Abstract
The homology and evolution of the archosaur ankle is a controversial topic that has been deeply studied using evidence from both extinct and extant taxa. In early stem archosaurs, the astragalus and calcaneum form the ancestral proximal tarsus and a single ossification composes the centrale series. In more recent stem archosaurs, the centrale is incorporated to the proximal row of tarsals laterally contacting the astragalus. This bone is subsequently lost as an independent ossification before the last common ancestor of birds and crocodilians, but the evolutionary fate of this element remains mostly unexplored. Here, we integrate embryological and palaeontological data with morphogeometric analyses to test the hypothesis of loss of the centrale or, alternatively, its incorporation into the archosaurian astragalus. Our results support the latter hypothesis, indicating that the astragalus developed ancestrally from two ossification centres in stem archosaurs and that the supposed tibiale of bird embryos represents a centrale. This conclusion agrees with previous embryological studies that concluded that the tibiale never develops in diapsids.
Highlights
The homology and evolution of the archosaur ankle is a controversial topic that has been deeply studied using evidence from both extinct and extant taxa
A single ossification originating from the four elements that are proposed to compose the centrale series of amniotes is retained in stem archosaurs[5]
Some previous authors have interpreted that the medial proximal tarsal of Archosauria is a result of the fusion between the centrale and astragalus that occur as separate ossifications in post-hatching individuals ancestrally in Archosauromorpha
Summary
The homology and evolution of the archosaur ankle is a controversial topic that has been deeply studied using evidence from both extinct and extant taxa. In more recent stem archosaurs, the centrale is incorporated to the proximal row of tarsals laterally contacting the astragalus This bone is subsequently lost as an independent ossification before the last common ancestor of birds and crocodilians, but the evolutionary fate of this element remains mostly unexplored. We integrate embryological and palaeontological data with morphogeometric analyses to test the hypothesis of loss of the centrale or, alternatively, its incorporation into the archosaurian astragalus Our results support the latter hypothesis, indicating that the astragalus developed ancestrally from two ossification centres in stem archosaurs and that the supposed tibiale of bird embryos represents a centrale. The potential recognition of the centrale as a component of the proximal tarsus of archosaurs has interesting evolutionary implications in the discussion of the homology of the primordial cartilages that form the astragalus in amniotes and the homology of the cartilaginous condensations of bird embryos
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