Flat-Footed Reptiles to High Stepping Chickens

May 18, 2010

Most scientists believe that birds evolved from small therapod dinosaurs. The key step was the development of feathers, turning animals that could walk or climb into animals that could fly. In today’s post, Dr. Bakker discusses the evolution of a key feature of Archaeopteryx, the first dinosaur to be discovered with preserved feathers.

Lizard Paws to Chicken Fingers

320 million years ago – the first reptiles evolve, the earliest vertebrates that can lay air-breathing eggs on land.  Legs are sprawling and flat-footed. The front paws have five fingers, all with claws.

250 million years ago – the first archosaurs evolve, the close kin of crocodiles and birds.  Legs are more upright but still flat-footed. The hands have only three claws. The outer two fingers are thin and claw-less.

225 million years ago – the first dinosaurs evolve. Legs are upright and the heel held high, off the ground. The outer two fingers are weak.

160 million years ago – the earliest raptor-like dinosaurs evolve. The outer fingers are gone. Hind-legs are used for running and ankles are tall and thin. The three clawed fingers are long. And there’s a swivel joint on the wrist to let the hand move quickly side-to-side.

150 million years agoArchaeopteryx evolves. Hind legs are like those of raptor-type dinosaurs. The hand is almost exactly like a raptor-dinosaur’s.

110 million years ago – the first modern-style birds evolve. Hind-legs have stiff ankle bones, all fused together. The wrist bones too are all fused together and no fingers have claws.

Want to know more about our current Archaeopteryx exhibit? Check out this article on PlanetEye Traveler.

Interested in learning more about paleontology? Check out our past blogs.

Authored By Bob Bakker

The Museum’s Curator of Paleontology, world-renowned Dr. Robert T. Bakker (or, as some call him, Bob) is the leader of the handful of iconoclastic paleontologists who rewrote the book on dinosaurs three decades ago. Along with other noted paleontologists, Bakker has changed the image of dinosaurs from slow-moving, slow-witted, cold-blooded creatures to — at least in some cases — warm-blooded giants well-equipped to dominate the Earth for 200 million years. Dr. Bakker can be found all over the globe, notably leading the Museum’s paleontology field program.

3 responses to “Flat-Footed Reptiles to High Stepping Chickens”

  1. Raptor Lewis says:

    I’m interested in the environmental processes that required such traits to emerge. Normally, mutations in the DNA sequence, when they affect the phenotype are spontaneous as there is a point mutation, that may or may not change the “reading frame,” occurs during Mitosis. Mutations like these are said to be the backbone for Evolution.

    So, is it behavior that disrupted Hardy-Weinberg Equilibrium? That is something you could look into, perhaps. Well, at this point, what is your opinion, Dr. Bakker?

  2. Steven says:

    Raptor Lewis, here is Dr. Bakker’s response. Thank you for your question!

    You’ve asked a fundamental question. It’s intriguing that the very first evolutionary paleontologist, Geoffroy St. Hilaire, had a hunch in the 1830’s that transformations occurred via changes in embryonic development. He thought he could see how a crocodile could become a human….he was close. He also thought a puppy could come from an embryonic squid by being turned upside down. Don’t laugh. Mollusk-vertebrate evolution did happen sorta like that.

    In the last ten years, we’ve learned a lot about how simple gene changes can alter the schedule of development of embryos. Take the loss of fingers. In the evolution of a bird wing from a lizard-oid paw, the outer two fingers disappear. Those outer fingers are the last to develop in a normal embryo. By suppressing the growth of the hand in the outer direction, a simple gene change can cancel the extra digits.

    Here’s another sort of digit cancellation: a primitive mammal forefoot, like today’s ‘possum paw, with five fingers, can transform into a horses’s hoof, with just one digit.

    A recent and very fine popular book is Neil Shubin’s “Your Inner Fish”, 2008, Pantheon Books. Shubin uses embryonic schedules to explain how a fish fin was remade into a primitive amphibian paw.

    Some anatomical evolution seems more complicated. Lizard-type reptiles have simple rectangular wrist bones. Raptor-type dinosaurs evolved a half-moon wrist bone that let the whole hand flip sideways very quickly.(You can see the swivel bone in the diagram in my post.) Birds have the very same flip-wrist and use it to flap their wings. What kind of embryonic changes happened in a primitive dinosaur, so the wrist became flippy? We don’t know….yet.

  3. Addison says:

    I don’t know why everyone is so obsessed with deriving a chicken from a dinosaur. The most likely scenario is that modern birds are descended from waterfowl-like ancestors. The evidence is all there: the oldest proto-birds that have many modern features all show evidence of a semi-aquatic existence, DNA analysis has suggested an earlier origin for anseriformes than for galliformes, the oldest proven neo-avian is a waterfowl type (Vegavis of the Antarctic), beaks are typical of filter-feeders who consequently are inhabitants of a watery environment, etc.

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