Dinosaur hand and forelimb posture might have been more diverse than previously hypothesized

Turn a doorknob and you are taking advantage of what anatomists call pronation and supination: the ability to rotate your hand palm-side down (pronation) or palm-side up (supination).  This ability stems from your bone geometry: the radius bone in your forearm is curved can pivot around your ulna, rotating your hand in the process.  Drop to the floor and crawl, and your hand is pronated by crossing the radius over the ulna just as it is for mammals which walk on all-fours like elephants, dogs, and cats.

Pronation and supination of the hand by rotating the radius bone over the ulna in humans. (c) 2013 M.F. Bonnan.

Pronation and supination of the hand by rotating the radius bone over the ulna in humans. (c) 2013 M.F. Bonnan.

In our paper published this week in PLOS ONE, my former student, Collin VanBuren (now a Ph.D. fellow at the University of Cambridge, UK) and myself suggest that most dinosaurs could not actively pronate their hands (that is, turn doorknobs) because their radius could not cross their ulna. Our conclusions were reached after analyzing the bones of nearly 300 specimens representing living birds, reptiles, mammals, and dinosaurs like Tyrannosaurus, Apatosaurus, and Triceratops.

Difference in radius bone geometry are correlated to some degree with forelimb posture.

Difference in radius bone geometry are correlated to some degree with forelimb posture.

Statistical analysis of radius geometry shows that dinosaurs most often have a straight radius bone with a non-circular head (the part that allows movement at the elbow), a shape similar to those of lizards, crocodiles, and birds.  These animals cannot actively pronate their hands, and in lizards and crocodiles this radius geometry is correlated with a non-erect forelimb posture.  In contrast, most land mammals show a curved radius geometry that enables the forelimb to be held erect and the hand to be pronated.  Mammals like ourselves have a well-rounded radial head that allows the radius to actively swivel around the ulna.  Tellingly, the only mammals in our sample that resembled reptiles, birds, and dinosaurs were the primitive, sprawling egg-laying duck-billed platypus and spiny echidna.

Our findings are significant in that they show dinosaur forelimb posture was not mammal-like and, possibly most importantly, more diverse than previously appreciated.  For example, radius shape suggests the forelimb posture and range of pronation in horned dinosaurs like Triceratops was more like those of a crocodile than a rhino.  In another example, the radius geometry of the giant, long-necked sauropods such as Apatosaurus don’t comfortably group with living reptiles, birds, or mammals, suggesting that their forelimb postures were achieved in anatomically novel ways.  Ultimately, our data strongly suggest that we must re-evaluate our conceptions of how dinosaurs could and could not use their forelimbs.

We can also breathe a sigh of relief: most predatory dinosaurs could not open our doors.

I  must give a big shout out and expression of gratitude to Collin — his dedication to this project, through several starts and stops, is what finally saw it through.  That we landed this research in a venue like PLOS ONE is that much more of a testament to his perseverance to get this science out there.  It means a lot to me that we got this out and into open-access: this represents the accumulation of some of my inferences and hypotheses on dinosaur forelimb posture since my graduate school days.  I also want to acknowledge the influence and inspiration of some fellow dinosaur forelimb fanatics, namely Ray Wilhite, Phil Senter and Heinrich Mallison.  All are colleagues and friends, and all have also in their own unique ways put dinosaur forepaws front and center — I encourage you to check out their research!

Read our paper, which is open access: http://dx.plos.org/10.1371/journal.pone.0074842

Apparently I’ve gone viral -or- Stick figures and evolution

I came back from teaching an introductory biology course yesterday (Friday, Sept 13, 2013), to find a whole lot of messages from friends and family waiting for me on Facebook.  It turns out, unbeknownst to me, that my stick figure evolution cartoon was picked up by I F**king Love Science and went viral!

Stick figure evolution, by me — an illustration I use to explain to my students and lay audiences how evolution is not a straight line but a family tree of life. This went viral on IFLS: https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-prn2/1185649_635460856485590_1073680004_n.jpg

First, I am honored — honored that anything I did would even remotely reach this kind of an audience, and honored that IFLS found it worthy.

Second, please keep in mind this is intended to be fun, simple, and, above all, a diagram.  I submitted it for a Stick Figure Science contest back in 2010 for Florida Citizens for Science: http://www.flascience.org/ss2010top10.html   I was pleased at that time that it made the top 10.  So, this cartoon of mine has been out and about for a while.  It was quite surprising to see it “discovered” and shared so rapidly just now.

