A Polar Radiation of Ray-Finned Fishes — The Austelliscus Lineage

Fossil discovery site of Austelliscus ferox

Estimated time period when Austelliscus ferox lived（白稜製作）

Among living bony fishes, two major evolutionary lineages dominate: the ray-finned fishes (Actinopterygii) and the lobe-finned fishes (Sarcopterygii). Of these, ray-finned fishes represent by far the most diverse and species-rich group of modern fishes. Earlier studies suggested that the divergence between these two clades occurred during the middle to late Silurian, earlier than approximately 419 million years ago. Yet the fossil record of ray-finned fishes during the Silurian and throughout the subsequent Devonian period remains extremely sparse, and the known fossils are largely restricted to low-latitude regions.

When Devonian fossils are compared, lobe-finned fishes appear far more diverse than ray-finned fishes, both in morphological disparity and species richness, and they are also represented by a greater number of specimens. Taken together, these patterns long suggested that lobe-finned fishes were the dominant group of bony fishes during the Devonian.

A discovery published in 2021, however, introduced an unexpected turn to this interpretation. Paleontologists working in the Paraná Basin of Brazil (Bacia do Paraná) recovered fossil remains of a ray-finned fish. Although the specimen is incomplete, it preserves enough diagnostic anatomical features to justify the designation of a new species:

Austelliscus ferox

Reconstruction of Austelliscus ferox (圖片由Julio the Artist繪製)

The fossil was discovered within the Malvinokaffric Realm, a biogeographic province that occupied parts of the Southern Hemisphere during the early and middle Devonian. This realm is characterized by relatively low overall diversity but a high degree of endemism, and its fossil assemblages are dominated primarily by invertebrates. During the Devonian, the discovery site lay within the Antarctic Circle, at paleolatitudes between approximately 73° and 84° south. This finding contrasts sharply with the long-standing assumption that Devonian bony fishes inhabited predominantly low-latitude environments.

The fossil is dated to roughly the Middle Devonian, around 387 million years ago, or possibly somewhat earlier, though the available evidence does not allow a more precise age determination. With an estimated body length of about 50–70 centimeters, Austelliscus ferox ranks among the largest known ray-finned fishes of the Devonian.

Earlier discussion noted that Devonian ray-finned fishes seemed to lag behind their lobe-finned contemporaries in terms of morphological diversity, species richness, and abundance. The discovery of Austelliscus, however, suggests a different possibility. Although the fossil material is limited, the morphology of its dentary bone is strikingly distinct from that of other Devonian ray-finned fishes, implying that the morphological diversity of Devonian actinopterygians may have been greater than previously recognized. Phylogenetic analyses further indicate that Austelliscus is not closely related to other known Middle Devonian ray-finned fishes; instead, it appears more closely allied with the Late Devonian genus Tegeolepis.

Within Late Devonian actinopterygian lineages, several groups cannot currently be traced back to the Middle Devonian or earlier, creating multiple “ghost lineages” in evolutionary reconstructions. These gaps highlight the severe incompleteness of the early ray-finned fish fossil record. The discovery of Austelliscus helps fill part of this gap and offers an important clue for future research. Most previously known Devonian actinopterygian fossils have been recovered from limited regions such as North America, Europe, and Australia—areas that occupied relatively low latitudes at the time. In contrast, Austelliscus was discovered within the Devonian Antarctic Circle, a high-latitude environment. This suggests that the center of early ray-finned fish diversification may have been located in these poorly studied polar regions, which could also explain the relatively low diversity of actinopterygian fossils recovered from lower latitudes.

Researchers therefore propose that although fossil preservation in the Malvinokaffric Realm is generally poor, increased exploration and collecting efforts in this region could significantly improve our understanding of early ray-finned fish evolution.

The discovery of Austelliscus carries considerable importance. Not only does it expand the known geographic distribution of Devonian ray-finned fishes, but it also provides valuable insight into the early evolutionary history of one of the most successful vertebrate lineages on Earth.

Author: Bai Leng

Reference:

Figueroa, R. T., Weinschütz, L. C., Friedman, M. (2021). The oldest Devonian circumpolar ray-finned fish?. Biology Letters.