Re: Lewisuchus, a Dinosauriform

[Dinogeorge@aol.com]

In a message dated 7/23/01 1:00:26 AM EST, Mickey_Mortimer11@msn.com writes:

<< True, but regardless of whether herrerasaurids and Eoraptor are dinosaurs,
 Pseudolagosuchus is more basal than either (Novas, 1996).  This is based on
 the absence of-
 - cervical epipophyses.
 - reduced tuberosity that laterally bounds the ligament of the femoral head
 - tibia overlaps the astragalar ascending process anteroproximally and
 posteriorly
 - calcaneum with concave proximal surface >>

I'm not at all convinced (especially by this roster of relatively trivial
anatomical features) that "lagosuchians" are basal to dinosaurs. Sauropods
have five-digit hind feet, but as I recall lagosuchians are derived relative
to this in having four-digit hind feet with a vestigial metatarsal V. So no
known "lagosuchian" can serve as a dinosaur ancestor. They would have had to
lose their fifth digit independently--which they might well have--but it's
more interesting to consider them as branching off the theropod lineage
between herrerasaurians and theropods. Lagosuchian feet were very
theropod-like in having long, slender, bundled metatarsals and a splint
metatarsal V. Of course, then the perforated acetabulum would have developed
independently in theropods from all other dinosaurs, but since it developed
independently in at least one other archosaurian clade, that's not such a big
problem.