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The Croc That Wanted to Be a Dinosaur



Alternate title, from M. Keesey: "Crocs: The Original Dinosaurs"

  The paper isn't quite out yet, but I figured I'd give this one a spin because
I have a few observations to make. And some of them have something to do with
current discussions, even. I will not, for the sake of the online publication,
use the name Nesbitt and Norell propose for their taxon, not because I consider
it useless but because I'd rather wait for the print version unless
_Proceedings of the Royal Society, series B_ has joined the digital publication
age in committing to digital nomenclature, as in _Naturwissenschaften_ and
_Palaeontologica Electronica_. Note that this paper is technically not
published in print, and the page numbers are set as 1-4 by default, but it is
unlikely this will be the final pagination. Thus all my page references will be
in brackets [1, and so forth].

  Of course, for the last few years, the authenticity of the distinctive
identity of a few Chinle archosaurs have been questioned, not lease of which
was *Chatterjeea* ever since someone found a toothless premaxilla in
association with a skeleton. Recently, Lehman and Chatterjee produced a study
relegating the Chinle and Dockum series as being part of a similar depositional
cycle, and representative of the western and eastern, respectively, equivalents
of the same essential stratigraphic period. Thus, the two "groups" or
"formations" are largely contemporaneous and homogenous. So too, then, should
their fauna, and demarkating the fauna in each "group" shows remarkable
correlations across the longitudes, as well as through time itself. This brings
us to the possibility of synonymy of eastern with western taxa because this
might be of special significance.

  Lehman, T. and S. Chatterjee. 2005. Depositional setting and
   vertebrate biostratigraphy of the Triassic Dockum Group of
   Texas. _Journal of Earth Systems Science_ 114(3):325-351.

Abstract:
  "Triassic strata of the Dockum Group in Texas comprise two major
   upward-fining alluvial-lacustrine depositional sequences. The
   two sequences are represented by the (1) Santa Rosa-Tecovas,
   and (2) Trujillo-Cooper Canyon Formations. The second sequence
   is much thicker than the first, and occupies a greater
   geographic part of the Dockum basin. Each sequence of alluvial
   and lacustrine sediment accumulation is characterized by
   sediment derivation from a different source terrain. The
   unconformable relationship between the two depositional
   sequences, the change in mineralogical composition and presumed
   source areas between these units, differences in paleocurrent
   orientation between units, and evidence for intervening
   episodes of local deformation indicate that the sequences are
   of tectonic origin. These strata are not the product of a
   single sediment dispersal system, such as the
   centripetally-drained lacustrine delta complex previously
   envisioned for the Dockum basin. Both Dockum sequences are
   comprised largely of two typical alluvial facies associations;
   stream channel facies, and overbank flood-plain facies, that
   are similar to those described in nearly all fluvial deposits.
   In addition, the Dockum Group contains a peculiar lacustrine
   facies that accumulated in local flood-plain depressions, and
   probably resulted from subsidence over areas of subsurface salt
   dissolution. Vertebrate fossil assemblages are found in all
   three Dockum facies associations. Five fossiliferous sites in
   the Dockum are discussed in the context of these three
   depositional settings. The Dockum tetrapod diversity is
   reviewed in a hierarchical phylogeny with remarks on the
   history of collection, stratigraphic distribution of genera,
   and their taxonomic status. The stratigraphic ranges of
   tetrapod taxa do not support the recently proposed successive
   Otischalkian, Adamanian, Revueltian, and Apachean biochrons
   within the Dockum Group. Instead, a few index fossils provide a
   broad framework for correlation of Late Triassic nonmarine
   strata of the Dockum with the Carnian and Norian Alpine marine
   stages."

