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Convergence and Macelognathus, a Perspective



  Thanks to Brad McFeeters for the paper, for which I am very grateful. Mickey
supplied me with Clark and Sues' paper on *Kayentasuchus* which I thought I had
and may have under another name, but now have for sure. My comments are based
on these papers:

  Göhlich, U. B., L. M. Chiappe, J. M. Clark, and H.-D. Sues. 2005. The
    systematic position of the Late Jurassic alleged dinosaur *Macelognathus*
    (Crocodylomorpha: Sphenosuchia). _Canadian Journal of Earth Sciences_
    402:307-321.

  Clark, J. M. and H.-D. Sues. 2002. Two new basal crocodylomorph archosaurs
    from the Lower Jurassic and the monophyly of the Sphenosuchia. _Zoological
    Journal of the Linnean Society_ 136:77-95.

  Göhlich et al. have recently redescribed the original material described by
Marsh as *Macelognathus vagans* (YPM 1415) based on five new specimens
comprising mostly postcrania and some cranial remains which affirm association
of the type mandibular fragments to a gracile crocodylomorphan postcrania. That
said, it is easy to see why one particular person referred to this material as
being very similar to caenagnathids, as the mandibular alveoli are very small
and arrayed on the margins of a shallow, elongated rostrum with a long though
unfused symphysis U-shaped on both rostral and caudal margins. In addition, a
portion of the skull roof and a partial maxilla with one tooth in situ are
present.

  The holotype mandible comes from the Brushy Basin Member of the Late
Jurassic, Kimmeridgian Morrison Formation at Quarry 9 at Como Bluff in Wyoming.
Five referred specimens come from the Fruita Paleontological Area locality,
near Fruita, Colorado, which comes from the same stratigraphic level. 

  The referred material consists of:

  LACM 5572/150148 ("a left dentary, two dorsal vertebrae, a left ilium, the
proximal half of the left femur and the distal halves of both femora, the right
tibia and the distal half of the left tibia, the proximal and distal ends of
the right fibula and the distal half of the left fibula, the left calcaneum,
portions of metatarsals, and indeterminate fragments of bone.");

  LACM 4684/128271 ("a right femur, the proximal halves of both tibiae and the
distal half of the right one, the distal half of the right fibula, the left
metatarsal II and most of the left metatarsal III, the proximal half of the
right metatarsal II, and the distal half of a metatarsal (possibly metatarsal
III)"), LACM 4684/128272 ("a single left femur");

  LACM 5572/150211 ("the proximal end of the right metatarsal III and both
calcanea.");

  and LACM 4684/133772 ("a portion of the right maxilla, both dentaries
including the symphysial region, and a fragmentary posterior portion of the
cranial roof.").

  The maxilla is complete from the first through to the seventh alvelous and
preserved the anteroventral corner of the lateral antorbital fenestra; the
ventral rim of the lateral antorbital fenestra is expanded into a ridge. The
medial, palatal ramus is broad and located well forward on the maxillary body;
there is a narrow foramen visible on the antorbital fossa halfway between the
anterior margins of the medial and lateral antorbital fenestra.

  The mandible is curved along its length and becomes more shallow rostrally
than caudally. There are not alveoli in the mesial half of the symphyseal
length of the jaw, which is long, and based on comparisons to *Kayentasuchus*
(Clark and Sues, 2002) may approximate the length of 4-5 alveolar positions.
Neither maxilla nor mandible exhibit interdental plates, only foramina placed
high or low, relatively. The referred mandibles LACM 5572/150148 and LACM
4684/133772 exhibit 5 alveoli in the length of the symphysis, and space for an
anterior 4 more, though such alveoli are not present; this differs from the
type jaw in which alveoli are much smaller, more rounded, and there is space
for up to 6-8 alveoli. The holotype mandible has a symphyseal length of 54mm
and a jaw depth at the posterior end of the symphysis of 13.3mm, compared to a
symphyseal length of 15mm and 10mm for the LACM jaws, respectively, and a jaw
depth at the same position of 5mm and 3mm, respectively. The referred jaws are
over 1/3 the length of the holotype, yet the holotype exhibits not fusion of
the jaws, and it is likely the rostrum elongated relative to the tooth room in
*Macelognathus* during ontogeny. This also explains the disparity in alveolar
size compared to jaw length, and the relative shape of the alveoli.

