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Marasuchus (formerly Lagosuchus)



nking.ucs@smtp.usi.edu wrote: 

> _Lagosuchus_ was close to, but not quite, a dinosaur.  I often see
> the character "perforated acetabulum" as defining dinosaur, but
> _Lagosuchus_ had that trait (and, yes--I know there are even other
> non-dinosaurian reptiles having perforated acetabula).  Then, I've
> seen three sacral vertebrae specified for dinosaur, but apparently
> _Herrerasaurus_ and _Eoraptor_ still had only two "fully
> incorporated" (as labeled by Sereno and Novas) sacral vertebrae.

> So, I'm not sure what to say it REALLY is that shows _Lagosuchus_ is
> not a dinosaur.  Can anyone help me with this one?  Thanks in
> advance.

First of all, Sereno & Arcucci (JVP 14(1):53-73) showed that
_Lagosuchus_ is most likely a _nomen dubium_ because the holotype
specimen of the type species _L. talampayensis_ is insufficiently
diagnostic and may even be a chimera of more than one taxon. What we
have been thinking of as _Lagosuchus_ is based on referred material
now considered to belong mainly to the species _Marasuchus lilloensis_
(formerly _Lagosuchus lilloensis_). Incidentally, this makes the
family name Lagosuchidae a nomen dubium, too. Perhaps it's time for a
different name for this family? (The ordinal or subordinal name
Lagosuchia can still stand, however, because it is not required to
have a type genus.)

Your doubts about the non-dinosaurian status of _Marasuchus_ (and
other lagosuchians) are well founded. Not only was _Marasuchus_ a
dinosaur, it was a primitive theropodomorph (i.e., in my system, a
primitive avian). It had the following combination of characters in
common with other primitive theropodomorphs (e.g., herrerasaurians):

(1) Narrow pelvic girdle (though broader than in later theropods)
(2) Fully erect hind limb (not just _nearly_ fully erect, as in
prosauropods and early ornithischians) with inturned femoral head,
although the head was not completely inturned at a right angle
(3) Ascending process on acetabulum (but a short one)
(4) Middle three metatarsals "bundled"
(5) Middle three metatarsals about twice the length of metatarsal I
(6) Metatarsal V vestigial
(7) Pedal digit V absent
(8) Cervical column upwardly inclined from a slightly upwardly inclined,
near-horizontal dorsal column

There are other dinosaurian characters present in _Marasuchus_, but
they're present in primitive phytodinosaurs as well as avians
(theropodomorphs) and could be interpreted as dinosaurian
plesiomorphies. The characters listed above are derived relative to
the states in the earliest phytodinosaurs and thus, if present in
later phytodinosaurs, appeared there independently. A furcula would
clinch the theropodomorph status of _Marasuchus_ beyond doubt, but so
far none is known. On the other hand, a pair of clavicles _not_ fused
into a furcula could support placement of _Marasuchus_ below the
common ancestor of the dinosaurs (I'd consider it a reversal,
however).

On this topic, Thomas_R_HOLTZ@umail.umd.edu (th81) wrote:

] _Lagosuchus_ (or _Marasuchus_) is not a dinosaur because the most
] parsimonious distribution of derived character states yields a
] position outside of the node joining birds and _Triceratops_ (or
] _Megalosaurus_ and _Iguanodon_).  It is very close to that group,
] but has not yet been demonstrated to lie within that clade (and
] hence is not a dinosaur).

Umm--is anyone else out there not entirely satisfied with this reply?