Did dinosaurs have any extra appendages or limbs?
Depictions of dinosaurs today may not be entirely accurate. The skeletal framework suggests a basic shape of the animal and provides enough rational for some scientific conclusions. However, examples from animals living today such as the Sperm Whale demonstrate how animal shape doesn't always conform to the skeletal structure such as a Blue Whale. Is it possible some dinosaurs may have had a spermaceti-type organ designed for a specific water or land-use during the period? The remains are not found because they decay like cartilage and flesh long before there is a chance of fossilization; literally leaving scientists and artists to fill in the gaps.
Modern animal fibrous models provide a general understanding of how an ancient animal might have appeared but do not guarantee by any means an entirely accurate reproduction. Another similar key understanding is that animals with extended sensory organs like an elephant's trunk decay away before locked into stone. In cases like an elephant trunk, clues to their existence in ancient animals come from the understanding of modern living animals and how they relate. This means animals discovered from dinosaur periods may actually look quite different from commonly accepted renditions.
Given the time-line between today's animal relatives and ancestors, it is not unreasonable to believe those ancestors might have had additional appendages or limbs which we may never know by scientific fact to have ever existed at all. Another great example of this is the T-Rex. The closest modern animal resembling a similar short upper arm configuration is the kangaroo. Given a top predator and carnivore, it's difficult to imagine a T-Rex posturing as kangaroos, and perhaps there was another significant purpose.
Could the small arms shown in nearly all Tyrannosaurus renditions actually be remnants of skeletal structure for a type of wing system or lower neck frill? Something like this would not preserve well in the fossil record, if at all, but the supporting bone structure remained. Pressure on the scientific community from the general public to produce visual reference of unearthed bone sets has the possibility of being rushed the process. Artistic interpretations have shaped common beliefs to become the accepted norm, even though they're artistic impressions without the actual specimen present for realism.
It would certainly be difficult and require a lot of time to determine if species of dinosaurs did have an extra apparatus, especially considering biological decay between flesh materials and bone without finding a completely intact specimen not completely lost to decay before fossilization.
† Article 8-10