  Teeth preserved have a constriction between root and crown, are strongly
recurved, with slight fluting of the crown in the whole preserved maxillary
tooth. Mandibular teeth are more strongly recurved than are the maxillary, and
all teeth are finely serrated. Caudal mandibular teeth are unknown, and no
premaxillary fragment or teeth are known. However, the jaw resembles
*Kayentasuchus* in which the skull has a typical spehnosuchian arrangement
(Paul's well-published skeleton of *Hesperosuchus* is available in various
publications). For comparison, the skull of *Gracilisuchus*, another
sphenosuchian, is available at
http://www.palaeos.com/Vertebrates/Units/270Archosauromorpha/Images/GracilisuchusSkull.gif

  The partial cranium is not figured, but comprises a portion of the parietals
and includes a sagittal crest and part of the occiput.

  The vertebrae have amphicoelous centra which are much longer than the neural
pedicels, and parapophyses dorsally located near the transverse processes; this
occurs even in anterior vertebrae. As Mickey noted, the neural canal is VERY
large, with an area greater than the area of the intercentral facets, but this
condition is known in other crocodylomorphans as well.

  The ilium is large and produces a large rounded anterior blade (preacetabular
ala) projecting forward and mediolaterally flattened rather than rod or
pyramid-like, as on basal dinosaurs. The pubic peduncle is deeper than
anteroposteriorly long and faces directly ventrally. The supracetabular crest
is eroded and details are unclear, except that as a lamina it extends onto the
posterior blade (postacetbaular ala); the ilium as a whole is thicker
anteriorly than posteriorly.

  The hindlimbs are very long, slender, and gracile. While the ilium is close
to 40mm long, the femur (75mm) and tibia (40mm) have a combined length of
115mm, close to three times the length; the tarsals add 8mm, and the preserved
metatarsals add 35mm, suggesting a limb length of ~200mm (assuming pedal digits
approximating metatarsal length) and a possible hip-height (with the knee bent)
around 140mm (just over 5 inches), so it was a very small animal. 

  The femur has a medially oriented caput with a developed trochanteric crest
and a large lesser trochanter well below the caput and on the lateral surface
of the femoral shaft; the fourth trochanter is very small and ridge like; the
femoral shaft is S-shaped in lateral view and straight in cranial view with
distal condyles angled around 25 degrees from the femoral long axis, implying a
naturally bent and strongly laterally everted femur, more splayed than in any
dinosaur known outside of birds and basal sauropods (such as *Isanosaurus* and
saltasaurids). The femoral condyle is also separated on a small neck and the
condyle is semi-spherical.

  The tibia has an inclined proximal end with a short and hook-like cnemial
crest and the medial proximal cotylus projects further caudally than does the
lateral one, unlike the more triangular proximal tibiae of dinosaurs as in
*Herrerasaurus* and *Lesothosaurus*. The distal end of the tibia possesses a
medial ventral process that projecs between the astragalus and the calcaneum
and there is a distinct notch on the anterior surface for a triangular and
likely pyramidal ascending process of the astragalus, unlike the condition in
basal dinosauromorphans in which it is either the caudal surface (as in
*Lagerpeton*) between the tibia and fibula (as in non-dinosaurian
dinosauriforms and some "herrerasaurs").

  The calcanea are large and preserve an expanded calcaneal tuber with a ridge
progressing from the corpus to the flange; the tuber, or "heel", is expanded at
it's distal end. The corpus is largely formed from a hemicyclindrical structure
that forms a slightly mobile, peg-in-socket joint with the astragalus; the size
of the distal tibia and fibular contacts suggest the astragalus was larger than
the calcaneum.

  The metatarsals are nearly the length of the tibia, and are half the length
of the femur in the same specimen (LACM 5572/150148); they are elongated,
closely bound to one another, and with interlocking hourglass-shaped proximal
ends; the distal ends are non-ginglymoid and bear a caudal intercondylar groove
which extends under most of the ventral surface, but no further.

  The jaw deserves extra comment:

  The margin of the edentulous symphyseal region is fluted and covered on its
external, lateral and ventral, surfaces with extensive foramina (a feature
which is not present in the referred mandibles nor is it present in
*Kayentasuchus*. The surface of the jaw is smooth yet traversed by grooves and
fossae around the foramina, as in some birds, turtles, etc., and the fossae are
elongated and confluent with the marginal fluting. This type of morphology has
been used by some to imply a beak of some sort as it appears in turtles, but it
also appears in fully dentigerous animals such as the rugose-snouted
tyrannosaurids, abelisaurids, and the lower jaws of "velociraptorine"
dromaeosaurids. That this morphology has been used to imply a keratinous snout
covering in abelisaurids, as in *Rugops* (Sereno et al., 2004) implies that the
highly vascular morphology may depend on the surface texture (smooth and
foraminated versus rough and foraminated) to determine a beak versus
close-fitting scales around the skull (as in some turtle skulls, e.g.
*Macroclemmys*). Thus, the lower jaw of *Macelognathus* may have a beak on it.
As for whether this was accompanied by an upper beak, the material only
preserves a maxilla for the upper jaw.

