How ‘Parrot Dinosaur’ Switched from Four Feet to Two as It Grew

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Tracking the growth of dinosaurs and how they changed as they grew is difficult. Using a combination of biomechanical analysis and bone histology, palaeontologists from Beijing, Bristol, and Bonn have shown how one of the best-known dinosaurs switched from four feet to two as it grew.


Psittacosaurus, the 'parrot dinosaur' is known from more than 1000 specimens from the Cretaceous, 100 million years ago, of China and other parts of east Asia. As part of his PhD thesis at the University of Bristol, Qi Zhao, now on the staff of the Institute for Vertebrate Paleontology in Beijing, carried out the intricate study on bones of babies, juveniles and adults.

Dr Zhao said: "Some of the bones from baby Psittacosaurus were only a few millimetres across, so I had to handle them extremely carefully to be able to make useful bone sections. I also had to be sure to cause as little damage to these valuable specimens as possible."

With special permission from the Beijing Institute, Zhao sectioned two arm and two leg bones from 16 individual dinosaurs, ranging in age from less than one year to 10 years old, or fully-grown. He did the intricate sectioning work in a special palaeohistology laboratory in Bonn, Germany,

Written By: Science Daily
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  1. Most of my dinosaur focus has been on the theropods, specifically the maniraptorans because of the link to birds. That really interests me, and as such I’ve paid relatively little detailed attention to other groups. But I’ve always loved Psittacosaurus. It’s like my dirty little Ornithischian secret.

    • In reply to #2 by Roedy:

      What are the advantages to a dinosaur of being bipedal? In there anything other than freeing up for forelimbs for mayhem?

      Bipedalism is actually basal to all dinosaurs. The first dinosaurs were bipedal, and some later groups eventually became quadrupedal. The giant sauropods (Diplodocus, Brachiosaurus, that kind of thing) became that way via the prosauropods of the Triassic, which were variously bipedal, quadrupedal and able to alternate between the two, such as Plateosaurus, which is an excellent transitional form between flighty Coelophysis-like dinosaurs and the large lumbering Apatosaurus-like sauropods.

      Psittacosaurus itself is actually one of the earliest ceratopsians. It’s descendants, or those of its close relatives, were giant quadrupeds such as Triceratops and Styracosaurus, reached via small intermediates like Protoceratops. Look at the skeleton of the Psittacosaurus again. It is still bipedal and lacks both horns and a frill, but its skull is a dead give away, especially the beak and jaw.

      The paper’s indication is that Psittacosaurus‘s ancestors were already quadrupedal and that it was regaining bipedalism. Bipedal creatures can often put on great bursts of speed, and many hadrosaurs appear to have been able to switch from quadrupedal walking to bipedal sprinting if necessary. Psittacosaurus was small, so being able to escape predators with such bursts may have been useful to it, which is my best guess. Evidentially this foray back into bipedalism was soon aborted and reversed, because later ceratopsians famously favour being very large and very pointy instead of being quick.

      • In reply to #3 by Callinectes:

        Bipedal creatures can often put on great bursts of speed, and many hadrosaurs appear to have been able to switch from quadrupedal walking to bipedal sprinting if necessary. Psittacosaurus was small, so being able to escape predators with such bursts may have been useful to it, which is my best guess.

        Some modern lizards can also rear up to a bipedal run.

        The forelimbs of lizards are often lifted from the ground when they start sprinting. Previous research pointed out that this is a consequence of the propulsive forces from the hindlimbs. However, despite forward acceleration being hypothesized as necessary to lift the head, trunk and forelimbs, some species of agamids, teiids and basilisks sustain running in a bipedal posture at a constant speed for a relatively long time. http://rsif.royalsocietypublishing.org/content/10/84/20130241.abstract

        As far as the OP contrast between adult and juvenile forms go, the amphibian tadpole/frog has a much larger change in its skeleton!

    • Bipedal might affect range: a combination of speed and efficiency.

      There must be some significant advantage because it shows up in other creatures, particularly marsupials. You can understand why flightless birds are bipedal. Not too many examples of flightless birds wings becoming adapted as limbs and eventually to become forelegs.

      Depending on the terrain it’s probably more relevant to scavengers and opportunistic predators than browsers. I don’t recall the engineering argument but it’s possibly similar to why motorcycles can be more efficient than small cars – different power to weight ratio and presumably less tyre surface are contact with the road. Plus why cars have better fuel economy when power is applied via 2 wheels rather than 4. Virtually all 4-wheel drive vehicles normally driven on conventional roads default to the 2 wheel drive mode. Traction is only spread over 4 wheels in unusual circumstances.

      With vehicles there’s also a lot of energy expended in rotating multiple heavy wheels up to speed, maybe something similar in animals with the reciprocating action of 2 legs versus 4 legs. But lots of energy is lost in tyre flexing for direction and speed changes. If there weren’t a sufficient energy loss from wheel or foot deformation then there wouldn’t be sufficient grip and therefore ineffective steering, traction, and braking – presumably the same for flexing in the foot surface contact area of animals.

      2 wheels, like 2 legs, is the minimum required for stability. The next best option might be a 1-wheeled motorcycle (which do exist thanks to software controlled power trains). A unipod 1-legged creature might be more efficient but so far only kangaroos (among the fastest and most efficient of all land animals) and some small birds seem to have evolved this approach. These birds mostly fly, so not very relevant. Plus they walk normally at very slow speed. Both legs become synchronised for hopping only intermediate ground speed purposes.

      Another example is that marine animals (that evolved from quadripeds) tend to have single large tails rather than 4 paddles.

