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Re: New Titanosaur (Maxakalisaurus) from Brazil
Matias Soto wrote:
Nope, Andesauridae has been recognized as a paraphyletic assemblage of
basal titanosaurs. Of the "andesaurids" you mentioned, *Andesaurus* is the
single non-titanosaurid titanosaur. In fact, one of the purported
synapomorphies (presence of hyposphene-hypanthrum) has been shown to be
lacking in *Argentinosaurus* (i.e., *Argentinosaurus* shares with other
"titanosaurids"/lithostrotians, or derived titanosaurs, the character-state
Yes, that's true. The confusion arose from the fact that _Argentinosaurus_
does show accessory articulations between the dorsal vertebrae. These were
originally interpreted at hyposphene-hypantrum articulations. However,
later studies regarded these accessory intervertebral articulations of
_Argentinosaurus_ as something different (and non-homologous): modified
centropostzygapophyseal laminae, not hyposphene-hypantra (Salgado and
Martínez, 1993; Sanz et al., 1999; Martinez et al., 2004) .
_Andesaurus_ and _Epachthosaurus_ have hyposphene-hypantra articulations in
their posterior dorsals, and in the latter (but not _Andesaurus_) they
extend into the tail. These are not present in more derived titanosaurs.
*Epachthosaurus*, in turn, it is one of the most basal titanosaurid with
procoelous mid and posterior caudal vertebrae (differing in this respect
from *Andesaurus*, whose mid and posterior caudals are amphyplatyan).
Fortunately, _Epachthosaurus_ is known from most of the postcranial
skeleton, making it very helpful for understanding basal titanosaur anatomy.
Martinez et al. (2004) regarded _Epachthosaurus_ as more derived than
_Andesaurus_, and the presence of strongly procoelous mid and posterior
caudal vertebrae would probably indicate this.
Nevertheless, this "key" titanosaurid character ("procoelous
middle-posterior caudals") may be more labile than previously thought.
After all, titanosaurs show an impressive range of caudal morphologies
(procoelous, amphicoelous, opisthocoelous, etc), and it seems that the
opisthocoelous condition might have arisen directly from the procoelous
condition more than once. So maybe the procoelous condition arose from the
amphiplatyan condition more than once too? In other words, titanosaur
evolution may be more complicated than previously thought.