Sauropod forelimbs -or- why I was wrong -or- why I do research

An in-press, open access paper by Joel Hutson extensively cites my Bonnan (2003) paper while developing a hypothesis that quadrupedal dinosaurs did not evolve fully pronated forearms.  Hutson suggests, correctly, that the hypothesis linking hand morphology and pronation in Bonnan (2003) is falsified.  I agree.  I also agree with Hutson that dinosaur forearms are best understood in the context of other non-mammal tetrapods, and I agree that mammalian-style (and chameleon-style) pronation of the hand was not possible in known quadrupedal dinosaurs.  But I take issue with the tone of Hutson’s paper, and for what I think he misses about the process of science.  To put this in context, first a little history:

As odd as it may seem, the forelimb posture of quadruepdal dinosaurs is anything but settled.  This is due to several reasons, chief among them being that a large amount of articular cartilage encapsulated the ends of the long bones (see here and here, for example).  Since this tissue is rarely preserved, determining how the elbows and shoulders of dinosaurs went together, let alone their possible ranges of movement, is difficult to determine at best.  This makes determining how the bones were oriented in life difficult to resolve.  Regardless of how much cartilage was or was not there, a dinosaur forearm and that of a large, quadrupedal mammal are different.  Without going into a long, drawn-out discussion on the subject, suffice it to say that, like other archosaurs, the radius and ulna of most quadrupedal dinosaurs lie parallel to one another.  If the forearm was held as a relatively vertical support structure, it is difficult to envision how the hand would be pronated so that it moved in synchrony with the foot.  Large mammals accomplish this by significant crossing of the radius over the ulna: this turns the hand palm-side down (pronation) and essentially allows it to work effectively in tandem with the foot to push the animal forwards.  In other words, an elephant hand and foot push in the same direction.

In graduate school (mid-to-late 1990s), I noted what I believed were inconsistencies: 1) sauropod trackways show that the manus is often pronated (although not quite as much as mammals and certainly the palm did not face directly backwards); 2) the forearm bones articulated like they do in other archosaurs, like alligators, that cannot assume an upright, columnar forelimb posture with a pronated hand; 3) quadrupedal dinosaur forelimbs were often restored with the radius crossing the ulna to some degree, which cannot occur when you articulate the bones together.  In essence, there appeared to be a mismatch between trackways and bone morphology.

It had been well-known that the hands of most sauropods were a vertically-oriented, tubular metacarpus (palm) with stubby fingers and sometimes a large thumb claw, whereas the hind feet were more what you might expect in a big animal: a large foot spread across a fat pad.  Why the difference?  I began to notice that when the radius was articulated with the ulna, it was cradled on either side by ulnar processes at the elbow.  One of these processes was not present in “prosauropods,” theropods (including birds), and crocs.  It occurred to me that, perhaps, the radius had shifted internally in the forearm relative to the ulna, and this “new” process (the craniolateral process) evolved to buttress the humerus where the radius once resided ancestrally.  If the radius had shifted medially, this would further “drag” the hand into pronation.  There was also a lot of cool Evo-Devo stuff going on at the time, and I was absolutely enraptured with the concept of the digital arch that forms the hand in embryos.  Since this arch forms from the ulna side and spreads to the radius side, I hypothesized that a shift in radius position internally could bend the hand into a U-shaped structure.

I published on this in the Journal of Vertebrate Paleontology in 2003, and it is one of my most cited papers.  It was, to the best of my knowledge at that time, the simplest “solution” to two “problems” — pronation of sauropod hands and their U-shape.

Needless to say, a lot has happened since 2003.  Many, many more sauropods and “prosauropods” have been discovered, and other well-known species have been re-described.  In my 2003 paper, I predicted that when the earliest sauropods were found, if they had an ulna with a craniolateral process that hugged the radius, they should also have a U-shaped hand.  You know what?  I was wrong.  My first excursion out to South Africa cinched it for me — I got to examine the forelimb of Melanorosaurus, either an almost-sauropod or a basal sauropod.  That one animal blew up my hypothesis — it had a craniolateral process on the ulna, but a flattened hand.  End of story. Done.

