Caudal spinous processes
In addition to limb structure and centre of mass, a
third key avian structure that might serve as an objective
adjudicator between dinosaur and bird is the height of
spinous processes on tail vertebrae. Theropods had tall
spinous processes used for muscle attachments, resulting in
powerful and finely controlled tail movements. But flying
birds generally have few or no spinous processes on their
caudal vertebrae. Therefore, somewhere along the supposed
evolutionary line, theropods dropped the spinous processes.
Those dramatic reductions in muscle attachments and in
mass surely reduced control of the tail. Wouldn’t its fully
tailed cousins outcompete it on tight turns while chasing
food? This means that natural selection would tend to keep
the heavy tail musculature instead of moving the centre of
mass forward for flight.
Admittedly, spinous processes
may not help classify all the feathered
dinosaur candidates, since some
extinct ground-based birds may have
had somewhat prominent ones. But
could they at least help identify flying
birds in the fossil record? For example,
Microraptor’s long, bony tail vertebrae
had no spinous processes, consistent
with a flying, not a primarily running
anatomy. It did have feathers connected
to its hind legs. It had bird feathers,
possibly bird fixed thighs, and non-theropod caudal vertebrae. So why not
classify it as a four-winged, extinct,
feathered, running/gliding bird?
Probably the most definitive
anatomical characteristic of dinosaurs
is their open (or ‘perforate’) acetabula.
The three hip bones ilium, ischium,
and pubis join at the acetabulum—
the socket into which the head of the
femur articulates. Other tetrapods,
including non-dinosaur reptiles,
mammals and birds, have a closed,
cup-shaped acetabulum, which derives
from the Latin for ‘little vinegar cup’.
All (and only) dinosaurs had no bone
at the back of their acetabula. Figure 3
shows two dinosaurs’ open acetabula.
Thus, a definitive feathered dinosaur fossil should show preserved,
branching feathers associated with a skeleton having an
open acetabulum. Archaeopteryx, Scansoriopteryx, and
Microraptor had partially open but largely closed acetabula7
unlike those completely open in dinosaurs and unlike those
completely closed in modern birds. In conformity to their
many other bird-like features such as feathers, these at least
seem better described as odd, extinct birds than as ‘feathered
dinosaurs’. Has anyone found a fossil with a completely
open acetabulum plus feathers? If so, why not simply let
that evidence cut to the chase of changing feathered dinosaur
Birds use flow-through, one-way lungs, connected
to air sacs and even to their hollow bones. This system
Figure 3. Perforate acetabula in dinosaurs. A) Steel armature travels through the perforate
acetabulum seen in an original fossil juvenile Edmontosaurus. Bones of the ornithischian hip girdle
are also labelled. B) Author’s (BT) fingers extend through the open acetabulum in a replica fossil adult
Stegosaurus (also order Onithischia). The acetabulum is proposed as an anatomical indicator of
feathered dinosaurs. Note also the prominent caudal spinal processes for tail muscle attachments.
Birds lack these processes.