Re: Archaeopterix

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>They could be the degenerate form of a flight-capable ancestral bird,
>having taken the first evolutionary steps toward complete flightlessness.
>
>That would be interesting, because there are affinities between Archaeopterix
>and the long-armed maniraptor dinosaurs, most of which date from AFTER
>Archaeopterix's appearance in Jurassic strata (don't they?)

Just because most of the FOSSILS of "maniraptoriform" theropods occur in
the Cretaceous doesn't mean that they weren't around in the Middle
Jurassic.  The Middle Jurassic is one of the least well known periods of
dinosaur history, especially for smaller species.  It may be that there are
many species of proto-bird, proto-dromaeosaurid forms waiting to be
discovered.

>Lets see where this takes us:
>In the early Jurassic, a branch of the theropods evolves feathers and flight,
>producing birds.  One branch of early birds GIVES UP flight, producing
>Archaeopterix and evolving into more highly developed flightless maniraptoran
>dinosaurs.  These guys survive the Jurassic-Cretaceous extinctions and move
>into the space vacated by the allosaurs.  In the mid-Cretaceous, a branch of
>the maniraptors gives up long arms and becomes gigantic.  Thus, in this
>scenario you could say Tyrannosaurus evolves from birds!

Contra Bakker, allosaurids did very well in the Early Cretaceous, thank
you.  Acrocanthosaurus and Chalantaisaurus (for example) show
tyrannosaur-sized allosaurids living in the mid-Cretaceous.  Gigantism in
the tyrannosaurid-line probably didn't begin until the beginning of the
Late Cretaceous.

>Is there any plausibility to this idea?  Could Archie's features be
>considered primitive w.r.t. mainstream maniraptorans?  Are there ancestors
>known for the earliest maniraptors, antecedent to the Solhofen deposit?
>
>Sorry if this is tediously naive speculation, but you must admit
>this idea would be interesting if true (and interesting anyway?)

This is basically Greg Paul's hypothesis in _Predatory Dinosaurs of the
World_: that dromaeosaurids, oviraptorids, ornithomimids, etc. are
descended from Archaeopteryx-like volant (flying) theropods.

However, not all of Archaeopteryx's characters are primitive.  Archie
shares many specialized characteristics with just dromaeosaurids and birds,
showing that it isn't ancestral to all other maniraptoraforms.  The
ancestral manriaptoraform may have been Archaeopteryx-like to a certain
extent, but would lack the transformations of the pelvis, the hindlimb, the
tail, etc.

As to there being earlier forms - very likely (actually, almost
certainly!), but they have yet to be discovered.  So, keep a lookout, the
next time your walking over Middle Jurassic sediments!!

                                
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.