BSCI392
11-09-07
The Fish in the Sea - Devonian Style

Near the end of the Devonian, gnathostomes hade diversified significantly. Many jawless craniates still lived in the oceans, including:

However the majority of fishy biomass had shifted to the gnathostomes, who occupied a vastly greater range of ecological niches. This is their diversity.

Gnathostome diversity: Three major groups of unknown relationships: Placodermi, Chondrichthyes, Osteichthyes. Placodermi: Silurian to Devonian armored gnathostomes experienced a rapid worldwide diversification and sudden decline.

Synapomorphies:

Chondrichthyes

Living chondrichthyan diversity cosists of:

Sampling of diversity:

  • Synapomorphy: It is often said that chondrichthyans are characterized by lack of bone. While true, it is not quite diagnostic. In fact, the unambiguous synapomorphy of Chondrichthyes is the presence of prismatic calcification of the cartilage. In contrast with bone, prismatic calcification takes the form of chains of tiny apatite crystals covering the surface of cartilage, likend together by collagen.
    Note: Chondrichthyans do not lack other hard tissues. Various groups make teeth, fin spines, and dermal armor out of dentine and enamel. Fossil Material: Sharks are arguably the oldest known gnathostomes, but their fossil record is poor because their bodies lack preservable hard parts that stay articulated when they die. Two notable exceptions:

    These structures are similar in histology (microstructure) and differ mostly in size. Teeth are generated in a conveyor belt of gum tissue which moves them into place, then causes them to be shed and replaced by the next member of their tooth family in line. Modern sharks do this very quickly, with individual teeth being functional for a few days or weeks. This ability has evolved slowly and earlier sharks grew and shed teeth at a lower rate.

    Chondrichthyes have occupied many of the ecological niches previously held by placoderms, and have added high-speed pursuit predators

    Osteichthyes - Bony fish

    Synapomorphies:

    Actinopterygii: (Dev - Rec)

    The vast majority of "fishlike" bony fish.