BSCI392
11-05-07
The conodnt animal and the vertebrate body-plan

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The other side of Deuterostomia is Chordata.

As with the echinoderm side, the chordate side of Deuterostomia contains enigmatic taxa that will be our focus. First, we review what is well established:

Major groups include:

Urochordata: (Unambiguous fossils Jurassic - Recent, but the record is very poor.)

Also known as "tunicates" or "sea squirts." Adults can be motile or sessile.

Urochordate diversity:

Cephalochordata: (Cambrian - Recent)

Represented by the living Branchiostoma. Fossils include Pikaia from the Burgess Shale and Yunanozoon from Chenjiang.

Craniata: (Cambrian - Recent). Sister taxon to cephalochordata. Includes all chordates with heads. Synapomorphies include:

Major groups

Hyperotreti (Hagfish) (Pennsylvanian - Recent, however record consists of one Penn. age fossil, Myxinikela)

Morphology:

Vertebrata (Animals with vertebral columns) - (Cambrian Recent): Obviously includes a great diversity. In the living world, most vertebrates are also members of Gnathostomata - the jawed vertebrates. During the Paleozoic, there was also a great diversity of jawless vertebrates.

Vertebrate synapomorphies:

For the moment, we will omit them and use as our example, the single remaining group of living jawless vertebrates.

Hyperoartia (aka Petromyzontida, aka lampreys) - (Devonian - Recent): Characterized by:

The oldest proper fossil lamprey is Priscomyzon riniensis of Devonian age, however suspiciously lamprey-like forms from much earlier include the Silurian Jamoytius . (Specimen.) Some researchers feel they can spot the annular cartilage in less lamprey-like critters.

Now, the problems....

Conodonts: Since 1856, paleontologists have been aware of minute (0.1 - 0.5 mm.) fossils made of apatite (calcium phosphate), the same mineral as vertebrate bone and teeth.

It was soon clear that conodonts came in four general varieties, and that these different types of elements must each have been present in the conodont element. We have:

These were always found as disarticulated clasts in marine sediment. This situation led to the very reasonable but misleading simplifying assumption that each type of element represented a different taxon.

At this point, speculation raged about: