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Re: Marasuchus (formerly Lagosuchus)



nking.ucs@smtp.usi.edu wrote:

> About _Marasuchus_, Dinogeorge wrote (4/18/96; 4:20p):

>> It [_Marasuchus_] had the following combination of characters in
>> common with other primitive theropodomorphs (e.g., herrerasaurians):

>> (6) Metatarsal V vestigial
>> (7) Pedal digit V absent

> I just read in The Dinosaur Society's Dinosaur Encyclopedia, and
> I've seen it elsewhere, that herrerasaurids had five toes.  So,
> actually, it's five metatarsals, and only four toes?

That Dinosaur "Encyclopedia" is notoriously inaccurate factually, but the
pictures are quite good...

I checked (it always pays to check--the old memory ain't what it used
to be) and in both _The Dinosauria_ and Novas 1993 (JVP 13[4]) the
foot of _Herrerasaurus_ is shown with a single vestigial phalanx on
pedal digit V.  Since the rest of the foot strongly resembles a
slender prosauropod foot in the proportions and conformation of its
metatarsals (the middle three metatarsals are not "bundled" as in
theropods and all five metatarsals overlap proximally), it is actually
_less advanced_ toward the theropod condition than in
_Marasuchus_(!). This makes sense in terms of the manus as well, which
is _Herrerasaurus_ retained a vestigial metacarpal V.  (Unfortunately,
the manus of _Marasuchus_ is unknown.) I would now put _Marasuchus_ a
shade higher up the avian portion of the dinosaur cladogram than
_Herrerasaurus_, something like this:

Dinosauria-------------------------------------------------Phytodinosauria
      "furcula present"
            Aves-------------------------------------------Longisquama
        "pedal digit V vestigial"
              \--------------------------------------------Herrerasaurus
          "bundled metatarsals II-IV; pedal digit V lost"
                \------------------------------------------Marasuchus
            "metatarsal I reversed"
                  \----------------------------------------Theropod groups
               "forelimb fully volant"
                    Avialae--------------------------------Extant birds

Talk about spur-of-the-moment cladistics...! Theropodomorphs: by their feet
shall ye know them.

> Also, are metatarsal I and its phalanges reversed?

No, metatarsal I in both _Marasuchus_ and _Herrerasaurus_ is ("still")
parallel to the longer metatarsals.

> What is the situation for _Eoraptor_?

I don't recall seeing clear figures of the foot. My guess is
_Eoraptor_ will fit in between _Herrerasaurus_ and
_Marasuchus_. (Anyone know what the foot was like?)