A field guide to mesozoic birds and other winged dinosaurs

Eudromaeosaurians
“True dromaeosaurs”, these mid-sized to giant flightless ornithodesmids
are well known from Late Cretaceous deposits, but fragmentary
fossils and isolated teeth show that the group originated in at least the Late
Jurassic. Their lower legs were greatly reduced, limiting running speed significantly,
but accommodating robust musculature to support the sickle
claw, which was employed in grappling and subduing large prey. The sickle
claws themselves were highly flattened and more suited to cutting and
hooking than the somewhat broader claws of most other ornithodesmids.
Almost all seem to have been hypercarnivores specializing in taking prey
larger than themselves. Based on footprint evidence and fossil assemblages
of multiple individuals, many seem to have been gregarious, travelling together
in small flocks. As in modern flightless birds relatively far removed
from their flying or gliding ancestors, their feathers had probably reverted
to open-vaned plumes with non-interlocking barbs in many cases, and large
species inhabiting hot, arid-environments may have lost feathers on some
parts of their bodies for more effective temperature regulation. In modern
birds, this loss of feathers helps facilitate heat shedding, but can also cause
overheating. In many cases, featherless portions of the body can be partly
or completely covered by the wing feathers when necessary.
Several lineages of eudromaeosaurians independently evolved large
to gigantic sizes (a similar trend is seen in the evolution of the giant unenlagiines
of the genus Austroraptor and giant itemirines like Itemirus). In both
instances, these huge species are found during the early Cretaceous period,
and may have competed with and ultimately lost out to deinodontids for
dominance of the giant carnivore niches left by the waning of the allosauroids
in the Northern Hemisphere.
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