Re: *Silesaurus*: the basalmost ornithischian after all?

["Jaime Headden" <ja_headden@qilong.8m.com>]

David Marjanovic (david.marjanovic@gmx.at) wrote:

<That's what I did. And it's Darren's comment which, for reasons that are 
unknown to me, states that *Agnosphitys* is the intended
spelling.>

  Yes, I went back and checked the reference in the archives. Fraser et al. 
have not, yet, published on being first revisor, so I'm
not sure when they distinguish, given the two published names, which will be 
set aside. However, I am completely happy with using
*Agnosphitys.*

<That's why I listed it as one character with several details, and counted it 
as one in the minimal list (of 6).>

  Not to twiddle here to much, or be TOO pedantic, but all these features as a 
suite is a functional complex, but all are valid
apomorphies. The fact is, none of the specifics really relate to 
ornithischians, as denoted. If one takes these as a functional
complex, then it must be herbivorous related, any other jaw-related herbivrous 
features should be included -- so when the lower jaw
and the "beak" was included in the comparative sources, then it is considered 
part of the complex. In fact, this feature, the
"hook," is the _only_ reason Dzik listed possible ornithischian characteristics 
or a possible basal "herbivorous" predecessor to
herbivorous dinosaurs, which Tom Holtz's exlaimed about not that long ago (to 
quote: "Phytodinosauria!").

I had written:

<<virtually identical to traversodont cynodontian and multituberculate molars.>>

to which David replied:

<Don't know what you mean, because those 2 look very different from each other.>

  In my observation, it was this: in *Chimaerisuchus,* multituberculate molars, 
and the traversodont molariforms, the teeth bear
multiple labial to lingual rows of multiple (3+) cusps. This is what I was 
relating to as being "virtually identical." I am not
saying finding a multicuspidate croc tooth from *Chimaerisuchus* will lead to 
confusion with a traversodont or a multi, but that
similar morphotypes both arise for similar reasons, and through similar 
processes, and this _does_ lead to problematic
identifications. *Silesaurus* has teeth quite unlike that of any ornithischian, 
and its not apparently similar enough to basal
ornithischians to the exclusion of basal saurischians to imply an ornithischian 
relationship. In this manner, teeth in basal
ornithischians and sauropodomorphans tend to be lanceolate, non-cingulate, with 
a distinct primary ridge and enamel ridges leading
to denticles. Teeth in *Silesaurus* lack these features, and are more similar 
to theropods in the sense of a conical crown with
carina and distinct recurvature in some crowns, as well as being 
non-ornithischian in the form of the arrangement of the teeth and
setting of the root.

<No, from the original. The 3rd isn't published yet!>

  This is clarifying, as your original quote appeared to lead you to resolve 
that Glut was speaking of *Silesaurus;* quoting David:
"But it shares with ornithischians, to the exclusion of (basal) saurischians, 
the following features (quotes from Glut's
encyclopedia, p. 62)[;]" ... so perhaps one can understand my confusion. 
Similarly, even if the work wasn't published, I had thought
perhaps this was an advance quote.

<I agree, considering the sheer number of beakless herbivores! Note it's the 
next character in my list, not included in the
herbivory complex.>

  Note above. Perhaps it should have been included in the list.

<More or less at the same time, I assume.>

  Not sure I would agree, but evidence of the formation of a keratinous beak in 
any vertebrate tends to be coincident with tooth
loss; the sequence is inferrable as teeth are lost, but this is theoretical. 
The argument for the presence of a beak in the upper
jaw of basal ornithischians like *Lesothosaurus* has argued its presence due to 
the lower beak, but the teeth would interact with
the predentary, rather than any "missing" beak, thus rendering any rhamphotheca 
useless if it was to conform to the shape of the
underlying bone as in birds, turtles, etc. That a large upper beak occurs in 
*Hypsilophodon* is favored by the edentulous tip of the
premaxilla, which is partially hooked, even though a tomion does not occur 
here; this would corroborate tooth loss/beak growth in
*Pelecanimimus* as well, but to the inverse of the direction seen in some 
ornithischians and oviraptorosaurs (rostral to distal).

