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I have looked over McDonald’s paper.
As it happens I have a paper in process on the subject.
Before I go further I am going to note that there is a really big
stratigraphic problem with working with Euroiguanodonts of the late Barremian
early Aptian (a problem I have been trying to get folks to pay attention to
since my Cretaceous Research paper which is at my website). It’s the famed
Bernissart quarry. Problem with the darn thing is that we don’t know its actual
age. We have very good data of the age of the holotype of M. atherfieldensis
because it is from nicely laid horizontal beds whose time of depostion is
well understood. But the Bernissart quarry was a fissure fill. According to a
series of papers cited in my CR paper and McDonal it can only be pinned
down to around 3 million years. This means those fossils are sort of floating
This is very important because there is lots of work coming out indicating
that dinosaur taxonomy is highly dependent upon specific stratigraphy.
Apparently what level a hadrosaur or ceratopsid came from in the Dinosaur Park
Formation that is just 80 m thick and took just a million or so years to
deposit has a lot do with what species it is. Three species within a genus or
appeared sequentially within this time. Something similar appears to be
going on in the Hell Creek/Lance.
This brings up something that is really interesting albeit little noticed
(so little I did not think to include it in my new paper). There is a
tendency to think that M. atherfieldensis and I. seelyi represent the same
No they don’t. The second is late Barremian and the first early Aptian.
Unless things did not change all that much for far longer periods of time in
England at that time than in the Dinosaur Park Formation they are members of
distinct faunas, and it cannot be assumed that they overlapped, unless much
better specimens than we now have indicate otherwise.
McDonald is correct that robust I. seelyi cannot be the same species or
genus as D. bampingi. However, I. seelyi is incomplete, probably too much so to
be assigned with confidence to another species. Has not been properly
described anyhow. Also, we not only do not know whether it is really
with I. bernissartensis, chances are they could be separated by a million
years or more. So as I noted in CR, I. seelyi should not be refered to I. bern
., and is best considered I. sp. until sufficient remains show up (don’t
hold your breath on that) to show whether the species is distinct and valid,
or that it really is not distinguishable from I. b.
The time problem is even worse between M. atherfieldensis and D. bampingi
than it is between I. seeleyi and I. bern. because M. a. is pretty solidly
early Aptian and the Belgian remains are most likely to be from somewhere in
the late Barremian. Ergo, the possibility that M. atherfieldensis and D.
bampingi are contemporary members of the same fauna is real low, and to show
that they are the very same taxon requires showing that they really are nearly
identical in the manner accepted in other dinosaur taxa.
In my newest paper I pull a little trick. I publish skeletal restorations
of Corythosaurus and Lambeosaurus. But -- being the taxonomic prankster that
I am -- I remove the crests! Try that yourself with skeletal restorations,
mix em up, and try to figure out which is which. These are widely accepted as
different species and even genera (the last is not a good idea), and they
are nearly identical in basic skull-skeletal morphology. Much more so than M.
a. and D. b.
There is another serious problem with comparing the M. a. and D. b.
holotypes. While the first skeleton is pretty good, it is missing the neural
spines, so we cannot compare this feature which is one of the ways to
iguanodonts (D. b. has taller spines than I. b. for instance). In principle
specimens that can be refered to M. a. can help with this, but this is only
true if the specimens are sufficiently complete to show they really are M. a
. versus something else (such as D. b. or D. sp.), and they need to be from
the same horizon as the type to show they really are the same species.
To get a better idea of why M. a. and D. b. cannot be the same taxon check
out my skeletal restorations in the CR paper or your handy-dandy field
guide. Do they really look like the same genus much less species? I cannot
of an example of a dinospecies in which there is so much variation excluding
differences in horns, bosses, crests and the like in a species from the
same horizon. There is no such example in the field guide even though I have
done multiple specimens for some species. To look at it another way, let’s say
I had never separated the two taxa. Say I did both skeletons, and published
them as both M. atherfieldensis. A lot of people would be saying wait a
minute, they don’t look like the same species or even genus, what’s the Greg
Paul guy doing trying to pass off them as the same species?! But of course he
would do that, he IS a known lumper after all (which I am really not, but
the widely held belief helps make the point).
Think about it. All of the American Tyrannosaurus specimens look pretty
much the same. I’ve done most of the skeletons – kind of boring after awhile.
If there was as much gross difference in the appearance of the two skeletons
as between M. atherfieldensis and D. bampingi they would be considered
distinct taxa. Even C. bauri are not that different – all C. bauri look like C.
bauri, they just differ in that some have been stretched and pulled on
certain parts compared to others.
