With the discovery that ornithomimids had wings, a new possibility for Pelecanimimus has emerged:
“Pelecanimimus does have large paired sterna with ossified ribs and possibly uncinates, so maybe basal ornithomimosaurs were more flight-y anyway.”
– Mickey Mortimer.
So far, no other explanations were offered for these skeletal structures, and honestly, I don’t think there are any other possibilities other than flapping adaptations.
Since WAIR is extremely controversial, given the relatively low angle the arms could be raised at (and even then, this is not well established), flight is actually the more sensible explanation; an animal capable of powered flight needs robust bones and extensive musculature, but not as much as an animal that practises WAIR.
If Pelecanimimus was a flyer, this opens a wide range of implications. For starters, it shows that flight evolved independently in at least two lineages of dinosaurs (or more, given that microraptorines, Archaeopteryx and Rahonavis might had become volant independently from birds, after all; however, do note that, as Mortimer proposes, ornithomimosaurs as non-paravians isn’t well established, so they might be highly derived archaopterygids), that the size limit for flying non-ornithurine dinosaurs was considerably larger than previously thought (and that flight development might also not be reserved for small animals), that pterosaurs indeed co-existed with flying dinosaurs for most of their temporal range, et cetera. To say nothing about the fact that Ornithomimus and kin really were ratite mimics.
If Pelecanimimus was indeed a flyer, it was almost certainly a soarer, as even it’s unique anatomy would perhaps be insuficient for a Galliforme-like extreme flapper. Given what we know of other flying non-avian dinosaurs, hindwings would had been very likely present. Hindwings were indeed quite vital for non-avian dinosaur flyers, as the inferred gliding models for Microraptor show that these animals could gain some altitude simply by raising them at an angle, and this could explain why their anatomy is often underdeveloped when compared to modern birds.
A terrestrial bustard/crane like lifestyle was likely, which would have been quite interesting as it lived in a time when chaoyangopterid and tapejarid pterosaurs were at their prime. This further indicates that niche partitioning between “birds” and pterosaurs ensured little direct competition, although the absence of chaoyangopterids in the area Pelecanimimus lived in and the low diversity of flying ornithomimosaurs seems extremely suspect.