<I don't consider the tips of the dentary homologous to the predentary! :-) How 
could I?!? I just entertain the possibility that the
rhamphotheca which covered them could be homologous. And this I base on another 
possibility, namely that the predentary could be an
"ossification of the beak", like the rostral.>

<Could be convergence in both cases. I just say it is one character, not less, 
not more.>

  If taken as a series of acquisitions, I would say it has a _major_ impact on 
the issue. "Four" vertebrae incorporated into the
sacrum, versus just "sacrals" touching the ilia, would be treated as a state of 
a separate character that has to be considered in
light of other taxa. In *Lesothosaurus,* only three scrals contact the ilia, 
and in *Herrerasaurus* and *Eoraptor* (apparently) only
two. In "prosauropods" it seems to be that only two sacrals touch the ilia, 
whereas towards the incorporation of a sauropodan
sacrum, it transforms to three, then four. But this is just one thing I am 
bringing up; the other is that sacral acquisition, even
if one wants to use different terms for the portions of the sacrum or call it a 
"synsacrum," *Silesaurus* still has an adult sacrum
of four vertebrae, unlime other basal dinosaurs.

<Of course. But because it is not impossible that the 5 sacrals of 
ornithischians evolved via such a condition (note my cautious
wording), I added it to the list.>

  Yet the conditions as they are are not the same, and such a hypothetical 
scenario shouldn't be used as an apomorphies, especially
since the orinithischian and *Silesaurus* sacra are distinct fomr one another 
under the listed condition.

<Sure it is. :-) OK: "the lack of phalanges on the 5th pedal digit". Better 
that way?>

  Yes. But then, retention of the tow could be a saurischian apomorphy, as 
well. This would be a valid feature, if one could find an
effective reason to support it. However, loss of digits and the occlusion of 
all primary metatarsals with one another, as in
*Silesaurus,* is a cursorial feature, corroborated in the pes but not the upper 
limb. Very peculiar. I would say it, like several
mammalian cursors, was a quadrupedal runner.

<It's commonly absent in ornithischians.>

  In derived most ornithischians, this bone is entirely lost. It is a nubbin in 
*Scelidosaurus,* *Scutellosaurus,*
*Heterodontosaurus,* *Psittacosaurus,* and absent in *Hypsilophodon* and up, as 
in eurypodan thyreophorans. In *Pisanosaurus,*
*Lesothosaurus,* and other ornithischians, the proximal end of the fourth 
metatarsal is expanded mediolaterally, as in basal
saurischians, but not as in *Silesaurus* or more primitive dinosauromorphans, 
and this also has implications for the articulation of
the proximal fifth metatarsal (rather than contact the tarsus, it contacts only 
the fourth metatarsal in all dinosaurs but some
basal sauropodomorphans. Reduced metatarsal epiphyses is another cursorial 
feature.

<Perhaps the loss of the 5th toe is a cursorial adaptation of Ornithischia 
including *Silesaurus*, and then *Silesaurus* added more
cursorial adaptations that the undoubted ornithischians didn't add.>

  While basal ornithischians reversed the other features, including convergent 
femoral anatomy with saurischians, the ascending
process shifted laterally ... again ... the calcaneum shrunk (being primitively 
large in basal ornithischians and saurischians but
tiny in *Silesaurus* as in tetanuran theropods with a semilunate aspect.

<Theropods don't, sauropods don't, crocodiles don't. Herrerasaurids don't 
either, AFAIK. Do basal sauropodomorphs? Has there been
enough research to tell?>

