of turkey-sized feathered theropod dinosaur from China.[1] Known from the Jianshangou bed of the Yixian Formation, it lived during the early Aptian age of the Early Cretaceous, approximately 124.6 million years ago.[2]

JEHOLORNIS



OVIRAPTOR

NOTE:
"Maniraptors" were either secondarily flightless avialae or secondarily flightless non-avialae paraves.
In either case, they descended from flying ancestors. They are not transitional between dinosaurs and Paraves.