McDonald dismisses the differences between M. a. and D. b. as mere
proportional divergence, but it is not. Nor is my restoration of the skull of
first “subjective.” The M. a. holotype is a large juvenile – centra not fused
to neural arches. But it is almost as large as the D. b. holotype. So
ontogeny does not explain the differences. This is especially true because the
posterior skull of M. a. is literally larger than that of D. b.! You can see
this in the same scale fig 4 in the CR paper. The two heads are those of very
different creatures. The highly elongated snout of D. b. is a very
distinctive feature, rare amoung iguanodonts.
The only alternative is sexual dimorphism, but such extensive variation
between boys and girls has not been shown to exist within other dinosaur
species. In order to demonstrate that the differences between the two holotype
specimens is really sexual would require the following. A series of quarries
from the same horizen would have to show that both types are always co-present
in fairly similar numbers. That would be highly compatible with but not
establish dimorphism. But if some other iguanodonts from differing horizons
showed the same pattern the sexual dimorphism hypothesis could become superior
to the two taxa hypothesis. Because this situation is far from present, and
because M. a. and D. b
probably differ substabtially in age, the dimorphism hypothesis is very
Basically, those who want to unite a number of specimens from two different
time stages and several formations think it is a good idea to consider a
lower tooth count, short snouted, short trunked and short armed so strongly
bipedal and, deep ilium and big hipped iguanodont from the lower Aptian the
same species or even genus as a lower tooth count, short snouted, short
trunked and short armed so strongly bipedal, deep ilium and big hipped
from the a higher tooth count, long snouted, long trunked and longer armed
so strongly quadrupedal bipedal, shallow ilium and smaller hipped iguanodont
probably a couple million years earlier from the Barremian.
I am not going to go into a long discussion of specific characters here, it
is discussed to a certain extent in the new paper. I will observe that
placing ilia as disparate and time separated as those in McDonald’s Fig. 2 is a
major stretch, it looks more like evolution at the species and perhaps genus
level within a conservative group. Also, I am pretty sure that both ilia of
the D. b. holotype are lower than in the "mantellisaurs," and the first
specimen was crushed laterally not dorso-ventrally, so that diagnostic
distinction between the taxa probably stands. In any case the current methods
using characters to designate taxa have problems if they end up putting such
disparate skeletons in the same species.
There is something else going on here that as a paleo/ntologist/artist I
find fascinating. None who are arguing that D. b. is the same as M. a. are
publishing side-by-side detailed skeletal restorations of both holotypes. I
suspect that is because the fact they obviously differ so much undermines the
hypothesis. What is even more cool is that those who do publish a “M. a.”
are publishing that of the D. b. holotype! That’s so cool that it’s bad.
Guys, if you are going to publish the skeleton of M. a., then publish that of
the holotype. That’s the only skeleton that for sure is M. a., and everyting
else stems from that. Using the D. b. 1551 holotype probably from a different
age as the premeire example of M. a. is getting people to think that is
what M. a. is, when 1551 is not at all a typical M. a. judging from the actual
holotype – in other words it is a form of circular logic in which since 1551
is used to illustrate M. a. then it is the same taxon as M. a.
McDonald as well as Norman are taking a lot of remains from a wide range of
horizons, much of them too incomplete to properly diagnose, with one major
quarry’s age not certain, and two holotypes that are quite distinct, and
throwing everything into just two species. This is not happening just in the
upper Wealden, but also much lower down in the Valanginian and Hautervarian.
Composite skeletons of supposed specific species are being assembled from
parts from different horizons, a no-no if one is trying to restore the known
appearance of as well as diagnose a single species. With what we now know
about the relationships between dinosaur species and stratigraphy, and how
evolution works, this possibility of this simplistic scenario being correct in
terms of the specimens we have on hand is very remote.
So I beg of you all. Please, please pay attention to the stratigraphy.
And we all need to stop taking incomplete specimens and automatically
chucking them into a species, even when the latter is based on a good holotype,
unless the remains are definitely from the same subhorizon of a formation as
the holotype, or it has been conclusively shown from a set of good specimens
that the species is found in more than one level of the formation and the
specimens are form one of those levels. For example labeling all Hell Creek
fossils that are attributable to Triceratops as T. horridus is looking to be
dubious, there may be a sequential series of species. If that is so then in
the future tri horned face remains from low in the formation will be T.
horridus, and from high up T. prorsus. When it comes to the upper Wealden
iguanos most specimens are at best assignable to a genus, and in many cases not
even that. The only stuff that should go into M. a. is that from the early
Aptian horizons. Putting remains that are not complete enough to show they are
anatomically the same species from higher up or lower down strongly risks
giving a false impression of the temporal span of the species. Not only is
there nothing wrong in not placing specimens of questionable identity in a
species, it is the most scientific thing to do.
And hey, since I am supposed to be such a lumper, then it follows that if I
say that M. a. and D. b are distinct then that’s the way it has got to be.
Sounds logical to me.
One thing that is disappointing is the difficulty described by McDonald in
examining key specimens because of how they are mounted. That’s not good.