  Contrary in these observations, theropods do have crossed unlae, as do 
"prosauropods." But perhaps I should clarify that when the
radius progressed from the lateral humeral condyle to the medial wrist, and the 
wrist is perpendicular to the distal humeral
transverse axis, the wrist in lateral view does not look crossed; this is still 
crossed when one looks in cranial view. In
quadrupedal animals such as thyreophorans and articulated *Heterodontosaurus* 
as in basal sauropodomorphans, the manus faced
cranially without any adequate alteration of the distal humerus to permit the 
radius _not_ to cross the ulna in lateral view as it
does in neosauropods. However, the condition is still plesiomorphic in all 
theropods save birds, in that in all taxa, the radius
does cross the ulna, even if they are only 15-20 degrees apart from being 
parallel to one another [but then, this angle decreases
the longer the bones are, so only works in similarly-sized animals with 
similarly-sized arm proportions]. Mobility of the wrist, as
in basal saurischians and ornithischians, is neccessary given their 
manipulative structures of the manus and their large, loose
carpals. The bowing of the radius is also aparent as a corrolary to 
pronation/supination in these forms, and this is evident in
*Eoraptor,* though I've not articulated the wrist myself to affirm or deny 
this, and the monograph is not out yet (soon...). Other
basal theropods, such as *Coelophysis,* have similar wrist anatomy.

  Crocodilian distal wrists bave bones between the radius and ulna (ulnare and 
intermedium) which are expanded and provide a carpal
block that prevents the radius from crossing the ulna. This is seen in 
lacertilians, amphibians, etc. As a secondary conclusion,
pronation of the wrist does not require a radius to cross an ulna, that's just 
how it's enabled when the distal ends contact one
another at the wrist, because of the articulation of the radius to the medial 
wrist and lateral humerus. This is clear in mammals as
well. Otherwise, the radius _always_ crosses the ulna in dinosaurs, including 
theropods and sauropods, just to a limited extent in
some, as neosauropods and apparently in *Marasuchus* as well as *Silesaurus.*

<<...[T]he long forelimbs thus argue for quadrupedal adaptations and, like the 
(obviously non-ornithischian *Marasuchus* which has
similar limbs, or
*Lewisuchus* with similar humerus, or *Scleromochlus* with similar limb 
structures as well).>>

<Yes?>

  I was hoping it was obvious that the limb structure is similar to 
non-dinosaurian archosaurs, and may allude to the plesiomorphic
nature of the limb structure, but this does not seem to have been the clearest 
case.

<What are, now, the synapomorphies of Dinosauria?>

  It has always been the argument that I am using the basal features of each 
group, including that of dinosaurs themselves.

<He doesn't do that. He uses e. g. the beak.>

  As an herbivorous quality relating to ornithischians. To quote, Dzik 
(2003:573: "These similarities to herbivorous dinosaurs mean
that (1) *Silesaurus* is an early member of the ornithischian lineage, (2) 
belongs to the lineage leading to both the Ornithischia
and Prosauropoda after its emergence from the ancestral carnivorous stock 
[...]"). The similarities Dzik uses involve almost
entirely the 'rather low, conical shape and worn tips" (pg. 573) and 'the 
presence of a horny beak on the lower jaw' (also pg. 573);
however, the osteological correllate is a convergent feature and in 
ornithischians, this region is supported by a separate bone that
*Silesaurus* lacks, so this feature has no anatomical 1:1 parallel. One would 
have to argue beyond the evidence to hypothesize that
the beak persisted, the dentary shortened so the teeth progressed back to the 
front of the jaw, and the keratin formed a calcified
core OR the edentulous portion fissioned off in early ontogeny to form a 
non-paired bone from the symphyseal region, which also
progressed caudally to a new site, altered the position of the symphyseal 
foramen, and so forth. Hmm. Maybe, maybe not. Maybe it
just formed a beak convergently and the jaw has nothing to do with its 
relationship to Ornithischia, if it had one exclusive of
other dinosaurs.

  And writing that, I had this odd, bizarre tought that had nothing to do with 
the jaws but has been regarded in the past. As the
oldest dinosauriform with a fully tridactyl pes, footprints regarded as Middle 
Triassic to early Late Triassic in age do not
neccessarily need to be theropodan. Oh, and that Avepoda now includes ALL 
dinosaurs and *Silesaurus,* if it's a non dinosaur, as it
possesses the first occurence in common with birds of a metatarsal I [or even, 
with a stretch, an mtV] that does not contact the
tarsus and a fully tridactly pes (I mean, hell, there may not even be a first 
toe anyway). Very peculiar.

  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)