RE: Titanosaurs and their relationship to Brachiosaurs and Diplodocoids.

[Michael Mortimer <mickey_mortimer111@msn.com>]

Mike Lima wrote-

> >From what I've read (online) Titanosaurs were once
> thought to be more closely related to Diplodocoids
> than Brachiosaurs. But now they are grouped under
> Macronaria with the Brachiosaurs. What led to the
> change?

Well, Malawisaurus was described in 1993.  As a basal titanosaur with 
spatulate teeth, it helped argue against placing diplodocoids and 
titanosaurs together due to peg-like teeth.
But the main force that turned the tide from Janensch's (1929) view was 
cladistics.  The first analyses had very few taxa (Gauthier, 1986; Yu, 
1990), and argued for a titanosaur-diplodocoid link.  Once more extensive 
cladistic analyses were performed though, the results almost always placed 
titanosaurs sister to brachiosaurs (Calvo and Salgado, 1995; Salgado et al., 
1997; Upchurch, 1998; Wilson and Sereno, 1998; Wilson, 2002; Upcurch and 
Barrett, 2004).  An exception was the early work of Upchurch (1993, 1995), 
but these lacked published matrices, and he later changed his view anyway.
The brachiosaur-titanosaur link has been strengthened with the recognition 
of intermediate taxa, like Chubutisaurus, Austrosaurus and the Texan 
"Pleurocoelus" material.

> Aside from the absence of a ungual claw
> (which I read was reduced in Brachiosaurids) I haven't
> read of anything that connects Titanosaurs and
> Brachiosaurids (but I acknowledge that this is
> probably due to poor sources).

Here's Wilson's (2002) list of macronarian and titanosauriform characters-
1. External naris, maximum diameter greater than orbital maximum diameter.
2. Coronoid process on lower jaw (surangular depth more than twice depth of 
the angular).
3. Surangular ridge separating adductor and articular fossae.
4. Dentary teeth 17 or fewer.
5. Posterior dorsal centra opisthocoelous.
6. Longest metacarpal-to-radius ratio 0.45 or more.
7. Metacarpal I longer than metacarpal IV.
8. Puboischial contact one-half total length of pubis.
1. Spongy presacral bone texture.
2. Mid-cervical centra elongate, length four times posterior centrum height.
3. Dorsal ribs with pneumatic cavities.
4. Anterior dorsal ribs plank-like.
5. Metacarpal I distal condyle undivided, phalangeal articular surface 
reduced.
6. Metacarpal I distal condyle oriented perpendicular to axis of shaft.
7. Iliac preacetabular process semicircular.
8. Femoral shaft with lateral bulge, proximal one-third deflected medially.

> Also, is the absence of a ungual claw a defining
> characteristic of Titanosaurs or not. I found a post
> from a couple years back (made by Mickey Mortimer I
> believe) in which all that defining characteristics
> were listed and I don't believe the absence of a claw
> was on it. Is this a more recent discovery? Or is it
> not a characteristic present in all Titanosaurs?

Wilson has it as an opisthocoelicaudine character, but with titanosaur 
phylogeny being as messy as it is, take that with a grain of salt.  An 
extensive literature search would be necessary to determine which 
titanosaurs have this, which would be hindered by preservational issues 
(e.g. diplodocoids were thought to lack calcanea until recently).

Mickey Mortimer