  Nesbitt and Norell have described a new, nearly complete archosaur from the
Chinle that corresponds so closely to a Dockum taxon, *Shuvosaurus*, that one
would be hard-pressed to find _differences_ between the two of them.
Nonetheless, the authors use this diagnosis to define their new taxon:

  "(e) Diagnosis
  "A suchian archosaur distinguishable from all other suchians
   except *Shuvosaurus* by the presence of an edentulous
   premaxilla, maxilla and dentary, a posteriorly long
   anterodorsal process of the premaxilla, a long preacetabular
   process of the ilium that connects to the posterior process by
   a large thin flange and a pubic boot that is 33% as long as the
   pubic shaft. Distinguished from *Shuvosaurus* by the presence
   of a dorsal and posterior process of the maxilla, small
   posterior process of the premaxilla, fossa on the posterior
   side of the lacrimal, absence of posterior process of the
   squamosal and a fossa on the posterior side of the squamosal."
   (page [1])

  The differences in the lachrymal could be glandular, not that I am worried
about it, but the features distinguishing the maxilla seem curious, in that the
maxilla of the skull of *Shuvosaurus* does not appear to be complete, with a
bizarrely short morphology apparently related to a vertical break in the bone
that has even been identified by Rauhut, though not as such, but which may be
attributed to an annatural fracture due to the crushing and distorted surface
of the original bone. The material indicates, rather, than the jugal and
maxilla may have had a different relationship than has been illustrated by
either Chatterjee or Rauhut, thus I am loathe to accept these features as
diagnostic in separating them until further work on the original skull reveals
breakage or the lack thereof. Nonetheless, two features of the squamosal seem
highly indicative of variation, and the absence of the posterior (paroccipital)
process of the squamosal is curious in itself, and possibly diagnostic. But I
am not sure we should honor with two divergent nomina (aka, genus AND species)
when it seems just as likely to retain the same information with a
*Shuvosaurus* [new species] monicker. But, then again, so too the converse.

  The postcranial skeleton reveals many features that will change our
interpretations of what to expect crocs to ever look like (hence the title of
this post). The legs are long, the forelimbs short, the neck long, and the
pelvis all make this an especially gracile-looking biped. The entire manus
seems to be shorter than the metatarsus, the femur is sigmoid in curvature in
lateral view, and the scapula was massive and vertically oriented. However,
several features reveal the crocodilian nature of the skeleton, and allows that
similar gracile bipeds can be explicitly distinguished between dinosaurs and
crocs: 

  (1) The pubis is articulated to the cranial margin of the pubic peduncle,
rather than the ventral, and the ischiadic component of the ventral acetabulum
is greater than the pubic component; 
  (2) the femoral caput it craniomedially oriented without a large greater
trochanteric crest, and sits inside the acetabular depression of the pelvis
such as to not be fully medially inserting as in dinosaurs; 
  (3) the ilium, while bearing both a long pre- and postacetabular blade, has a
large muscular-supporting crest above and caudal to the acetabulum, and the
anterior cuppedicus shelf extends the length of the preacetabular portion of
the ilium and onto the pubic peduncle, indicating a very massive component of
the medial thigh musculature, unlike in dinosaurs; the tibia is shorter than
thge femur, unlike typical dinosaurian bipedal cursors; 
  (4) the astragalus and calcaneum form a crocodile-normal type ankle, and the
calcaneum bears a very large hatchet-shaped tuber;
  (5) the calcaneum is nearly as broad, and is certainly deeper proximodistally
than the astragalus, and the proximal tarsus clearly lock and separate the
shafts and the distal ends of tibia and fibula, suggesting some flexibility in
the crural aspects of the leg;
  (6) the metatarsus bears a very large, and very distinctively L-shaped
metatarsal V, and is over 40% the length of mtIV.

  It should at this point be very easy to distinguish these features from most,
if not all, avowed dinosauromorphs, including *Silesaurus*, *Marasuchus*, and
so forth.

  It is also notable that many of tese features, as suggested by Nesbitt and
Norell, are found in various Chinle archosaurs, such as the "chatterjeeids".
They go on to produce a cladistic analysis

--+--Euparkeria
  `--+--Proterochampsidae
     `--+--+--Pterosauria
        |  `--+--Lagerpeton
        |     `--+--Marasuchus
        |        `--+--Ornithischia
        |           `--+--Sauropoda
        |              `--+--Coelophysis
        |                 `--+--Tyrannosaurus
        |                    `--Gallimimus
        `--+--Parasuchia
           `--+--Stagonolepididae
              `--+--Crocodylomorpha
                 `--+--Ornithosuchidae
                    `--+--+--Postosuchus
                       |  `--Saurosuchus
                       `--+--Arizonasaurus
                          `--+--Lotosaurus
                             `--+--Shuvosaurus
                                `--[new taxon]

  20 taxa were coded for 81 characters, producing a single tree with 156 steps.

