When Balaur was described in 2010 it was initially thought to be a dromaesaurid closely related to Asian forms like Velociraptor. With its particularly stocky legs built for strength rather than speed, two-fingered hands, and two large sickle claws on each foot, it was interpreted as a weird highly specialized predator terrorizing the other Hațeg Island species at the end of the Cretaceous. Although only 1.8m long (5’10”), it was hypothesized to have taken down prey much larger than itself with powerful slashing kicks.
But later analyses cast doubt on this interpretation.
A lot of the anatomical features of Balaur’s skeleton were odd for a dromaeosaurid, but matched those of avialans – a group of close evolutionary “cousins” to the dromaeosaurids, containing Archaeopteryx and the common ancestors of all modern birds. And, by 2015, multiple studies had confirmed Balaur wasn’t really a “raptor” but instead a little further along on the bird lineage.
So now our picture of this dinosaur is very different: a chunky-bodied island bird, grown large and secondarily flightless sort of like a Cretaceous equivalent to the dodo. Its double sickle claws were probably adaptations for climbing and perching in trees, and based on similar avialans it was likely a herbivore rather than a hypercarnivore.
By far the biggest island in the Late Cretaceous European archipelago, the Ibero-Armorican island (sometimes also known as the Ibero-Occitan island) was made up of most of the Iberian Peninsula and France and was larger than modern-day Madagascar.
Around 73-71 million years ago one of the residents of this island was the aptly-named Gargantuavis – the largest known Mesozoic bird, and probably an example of island gigantism.
Although only known from a few isolated bones, it’s estimated to have been slightly larger than a modern cassowary, somewhere in the region of 2m tall (6′6″). At that size it would have also been secondarily flightless, which is surprising for a bird that was living alongside larger fast-moving theropods like abelisaurs.
Not much else is known about it due to the scarce remains, but it seems to have had a long slender neck and probably had a small head. Its hips were fairly broad, suggesting it wasn’t capable of running very fast, and it was likely a slow-moving herbivore that was a fairly rare member of its ecosystem.
Exactly where it belongs in the bird evolutionary tree is also unclear, with the best current guess being “some sort of euornithean”.
Enantiophoenix electrophyla, an enantiornithean bird from the Late Cretaceous of Lebanon (~95 mya).
It was similar in size to a modern starling, around 20cm (8″) long, and although only known from a fragmentary fossil it had fairly chunky leg bones with large claws. It was probably a strong percher like most other avisaurid enantiornitheans.
Several tiny pieces of amber were also found within the fossil, which have been suggested to be stomach contents. This could perhaps be evidence of Enantiophoenix feeding on tree sap like modern sapsuckers, but without a known skull it’s hard to tell for certain whether it was specialized for that sort of diet or not.
Longipteryx chaoyangensis, an enantiornithine from the Early Cretaceous of China, about 120 million years ago. With a body length of only around 15cm (6″), it had a long snout tipped with a few hooked teeth and feet capable of perching – features that indicate it may have lived very similarly to modern kingfishers, feeding on fish and small invertebrates in its swampy forest habitat.
The enantiornithines were a sort of “cousin” lineage to modern birds. Most had toothy jaws and clawed wings, and the wide variety in their skull shapes suggests that they were specialized for many different dietary niches. The entire group went extinct during the K-Pg mass extinction and left no living descendants, but during the Cretaceous they were the most widespread and diverse group of birds*, with fossils currently known from every continent except Antarctica.
* Depending on how you define “bird”.