Question

why are sharks and rays the biggest fishes?

what synapomorphy allowed them to outgrow all other fishes in the ocean?

Answer 1

Sharks and the rays are collectively known as Chondrichthyes.

They are estimated to have lived in the world’s oceans for more than 400 million years.

The species that remain in our waters today many of which are living fossils, belong to groups that first appeared ~250 million years ago.

They are a diverse group of fishes with a cartilaginous skeleton that gives them flexibility and buoyancy.

Species from this group have successfully adapted to almost all aquatic ecosystems from lakes and rivers to the deep sea, including estuaries, coastal lagoons and coral reefs.

Grogan and Lund define the two synapomorphies of the Chondrichthyes as having skeletons of cartilage and having pelvic fins in the males modified as claspers.

The most sharks, rays and chimeras have a heterocercal tail, large pectoral fins, relatively small pelvic fins, and an obvious first dorsal fin.

Another characteristic is the tooth-like placoid scale which is made of dentine with a pulp chamber on the inside and a layer of enamel on the outside.

The structure of the placoid scale is similar to the scales of the thelodonts.

The cartilaginous fishes date from the Devonian to the present, but typical shark teeth and scales occur earlier in the Silurian.

Chondrichthyes has a long fossil history with at least two major periods of evolutionary radiation, today the diversity has been reduced to two groups the elasmobranchs (the sharks, skates, and rays) and holocephalians (the chimeras or ratfish).