Where are the Bipeds in the Reptile Tree?

A new paper by Kubo and Kubo (2012) discussed bipedalism in archosaurs. They found basal dinosaurs and Poposaurus to be definite bipeds. Possible bipeds in their study included Euparkeria, Crocodylomorpha, Gracilisuchus, Ornithosuchidae and Pterosauromorpha (pterosaurs + Scleromochlus). Seeking to find a phylogenetic connection, Kubo and Kubo (2012) recovered over 12,000 MPTrees. They avoided pterosaurs and most of the bipdal croc-types without comment. They considered Poposaurus a “crurotarsan,” which is traditional thinking that has come into question here.

reptile tree with bipeds

Figure 1. Click to enlarge. This is the large reptile tree with bipeds highlighted in white and possible bipeds in pink.

The large reptile tree (Fig. 1) includes 300+ taxa and recovers a single tree (when the two poorly known taxa are excluded). Several taxa were facultative bipeds. Others were obligate bipeds (highlighted in white).

In the new Lepidosauromorpha, only one clade was bipedal, the Fenestrasauria (pterosaurs and kin). However 19 living lizards (not listed) are facultative bipeds.

In the new Archosauromorpha several clades were at least faultatively bipedal, from Eudibamus and Lagerpeton + Tropidosuchus to Smok + Postosuchus and basal archosaurs, most of which (excepting theropods) had descendants that reverted to a quadrupedal configuration. In the large reptile tree the only two ornithischian dinosaurs reverted to quadrupedalism and both were armored. Lotosaurus had a finback, reason enough.


Figure 1. Silesaurus as a biped and occasional quadruped. Click for more info.

Kubo and Kubo (2012) considered Silesaurus a quadruped. I think it was rarely a quadruped. They discussed comparisons with certain mammals, which developed more propulsive powers of the forelimb, such as a mobilized scapula. Dinosaurs, with their heavy tails, had a center of balance further back than in mammals. All good thoughts.

By the large reptile tree it appears that reptiles like Lewisuchus and Gracilisuchus originated bipedal locomotion in the Archosauria. Bipedalism conferred certain advantages. Those advantages include breathing while running (avoiding Carrier’s restraint), elevating the skull above obstructions in order to see further, enabling the forelimbs to perform other tasks (such as flapping in Cosesaurus). Increased speed is not associated with bipedality. That many clades reverted to quadrupedalism is just in the nature of evolution, like losing limbs after they had evolved from fins.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Kubo T and Kubo MO 2012. Associated evolution of bipedality and cursoriality among Triassic archosaurs: a phylogenetically controlled evaluation. Paleobiology 38(3):474-485.

4 thoughts on “Where are the Bipeds in the Reptile Tree?

  1. But why did they become bipedal, and, more importantly, why did they (or their descendants) become obligatory bipeds? Many modern reptiles are facultative bipeds, but none is obligatory. How and why did obligatory bipedality evolve in the archosaurs? To say that obligatory “conferred advantages” merely begs the question, because there are mechanical issues involved, e.g. static stability. In other words, obligatory bipeds couldn’t reap the benefits of obligatory bipedality without first becoming obligatory bipeds. At some point, a certain reptile or reptiles remained permanently bipedal and no longer reverted to a quadrupedality. How and why did that happen? It’s a classic chicken-and-egg question.

  2. The old ‘why’ question is a tough one when natural selection involves randomness and all sorts of influences from disease to weather. From what I can see, the bipeds started out small, descending from ever smaller parents. Some later got bigger in the new configuration. Perhaps some insight into the ‘why’ question can come from living lizards that occasionally go bipedal. Others (Snyder 1954) have quantified what bipedal lizards have that other lizards don’t. Jayne (on YouTube) puts his lizards on treadmills and you can see them ‘shift gears’ in a narrow-gauge, digitigrade configuration, totally unlike the sprawling plantigrade configuration. Thanks, for asking, Bill. Hope this helps.

    • Randomness creates variability, but whether those randomly generated variations become “fixed” genetically depends on the survival of the individual animals that possess them. Evolutionary success obviously involves environment (i.e. external factors, including not only climate etc. but also predators and competitors) and/or behavior (internal factors, body size and shape, how the individuals actually use the randomly generated feature) . But that’s the beauty and mystery of natural selection: individual animals — this animal or that animal living in the “present” (the here-and-now, or there-and-then) — must actually survive and propagate in order for the new feature/behavior to have the opportunity to be passed down to their descendants. Evolution is indeterminate; there is no grand plan and nothing is inevitable.

      So some combination of environment and behavior must have conspired to foster obligatory bipedality in the archosaurs in the Mid and Late Triassic. To my mind, the bioenergetics of gait selection was the key factor in the adoption of obligatory bipedality. But why was it so advantageous in the Mid-to-Late Triassic, and apparently only then, and apparently in several lineages? And if it was so advantageous back then, then why not before or since? After all, modern crocodiles are not all that different from Triassic thecodonts.

  3. It happens gradually and rarely if you look at all the primates that can be occasionally bipedal and one, us, that took it to the highest level. Think of the advantages that gives hominids and see if any of those were acquired by convergence in Triassic proto-dinos. I also consider bats bipeds, just upside down, but that’s another story entirely.

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