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Re: IGUANODON WARS [Attack of the Clades?]



> Hi Jon, thanks for your thoughts on iguanodontian
> systematics (am ccing this to DML).

    Actually, this makes me a little grumpy, as my response was intended to
be FYEO. I am now placed in the uncomfortable position of conducting a
discussion of an offhand comment in an unpublished paper in public, in front
of and involving the editor of the volume for which the paper was intended.
I cannot stress to the list enough the importance of CHECKING TO SEE IF IT
IS OK WITH ALL PARTIES BEFORE BRINGING PRIVATE DISCUSSIONS INTO PUBLIC. Any
grouchy comments below can be traced to this issue. I tried to edit them all
out, Darren... sorry. :)


> Re: your suggestion that _I. atherfieldensis_ and _I.
> bernissartensis_ might be growth phases of the same
> species, this is interesting and thanks for the data but right

    Not what I said. The gist of the passage I relayed was that, given the
range of variation we observed in hadrosaurs, the two morphotypes of
Iguanodon would be considered the same species. There appears to be
substantial anisometry with respect to size in I. bernissartensis; "I.
atherfieldensis" specimens plot where the Iguanodon trend intersects the
hadrosaur trend. The ultimate point being that you need to be careful in
coding OTUs lest you bias your study with morphologically immature
specimens. There certainly is a strong *implication* in this discussion that
the two forms may be conspecific, but I feel that such a determination is
beyond the scope of our analysis and requires a close look at the actual
material.

    To clarify against the almost inevitable (on the Dino List, anyway)
interpretation: I am not criticizing any aspect of Norman's work. I hold it
in the highest regard, and I have used it as an example of the level of
scholarship I believe is worth striving for.


> now I find it very hard to believe: the two species aren't just
> different in the features you state [...] but in others that seem less
logical
> in terms of ontogeny.

    Having considered the subject in a little depth, I am currently quite
uncertain regarding what (if anything) is logical about ornithopod ontogeny,
beyond the general observation that "things get bigger." There seems to be a
good amount of individual variation within what we believe to be species,
and that could heavily overprint any ontogenetic pattern. There is certainly
variation in the pattern of the ontogeny of vertebrates itself, such as in
the ossification sequence of carpal elements in tapirs (Matt Colbert, in
prep.) and attainment of characteristics of osteological maturity in
crocodylians (Brochu's paper). I'm very interested in how we should approach
this issue in the fossil record.


> For example, they differ in number of
> caudal vertebrae (45 in _I. atherfieldensis_, over 50 in _I.
> bernissartensis_)

    Quoting Norman (1980, p. 42, emphasis mine), "Several of the specimens
from Bernissart possess ALMOST complete articulated tails... In one of the
more complete specimens (IRSNB 1726) the tail, as preserved, comprises
forty-six articulated vertebrae. It seems unlikely there were many more than
this in the tail of _I. bernissartensis_, and fifty would seem a reasonable
estimate..." And, Norman (1986; p. 310), "The remainder of the tail (34-?45)
is not preserved in any specimen." I therefore do not find your point
particularly compelling, based on the evidence as I read it.
    On the other hand: anotomical authorities I have consulted have
communicated their expectation that caudal vertebrae will vary both
individually and ontogenetically. I am afraid I have been unable to find the
expected numerous supporting references concerning variation in caudal
vertebral count in the literature. I also can't cite much in the way of data
from other ornithopods, thanks to their notorious habit of not preserving
complete tails. In the ornithopod literature, Gilmore (1946) expresses his
expectation that caudal count will vary both individually and
ontogenetically in reptiles. I won't feel comfortable until I find more
concrete references, but I am discomforted by the notion than you apparently
are.


> and the shape of the quadrate (pillar-like
> in _I. bernissartensis_, curving in _I. atherfieldensis_).