  Various features indicate that *Macelognathus* is not a dinosaur, including
various sphenosuchian apomorphies and the diagnostic elongated mandibular
symphysis. Göhlich et al. note about Sphenosuchia: "The Sphenosuchia are a
problematic group. It includes 14 valid taxa mostly represented by incomplete
specimens that were previously placed in the ?Pedeticosauria? or ?Thecodontia?
(Bonaparte 1972; Walker 1968, 1970, 1972). Together with the Crocodylia
(Crocodyliformes, sensu Clark 1986), Sphenosuchia form the Crocodylomorpha
(Walker 1968). Sphenosuchia was initially considered to comprise a series of
sister-taxa of the Crocodyliformes by Benton and Clark (1988) and Parrish
(1991), but later workers have differed as to whether sphenosuchians themselves
constitute a para- or monophyletic group (Sereno and Wild 1992; Wu and
Chatterjee 1993). Recent cladistic analysis of crocodylomorphs by Clark et al.
(2000) and Sues et al. (2003) recovered a monophyletic Sphenosuchia, although
support for this hypothesis was weak, and a similar analysis by Clark and Sues
(2002) did not find support for this clade. The recent description of
*Junggarsuchus* from China (Clark et al. 2004) was accompanied by an analysis
resulting in a paraphyletic ?Sphenosuchia? with this genus the sister-taxon to
Crocodyliformes." 

  *Macelognathus vagans* is diagnosed by the following: "A sphenosuchian
crocodylomorph with dorsoventrally much flattened and anteriorly edentulous
dentary, with heterodont dentition, and tooth crowns devoid of mesial and
distal serrations; dentary lacking caniniform teeth; maxilla with laterally
concave and ventrally sinuous alveolar margin; lateral longitudinal ridge above
alveolar margin of maxilla; at least two enlarged anterior maxillary teeth;
enlarged maxillary teeth serrated only distally; presacral vertebrae with large
neural canals (almost as large as the centrum); ilium without supraacetabular
crest; ventral margin of preacetabular process of ilium thickened and medially
projecting; round femoral head oriented medially and separated from proximal
end by a distinct neck; proximal facet of tibia distinctly slanted laterally;
longitudinal groove on proximoanterior end of fibula; calcaneum with a
completely flat distal surface; strong medioplantar crest on medial base of
calcaneal tuber; overlapping proximal ends of metatarsals."

  As I noted above, the supracetabular crest is not preserved and the figures
indicate broken material where the base would be, but the authors imply too
little space to allow for a crets; the description of the ilium does not
describe the feature. Maxillary teeth are only distally serrated while the
mandibular teeth are non-serrate, but this may be a juvenile feature, if the
size variation of specimens is indicative of ontegenetic variation. The authors
note a medial crest from the anterior blade, which corresponds to the
cuppedicus shelf, which is not present in other sphenosuchians.

  Comparisons are made to the ambiguous "sphenosuchian" *Hallopus*, which has
been strongly suggested to be from the Morrison Formation. Ilium, femur, tibia
and metatarsals all compare very closely to *Macelognathus* in lacking a
supracetabular crest, having a medially oriented femoral caput with spherical
femoral condyle, and in general features, though the authors note the calcaneum
in *Hallopus8 is more slender. I am of the suspiction that Göhlich et al. have
described postcranial remains they may indicate that *Macelognathus* and
*Hallopus* are the same taxon, in which case *Hallopus victor* Marsh, 1877
would have priority over *Macelognathus vagans* Marsh, 1884. This is, however,
a very curious and enlightening specimen, and the posibility of variation
between individuals or sexes may result in the hindlimb variation noted by
Göhlich et al. So all in all, very much an excellent discovery and it may
indicate new unforeseen ecological variation and possible trophic
ecomorphology. Compressed, recurved upper teeth coupled with smaller curved
teeth and an edentulous rostrum of the lower jaw (the edentulous mandibular
rostrum occurs in artiodactyls but the teeth are vastly different) may suggest
a unique trophic function.

  Cheers,

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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