Osteichthyans, or bony vertebrates, gave rise to two primary branches: one excelled in aquatic environments, while the other, approximately 380 million years ago, evolved into tetrapods, including humans. However, our understanding of Silurian bony fishes was limited to isolated scales and bone fragments, Such scarcity posed challenges in comprehending their overall body structure, phylogenetic status, and crucial features like feeding morphology.
Here we present a remarkably preserved specimen of Megamastax amblyodus, the largest Silurian vertebrate and potentially the earliest apex predator among vertebrates. The specimen was recovered from the Kuanti Formation, dating back to the Late Ludlow period, approximately 423 million years ago, in Yunnan, China. High-resolution computed tomography (HRCT) scanning has unveiled intricate details of various anatomical components, including the neurocranium, palatoquadrates, dermal skull roof, cheek bones, operculum, and dentition.
Notably, the inner dentition comprises a series of structures called "tooth cushions," a distinctive form of osteichthyan dentition previously documented only in isolated micro remains of Lophosteus from the Silurian Baltic region. The braincase, remarkably, lacks an intracranial joint and features enclosed aortic canals reminiscent of chondrichthyans. However, the dermal cheek bones of Megamastax exhibit similarities to those of primitive osteichthyans like Psarolepis and basal actinopterygians. Such pattern of the skull roof diverges significantly from known osteichthyans and instead shares similarities with the "maxillate placoderm" Entelognathus.
This unique combination of chondrichthyan, osteichthyan, and "maxillate placoderm" features suggests that Megamastax occupies a position proximate to the origin of the gnathostome crown group. The new discovery offers new insights into the evolutionary pathways through which early osteichthyans developed their body plan.