ON DINO-BIRD HANDS (long)

[GSP1954@aol.com]

I have now read the Burke-Feduccia Science paper on the alledged nonhomology
of theropod and bird hands - a subject I have worked upon (1984a) - and the
supporting viewpoint by Hinchliffe. As I suspected, it gives no reason to
challange the fact that birds are as much glorified dinosaurs. Instead, we
are seeing the last gasps of a dying hypothesis. 

To start with, the fossil evidence clearly shows that the outer digits are
lost theropods, leaving only 1-II-III in avetheropods, with I always being a
strong, big clawed thumb weapon (except when the arm is hyper-reduced). In
herrerasaurs V is a wee splint and IV is just two small bones, there is no
practical way to make IV into a strong finger with five bones including a
large claw, while losing the big thumb, etc. 

The anti-dinosaur group keeps stressing that digits are normally reduced
symmetrically, and that this should be true of birds as well. Of course in
the next breath they acknowledge that dinosaurs lost only the outer fingers,
so there is no reason the other group of bipedal archosaurs, the birds, could
not have done the same. 

The basic problem is that even in the earliest bird embryos there are only
four digits, the other is entirely lost. If there were five digits and we
could watch which ones were lost there would be no problem. At it is there is
currently no way to reliably number the digits in bird embryos. Doing so
requires a number of untestable assumptions. 

At the same time, we do not have any avetheropod embryos to examine. It is
quite possible that they grew their fingers in exactly the same manner as
baby birds, with the well developed digit opposite the ulna being III, rather
than IV as common in other tetrapods. After all, in adult avetheropods the
digit opposite the ulna IS number III. 

Why the avetheropod-bird clade would initially emphasize the development of
III rather than IV is obvious. Loss of digit one in even the embryos would
leave a big gap between the pisiform in the side of the wrist and metacarpal
IV, unless the other digits shifted laterally. So IV would be were V was, and
III would be were IV was. If the shift is not made from the get go, it is
going only going to have to occur at some point later. Also, IV will be
entirely lost. To follow the usual tetrapod finger growth pattern would
require IV to grow large in embryos, then completely lose it later on (Burke
& Feduccia say that some lizards sharply reduce the size of IV, but it is not
completely lost. Does anyone know what happens to digit IV in horse embryos?)
 This would be a waste of growth energy, and natural selection does not work
to make finger burds convenient for embyrologists to count, but to maximize
efficiency of growth. The severe asymmetry of finger growth in
theropods-birds should have forced them to reconfigure the growth pattern, so
that III is initially emphasized rather than IV, and the latter is never more
than a stub before it is eliminated. 

The problem is that some embryologists expect digit IV to be large because it
is so in animals with symmetrical finger reduction, and some want it to be
IV, so they say it is.  Even though strong asymmetric finger growth could be
expected to result is important changes in embryonic growth. As it is, there
is no conclusive evidence that birds retain digits II-IV rather than I-III,
there is no way to compare avian and dinosaur hand embryology, so the problem
is untestable. 

Another thing the anti-dinosaur group does not have is a fossil record that
in any way supports symmetrical reduction of fingers in protoavians. Of
course this is because nondinosauruan ancestors of birds did not exist! There
is a wonderful fossil trail of asymmetrical finger reduction in dinosaurs
leading to the avian condition. 

Science is always partly political, and it is important to understand the
deep bias of some against birds being dinosaurs. For example, Feduccia (1996)
claims that the hand of Archaeopteryx "does not closely resemble that of a
theropod dinosaur". This when the urvogel's long raptorial hand with a
semi-lunate carpal block, and three gracile digits, is clearly a diminutive
version of a dromaeosaur manus! Feduccia cannot point to any nondinosaur that
has a hand anything at all like those of birds. 

There are other errors in the B&F paper, and the Hinchliffes note. In
defining Aves B&F cite the presence of feathers, opisthopubic pubis with
hypopubic cup and scapula articulating with coracoid at 90 degree angle. 

The most strongly reflexed coracoids to be found outside Aves are in
dromaeosaurs, troodonts and oviraptors, in which the coraocoid is large,
vertical, the outer surface faces anteriorly, and it articulates with the
anterior edge of the broad sternal plate via transverely long articulations
(Barsbold 1983, Paul 1988, Russell & Dong 1993, Norell et al 1997). This is
the avian condition, and the possession of a large sternal plates in these
"dinosaurs" makes them more avian than Archaeopteryx! 