    This has been a sticking point for me for a number of years. Recently, I
have been made aware of authorities who plan to synonymize certain
ornithopod taxa that differ in the degree of curvature of their quadrate,
and I am myself coming to the conclusion that there may be unrecognized
problems with this character. I also have some doubts that the difference in
Iguanodon is as pronounced as it appears at first. The caudal margin of the
bone, where I think curvature is best appreciated, does not appear to differ
as much as the rostral margin in these morphotypes as restored by Norman
(1980; 1986). The rostral margin, where the difference appears more
pronounced, varies in thickness somewhat among apparent conspecific
hadrosaurs, especially above the quadratojugal articulation. My hunch is
that at least some of this is due to slight differences in the position of
the element relative to the rest of the skull, but it is certainly possible
that there is true variation as well. This is something I actually plan to
look at in detail for hadrosaurs some day. It would be truly marvelous to
get a chance to examine Iguanodon in detail as well. Hopefully the specimen
HP Carpenter refers to will shed some light on this.


> Also, it would be nice to have the figures for this but surely
> there are some _I. atherfieldensis_ individuals that overlap
> in size with some _I. bernissartensis_ individuals, plus there
> are (to my knowledge) no specimens that are intermediate.

    Doesn't bother me in the least. It  is not my contention that "I.
atherfieldensis" is necessarily either A) a "dimorph" or B) an ontogenetic
stage of  I. bernissartensis. In at least some features, Ia falls on the
hadrosaur growth curve as well. My point was that, from what little data are
available, they appear to fall on the same growth curve. The truth could be
neither A nor B, it could well be both. Since I have not been able to
conduct a proper study of the material, it would not be proper for me to
express an definite opinion on this matter.
    Entering the realm of speculation, a continuous pattern of morphological
variation is suggested by the data in some cases of "sexual dimorphism"
among dinosaurs (e.g., Protoceratops, Chasmosaurinae). It is certainly
possible that specimens of Iguanodon "atherfieldensis" are those few
individuals in the species that represent the extremes of more than one
morphological continuum (e.g., the most "gracile" and the most "feminine"),
with juveniles resembling the endmembers of one or both of these
morphoclines. Exploration and documentation of this sort of pattern of
variation is actually a long-term research goal of mine. Anyway, right now
all this amounts to speculative crap, but I thought I'd bring it up just to
suggest that there need not be just one morphocline in a species, and that
the distribution of morphotypes need not superficially be bimodal, highly
kurtotic, or unskewed.


> Sorry to pull in anecdotal evidence, but fragmentary bits
> (MIWG/IWCMS metatarsi) indicate there also seem to be
> gracile _I. atherfieldensis_ that reached the size of a big _I.
> bernissartensis_.

    Anecdotal evidence is ok. I wonder, however, what apomorphies are
present in the metatarsus of Iguanodon that allow this referral? I don't
recall any myself. Anyway, what few data I have suggest variation in the
proportions of the metatarsals among hadrosaur specimens believed to be
conspecific, especially in metatarsals II and IV. Some of this variation
appears to be as extreme as that observed in Iguanodon. We really need more
data on this, though.


> It also seems unlikely to me that the
> diagnostic dorsoventrally deep plate-like prepubic proc. of
> the pubis in _I. atherfieldensis_ could ontogenize (new
> word) into the shallower, more prong-like process of _I.

    If you'll recall the passage I sent you, the suggestion was that this
may be individual variation, not neccessarily ontogenetic variation. The
argument is that a level of variation *nearly* equivalent to this is seen in
some hadrosaurs, and the incompleteness of the marginal bone in the elements
from Bernissart indicates that the full dimensions of some of the compared
elements are not preserved (indicated by Norman's illustrations). However,
this remains an issue I am not entirely comfortable with. Certainly, such
differences in hadrosaurs are often associated with taxonomic boundaries.
Interestingly, although the prepubis is absolutely deeper in Ia, the ratio
of neck to blade depth (which is pretty consistent wihtin putative hadrosaur
species, and appears to be systematically informative) is similar in the two
species. I don't know what to do with that, we need more data. It is
possible that overall pubis depth varies at the species level generally.


> bernissartensis_. However, I see that your theory is a new
> idea based on your work on hadrosaurs: hopefully this will
> prompt someone in future to look into it in more depth.
    If someone has $15,000 to donate and a pair of ginormous callipers, I'll
ahppily go and do it myself!


> Incidentally, one should not talk of 'crown' hadrosaurs
> seeing as, by definition, crown groups only encompass
> extant representatives of clades. A common error (the most
> notorious of which might be Sullivan's 'crown-group
> Ankylosauridae').

