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TWIST THUMB



Ken Kinman wrote:
<<Someone mentioned the twist-thumb synapomorphy.  Is it a synapomorphy
of Dinosauria or something more or less inclusive?  Segnosaurs apparently
lack it, so assume there are a lot of other dinosaurs that lack it as well.>>

Tim Williams responded with:
<<The only "twist-thumb" I know of is a synapomorphy of the Prosauropoda.>>

The twist-thumb I was refering to was the divergent first manual digit that
is seen in basal members of Sauropodomorpha, almost every member of Theropoda
(the , and aparently Heterodontosaurus tucki.

A quote from Bakker concerning H. tucki:
"There's a tale to tell about the famous clawed hand of Heterodontosaurus
found by Fuzz Crompton.  Pete Galton and I published a diagram in Nature
showing the sharp twist to the thumb claw, and, indeed, the specimen showed
this twist clearly then.  But the hand bones were broken by accident later,
and glued back incorrectly later so the first finger joint lost its twist
(you can see the mismatched glued ends in specimens and in casts).  Some
scholars have been mislead into thinking that there was no twist in this
animal." (page 472)

And a quote from Bakker and Galton:
"The hand of Triassic saurischian dinosaurs is distinctive (Fig 1) with a
very short stout metacarpal I; a long, strong thumb phalanx 1; a powerful,
curved trenchant thumb claw; thumb articulations forcing the claw to diverge
and point inwards during extension, converge with digits II and III and point
downward during flexion.  Manual digits II and III, subequal and subparallel,
bearing trenchant claws; and reduced digits IV and V (29).  The hand of the
better known primitive ornithischians (fabrosaurids and hypsilophodontids)
differed from the Triassic saurischians in lacking the long thumb and having
blunt hooves instead of claws on digits I to III (30).  Recently, a complete
articulated skeleton of Heterodontosaurus, a Triassic ornithischian that was
collected by A. W. Crompton, now at Harvard for the South African Museum.  
The hand is virtually identical to those of Triassic saurischians in all six
of the features cited above.  Although specialised cranially (31),
Heterodontosaurus probably represents the original ornithischian hand
pattern, inherited from saurischians, where the thumb was specialised for
defence and digits II and III were used for defence, digging and support
during slow, quadrupedal locomotion."

By the way, Santa Luca is surely the mislead scholar Bakker was refering to
in the first quote, as his 1984 description states that the thumb twist is
not present.

Pete Buchholz
Tetanurae@aol.com

--------------------------------

Bakker, R T.  1986.  The Dinosaur Heresies.  Kensington Publishing Corp, New
York.  Pp 1-481.

Bakker, R T and P M Galton.  1974.  Dinosaur monophyly and a new class of
vertebrates.  Nature 248:168-172.

Santa Luca, A P.  1984.  The postcranial skeleton of Heterodontosaurus tucki
(Reptilia, Ornithischia) from the Stormberg of South Africa.  Annals of the
South African Museum 79(7):159-211.