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Candidate Transitional Forms Between Invertebrates and Vertebrates (Sidebar 3)

If evolution is true, then there must be some transition forms between the invertebrates and vertebrates. Three phyla have traditionally been considered candidates: Arthropoda, Annelida, and Echinodermata.

Arthropoda Annelida Echinodermata
Anthropoda
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Annelida Echinodermata
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The gap to be bridged is between organisms that do not have the four distinguishing characteristics (notochord, pharyngeal pouches, dorsal hollow central nervous system, and vertebrae) and those that do. Ostracoderms (a type of fossil fish) are considered to be the first vertebrate to appear in the fossil record (Ordovician/Paleozoic - Period/Era).

Ostracoderms


They had no jaws or paired fins and fed by filtering food from that freshwater environment. Their skin was covered with bony plates.

Below is a hypothetical evolutionary tree of how this process may have taken place. Keep in mind that this is a mental construct that attempts to show relationships that have not been proven. Evolving new organs and body plans is easy to talk about but practically impossible to demonstrate. Thus, a series of subphyla of the Phylum Chordata have been proposed as invertebrate intermediates. These are not on direct line to vertebrates, but indicative of how various innovations may have been added to the fundamental plan of say larval echinoderms to make permanent bilateral symmetrical animals with the four primary characteristics of all vertebrates.

tree

Definitions:
Bilateral symmetry:
matching structures on both sides of an organism

Ciliated larvae:
larval forms that have cilia on their surface for locomotion

Coelomate:
having a body cavity

Dermal bone:
bony deposits or plates that are located in the skin

Deuterostome:
an animal that uses the blastopore as an anus and forms a second mouth

Dorsal and ventral nerve cords:
nerve strings positioned either on the back or front of an animal

Dorsal hollow nerve cord:
bundle of nerves that have a central fluid filled channel

Gill slits:
openings between pharyngeal pouches

Mineralized mesoderm:
bony deposits in the middle tissue of the body

Notochord:
a specialized tissue for support of the body

Sessile:
attached to the substrate

Radial symmetry:
structures projecting out from a center hub


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