Actinopterygii, the ray-finned fishes, are the most abundant and diverse group of vertebrates, comprising over 50% of all living vertebrate species. The study of their evolutionary history, or phylogeny, reveals a complex story of adaptation spanning hundreds of millions of years. Understanding this phylogeny helps explain why this class of bony fish has achieved such widespread success, populating nearly every aquatic habitat from deep-sea trenches to high-altitude mountain streams.
Defining Features of Ray-Finned Fishes
The name Actinopterygii means “ray-finned,” which refers to the group’s defining feature: fins made of skin webs supported by flexible, bony spines called lepidotrichia. This structure contrasts with the fleshy, bone-supported fins of their sister group, Sarcopterygii (lobe-finned fishes). The ray-fin design allows for high maneuverability, as the fins can be folded and adjusted with precision.
Another characteristic is a single dorsal fin, unlike the two dorsal fins of sarcopterygians. Their bodies are covered in scales that have evolved over time. Early forms had heavy, interlocking ganoid scales for armor, while more recent groups feature thinner leptoid scales (cycloid and ctenoid), which allow for greater flexibility and speed.
These modern cycloid and ctenoid scales are derived from the heavier ganoid scales of their ancestors. This transition involved the loss of the hard, enamel-like ganoine layer and a thinning of the underlying bone. This change reflects a broader evolutionary trend towards lighter, more agile body plans.
Basal Actinopterygian Lineages
The evolutionary tree of ray-finned fishes begins with basal lineages that diverged early and retain many ancient characteristics. The most basal living lineage is the subclass Cladistia, which includes the bichirs and reedfish of Africa. These fish have a dorsal fin composed of multiple separate finlets and use their gas bladder as a lung for breathing air.
The next major branch is the subclass Chondrostei, which includes sturgeons and paddlefishes. This lineage is known for its largely cartilaginous skeletons, a trait that evolved from a bony ancestry. Despite this, they retain the heavy, ganoid scales of early fish, making them living fossils that provide clues about Paleozoic actinopterygians.
Fossil and molecular data suggest these primary lineages—Cladistia, Chondrostei, and the subsequent Neopterygii—split during the Devonian and Carboniferous periods. The distinct traits of these early groups highlight the initial evolutionary experiments that occurred before the emergence of more modern fish forms.
The Rise of Neopterygii
An evolutionary divergence gave rise to the Neopterygii, or “new fins,” a group encompassing the vast majority of living ray-finned fish. Appearing in the fossil record during the Late Permian, this lineage introduced modifications to the jaw structure and fin suspension. These changes allowed for more powerful feeding mechanisms and improved speed and agility.
The Neopterygii are divided into two infraclasses: Holostei and the much larger Teleostei. The Holostei are an intermediate group now survived by only gars and the bowfin. Gars are recognizable by their torpedo-shaped bodies and hard ganoid scales, while the bowfin has a long dorsal fin and a more rounded tail.
Recent molecular and morphological studies have confirmed that the Holostei form a monophyletic group, meaning they share a single common ancestor. This group is the sister lineage to the Teleostei. The emergence of Neopterygii, and the subsequent split between these two groups, set the stage for an unprecedented radiation of fish forms.
Teleostei: An Evolutionary Explosion
The Teleostei represent the height of ray-finned fish diversification, accounting for approximately 96% of all living fish species. Their success is linked to anatomical innovations that allowed them to exploit a vast range of ecological niches. One of the most important changes was the development of a mobile premaxilla, which enabled teleosts to protrude their jaws and create suction to capture prey.
This feeding mechanism was complemented by the homocercal tail, a symmetrical tail fin where the vertebral column ends at its base rather than extending into the upper lobe. The homocercal design is hydrodynamically efficient, providing powerful forward propulsion for rapid swimming and complex maneuvering. Together, these features unlocked new feeding strategies and lifestyles, fueling an adaptive radiation.
The diversity within Teleostei is immense, with the group branching into several major superorders. Early diverging lineages include Osteoglossomorpha (bony-tongues), Elopomorpha (eels and tarpons), and Clupeomorpha (herrings and anchovies). These are followed by the enormous group Euteleostei, or “true teleosts,” which includes everything from salmon and trout to reef fishes. This evolutionary explosion established teleosts as the dominant vertebrates in aquatic ecosystems.
Methods of Phylogenetic Reconstruction
Reconstructing the evolutionary history of ray-finned fishes requires analyzing evidence to build a phylogenetic tree. The primary data sources are morphology and molecular genetics. Morphological analysis involves comparing anatomical features like bone structures and scale types, which is useful for incorporating extinct species from the fossil record.
Molecular phylogenetics uses genetic data, such as DNA sequences, to infer relationships. By comparing the genetic codes of different species, scientists can determine how closely related they are, as fewer differences in DNA imply a more recent common ancestor. These methods have provided new insights and helped resolve long-standing questions about fish group relationships.
Both data types are often analyzed using cladistics, which groups organisms based on shared derived characters inherited from a common ancestor. By combining evidence from fossils and genes, researchers can piece together the branching history of Actinopterygii from its ancient origins to its modern diversity.