Re: Lewisuchus, a Dinosauriform

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

Ken Kinman wrote-

>      Well, some workers still think herrerasaurs are basal dinosauriforms
> rather than theropods, and that even Eoraptor may be more derived than
> herrerasaurs.

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

> P.S.  I could be wrong, but I don't think it matters if Lagosuchus is a
> nomen dubium.  Family Lagosuchidae could still be the valid family name.

Hmm.  I'm actually unsure of what the ICZN says about this.  Is it required
to have a diagnostic eponym for a group?
In any case, Lagosuchidae certainly shouldn't be used yet.  As Lagosuchus
has no certain phylogenetic position within Dinosauromorpha, using a family
based on it would be pointless.  Mike Keesey said it perfectly- "In a
cladistic taxonomy, there would be no reason for a Lagosuchidae, since
Lagosuchus is not known to clade with any other genus".
Of course, we must ask if Lagosuchus is even indeterminate or not.  The
holotype consists of vertebrae, hindlimbs and a probably associated
forelimb.  The forelimb is certainly distinct from Marasuchus (narrow
scapular blade; radius subequal to humerus; ulna very broad distally), so
belongs to a different taxon (Sereno and Arcucci, 1994).  The problem is
that it may not be correctly associated with the vertebrae and hindlimbs.
Sereno and Arcucci do not mention that the hindlimb proportions are
different from Marasuchus as well (tibiofemoral ratio 5-6% less;
metatarsofemoral ratio 7-11% less; metatarsal I/III ratio 10% more).
Lagosuchus is only 3% smaller than a Marasuchus specimen measured for these
ratios, so ontogeny can't explain it.  Perhaps this indicates the hindlimbs
are also not referrable to Marasuchus, though I would need more evidence to
be certain.   Forelimbs of Lagerpeton and Pseudolagosuchus are unknown, but
the pes clearly lacks the derived asymmetrical morphology of the former
genus.  Comparison to Pseudolagosuchus is difficult, but the tibiofemoral
ratio is 8% more.  Lagosuchus is only 36% as long as Pseudolagosuchus, so
ontogeny certainly could be a factor in their differences.  Luckily, we can
tell that the pubis of Lagosuchus is shorter than the
Pseudolagosuchus-Dinosauria clade, being less than 40% of femoral length.
The distal limb elements are much too short to be Saltopus (tibiofemoral
ratio 24% less; metatarsofemoral ratio 28% less).  If the forelimb belongs
with the hindlimb, I say it's a valid taxon that can be distinguished from
other related genera.  I would tend to say the same thing about the hindlimb
alone, but am wary about distinguishing it from Marasuchus.

Mike Keeesey wrote-

>That I have not seen. I've always seen _Eoraptor_ more basal than
>_Herrerasauridae_, whether those two taxa are inside _Dinosauria_ or
>outside. Who placed _Eoraptor_ "above" _Herrerasauridae_?

Tom Holtz listed these in 1998-
Characters found in Eoraptor and Neotheropoda, but not Herrerasauridae:
Reduced contact between premaxilla and maxilla
Distal carpal V absent
Reduced overlap of dentary onto postdentary bones
Pubic blade greater than six times as long as broad
However, in his 2000 GAIA paper, he writes "No characters were observed
which were present in Eoraptor and basally within Neotheropoda that were not
also present in herrerasaurids."
Maybe Tom can explain......

Mickey Mortimer