Largest Silurian fish illuminates the origin of osteichthyan characters

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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 fossils1-4. 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 placoderms6-8. Crucially, the discovery of an inner dental arcade of discrete tooth cushions on individual attachment bases aligns Megamastax with the fragmentary genera Lophosteus and Andreolepis2-4, corroborating the previous interpretation of isolated tooth cushions as part of the jaw dentition3,9 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.

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