New embryological and palaeontological evidence sheds light on the evolution of the archosauromorph ankle

BLANCO, MARÍA VICTORIA FERNANDEZ; EZCURRA, MARTÍN D.; BONA, PAULA

Scientific Reports

Nature Publishing Group

Año: 2020 vol. 10

The homology and evolution of the archosaur ankle is a controversial topic that has been deeplystudied using evidence from both extinct and extant taxa. In early stem archosaurs, the astragalus andcalcaneum 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 laterallycontacting the astragalus. This bone is subsequently lost as an independent ossification before the lastcommon ancestor of birds and crocodilians, but the evolutionary fate of this element remains mostlyunexplored. Here, we integrate embryological and palaeontological data with morphogeometricanalyses to test the hypothesis of loss of the centrale or, alternatively, its incorporation into thearchosaurian astragalus. Our results support the latter hypothesis, indicating that the astragalusdeveloped ancestrally from two ossification centres in stem archosaurs and that the supposed tibialeof bird embryos represents a centrale. This conclusion agrees with previous embryological studies thatconcluded that the tibiale never develops in diapsids.