Shark phylogeny is the study of the evolutionary history and relationships among sharks, mapping their family tree to understand how modern species evolved and relate to one another. This history is immense, with sharks appearing in the fossil record hundreds of millions of years ago. Their lineages predate dinosaurs and even trees, underscoring their profound evolutionary persistence.
The Ancient Origins of Sharks
The story of sharks begins over 450 million years ago in the Late Ordovician Period, with the earliest evidence being a few fossilized scales. Scientists debate whether these first shark-like animals were true sharks, as no teeth from this period have been found, suggesting they may have been toothless. By the Devonian Period, more recognizable shark-like forms began to appear. One notable example is Cladoselache, a predator that swam the ancient oceans around 380 million years ago.
Following a massive extinction at the end of the Permian Period, a few shark lineages managed to survive. These survivors gave rise to new forms during the Mesozoic Era. A prominent group from this time was the hybodonts, such as Hybodus, which lived alongside dinosaurs. While once thought to be direct ancestors of modern sharks, many scientists now consider hybodonts a distinct side-branch in shark evolution.
The Jurassic Period marked a significant radiation of shark evolution, with the appearance of the first modern shark groups. The oldest of these, the Hexanchiformes or sixgill sharks, emerged around 195 million years ago. This era saw the development of more flexible, protrusible jaws, a feature that allowed sharks to consume prey larger than themselves and contributed to their success as predators, setting the stage for their diversification.
Constructing the Shark Family Tree
Scientists reconstruct the evolutionary relationships of sharks using two primary forms of evidence: morphology and molecular data. Morphological analysis involves comparing the physical features of both living sharks and fossilized remains. Anatomical details like the shape and structure of teeth, the design of fins, and the number of gill slits provide clues about how different species are related. Because cartilage rarely fossilizes well, shark teeth are often the most abundant pieces of evidence from the ancient past.
Molecular data offers a complementary approach. By comparing the DNA and RNA sequences of living shark species, researchers can measure genetic similarity and infer evolutionary closeness. The rate at which genetic mutations accumulate over time acts as a “molecular clock,” allowing scientists to estimate when different lineages diverged from a common ancestor. This genetic information has been useful in clarifying relationships that were ambiguous based on physical traits alone.
A well-known example is the debate over the ancestry of the Great White Shark (Carcharodon carcharias) and its connection to the extinct Megalodon (Otodus megalodon). For a long time, their large, serrated teeth led scientists to believe they were closely related. However, more detailed morphological analysis of tooth structure and molecular evidence from living sharks suggest they belong to separate lineages. The Great White is now thought to have descended from a mako-like ancestor, while Megalodon represents the end of a different lineage.
Major Modern Shark Lineages
Modern sharks are categorized into two major superorders based on the presence or absence of an anal fin. The Galeomorphii are sharks that possess an anal fin, a group that includes many of the most widely recognized species. In contrast, the Squalomorphii are sharks that lack this fin.
Within the Galeomorphii, several orders contain familiar sharks. The Lamniformes include powerful, fast-swimming predators like the Great White and Mako sharks. The Carcharhiniformes, or ground sharks, is the largest order and encompasses species like the Tiger, Bull, and Hammerhead sharks. Another group is the Orectolobiformes, which includes the Whale Shark and the Nurse Shark.
The superorder Squalomorphii is also diverse. The order Squaliformes, for example, includes various species of dogfish and the long-lived Greenland Shark, which inhabits deep, cold waters. Another order, the Hexanchiformes, contains the sixgill and sevengill sharks.
Relationships with Other Cartilaginous Fish
Sharks belong to a larger class of fishes known as Chondrichthyes, distinguished by their skeletons made of cartilage instead of bone. This class is divided into two main evolutionary branches. The first is the Holocephali, a group whose only living representatives are the chimaeras, or ghost sharks. Genetic evidence suggests that the lineage leading to chimaeras diverged from the rest of the cartilaginous fishes around 420 million years ago.
The second branch is the Elasmobranchii, which includes all modern sharks, skates, and rays. Within this subclass, sharks (superorder Selachimorpha) and the rays and skates (superorder Batoidea) are considered sister groups, meaning they are each other’s closest living relatives. They share a more recent common ancestor with each other than they do with the chimaeras.
The primary physical distinction between sharks and batoids is their body plan. Sharks have a streamlined, torpedo-shaped body with pectoral fins that are not fused to their head, whereas skates and rays have flattened bodies with large pectoral fins that are fused to the sides of their head, giving them a wing-like appearance. This close relationship highlights that a Great White Shark is more closely related to a stingray than it is to a chimaera.