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Re: Tyrannotitan, new carcharodontosaurid from Argentina

Tim Williams (twilliams_alpha@hotmail.com) wrote:

<Thirdly, as mentioned in a previous post, Novas et al. (2005) overturn 
Sereno's referral of _Sigilmassasaurus brevicollis_ ("Spinosaurus B") to
_Carcharodontosaurus_.  If _Sigilamassasaurus_ and _Carcharodontosaurus_ are 
indeed distinct, this would vindicate Russell's (1996) original judgement that
_Sigilmassasaurus_ is a valid taxon.  Russell (1996) even thought
_Sigilmassasaurus_ was distinct enough to warrant its own family, the
Sigilmassasauridae.  However, Mahler (2005) referred _Sigilamassasaurus 
brevicollis_ to _Spinosaurus maroccanus_.>

  Mahler's reasons for referral of *S. brevicollis* to *S. maroccanus* were not
given, but I suspect this is a possible lapsus; Luke Mahler worked with Paul
Sereno on this project, and Paul had previously referred *S. brevicollis* to
*C. saharicus* due to the nearly identical cervical anatomy. The cervicals of
*S. maroccanus,* on the other hand, are _extremely_ different (for example, the
cervical centra in *S. marocannus* are not lenticular in cranial aspect and
there is no hypapophysis; the neaural arch (sans the neural spine) and centrum
are taller than wide and much longer than tall, whereas in *S. brevicollis* and
*C. saharicus* the centra are short and low and the neural arch/centrum [sans
the neural spine] height is about as broad as high or broader than high).
However, a message to Mahler is asking for clarification. I will personally
agree with Paul that the Tafilalt vertebrae Russell described as
*Sigilmassasaurus* come from the same kind of animal as the skull that Paul
described in 1994; however, given that the holotype tooth and the referred
Baharija material Stromer described in 1931 are both lost, a neotype should be
determined, and it would likely end up being the Kem Kem skull. Note that Novas
et al. mistakenly refer to Stromer's material as the holotype, which is not the
case (Stromer indicated his species was from Dépéret and Savornin's species
[Megalosaurus saharicus*] which he named a new genus to differentiate it from
*Megalosaurus* with).

  Novas et al. do not describe the differences between *S. brevicollis* and *C.
saharicus,* but rather distinguish *S. brevicollis* from the Patagonian taxa
alone and only allude to differences with *C. saharicus*:

  "*Tyrannotitan* helps to clarify the confusing aspects of the skeletal
   of its close relative *Carcharodontosaurus*. This Saharan taxon was recently
   diagnosed (Sereno et al. 1996) and reconstructed (Currie 1996) on the
   supposed overlapping characters of specimens of *Carcharodontosaurus
   saharicus* (Stromer 1931) and the problematic theropod "*Spinosaurus* B"
   (Stromer 1934). Pivotal in the purported overlap is a stout cervical
   vertebra, characterized by its low and very broad centrum, strong ventral
   keel, and reduced neural spine (Sereno et al. 1996). However, this vertebra
   (not found in association with specimens of *C. saharicus*; P. Sereno,
   personal communication) shows clear distinctions with cervicals of
   *Tyrannotitan*, *Giganotosaurus*, and the holotype specimen of *C.
   saharicus*. On the contrary, the cervical in question closely resembles that
   of *Sigilmassasaurus* (Russell 1996), a theropod of uncertain phylogenetic
   relationships. Besides, the flattened and acuminate pedal unguals of
   "*Spinosaurus* B" (Stromer 1934) purportedly referred as to
   *Carcharodontosaurus* by Sereno et al. (1996, 1998), are sharply different
   from the robust and curved ones of *Tyrannotitan* (Fig. 1, ph). Such
   differences do not correspond with digit position. Also, femur, tibia,
   and caudal vertebrae originally referred as to "*Spinosaurus* B" show clear
   distinctions from those of *Tyrannotitan* and *Giganotosaurus*. In sum,
   diagnosis and reconstruction of *Carcharodontosaurus* recently offered
   (Sereno et al. 1996; Currie 1996) are based on the chimaeric association of
   specimens corresponding to different theropod clades. In this context, we do
   not regard *Sigilmassasaurus brevicollis* as a subjective junior synonym of
   *C. saharicus*, as recently proposed (Sereno et al. 1998)."

  Novas et al. contend that Sereno's *Carcharodontosaurus* is a chimaera due to
inclusion of some of the material Russell used for *Spinosaurus maroccanus*,
but did not determine that apomorphies may exist within a clade and that the
unguals were NOT carcharodontosaurine in nature. Indeed, they were only
compared to the carcharodontosaurs in question in the passage above (other
comparisons that may have been made were not included). Absence of clear
differentiation of *S. brevicollis* from *C. saharicus* does not support Novas
et al's conclusion, though it IS possible. The key of note here is that Sereno
compared Stromer's EGYPTIAN cervicals with that of an isolated cervical from
Kem Kem and Russell's Tafilalt cervicals; as Novas notes, these and
*Tyrannotitan* are identical, but it is *Giganotosaurus* that is different, and
this only argues that *G. carolinii* is the odd-man out.

Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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