Well, sort of.  Adam Yates and I published on the forelimb of Melanorosaurus in 2007, and we drew attention to this issue.  We suggested that the radius might still have shifted proximally at the elbow, but that it did not directly and radically effect the hand.  We suggested that the U-shaped hand seen in most “classic” sauropods evolved after this shift and may have enhanced pronation by assuming a U-shape.  But we definitely stated that the Bonnan (2003) hypothesis linking the possible shift in the radius and the U-shaped hand was falsified.  As we stated in the abstract for that paper:

The forelimb morphology of Melanorosaurus suggests that pronation of the manus occurred early in basal sauropods through a change in antebrachial morphology, but that changes to the morphology of the manus followed later in eusauropods, perhaps related to further manus pronation and improved stress absorption in the metacarpus. Thus, we conclude that changes to antebrachial morphology and manus morphology were not temporally linked in sauropods and constitute separate phylogenetic events.

So, to return to Hutson’s paper, I was surprised that he is apparently unaware of the Bonnan and Yates (2007) paper on Melanorosaurus where we clearly say, yes, there probably was no direct link between pronation and U-shaped hands.  Again — the hypothesis put forward in Bonnan (2003), based on what was available and known at the time, is falsified, so far as the U-shaped hand and radius-shift are concerned.

I was also surprised that Hutson claims, for example, that I formulated my original hypothesis within a restricted phylogenetic context.  At the time, I had dissected and studied bird and reptile forelimbs, and also examined and articulated where possible the forelimbs of “prosauropods” and theropods, and had examined a variety of mammalian forelimbs — keep in mind, this is all before it was feasible to easily digitize and manipulate sauropod dinosaur skeletons.  I reference all of these taxa in additional to numerous sauropods in my study.  To suggest my hypothesis was developed within a restricted phylogenetic context is specious.  Hutson also suggests that I was unaware of the plesiomorphic condition for pronation in tetrapod forelimbs.  I will leave that to my readers and to the scientific community to judge.

Throughout the paper, Hutson uses phrases like “Bonnan reasoned …,” “Bonnan relied upon a suggestion …,” and so forth that imply I did not examine material first-hand.  I did, and spent many many months and years agonizing over what I had examined, articulated, and dissected.

I could go on, but my point is this.  Science proceeds by making hypotheses, testing them, putting that through the process of peer-review, and the allowing the scientific world community to continue to test and modify those hypotheses.  As a scientist, you are going to be wrong, and wrong a lot.  Over time, new data are going to emerge, new approaches will crop up, and new eyes will look at old bones.  You do the best you can with what you have, but you can’t let perfection be the enemy of progress.  No paper and no study is perfect — hypotheses will be overturned.  If we waited to publish when everything was perfect, nothing would be.

When your hypotheses have been falsified, it is okay to admit that.  In 2007, that is precisely what Adam Yates and I did — we said, yep, Bonnan (2003) got some things wrong because we now have better data, and the data don’t agree with that hypothesis anymore.  And you know what?  That is going to keep happening — scientists evolve past their older papers, and science is self-correcting.  If I were still trumpeting from the hills that my Bonnan (2003) article was totally correct and unassailable, the scientific community would be right to castigate me in light of all the new data.

So I think Hutson misses the point.  There are statements in his paper such as, “Unfortunately, pronation research has suffered from a lack of awareness that semi-pronated forearm anatomy is plesiomorphic to Archosauria, and indeed all tetrapods.”  I know many colleagues who spend an inordinate amount of time carefully collecting and examining data from fossils and living animals.  The issue is not one of ignorance or lack of awareness, but one of difficulty — it is damn hard to elucidate evolutionary patterns of forelimb posture because of so many contingencies.  I have grown to appreciate these even more as I’ve ventured into collecting kinematic data on live animals.  It ain’t easy, and it never will be perfect.

I wish nothing but the best for Hutson and his future studies on what is admittedly an intriguing evolutionary history among the archosaurs. I do hope that he remembers, when his hypotheses are ultimately changed or falsified, that this is the process of science — and that that’s okay.


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