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Hidden morphological diversity among early tetrapods
Univ Calgary, Dept Comparat Biol & Expt Med, 3330 Hosp Dr, Calgary, AB T2N 4N1, Canada..
Univ Calgary, Dept Biol Sci, 2500 Univ Dr, Calgary, AB T2N 1N4, Canada..
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Organismal Biology, Evolution and Developmental Biology.
Univ Calgary, Dept Comparat Biol & Expt Med, 3330 Hosp Dr, Calgary, AB T2N 4N1, Canada..
2017 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 546, no 7660, 642-645 p.Article in journal (Refereed) Published
Abstract [en]

Phylogenetic analysis of early tetrapod evolution has resulted in a consensus across diverse data sets(1-3) 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 terrestrial(4,5), whereas the crown group demonstrates marked morphological diversity and disparity(6). The oldest fossil attributed to the tetrapod crown group is the highly specialized astopod Lethiscus stocki(7,8), 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 astopod, Coloraderpeton, although the details differ. Phylogenetic analysis, including critical new braincase data, places astopods 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.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP , 2017. Vol. 546, no 7660, 642-645 p.
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Biological Sciences
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URN: urn:nbn:se:uu:diva-329631DOI: 10.1038/nature22966ISI: 000404332000045PubMedID: 28636600OAI: oai:DiVA.org:uu-329631DiVA: diva2:1143588
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2017-09-21Bibliographically approved

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