Phylogenetic analysis of early tetrapod evolution has resulted in a consensus across diverse data sets1,2,3 in which the tetrapod stem group is a relatively homogenous collection of medium- to large-sized animals showing a progressive loss of ‘fish’ characters as they become increasingly terrestrial4,5, whereas the crown group demonstrates marked morphological diversity and disparity6. The oldest fossil attributed to the tetrapod crown group is the highly specialized aïstopod Lethiscus stocki7,8, which shows a small size, extreme axial elongation, loss of limbs, spool-shaped vertebral centra, and a skull with reduced centres of ossification, in common with an otherwise disparate group of small animals known as lepospondyls. Here we use micro-computed tomography of the only known specimen of Lethiscus to provide new information that strongly challenges this consensus. Digital dissection reveals extremely primitive cranial morphology, including a spiracular notch, a large remnant of the notochord within the braincase, an open ventral cranial fissure, an anteriorly restricted parasphenoid element, and Meckelian ossifications. The braincase is elongate and lies atop a dorsally projecting septum of the parasphenoid bone, similar to stem tetrapods such as embolomeres. This morphology is consistent in a second aïstopod, Coloraderpeton, although the details differ. Phylogenetic analysis, including critical new braincase data, places aïstopods deep on the tetrapod stem, whereas another major lepospondyl lineage is displaced into the amniotes. These results show that stem group tetrapods were much more diverse in their body plans than previously thought. Our study requires a change in commonly used calibration dates for molecular analyses, and emphasizes the importance of character sampling for early tetrapod evolutionary relationships.
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We thank S. Pierce, J. Cundiff, the late F. A. Jenkins, C. Schaff, the late W. Amaral, D. Berman, A. Henrici, P. Holroyd, A. C. Milner, J. A. Clack, J. Bolt and W. Simpson for access to specimens, and J. Bolt, R. Carroll, J. A. Clack, M. Coates, N. Fröbisch, D. Germain, A. Huttenlocker, M. Laurin, H. Maddin, D. Marjanović, J. Olori, R. R. Reisz, and R. Schoch for discussions. We particularly thank D. Germain for first suggesting reanalysis of the Lethiscus data set with current computational tools. This research was supported in part by a Natural Sciences and Engineering Research Council of Canada Discovery Grant to J.S.A.
Extended data figures
3D yaw of main skull block of Lethiscus stocki (MCZ 2185). Scale is in millimeters.
3D pitch of main skull block of Lethiscus stocki (MCZ 2185). Scale is in millimeters.
3D yaw of braincase of Lethiscus stocki (MCZ 2185). Scale is in millimeters.
3D yaw of cranial endocast of Lethiscus stocki (MCZ 2185). Scale is in millimeters.
3D yaw of skull of Coloraderpeton brilli (CM 47687). Scale is in microns.
3D roll of braincase of Coloraderpeton brilli (CM 47687). Scale is in microns.