Shark Phylogeny, Part 2: A Phylogeny Tree for Sharks

Sharks and rays are known as elasmobranchs (subclass Elasmobranchii), and they share a common ancestor separate from that of chimaeras (subclass Holocephali). Living elasmobranchs descended from a group called Ctenacanthoidea, which went extinct in the early Triassic period, around the time the dinosaurs first evolved.

The descendants of the Ctenacanthoidea are the extinct group Hybodontoidea and the lineage that includes modern sharks, Neoselachii (literally, "new sharks"). Neosalachii radiated into two major lineages during the Mesozoic era: Galeomorpha (one representative is the modern great white shark) and Squalea, which in turn consists of Squalomorpha (one representative is the modern dogfish shark) and Hypnosqualea (one representative is the modern manta ray). Great whites and dogfish are both commonly called "sharks" and superficially resemble each other more than either resembles a ray, so traditionally they were classified together in superorder Selachimorpha (literally, "shark shaped"). Recent analysis using cladistics shows that, in terms of phylogeny, rays and dogfishes are actually more closely related to each other than either is to great whites. The group known as Selachimorpha is, therefore, a paraphyletic group. It can still be found in some classification systems, but in systems that use phylogeny exclusively, it is considered invalid.

The most noticeable difference between the two lineages of sharks is that animals in the lineage that also contains rays, Squalea, have no anal fin, while animals in Galeomorpha do have an anal fin. The anal fin is a synapomorphy (new adaptation) in Galeomorpha, not a secondary loss in Squalea. It acts like a keel, located on the underside of the fish just behind the cloaca (anus). Squalea contains mostly bottom dwellers, so an anal fin would get in the way of their lifestyle. Galeomorpha contains mostly active swimmers, some of which will actually suffocate if they stop swimming because the forward motion is what causes water to pass over their gills. An anal fin is very useful to provide stability to actively swimming fishes.

Here are some of the major branches of the shark phylogeny tree, listed in chronological order.

• Chondrichthyes split into Holocephali (which includes modern-day chimaeras) and Elasmobranchii (which includes living and extinct sharks, rays, and skates).
• Elasmobranchii split into Hybodontoidea, a successful but long-extinct group, and Ctenacanthoidea.
• One lineage of Ctenacanthoidea developed into Neoselachii, which split into Squalea and Galeomorphii, the two lineages to which all modern sharks belong.
• Squalea arose in the Triassic or early Jurassic period. It split almost immediately (in the early Jurassic) into the lineages Squalomorpha (including modern-day dogfish, bramble sharks, and others) and Hypnosqualea, which includes the diamond-shaped, flattened, ray-like angel sharks (Squatina) as well as rays, skates, and sawfish (Rajiformes).
• Galeomorphii arose in the early Jurassic period. Its first branch is Heterodontiformes (Port Jackson sharks) and a sister lineage. This other lineage branched into Orectolobiformes (wobbegons and nurse sharks) and a sister lineage, which in turn branched into Carcharhiniformes (tiger sharks, bull sharks, hammerheads, and many others) and Lamniformes (basking sharks, great whites, and the giant extinct Megalodon).

The information in a cladogram could, in principle, be displayed just as easily as nested groups, with each group containing the ancestor of all the subgroups within it. When describing phylogeny in plain text without a chart, these subgroups are nested together using parentheses, following the name of the parent group and a colon. A plus sign (+) links sister groups. Using this format, shark phylogeny could be written like this:

• Chondrichthyes: Holocephali + Elasmobranchii (Hybodontoidea + Neoselachii (Squalea + Galeomorphii))
  • Squalea: Squalomorpha + Hypnosqualea (Squatina + Rajiformes)
  • Galeomorphii: Heterodontiformes + sister group (Orectolobiformes + sister group (Carcharhiniformes + Lamniformes))

Shark phylogeny is counter-intuitive, with similar-looking groups only distantly related. This phylogeny is an example of how cladistics is a powerful tool to show evolutionary relationships when superficial appearances are deceptive.