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.
- Distinct cranial and thoracic armor with unique ossification pattern.
- Jaws lined with self-sharpening occluding bony plates . Traditionally regarded as toothless, it now develops that some have teeth. Is this convergent evolution or were they present ancestrally and lost in most placoderms?
- Adductor muscles (that close the jaw) pass medial to palatoquadrate.
Placoderms occupied a wide range of ecological roles. Their specializations included:
Fossil record: Fragmentary records of placoderms appear in the Middle Silurian. this is followed by a rapid diversification. During the Devonian, placoderms were the dominant vertebrate group. Both marine and fresh water forms are recorded with a worldwide distribution except for puzzling absence in South American sediments. Placoderm diversity was greatly reduced by an extinction event in the Late Devonian. They were completely extinguished by the mass extinction event at the end of the Devonian. Thus, entire radiation took up only about 50 million years, but while it lasted, it was spectacular.
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
- Unique pattern of dermal bones of head and pectoral girdle. (clavicle, interclavicle, cleithrum, parasphenoid, etc.)
- Dermal skull bones with internal descending laminae that interact with endochondral elements.
- Branchiostegal rays present: These are the narrow elements that cover the throat and part or all of the gill covers.
- Lungs: The presence of lungs - outpouchings of the esophagus used to obtain suplimentary oxygen from "swallowed" air. As we will see, Actinopterygii and Sarcopterygii exploit this synapomorpy in very different ways.
- Operculum: A large plate of dermal bone suspended from the hyoid arch covering and protecting the gill arches. (Ancestrally consists of three bones, the preopercular, opercular and subopercular.)
- Major groups:
- Actinopterygii: Ray-finned fish
- Sarcopterygii: Lobe-finned fish
Actinopterygii: (Dev - Rec)
The vast majority of "fishlike" bony fish.