Abstract
The synsacrum is an important element of the axial skeleton in birds, both volant
and flightless. Little is known about the maturation of this complex bone in penguins. In this
work, the supposedly ontogenetically youngest known synsacrum of early penguins was
described. The analysis of this specimen, collected within the Eocene La Meseta Formation
of Seymour (Marambio) Island, Antarctic Peninsula, revealed that this bird had attained at
least the fledging stage of growth. Studies of three mature synsacra recovered from the same
formation focused on the synsacral canals and, using indirect reasoning, their contents.
These analyses revealed that the lumbosacral intumescence of the spinal cord and its extensions, the transverse canals, had been developed roughly like those in extant penguins (and
also swifts and cormorants). The neural spine extensions (a non−nervous tissue) tracing the
transverse grooves of the dorsal wall of the synsacral canal are currently considered as involved in the control of walking. The presented data suggest that such a sense organ gained
its current penguin configuration by the late Eocene.
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