_Much_ more on Digit Loss in Theropods

["David Marjanovic" <david.marjanovic@gmx.at>]

----- Original Message -----
From: <NJPharris@aol.com>
Sent: Monday, October 22, 2001 11:54 PM
Subject: Re: More on Digit Loss in Theropods

> My memory is that the prepollex and prehallux are retentions of the digits
> derived from the line of development that passes through the radius (all
> digits in most tetrapods are derived from a line of development passing
> through the ulna), which are resorbed early in other tetrapods.

AFAIK they are just the results of the phenomenon that the line of
development that passes through the ulna doesn't stop growing as early as
"usual". However, I've found something on the embryonic praepollices of
_*BIRDS*_:

Unfortunately I don't know the ref because I got the pages as photocopies.
The book is from 1979, in German, and I'll translate from the chapter "Der
VogelflÃgel" (The bird wing) from p. 619 on. I'll look for the book in the
biosciences library.

"It was debated for a long time which fingers the three rays of the bird
hand correspond to [note the past tense in 1979]. Embryological and
paleontological finds [dinosaurs are automatically assumed to be the _sister
group_ of birds or suchlike later because the author wants arboreal bird
ancestors] have shown that the preserved fingers correspond to the rays I,
II and III (fig. 4729. In the embryonic bird hand all five rays can be found
(STEINER). The anlage of the bird hand looks equal to the embryonal reptile
hand. The radius anlage lies at an angle to the ulna anlage and continues
into a praecarpale (praepollex). The fourth finger is developed most
strongly in the early embryonic hand [so far Feduccia is right]. Metacarpal
IV remains for a long time and becomes cartilaginous. It can probably still
be included into the ossification of mc III. The fifth finger ray is clearly
visible in embryos of *Anas* and *Anser* on the 6th -- 7th day of brooding
(fig. 472). However, it remains [...] [whatever "blastematÃs" means] and
doesn't become cartilaginous at first. The basal [ = proximal] part of the
fifth ray [...] [baroque wording] is secondarily included in the carpal
skeleton.
        The definitive carpal skeletal elements of birds don't correspond to
the primary carpals (STEINER). The primarily [...] [hm, "angelegt".
Differentiated? Begun?] carpals show the typical tetrapod pattern. The
intermedium branches from the ulnar ray and continues into the Ist and IInd
fingers. The radial, two centralia and the ulnare have been shown to be
present. The intermedium is completely reduced early. The radial and the
centrale radiale fuse and form the radial carpal element of the definitive
wing (fig. 473). The centrale ulnare and the ulnar are likewise reduced.
Four distal carpals occur in ontogeny. First the distal carpals 1 and 2 as
well as 3 and 4 fuse. Then the carpals 1 and 2 fuse to the proximal end of
metacarpal I and disappear in it. Carpals 3 and 4 join metacarpal III. The
unified skeleton piece finally fuses with metacarpal II to form the
so-called os metacarpi. The basal part of the fifth ray is preserved and
grows bigger. It moves plantarly from the ulnar forwards, chondrifies [if
that word exists :-) ] and ossifies afterwards and becomes the ulnar carpal
element of the definitive wrist (Fig. 473). These changes of the primary
carpus of birds can be read directly from ontogeny. The old interpretation
of the radial carpal bone as an intermedio-radiale is therefore [...]
[superfluous, wrong, falsified]. The loss of the intermedium has begun early
in the archosaur stem (â*Protorosaurus*). Coossification of the distal
carpal elements to at first two bones is also present in ancestral
archosaurs [strange, never heard of that]. Likewise the reduction of the 4th
and 5th fingers begin long before the evolution of flight. In the Dinosauria
similar changes [...] have evolved in parallel evolution independently from
the bird stem [sic]. There too the first three fingers are developed much
more strongly than the 4th and 5th. The fifth ray is reduced, but its basal
part is short and thick in â*Thecodontosaurus* and â*Plateosaurus* and begin
a new differentiation. The fusion of the radial and the centrale radiale
begins early in reptiles, too [no word where exactly]. Thus the phylogenetic
development runs the same way as the ontogeny of the skeleton in the bird
wing. In â*Archaeopteryx* the reduction of the 4th and 5th fingers is
already reached [big surprise]. Finger I is strongest, finger II longest.
The metacarpalia I to III are still free. [Rhymes not intended.] The number
of phalanges is still 2-3-4 in â*Archaeopteryx* as in primitive forms. It is
unknown whether a rudiment of the 4th metacarpal is still preserved [in the
living animal, not the fossil, I think]. The carpus isn't fully preserved in
the remains of â*Archaeopteryx*. A distal piece is thought to be the fusion
product of the distal carpals. A rudiment of the fused radial and centrale
radiale is likewise preserved. Whether a free ulnare was still present is
unknown."

Too bad I can't send the figures (but if I get the scanner to work again I
can send them offlist).
Fig. 472 shows the hand skeletons of goose embryos at 7 1/2 and 8 1/2 days,
"redrawn from STEINER, 1934". At 7 1/2 days radius, ulna, radial, centrale
ulnare and radiale, intermedium, ulnare, distal carpals 1 -- 4 (in an
unambiguous row) and mc II and III are cartilaginous, while I and V are
visible ("blastematÃs"). Mc II is already in a straight line with the ulna.
Directly proximally to the radial there is an empty space labeled prepollex,
while the thumb lies farther laterally. At 8 1/2 days mc I and IV (very
small), phalanges II-1 and III-1 and a bone labeled mc V have become
cartilaginous in addition; mc V is a small grain of cartilage that lies
_proximally_ to the ulnare, so I wouldn't be surprised if it turned out as
the distal carpal V or the pisiform. The similar-sized intermedium touches
the ulna.
Fig. 473 shows the hand skeleton of an embryonic *Sturna* at beginning
ossification and that of *Archaeopteryx*. In the former radius, ulna,
radial, ulnar, dc 1 + 2, dc 3, mc I, II, III and V (not IV which is
apparently assumed to be fused to III) and phalanges I-1, II-1, II-3 and
III-"1" are ossified and labeled; I-2 and II-3 are shaded in grey, don't ask
me why. The 2 preserved carpalia of Archie are labeled dc 1 + 2 and radial.