The evolution of Osteichthyes began with a split into two major lineages: Sarcopterygii (lobe-finned fishes) and Actinopterygii (ray-finned fishes). In one lineage—sarcopterygians—some groups evolved robust internal bones and limb-like fins and ultimately gave rise to semi- and fully terrestrial tetrapods; the other lineage—actinopterygians—remained primarily aquatic and later radiated into the diverse teleosts. Repeated mass extinction events and ongoing genetic divergence allowed novel functions and new niches to be exploited, a pattern especially evident in recent analyses of teleost diversification. Lobe-finned fishes characteristically possess an endoskeleton fin architecture, whereas ray-finned fishes bear dermal fin rays built on a different structural plan. Primitive Osteichthyes also show an early origin of paired air-spaces (lungs), but many derived actinopterygians modified this ancestral condition into a dorsal swim bladder. Imagining a world without sarcopterygians or tetrapods highlights how teleosts might have convergently colonized many terrestrial-associated niches; although significant developmental and structural hurdles would have made such a transition challenging, this thought experiment underscores the cascading ecological consequences that the loss of a major clade can produce. Ecosystems thrive on diversity and adaptability, and episodes of environmental upheaval—such as the Silurian and Devonian extinctions—often catalyze rapid evolutionary change.
Fritzsch et al. (Thu,) studied this question.