The axial skeleton of Tiktaalik roseae
Abstract
The axial columns of the earliest limbed vertebrates show distinct
patterns of regionalization as compared to early tetrapodomorphs.
Included among their novel features are sacral ribs, which provide
linkage between the vertebral column and pelvis, contributing to body
support and propulsion by the hindlimb. Data on the axial skeletons of
the closest relatives of limbed vertebrates are sparce, with key
features of specimens potentially covered by matrix. Therefore, it is
unclear in what sequence and under what functional context
specializations in the axial skeletons of tetrapods arose. Here, we
describe the axial skeleton of the elpistostegalian Tiktaalik roseae and
show that transformations to the axial column for head mobility, body
support, and pelvic fin buttressing evolved in finned vertebrates prior
to the origin of limbs. No atlas–axis complex is observed; however, an independent basioccipital–exoccipital complex suggests increased
mobility at the occipital vertebral junction. While the construction of vertebrae in Tiktaalik is similar to early tetrapodomorphs, its ribs
possess a specialized sacral domain. Sacral ribs are expanded and
ventrally curved, indicating likely attachment to the expanded iliac
blade of the pelvis by ligamentous connection. Thus, the origin of novel
rib types preceded major alterations to trunk vertebrae, and linkage
between pelvic fins and axial column preceded the origin of limbs. These
data reveal an unexpected combination of post-cranial skeletal
characters, informing hypotheses of body posture and movement in the
closest relatives of limbed vertebrates.
Open access:
https://www.pnas.org/doi/10.1073/pnas.2316106121
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