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Re: Acetabulum

At 03:08 PM 12/6/98 -0500, Alan Edels wrote:
>    1)    Do any other archosaurs (non-dinosaurs) have the acetabulum (a
>hole in the hip girdle, created by the intersection of the three hip bones)
>as a character?

Mixup in the terminology here: acetabulum = hip socket.  We've got 'em,
frogs go 'em, crocs got 'em, any tetrapod with hindlimbs attached to the
pelvis has got 'em.

What dinosaurs have is a perforate acetabulum.  This is much rarer.  Despite
early reports to the contrary, basal dinosauromorphs like _Lago/Marasuchus_
does not have a perforate acetabulum.  _Herrerasaurus_ has sort of a
"semi-perforate" acetabulum.

>    2)    Does the description/definition of a dinosaur indicate that the
>supra-acetabular crest must be present?

Definition, no.  Definition is "all descendants of most recent common
ancestor of birds and _Triceratops_".

Even if it is part of the diagnosis (still debated), NO character MUST be
present in any taxon.  Membership in a taxon is based on ancestry, not
diagnosis.  If a mammal were, through evolution, to lose lactation, it would
still be a mammal.

>    3)    Are there instances in dinosauria where the acetabulum has
>closed/sealed up (i.e fused the hip bones together), as we see the closure
>of fenestrae in Ceratopsians, and Ankylosaurs?

Ankylosaurs are a good example where it did close over.

>    4)    Did ALL dinosaurs have digitigrade stance (i.e. they walked on
>"tiptoe")? - Or were some "flat-footed"?
All known dinosaurs were digitigrade skeletally, although some (sauropods,
instance) cheated by adding fleshy tissue under the hindfeet, and others
walked plantigrade at least some of the time.

Remember, everyone: evolution happens.  All characters are subject to
further modification.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661