Saturday, May 28, 2016

[PaleoIchthyology • 2016] Meemannia eos • The Oldest Actinopterygian Highlights the Cryptic Early History of the Hyperdiverse Ray-Finned Fishes

Meemannia eos 
Life restoration by Brian Choo

• Once considered a lobe-fin, Meemannia is the oldest ray-finned fish
• MicroCT reveals ray-fin characters including lateral cranial and spiracular canals
• Meemannia revises hypotheses of bone histology in the ancestor of bony fishes
• “Cosmine”-like tissues are also present in the ray-fin Cheirolepis

Osteichthyans comprise two divisions, each containing over 32,000 living species: Sarcopterygii (lobe-finned fishes and tetrapods) and Actinopterygii (ray-finned fishes). Recent discoveries from China highlight the morphological disparity of early sarcopterygians and extend their origin into the late Silurian. By contrast, the oldest unambiguous actinopterygians are roughly 30 million years younger, leaving a long temporal gap populated by fragments and rare body fossils of controversial phylogenetic placement. Here we reinvestigate the enigmatic osteichthyan Meemannia from the Early Devonian (∼415 million years ago) of China, previously identified as an exceptionally primitive lobe-finned fish. Meemannia combines “cosmine”-like tissues taken as evidence of sarcopterygian affinity with actinopterygian-like skull roof and braincase geometry, including endoskeletal enclosure of the spiracle and a lateral cranial canal. We report comparable histological structures in undoubted ray-finned fishes and conclude that they are general osteichthyan features. Phylogenetic analysis places Meemannia as an early-diverging ray-finned fish, resolving it as the sister lineage of Cheirolepis plus all younger actinopterygians. This brings the first appearance of ray-fins more in line with that of lobe-fins and fills a conspicuous faunal gap in the otherwise diverse late Silurian-earliest Devonian vertebrate faunas of the South China Block.

Fig.1 Cranial anatomy of Meemannia eos based on High-Resolution Computed Tomography.
A Dorsal view; B Ventral view; C Endocast in dorsal view; D Endocast of Mimipiscis in dorsal view
(Image by LU Jing).

Fig.3 Summary phylogeny, simplified from the strict consensus tree, and the evolution of key Actinopterygian features
(Image by LU Jing).

Jing Lu, Sam Giles, Matt Friedman, Jan L. den Blaauwen and Min Zhucor. 2016. The Oldest Actinopterygian Highlights the Cryptic Early History of the Hyperdiverse Ray-Finned Fishes. Current Biology. DOI:  10.1016/j.cub.2016.04.045 

Oldest Actinopterygian from China Provides New Evidence for the Origin of Ray-Finned Fishes

Min Zhu, Xiaobo Yu, Wei Wang, Wenjin Zhao and Liantao Jia. 2006. A primitive fish provides key characters bearing on deep osteichthyan phylogeny. Nature. 441, 77-80. DOI: 10.1038/nature04563

1st Min Zhu, W. Wang and Xiaobo Yu. 2010. Meemannia eos, a basal sarcopterygian fish from the Lower Devonian of China –expanded description and significance. in D.K. Elliott, J.G. Maisey, X.-B. Yu, D.-S. Miao (Eds.), Morphology, Phylogeny and Paleobiogeography of Fossil Fishes, Verlag Dr. Friedrich Pfeil; 199–214.