Intermedium reappears in birds

Ossa-Fuentes L, Modozis J and Vargas AO 2015
discover a detail of interest in bird osteology and ontogeny.

They report, “This work has revealed that the ascending process [of the ankle] does not develop from either the heel bone or the ankle bone, but from a third element, the intermedium. In the ancient lineage of paleognath birds (such as tinamous, ostriches and kiwis) the intermedium comes closer to the anklebone, producing a dinosaur-like pattern. However, in the other major avian branch (neognaths), which includes most species of living birds, it comes closer to the heel bone; that creates the impression it is a different structure, when it is actually the same.”

And that’s not all… They continue: “More remarkably, however, this finding reveals an unexpected evolutionary transformation in birds. In embryos of the land egg-laying animals, the amniotes (which include crocodilians, lizards, turtles, and mammals, who secondarily evolved live birth) the intermedium fuses to the anklebone shortly after it forms, disappearing as a separate element. This does not occur in the bird ankle, which develops more like their very distant relatives that still lay their eggs in water, the amphibians. Since birds clearly belong within landegg-laying animals, their ankles have somehow resurrected a long-lost developmental pathway, still retained in the amphibians of today – a surprising case of evolutionary reversal. The study also presented fossil evidence from juvenile specimens of toothed birds from the Cretaceous period. These show that, at this early stage of bird evolution, the ascending process already developed separately.”

On a similar note,
as you may recall from this earlier blog post, the pre-amniote, and almost pre-tetrapod, digit zero, the manual digit medial to the thumb, which is absent in almost all derived tetrapods, also appears on Limusaurus and caused the phase shift confusion noted earlier.

References
Ossa-Fuentes L, Modozis J and Vargas AO 2015. Bird embryos uncover homology and evolution of the dinosaur ankle. Nature Communications. DOI: 10.1038/natcomms9902
Diaz RE & Trainor PA 2015. Hand/foot splitting and the ‘re-evolution’of mesopodial skeletal elements during the evolution and radiation of chameleons. BMC evolutionary biology, 15(1), 184.
https://paleobiologia.wordpress.com (blog en español)
http://www.nature.com/ncomms/2015/151113/ncomms9902/full/ncomms9902.html

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