I used to illustrate Archaeopteryx with a strongly retroverted pubis, but all
the new specimens show it ain't so, the pubis was nearly vertical. The pubis
was more strongly retroverted in dromaeosaurs (Barsbold 1983, Paul 1988). The
pubic shafts of dromaeosaurs and Archaeopteryx are thin plates than meet at a
shallow angle, a feature found in no other archosaurs. 

As for the feathers, Feduccia in 1996 (under one of my illustrations) said
that "there is no evidence that any dinosaur possessed feathers" which was
true at the time, but then there was no evidence that they did not. He
continued "feathers are absolutely unique to birds". That was an
unsubstantiated opinion based on a lack of fossil evidence. If and when it is
shown that theropods did have simple but true feathers then all the fuss
about digit counts will fade away. 

Hinchliffe dredges up the old saw that theropod arms were too short for them
to be protobirds. Never mind that the arms of Deinocheirus are 2 meters(!)
long, or that in dromaeosaurs and oviraptors the long arms are as long
relative to the hindlimbs as in Euparkeria and other small early archosaurs.
Never mind that the only long armed obligatory bipeds other than birds are
theropods. 

Hinchliffe then states that semi-lunate carpals are rare in dinosaurs.
Actually they are known in Coelurus, dromaeosaurs, troodonts, oviraptors, and
therizinosaurs. Even if they were rare, what does that have to do with the
price of tea in China? That would only mean that those few theropods are the
most probable relatives of birds. Notice how Hinchliffe takes an avian
feature of theropods, the lunate carpal, and manages to use misleading
rhetoric to turn it into a reason to think that dinosaurs are not close to
birds! Using that kind of illogic, they will soon have birds allied with
flying fish!  

Then Hinchliffe repeats that the most bird-like dinosaurs are Cretacous forms
that appear after Late Jurassic Archaeopteryx. Well, there is Coelurus with
that "rare"  semi-lunate carpal in the Late Jurassic. Troodont teeth have
been identified from the same period (Chure 1994). A few dromaeosaur -like
bones are known from the same time (Jensen & Padian 1989). Possible
dromaeosaur teeth have been found in the Middle Jurassic (Evans & Milner
1994). Gosh, could have been a whole bunch of little dino-birds skittering
around the last half of the Jurassic for all we know. But small predators
near the top of the food chain are rather scarce and difficult to preserve -
that old fossil record is spotty, after all. 

As the last subject points out, it is apparent that those who oppose
dinosaurs as bird ancestors are not sufficiently familiar with the record and
anatomy of theropods. I suggest that they compare the paraccipital process of
Archaeopteryx to Dromaeosaurus, and note the extreme similarity in the twist
of the process and the subrectangular distal expansion that results in a deep
auditory meatus not observed in other archosaurs (Currie 1995). This is just
convergence? No, Archaeopteryx is a small, flying dromaeosaur. 

On a related subject, just read Hecht's article in New Scientist Oct 18 on
the proof that the short armed (relative to Archaeopteryx) dino-bird was
feathered, and the possibility that it was secondarily flightless.
Considering that it had contour feathers (that may have first evolved for
flight) but was flightless makes this a good possibility. Of course, moi was
the first one to argue that some Cretaceous dinosaurs were secondarily
flightless (1984b, 1988). 

GSPaul

Barsbold R 1983 Joint Soviet-Mongolian Palaeont Exp 19:5
Chure D 1994 BYU Geology Studies 40:11
Currie P 1995 J Vert Paleo 15:576
Evans S & Milner A 1994 p 303 in Frase & Sues eds, In the 
 Shadow of the Dinosaurs
Feduccia A 1996 The Origin and Evolution of Birds
Jensen J & Padian K 1989 J Paleont 63:364
Norell M et al 1997 Nature 389:447 see fig 1
Paul G 1984a Nature 310:732
Paul G 1984b p 175 in Reif & Westphal eds, Third Symp 
  Mesozoic-Terrestrial Ecosyst 
Paul G 1988 Predatory Dinosaurs of the World
Russell D & Dong Z 1993 Canadian J Earth Sci 30:2163 see figs  
  1a & 2a