Oldest actinopterygian from China provides new evidence for origin of ray-finned fishes
Osteichthyans, or bony fishes, comprise two categories, each containing over 32,000 living species: Sarcopterygii (lobe-finned fishes and tetrapods) and Actinopterygii (ray-finned fishes). Nevertheless, actinopterygians have an obscure early evolutionary history. The earliest definitive actinopterygian is the Middle Devonian (Eifelian) Cheirolepis, with earlier candidates generally represented by fragments subject to differing phylogenetic interpretations. By contrast, earliest Devonian deposits yield a diversity of lobe-finned fishes and recent discoveries from China extend their origin into the late Silurian.
|Life restoration of Meemannia eos [Credit: Brian Choo]|
Meemannia is the newest--and least understood--member of this fauna. Represented by four isolated skull roofs and a referred jaw, Meemannia presents an intriguing mosaic of characteristics: histology interpreted as a precursor to the "cosmine" of rhipidistian sarcopterygians (lungfishes plus tetrapods) combined with an undivided braincase and skull roof resembling that of actinopterygians. Previous phylogenetic analyses placed Meemannia as the earliest-diverging sarcopterygian, based on histological features.
|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 [Credit: LU Jing]
Researchers revised hypotheses of bone histology in the ancestor of bony fishes, and found that "cosmine"-like tissues, previously thought to unite Meemannia with lobe-fins, are widely distributed among early bony fishes, including the ray-fin Cheirolepis. This finding revealed that Meemannia, once considered a lobe-fin, is the oldest ray-finned fish, providing new evidence for the origin of ray-finned fishes.
|Summary phylogeny, simplified from the strict consensus tree, and the evolution |
of key Actinopterygian features [Credit: LU Jing]
"Phylogenetic analysis places Meemannia as an early-diverging ray-finned fish, resolving it as the sister lineage of Cheirolepis plus all younger actinopterygians," said study co-author Dr. Sam Giles, of the University of Oxford. "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."
"The rarity of early ray-fins supports a 'long-fuse' model for actinopterygian diversification," said Dr. ZHU. "Actinopterygians persisted at low levels of numerical abundance, taxonomic richness, and morphological disparity for millions of years before undergoing apparently explosive diversification in the early Carboniferous after the end-Devonian Hangenberg Event. Meemannia provides an anatomical snapshot of the earliest stages of ray-finned fish evolution, at a time when their rarity and limited ecological variety gave no indication of the dominant role they would play in aquatic vertebrate ecosystems of the future."
Source: Chinese Academy of Sciences Headquarters [May 19, 2016]