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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 "hyposphene-
hypanthrum absent)".

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.