    We address this explicitly earlier in the paper; sorry I didn't send
that bit. Using "stem" and "crown" avoids the somewhat contentuous issue of
where on the tree the term Hadrosauridae should be applied. I favor a very
strict definition (as do certain noteable ornithischian specialists).
However, since others have argued strenuously (if not effectively) to
include their favorite "basal" hadrosaurs in the definition, we opted to
avoid this issue by picking a very broad definition of Hadrosauria (so that
most everything of interest is a "hadrosaur"), and dodged the issue of the
"family" definition altogether. Since the "crown" term is being used
(inappropriately) in some circles, and it proceedurally amount to the
concept we wanted, the group of well-known, well-characterized, unequivocal
members of an established taxon (for taxa with extant members, this is the
true crown), we felt it was better than adopting a new term for a clade that
already suffers from nomenclatural proliferation.


> ---------------------
> Actually, I. atherfieldensis comes out closer to haddies...
> just as juvenile chimps might code closer to humans in a
> morphological analysis.
> ---------------------
> They probably wouldn't seeing as the _Homo_-like
> characters of baby chimps (proportionally larger cranium,
> smooth brow ridges, flatter face) are not seen in hominins
> closer to _Homo_ than to _Pan_. Baby chimps instead have
> the intermembral indeces, palatal shape, long dentary
> symphysis, dorsoventrally long ilia, opposable hallux and
> laterally round (rather than oval) femoral condyles of non-
> hominin hominids so surely would group close to _Pan_
> even if coded as a separate OTU.

    Certainly a good point, from what I understand of hominid systematics.
My comment was an off-the-cuff comparison regarding the dangers of coding
different ontogenetic stages in an analysis. Since I (obviously) don't know
anything about hominid morphology, perhaps you would enlighten the group on
some of these characters... I was under the distinct impression (from
childhood trips to the zoo) that an opposable hallux was ancestral for the
great apes. Is this, and the other characters, really a synapomorphy of
_Pan_? It sounds, from your description, like the characters you listed are
plesiomorphies, and therefore would not result in a _Pan_ + juvie _Pan_
clade in the analysis. You should then have a trichotomy of _Pan_, juvenile
_Pan_, and Homininae, which might, with poor sampling of the latter taxon,
result in selection of a different ingroup tree for hominines, and different
interpretations of the ancestral states. Which was where I was going in the
first place.
    Obviously, the tenor of the above is a bit of a combative, grouchy
response to a nitpicky point (such picking of nits from HP Naish is always
good-natured and well-informed and therefore very welcome). I would not be
suprised to be told there are apomorphies of _Pan_ that develop relatively
early in ontogeny. These would probably be recognized as such by specialists
and would be applied to the problem, resolving the issue nicely.
    As far as I can tell, Hominids are a realtively well-sampled and
well-understood group (by paleontological standards). The "viscinity" of
Iguanodon on the ornithopod tree is, judging by the recent pronounced
increase in named taxa, probably relatively poorly sampled and poorly
understood (from uncertainty in recent analyses, including my own, and from
this very discussion). Coding a morphologically immature specimen might be
especially misleading under these circumstances. That's all.


> ---------------------
> Much as I hate genera, hate generic proliferation, and
> thoroughly despise monotypic "higher" taxa, my vote is for
> _Mantellodon_.
> ---------------------
>
> Hmm, well it's by no means clear that Mantell's teeth were
> from _I. atherfieldensis_, so this might not be so cool:)

    The idea was just to give the man SOME credit in the name of a valid
taxon. Normansgiantbitchincoolmonographosaurus is a bit long, and hardly
"euphonious." ;)

Wagner


REFS:

Gilmore, C. W. 1946. Notes on recently mounted fossil skeletons in the
United States National Museum. Proceedings of the U.S. National Museum,
96(3196):201-203.

Norman, D. B. 1980. On the ornithischian dinosaur Iguanodon bernissartensis
of Bernissart (Belgium). Memoires de l'Institut Royal des Sciences
Naturelles de Belgique, 178:1-103.

Norman, D. B. 1986. On the anatomy of Iguanodon atherfieldensis
(Ornithischia: Ornithopoda). Bulletin de l'Institut Royal des Sciences
Naturelles de Belgique. Sciences de la Terre, 56:281-372.
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