中国の古代魚化石が「魚から人へ」の進化の重要な空白を埋める (Ancient Chinese Fish Fossils Fill Critical Gap in "Fish to Human" Evolution)

2026-03-06 中国科学院（CAS）

This photo taken on March 3, 2026 shows the fossil of the Megamastax amblyodus at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences (CAS), in Beijing, capital of China. (Xinhua/Jin Liwang)

＜関連情報＞

• https://english.cas.cn/newsroom/cas-in-media/202603/t20260306_1151839.shtml
• https://www.nature.com/articles/s41586-025-10008-y
• https://tiisys.com/blog/2026/03/06/post-187257/

シルル紀最大の魚類が硬骨魚類の特徴の起源を明らかにする Largest Silurian fish illuminates the origin of osteichthyan characters

Jing Lu,Brian Choo,Wenjin Zhao,You-an Zhu,Xindong Cui,Zhaohui Pan,Donglei Chen,Xiaoyue Liu,Yilun Yu,Tuo Qiao,Qiang Li,Liantao Jia,Per Ahlberg & Min Zhu
Nature  Published:04 March 2026
DOI:https://doi.org/10.1038/s41586-025-10008-y

Abstract

Osteichthyans (bony fishes and tetrapods) encompass 98% of modern vertebrate species. However, our understanding of the sequence of character evolution among stem osteichthyans has been substantially limited by the fragmentary nature of known stem osteichthyan fossils^1,2,3,4. Here we investigate newly discovered articulated head and trunk material of Megamastax amblyodus⁵, which yields previously unseen morphological details of a Silurian stem osteichthyan. Megamastax—previously interpreted as a lobe-finned fish⁵—exhibits distinct osteichthyan traits in the dermatocranium, such as resorptive tooth shedding and the presence of extrascapular bones. However, the arrangement of its dorsal aortae is reminiscent of crown-group chondrichthyans. The premaxilla with extensive palatal lamina and the elongated post-hypophyseal region of the braincase recall the condition in maxillate placoderms^6,7,8. Crucially, the discovery of an inner dental arcade of discrete tooth cushions on individual attachment bases aligns Megamastax with the fragmentary genera Lophosteus and Andreolepis^2,3,4, corroborating the previous interpretation of isolated tooth cushions as part of the jaw dentition^3,9 and verifying their identity as stem osteichthyans. Phylogenetic analysis places Megamastax within the osteichthyan stem, near the osteichthyan crown-group node, and provides a framework for exploring the sequence of character acquisition along the osteichthyan stem. Together, these new findings help to bridge the morphological gap between stem gnathostomes and modern osteichthyans, offering insights into the sequence of early evolutionary steps that shaped the osteichthyan lineage.