Sarcopterygii

Guiyu BW

Latimeria Paris

Barramunda

Tiktaalik restoration (side view) by ObsidianSoul 02

Onychodus

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Life restoration of Sparalepis tingi.tiff

Sarcopterygii or lobe-finned fish represent a clade of the Osteichthyes (bony fish) that is distinct from the Actinopterygii, or ray-finned fish. The Sarcopterygii, alongside the Actinopterygii, form a superclass within the bony fish known for their unique limb-like fins, which are pivotal in the study of the evolution of tetrapods, the four-limbed vertebrates including humans.

Characteristics

Sarcopterygii are characterized by their lobe-finned appendages, as opposed to the ray-finned appendages of the Actinopterygii. These fins are supported by a single bone, followed by a series of smaller bones, and muscle tissue, allowing for a greater range of movement compared to the fins of ray-finned fish. This anatomical feature is significant in the evolutionary transition from aquatic to terrestrial life forms. The skin of Sarcopterygii is often covered in small, hard scales, and they possess a unique pattern of skull bones, distinguishing them from their ray-finned counterparts.

Evolution

The Sarcopterygii are critical to understanding the evolution of vertebrates onto land. Fossil records indicate that the first sarcopterygians appeared in the late Silurian period, about 420 million years ago. The most famous of these early sarcopterygians is Tiktaalik, which exhibits both fish-like and amphibian-like features, suggesting it could live both in water and on land. This evolutionary milestone highlights the transition phase from aquatic sarcopterygians to terrestrial tetrapods, leading to the diverse range of land vertebrates present today, including mammals, birds, reptiles, and amphibians.

Classification

The Sarcopterygii are divided into two main groups: the Coelacanthiformes, which include the living coelacanths, and the Dipnoi, or lungfish, which are characterized by their ability to breathe air. A third group, the extinct Rhipidistia, is believed to be the ancestor of tetrapods. The classification of Sarcopterygii has undergone significant changes with the advent of molecular phylogenetics, providing a clearer understanding of the evolutionary relationships between these ancient fish and modern vertebrates.

Significance

The study of Sarcopterygii offers invaluable insights into the evolutionary transition from water to land, a pivotal event in the history of life on Earth. Their unique anatomical features, particularly their limb-like fins, provide a direct link to the earliest tetrapods and, by extension, to all terrestrial vertebrates. Understanding the evolutionary history and biology of sarcopterygians sheds light on the adaptive changes that enabled the colonization of land, including developments in locomotion, respiration, and reproduction.

Current Status

Today, the Sarcopterygii are represented by a small number of species, including the critically endangered coelacanths and several species of lungfish. Despite their limited diversity in the modern era, their evolutionary legacy is vast, underpinning the development of the rich and complex ecosystems found on land.