Re: Lewisuchus, a Dinosauriform

["Mickey Mortimer" <Mickey_Mortimer11@msn.com>]

George Olshevsky wrote-

> 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.

First of all, those characters were only those which are known to be absent
in Pseudolagosuchus, but present in dinosaurs.  The actual list is
longer.......
Characters absent in Lagerpeton, but present in Marasuchus, Pseudolagosuchus
and dinosaurs-
1. fossa trochanterica in femoral head
2. anterior trochantor present
3. trochanteric shelf
4. cnemial crest present
5. distal tibia quadrangular, with medial and posterior surfaces forming
obtuse angle distally
6. distal tibia with lateral longitudinal groove
Characters absent in both Lagerpeton and Marasuchus, but present in
Pseudolagosuchus and dinosaurs-
7. pubis more than 70% of femoral length
8. pyramid-shaped ascending process of astragalus, with posterior
subvertical facet and an elliptical depression behind
9. metatarsal IV sigmoidally curved in anterior view
Characters absent in Lagerpeton, Marasuchus and Pseudolagosuchus, but
present in dinosaurs.
10. cervical epipophyses
11. deltopectoral crest extends more than 30% down humeral shaft
12. perforate acetabulum
13. brevis shelf present (not in herrerasaurids)
14. elongate ischial shaft with ventral keel restricted to proximal third
15. reduced tuberosity that laterally bounds the ligament of the femoral
head
16. tibia overlaps the astragalar ascending process anteroproximally and
posteriorly
17. calcaneum with concave proximal surface
18. distal tarsal IV proximodistally depressed and triangular proximally
All of the above are from Novas (1996).  Sometimes characters aren't known
for Pseudolagosuchus, and all except 10 and 11 are known to be absent in
Lagerpeton.  Dinosaurs include Eoraptor and herrerasaurids in the lists
above, although these taxa may be outside of Dinosauria sensu stricto.
Now, you're correct in stating that "lagosuchians" (Lagerpeton, Marasuchus)
lack phalanges on pedal digit V, while basal dinosaurs (Eoraptor,
herrerasaurids, basal sauropodomorphs, sauropods) have a phalanx.  Of
course, the loss of this phalanx in many other lineages (Pterodactyloidea,
Scleromochlus, Guaibasaurus+"Eutheropoda", Genasauria) shows that such an
occurance is not rare.  I wouldn't doubt Lagerpeton and Marasuchus lost the
phalanx due to their cursorial lifestyle.  However, it is still a valid
character.  Unless you can find more characters supporting such a
relationship, the eighteen above easily outmatch your hypothesis
parsimony-wise.  As a side note, no one is suggesting Lagerpeton or
Marasuchus are dinosaur ancestors, simply sister groups to various clades
including dinosaurs.  So it doesn't matter which characters they develop
once they split from the "main branch".  And even if basal dinosauromorphs
do have reduced fifth pedal digits all along the lineage, it could always
reverse at Dinosauria.

Mickey Mortimer