Hone et al. (2005) on dinosaur body size evolution

[David Marjanovic <david.marjanovic@gmx.at>]

For technical reasons I can't work today, so I'll launch a great & mighty 
nitpicking attack.

Recently I said that the methods used by the following study were "far too 
simple":

D. W. E. Hone, T. M. Keesey, D. Pisani & A. Purvis: Macroevolutionary 
trends in the Dinosauria: Cope's rule, Journal of Evolutionary Biology 18, 
587 - 595 (2005)

David Hone then wrote me a nice little e-mail explaining why he thought 
the methods weren't so simple at all. It turned out that I had last read 
the paper at least half a year ago...

I read it again last evening, and here are my comments:

p. 587:
"Cope's rule, where found, is a result of one or more of three processes: 
(i) necessary increase in average size when founders of clades are small 
organisms (Cope himself suggested that the trends he observed were the 
result of ancestors being small [...]; (ii) genuine natural selection 
where teh advantages of larger size act to produce progressively larger 
descendants; or (iii) clade selection where larger taxa within a lineage 
tend to survive or proliferate in preference to smaller taxa".

IMHO (i) would not be the operation of any "rule" but the inevitable 
result of body size diversification restricted by a minimum size near 
which the ancestor is. So Cope's rule seems to be defined as "everything 
that, however superficially, looks like Cope's rule".

Maybe, however, this is just the difference between Cope's rule and 
Depéret's rule; I'm more familiar with the latter. (Depéret was extreme. 
To him any evolutionary increase in body size was absolutely irreversible. 
He insisted that even the small size of the (sub)fossil dwarf elephants of 
various mediterranean islands was plesiomorphic!)

My study will be simpler in this respect. I will most probably not try to 
distinguish (ii) and (iii).

p. 588:
"We used total adult body length in metres [...] to indicate body size 
[...]."

Many of these lengths are potentially quite shaky estimates. For example 
*Sauroposeidon* was used even though the only known fossil consists of 
nothing but 4 cervicals. Additionally length and mass may not be 
correlated very well. For example much of the length variation in 
sauropods comes from the tail, which is rather thin in the part the length 
of which varies most, and the neck, which consists mostly of air.

I use several osteological measurements instead -- but I can't guarantee 
that these will be better correlated to mass! Most or all of them can vary 
independently of the size of the rest of the body, and the program can't 
combine them.

"We [...] constructed phylogenetically independent comparisons between 
related taxa of different ages,"

Here begins the description of the first method: independent phylogenetic 
contrasts. While laudably taking phylogeny into account, it does not 
include any attempt to infer the body size of any ancestor. Instead it is 
forced to assume that the old taxa are direct ancestors of their closest 
known young relatives. The program I'm using gets around this constraint 
and even takes branch length into account (after all a lot of evolution 
can happen in tens of millions of years).

"using a recent supertree of 272 genera (Pisani et al., 2002), the most 
inclusive dinosaur phylogeny available."

A supertree is a time-averaged consensus of opinions about phylogeny, no 
matter (in this case) how well supported those opinions were when they 
were produced, let alone how well supported they are now after the 
discovery of new data. Consequently I went through the "trouble" of 
compiling my own supertree by hand, using only the most recent sources. In 
addition to cladograms I used the accompanying text; for example the 
sauropod chapter of The Dinosauria II allowed me to place a few 
titanosaurs which are not included in the cladogram of the same chapter. 
Consequently my database is very, very big.
     The only drawback is that the program I'm using can't deal with 
(soft) polytomies. In cases where the phylogenetic position of a species 
wasn't so uncertain (smeared over several nodes) that I wasn't able to 
include it (this is the case for plenty of ankylosaurs, for example), I 
resolved the polytomy by stratigraphy, and where that was impossible, I 
resolved it based on what feels best. Clearly this is badly defensible, 
even though it may not be any worse than any published resolution of the 
Avebrevicauda-*Archaeopteryx*-*Rahonavis*-Dromaeosauridae sensu latissimo-
Troodontidae polytomy.

"A comparison in which the later taxon is larger than the earlier taxon 
supports Cope's rule. Under the null hypothesis of no body size trend, 
around half of such comparisons should show a size increase and the 
remainder a size decrease, with the mean change across all comparisons not 
differing significantly from zero."

This null hypothesis works only if there's no minimal body size. IMHO 
there are several different such minima. For examples theropods may have 
been limited by competition with mammals, lizards and so on (the birds 
have flown around this barrier), while sauropods were certainly limited by 
their graviportal build -- an animal the size of, say, a dog is in grave 
danger if it can't run, isn't armored like a turtle, doesn't live in an 
environment without predators of comparable or larger size, and yet isn't 
small enough to hide in some hole.
     (The mini-stegodonts on Flores that were preyed on by the Komodo 
monitor may be a borderline case -- the monitor isn't good at running 
either. However, I really wonder what baby sauropods did.)

p. 589:
"In comparisons where the bauplan is varied (e.g. within the Stegosauria)"

Stegosauria???

fig. 1: *Patagonykus* is shown as Kimmeridgian or earlier. IIRC it's as 
young as *Alvarezsaurus*. *Avimimus* is shown as the sister-group of 
Alvarezsauridae because not enough studies where it's an oviraptorosaur 
were included in the supertree. The same goes for *Caudipteryx* and 
probably the misspelled *Protarchaeopteryx* (which was given an extra o).

"We explored patterns of size change in two other ways. First, Jablonski 
(1997) provided a graphical method of representing Cope's rule or other 
trendds in size over time. This uses deformed cylinders to represent the 
variance of size changing over time within taxa. We have adapted this 
approach to demonstrate Cope's rule with a 'Jablonski polygon'. Simply, 
the sizes of the taxa under study are plotted against time (or 
stratigraphy). A line is then drawn between the largest and the smallest 
taxa at the earliest and latest time. If Cope's rule has been operating, 
this should provide a right-leaning rhombus, as both the smallest and the 
largest taxa at a later evolutionary time should be larger than their 
respective counterparts at an earlier time."