Third, the comments section under this cartoon has been interesting to read.  I wanted to tackle some of the recurrent themes here and put to rest any misunderstandings.

First, I will tackle the whole relationship aspect of the questions:

  1. There a lot of comments that suggest the human family tree is wrong — that I should have aunts and uncles where I have cousins.  The confusion here seems to be the conflating of “relatives” in a general sense and common ancestry.  Certainly, you share DNA with your aunts and uncles, and no doubt they descended from your grandparents.  But remember – your have aunts and uncles on your mom’s side and on your dad’s side.  Therefore, they are related to your parents and to you, but they are not your direct common ancestors.
  2. Regarding your relationship with your cousins – the only direct common ancestor you have with your cousins are your maternal and paternal grandparents.  Why?  This is because, unless there is a lot of close family intermarriages, your aunts and uncles, from which your cousins are descended, likely married someone not related to them.  So your cousins share some but not all of their DNA with you and your parents.  To find a common ancestor – a group of people whom you and your cousins could all call blood relatives – we have to look to your grandparents.
  3. Regarding second-cousins — as it turns out, there are many colloquial and law-based ways in which this word is used.  When I made the cartoon, I was simply thinking of the children of your great aunts and uncles, to which you would share a last common ancestor with your great grandparents.

Now, I want to clear up biological evolution and comments related to how I drew the “family tree”:

  1. The point I am making with the cartoon is that if you can appreciate that you have descended from common ancestors you call your parents, grandparents, and so on, you can understand biological evolution at it’s most basic level.  Darwin said that biological evolution is a theory that explained the diversity of life because of “Descent with modification” from a single, common ancestor.  All living things on earth, and all those we find as fossils, are/were part of a great family tree of life.  That is the sum total of what biological evolution means to a scientist.
  2. Saying you “evolved” from a cat or chimp or whatever else is as incorrect and ridiculous as saying your morphed from your dad to you.  No serious scientist would accept that.  But saying that we have a closer common ancestor with other mammals, like a cat, than we do with a salamander is what the data support.  As an example, the theory of biological evolution predicts there would be a common ancestral population of mammals from which all of we milk-giving fur balls have descended.  But no monkeys are mutating into humans, I promise.
  3. I stress BIOLOGICAL EVOLUTION – that is because the word “evolution” means a lot of things and is used in various ways to describe change over time, star development, the development of cultural ideas, etc.  BIOLOGICAL EVOLUTION strictly means “Descent with modification from a single, common ancestor.”

Finally, there has been a “fishes” controversy:

  1. Please keep in mind that our common use of words doesn’t always apply in science.  Theory is a great word that outside of science means a guess or hunch, but in science a theory is a powerful tool – an over-arching explanation of laws, hypotheses, and observations that can be tested, falsified, and has predictive power.
  2. Yes, yes, I know — we were all scolded by our English teachers that “fish” is singular and plural.  Except that in biological and paleontological science, “fish” is, well, “fishy.”  Based on common ancestry, for example, we humans are highly derived bony fish!  Why?  Because we share homologies with everything that has an internal bony skeleton that set us apart from non-bony backboned (vertebrate) animals.  Sharks are a type of “fish,” technically called chondrichthyans; the fish we consume tend to be ray-finned bony fishes or actinopterygians, etc.  In science, it is perfectly acceptable to use the term “Fishes” to acknowledge that “Fish” is not a catch-all for everything with fins, scales, and gills.  Don’t believe me?  Fine, see Berkeley’s evolution website right here: http://evolution.berkeley.edu/evolibrary/article/fishtree_01

Still confused?  Please remember that what you were taught in high school for biology is often simply a list of beasties with name tags — the good old Linnean classification system: Kingdom, Phylum, Class, and so on.  The Linnean system is fantastic for being a universal labeling system, but it does not often reflect evolutionary relationships, just as your name and family name don’t always accurately reflect your DNA and actual ancestry.  Why else would National Geographic and other entities be so popular at tracing people’s lineages through DNA?  Modern biology and paleontology use a tool called cladistics which focuses on unraveling relationships through special, shared traits called homologies — however, most of the non-career-science public is not familiar with this science, and that understandably causes confusion.

Finally — yes, technically what I have drawn in the cartoon are over-simplified cladograms.  They still reflect the fundamental principle of BIOLOGICAL EVOLUTION — a family tree of life, descent with modification from a single, common ancestor.  If you can communicate this family tree concept to a lay audience with stick figures even more elegantly in a single panel, I would love to see it and cherish using it in my teaching and outreach.

If you want to learn more, I have a whole audio series and other “resources” on evolution.