  Note that not only is *Lagerpeton* included is a phylogeny testing the early
placement of various crurotarsans, but so is *Marasuchus* and both are
dinosauromorphans. Technically, so is Pterosauria.... Crocodylomorpha in this
case is "Sphenosuchia", *Erpetosuchus*, *Gracilisuchus*, and Crocodyliformes,
so this topology doesn't differ from that of most other recent topologies on
crurotarsan phylogeny except that ornithosuchids and the
"poposaurids/prestosuchids" are normally considered successive ingroups outside
of Crocodylomorpha. It should also be worthwhile to note that *Lotosaurus*, a
semi-finback with edentulous jaws from China, is the sister taxon to
*Shuvosaurus* + the new taxon, which are nested as "ctenosauriscids" (when
referring to Nesbitt's work on *Arizonasaurus*).

  Finally, Nesbitt and Norell note that this data provides evidence that the
cranial elements of *Shuvosaurus* belong to postcranial elements described as
*Chatterjeea*. While they suggest synonymy between the two, I would caution
this should wait until just such a skeleton appears to indicate they ARE the
same animal. This specimen from a different formation makes this referral
currently suggestive, entrancing, but weak because of the lack of diagnostic
overlap. It is clear that this specimen itself is not identical to the type
species of *Chatterjeea*, and thus we should use the same caution.
*Shuvosaurus* and the new taxon are also comparable 1:1 in the data matrix
provided, and this is further data for an inclusive beaky clade inside the
Rauisuchia, separated from the toothed Prestosuchidae and likely to be named
Ctenosauriscidae due to the presence of a group of finback rauisuchians with
teeth, which include a group of finbacks without teeth, and a group of
toothless non-finbacks within that. Such wierd and wonderful evolution.

  But this enhances the already notable similarities between the western and
eastern SW USA faunae, and while biochronological segregation in the Dockum is
unlikely (a summary of the decision is in Hunt et al., 1998, ref below), the
diversity and thus interchange between the two regions through time seems
largely continuous and may thus permit substantial reduction in east-west fauna
distinction by synonymizing taxa. One may, if one wishes, simly start lumping
"genera" that may be nothing so different as form genera on the basis of the
similarities between the New Mexico, Arizona, and Texas Late Triassic animals.

Refs:

  Chatterjee, S. 1993. *Shuvosaurus*, a new theropod. _National
   Geographic Research and Exploration_ 9:274-285.

  Hunt, A. P., S. G. Lucas, A. B. Heckert, R. M. Sullivan & M. G.
   Lockley. 1998 Late Triassic dinosaurs from the Western United
   States. _Geobios_ 31:511?531. (online,
   doi:10.1016/S0016-6995(98)80123-X)

  Lehman, T. and S. Chatterjee. 2005. Depositional setting and
   vertebrate biostratigraphy of the Triassic Dockum Group of
   Texas. _Journal of Earth Systems Science_ 114(3):325-351.

  Nesbitt, S. J. & M. A. Norell. 2006 (online). Extreme
   convergence in the body plans of an early suchian (Archosauria)
   and ornithomimid dinosaurs (Theropoda). _Proceedings of the
   Royal Society, series B_ (4 pp.) [published online:
   doi:10.1098/rspb.2005.3426]

  Rauhut, O. W. M. 1997. Zur schadelanatomie von *Shuvosaurus
   inexpectatus* [On the cranial anatomy of *Shuvosaurus
   inexpectatus*], pp. 17-21, in Sachs,  S., O.W. M. Rauhut & A.
   Weigert (eds.) _Treffen der deutschsprachigen
   palaeoherpetologen_ [Meeting of the German-Speaking
   Palaeoherpetologists]. (Germany: Alfred-Wegener-Stiftung.) [in
   German]

  Cheers,

Jaime A. Headden

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

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