>Lemurs (lemuriforms) ARE primates. But indeed some other mammals have opposable
>thumbs, such as the opossum (which is a marsupial). I never heard about
>Dilophosaurus having an opposable thumb. To my knowledge, the only theropods
>with opposable digits are troodontids and possibly ornithomimosaurs (other
>small theropods maybe?). Any one care to comment on this? Tom? Tony? Stan?
>Anybody?
Reports of opposable thumbs in theropods have been highly exaggerated, and
mostly based on incorrect restoration of troodontid, ornithomimiosaur, and
dromaeosaur hands. In fact, the average "maniraptoran" condition (found in
troodontids, dromaeosaurids, elmisaurids, oviraptorids, Archaeopteryx,
etc.) is one where the thumb (digit 1) flexes on only a slightly different
plane than digits 2 and 3. In ornithomimosaurs, the digits all flex in the
same plane, forming the "hook-and-clasping" structure various authors have
described. Although they are not truely opposable, many theropod hands
(including Dilophosaurus) could grasp.
The dinosaurs that do have opposable digits are derived iguanodontian
ornithopods (Iguanodon, Ouranosaurus, Hadrosauridae, and a few others). In
these forms, it is digit 5 (the "pinky) which is long, flexible, and could
be brought in opposition with the "palm". Digits 2, 3, and 4 form the
hooved "mitten" the animals would occasionally stand upon, and digit 1 was
either a spike (in Iguanodon, Ouranosaurs, etc.) or lost altogether (in
Hadrosauridae).
Thomas R. Holtz, Jr.
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile Phone: 703-648-5280
U.S. Geological Survey FAX: 703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA 22092
U.S.A.