Can We Get A Hand?
Or at least some measurements of hands? One of the things we’d love to do with the ODP data is investigate the way that the proportions of the fingers and toes, phalanges and unguals, metacarpals and metatarsals, changed through evolution. Do quadrupeds do something funky with their hands, and do they all do it in the same way?
For a little bit of data exploration, I decided to create a visualization of the proportions of the various bones making up the third digit of the hand. The third digit (which we know as the middle finger) is usually the central one in dinosaurs, or at least the longest one. Thus, it might be inferred to be most important functionally. The graph below shows the relative proportions of each phalanx and metacarpal in digit III, for a variety of dinosaurs. All have been scaled to the same length.
There are certainly some interesting patterns. First, the little ornithischian Heterodontosaurus and the saurischian Herrerasaurus (our outgroup comparison here) have very, very short metacarpals relative to the rest of the finger. Second, hadrosaurs (the duck-billed dinosaurs, including Kritosaurus, Parasaurolphus, and Corythosaurus) have really, really long metacarpals. Without exception, around 2/3 of their hand is metacarpal (equivalent to our palms). Note also that these hadrosaurs have lost phalanx four, and the penultimate (second-to-last) phalanx is really, really short. Ceratopsians (the horned dinosaurs, including Protoceratops, Styracosaurus, and Centrosaurus) and Hypsilophodon are somewhere in between.
How might I interpret this (cautiously, because we have so few data here)? Without a doubt, hadrosaurs are funky. Is this an adaptation for their unique brand of quadrupedality? Maybe. . .it would be nice to compare with more basal iguanodonts. I would like to say that the intermediate proportions of the ceratopsians (between Heterodontosaurus and hadrosaurs) are adaptations for quadrupedality, too, but the similar proportions of Hypsilophodon (usually assumed to be bipedal) add an unwelcome reality check on this hypothesis. Assuming that Heterodontosaurus and Herrerasaurus are both basal, perhaps we can interpret their digits as the ancestral condition for dinosaurs. These are interesting hypotheses, and ones that could be tested.
Unfortunately, we are running up against a problem with the goal of thoroughly documenting this aspect of ornithischian evolution: there just aren’t many measurements of fingers and toes out in the literature. I suspect the reason for this is two-fold: 1) fingers and toes were often the first thing to wash away prior to fossilization, or are so disarticulated that it’s impossible to identify which is which; and 2) authors present measurements for the humerus, radius, ulna, and sometimes metacarpal or metatarsal III, but don’t measure the rest of the limbs.
Conspicuously absent from the list: measurements of hands and feet for ankylosaurs, stegosaurs, basal ceratopsians (including psittacosaurs, for which there are literally dozens of articulated hands and feet!), and non-hadrosaurian ornithopods (with some many good, articulated iguanodont skeletons out there, you think someone would have published measurements). Even clades with a decent number of good hands and feet–such as hadrosaurs and ceratopsians–are missing measurements for many of the key specimens. So here’s your assignment: 1) if you have access to original material catalogued in public collections, spend a few minutes to measure some hands and feet, and send the data our way; and 2) if you have access to the literature, see what you can find for measurements there.