      There’s also the concept of regenerative braking and suspension, which requires a means of storing a pulse of energy. In land animals it might be that fewer but larger spring-like tendons are more efficient energy stores. Which might favour bipeds over quadripeds.

      In reply to #2 by Roedy:

      What are the advantages to a dinosaur of being bipedal? In there anything other than freeing up for forelimbs for mayhem?

      • In reply to #8 by Pete H:

        Bipedal might affect range: a combination of speed and efficiency.

        There must be some significant advantage because it shows up in other creatures, particularly marsupials. You can understand why flightless birds are bipedal. Not too many examples of flightless birds wings becoming adapted as limbs…

        All else being equal, a quadruped is usually faster than a biped, but it very much depends on factors such as gait, method of locomotion, and body size. For instance, the fastest human can still be outrun by a determined baboon, and is certainly no match for a dog or a gazelle in a straight race, but that’s probably because human bipedalism was better designed for energy conservation over long distances rather than for explosive speed.

        Hadrosaurs were believed to alternate between quadrupedal and bipedal stances depending on whether they wanted to walk at a cost-effective slow pace from food to food or wanted to flee an approaching predator. Maybe Psittacosaurus was camouflaged in its youth to hide it from predators, but would need a faster means of escape as it got bigger and less inconspicuous.

        • In reply to #9 by Zeuglodon:

          Definitely energy conservation is the advantage of bipedalism. Plus the ability to free the forelimbs for other flexible purposes – flying, climbing, carrying, fruit picking, digging, tool making, or just accessories for slow speed foraging.

          Reminds me of the riddle about the mythical creature that begins on all fours, then 2 legs, but ends with 3.

          I recall reading somewhere many years ago, possibly in a Scientific American article in the 1970s, that human walking can outpace horses over long journeys. For a very long time the fastest humans in the world rode horses, so we all just assume that human walking is some kind of inferior mode of transport.

          The reason humans employed horses for transport rather than walking is not speed but to carry extra stuff around. Owing to the difference in burden capacity from the relative size of horses compared to humans. You could think of the relative weight of a near term foetus plus placental fluid etc. as being the natural upper limit of long haul carrying capacity of any mammal, perhaps except the camel and the kangaroo – which might explain kangaroos extraordinarily greater foraging range.

          Soldiers in Roman armies on the March walked everywhere but carried only basic gear. The horses hauled the heavy engineering materials and camping kit. Which allows the Roman armies much greater flexibility and speed of deployment.

          To enhance the long haul performance of horses complex stabling networks are established. If not for staging, or long breaks with stabling at Inns on long trips, then when humans and horses are compared directly in physiological performance the human on foot comes out ahead. It’s like the tortoise and the hare.

          Reason is that browsing and grazing animals need to spend an immense about of time obtaining and processing sufficient food energy for long haul travel. In their natural environment horse travel would mostly have been migrations associated with slow seasonal changes in vegetation. In contrast humans scavengers evolved to obtain immense amounts of highly concentrated energy by eating other animals which themselves spent an immense amount of time harvesting vegetation food sources. Early human scavengers would frequently and routinely employ long haul travel to obtain that food. Basically outcompeting other scavengers just by covering much more ground. While also being able to defend against larger predators and scavengers by group cooperation and the use of tools.

          Human endurance capacity very much depends on the nutritional state. The endurance required for long haul scavenging is of the ultra-marathon type. (But without access to the incessant snacking normally associated with extreme endurance sports.) Something that would be impossible for most people on the traditional supermarket diet. Interestingly the capacity for this kind of extreme endurance increases the less well fed is the human. Counter-intuitive but makes some kind of evolutionary sense.

          You can try it at home. Get yourself walking fit by fast walking for several hours a day for several weeks. Then attempt an excessively long, all day walk and note the time and distance until you reach the point of collapse. (Take a mobile phone and stay near the road!) Then switch to some kind of ketogenic diet for several weeks or months to trigger the physiological starvation response. Repeat the long hike experiment, this time without eating anything that day or the previous day, and you’ll be astonished at the difference.

          BTW I’ve also experimented with a form of semi-quadriped locomotion on myself. You can use light hiking sticks, like ski poles. Theory is that hiking sticks improve thrust and stability by engaging the upper body muscles. Kind of like the 4WD transmission in my car – automatically switches torque to share the thrust in varying proportions between front and rear wheels. But I found the hiking sticks don’t work too well – except at very slow speed or when ascending steep tracks when I’m still recovering from knee injuries. One single stick is definitely easier. Reason might be the extra mental processing demand on visual and other coordination for placement of the sticks.

          Though should be quite useful eventually in my frail old age. Apparently there was a time when all gentlemen and famous wizards were expected to always carry a walking stick.

  2. Psittacosaurus actually has quite a few claims to fame in palaeontology. It’s the genus with the largest number of confirmed and valid dinosaur species, it’s currently the dinosaur with the largest time range (about 40 million years, I think), and it’s one of the few ornithiscian dinosaurs whose fossils contain evidence of feathers or feather-like structures. The implication is that it was one of the more successful kinds of dinosaur despite its unassuming appearance.

    I am quite surprised to learn that it descended from quadrupedal precursors, especially since its most likely ancestors were bipedal ornithiscian cerapods, presumably something like Hypsilophodon or Dryosaurus. Also, most other early ceratopsians and marginocephalians were bipedal, like Archaeoceratops, Chaoyangsaurus, and Yinlong. I think there’s an undiscovered quadruped fossil waiting to be unearthed…

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