If the difference in size of the earliest and the last smallest taxa is 
not significant, we are more probably dealing with diversification away 
from a left wall. Indeed 3 or 4 of the 5 Jablonski polygons in fig. 4 have 
vertical or nearly vertical left boundaries.

If, however, the left boundary is strongly inclined, as is the case in one 
of these polygons (apparently that of Jurassic sauropods), then we have 
two possibilities: Either Cope's rule has acted -- or the taxon first 
diversified (with or without a left wall!) and then the small-bodied 
clades died out, leaving only the big ones.

My advisor and I have already identified one case of the latter phenomenon 
(submitted... IIRC), using the published method to tell apart the effects 
of time (which Cope's or at least Depéret's rule would be) and the effects 
of phylogeny (diversification):

Y. Desdevises, P. Legendre, L. Azouzi & S. Morand: Quantifying 
phylogenetically structured environmental variation, Evolution 57, 2467 -- 
2652 (2003)

Take-home message: "Nothing makes sense in evolution, except in the light 
of a good phylogeny!"

p. 590: "Phylogenetic analyses of the fossil data require that both the 
phylogeny and the fossil record are adequate for the purpose. We used the 
method of Benton (1995) to test the goodness of fit between the phylogeny 
and the stratigraphy."

This looks better that what I'll do -- I won't do any such test. But it 
isn't necessarily any better. If both the phylogeny and the fossil record 
are good, they'll fit; if both are bad, they can still fit by chance; if 
one is good and the other bad, it doesn't tell us which is what.

"[...] we tested both orders and each family and superfamily 
independently."

A bit too much reliance for my taste, but it can't matter much -- all 
those taxa are monophyletic after all.

"Notably, the size increases occur at various starting sizes and so the 
size change is not just a result of beginning with small genera."

This would be true if there were either no minimum size for dinosaurs or 
if the minimum size were the same for all dinosaur clades. I can't imagine 
that either of these possibilities is the case, see above.

Minor nitpicking: *Archaeopteryx* is Late Jurassic as shown in fig. 1, not 
"mid" as mentioned on p. 590, and "Late Cretaceous" starts with a capital 
letter.

Table 1: "Sixty-five phylogenetically independent matched-pairs 
comparisons selected for analysis based on the phylogeny."

I gather it doesn't matter much that I count no less than nineteen typos 
and similar misspellings (like "Iguanadon") in this table. But the 
following things certainly matter:
- As mentioned above many of these taxa are so incomplete that I wouldn't 
dare estimate a total length. *Unenlagia*, *Therizinosaurus* and 
*Antarctosaurus* for instance.
- The size of *Dilophosaurus* is not known. The only known specimen is 
subadult. Thus *D.* was most probably not slightly smaller but quite a bit 
bigger than *Liliensternus*.
- Comparing *Alvarezsaurus* with *Avimimus* relies on a probably wrong 
phylogeny (see above).
- Why compare *Gasosaurus* and *Marshosaurus*? Maybe both were listed as 
"megalosaurids" (which used to be a horrible wastebasket!) in the source 
the supertree used?
- The age difference of *Carnotaurus* and *Majungatholus* is given as 28.2 
Ma. Off the top of my head this assumes that *Carnotaurus* is Early 
Cretaceous. Indeed this was the age published in the description, but, as 
has been known for many years, *C.* comes from a different formation that 
is much younger, IIRC Campanian, so the difference to the Maastrichtian 
*Majungatholus* shrinks to some 10 Ma or less.
- Comparing *Ligabueino*, a noasaurid, to *Abelisaurus* is suboptimal. 
Abelisaurids the age of *Ligabueino* (and many times its size) are known. 
Besides, isn't *L.* juvenile or something? (I can't look that up at the 
moment...)
- *Alamosaurus* as currently understood ( = any sauropod from the 
Campanian or Maastrichtian of North America) could contain any number of 
"genera" of different sizes.
- The comparison of *Euhelopus* and *Nemegtosaurus* is not independent of 
some or all of the three involving *Argyrosaurus*. *Nemegtosaurus* seems 
to be a highly derived titanosaur, close to *Antarctosaurus*, while 
*Euhelopus* is at best a _very_ basal titanosaur (and at worst, of course, 
a non-neosauropod that has convergently evolved somphospondylous 
vertebrae). Apart from that, *E.* has a preposterously long neck (like 
*Mamenchisaurus* which is shown in fig. 4), while *N.*, only known of a 
skull, can be expected to have had more usual proportions based on its 
relationships.
- *Euskelosaurus* is in the process of being split into at least two 
"genera" of which some may be prosauropods and others sauropods...
- If we're out of luck, the comparison of *Camptosaurus* and 
*Muttaburrasaurus* is not independent of that of *Hypsilophodon* and 
*Parksosaurus*.
- Same for *Yandusaurus* and *Othnielia*.
- Same for *Bactrosaurus* and *Pararhabdodon*; apart from that *B.* (IIRC 
it's from the Iren Dabasu Fm, is it?) may be much younger than usually 
thought.

fig. 4:
What is the circle in the Triassic theropod(?) polygon? Circles are 
supposed to be reserved for eumaniraptorans...
The silhouette of *Argentinosaurus* is a fantasy portrait, except for a 
few things like the length of the lower legs. I think it was made for 
*Apatosaurus* and scaled up. See above for *Alamosaurus*.

The study of the evolution of body size is in its infancy. Every paper in 
this field is much better than all its predecessors, and this state of 
affairs is likely to continue for several more years.

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