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

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The other side of Deuterostomia is Chordata.


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.)


From The Lancelet

Also known as "tunicates" or "sea squirts." Adults can be motile or sessile.
  • Anatomy:
    • Their bodies consist of a large basket-like pharynx perforated by numerous slits. On one side, the endostyle secretes mucus which is transported across the inner surface of the pharynx by cilia, capturing food items as it goes.
    • Entire body enclosed by a "tunic" made primarily of tunicin, a unique complex sugar. Water enters pharynx through a "mouth" and, after filtration, enters an atrium from which it is expelled through a separate opening, the atriopore.
  • Larvae: Before settling on a hard surface, the larva is tadpole-shaped, and swims about searching for a suitable attachment place. The larva swims by means of a tail which degenerates after attachment. But note, the tail has a notochord. Furthermore, although in the adult, terms like dorsal and ventral have little meaning, in the larva it is clear that the nerve cord is dorsal.
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:

  • The skeleton consists of the notochord and specialized cartilages of mouth. The braincase is made primarily of connective tissue.
  • A two-chambered heart is present.
  • Single olfactory pouch communicates with the pharynx.
  • Eyes are present but very small, with no extrinsic muscles.
  • Otic (inner ear) capsules have only one semicircular canal (as opposed to three in jawed vertebrates).
  • Feeding apparatus. Keratinous "teeth" on paired protrusible plates used to grasp small prey or rasp pieces off of carcasses of larger animals.
  • An adult hagfish is much too big to achieve gas exchange by simple diffusion. Its gill slits are, therefore, lined with thin pleats of heavily vascularized tissue - proper gills - which serve as breathing organs.
  • Mucus glands secrete copious amounts of mucus as defense. Yuck.


From California diving

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:
  • a large sucker surrounding the mouth armed with keratin "teeth.", strengthened by annular cartilage
  • Piston cartilage supporting a protrusible "tongue" armed with more keratinous denticles
  • unique among extant vertebrates in having a median dorsal "nostril" but some other fossil vertebrates also display the same structure.
  • Undergo metamorphosis from suspension-feeding ammocoetes larva to parasitic adult.

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.


From Ichthyology at the Florida Museum of Natural History

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.

  • (Cambrian - Triassic)
  • Highly diverse and rapidly evolving, thus excellent index fossils.
  • Originally proposed to be the teeth of some unknown fish, but paleontologists soon determined they were were clueless about:
    • What kind of animal they were from
    • What part of the animal they represented.
    Thus, the word "conodont" was used to refer to the elements, themselves. The unknown creatures that made them were called "conodont animals.

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:

  • Which major taxon the elements belonged to
  • How they were used. To some they seemed likely to be used in prey capture, as in similar sturctures in chaetognaths (arrow-worms)
  • To others, they seemed like mineralizations of the suspension feeding apparatus of cephalochordates.
  • Or perhaps they were for internal support and not for feeding at all.

In the 1960s the situation was clarified somewhat by the discovery of articulated groups of conodonts. For the first time it became clear that these elements (or most of them) worked together as part of a conodont apparatus.

Moreover, different elements began to be distinguished by the manner in which they formed:

  • Protoconodonts (Cambrian) - internal addition only.
  • Paraconodonts (Cambrian) - both external & internal addition.
  • Euconodonts (Cambrian - Triassic) - external addition.
Since these seemed to appear in a stratigraphic (chronological) sequence, speculation was that these growth forms represented evolutionary stages. Not quite.

In 1988, single cone elements were eliminated from the roster of euconodonts by the discovery that they were the fossils of early chaetognaths. In living chaetognaths, similar elements are made of calcium carbonate and are used in prey capture. Indeed, all articulated conodont apparatuses were made of euconodont elements.

Moreover, different elements began to be distinguished by the manner in which they formed:

  • Protoconodonts (Cambrian) - internal addition only.
  • Paraconodonts (Cambrian) - both external & internal addition.
  • Euconodonts (Cambrian - Triassic) - external addition.
Since these seemed to appear in a stratigraphic (chronological) sequence, speculation was that these growth forms represented evolutionary stages. Not quite.

The conodont animal unveiled...

Are you ready?

In 1983, Simon Conway-Morris (of Burgess Shale fame) published on Cladygnathus, a Mississippian age eel-shaped creature in which he noted:

  • Chordate-like V-shaped segmented muscle blocks
  • Midline fins supported by fin rays
  • The conodont apparatus in an anterior position, suitable for use in feeding.
  • Notochord
  • A head a brain and two capsules for special senses, thought to be very large eyes and smaller otic capsules.

Ten years later, we got Mark Purnell reporting on Typhloesus wellsi from the Mississippian of Montana.

We now have an emerging consensus on what the "conodont animal," now the monophyletic group Euconodonta, looked like - a small, eel-shaped chordate.

But where does it go on the chordate cladogram? The presence of a phosphatic skeleton arguably places it within vertebrata, closer to the jawed vertebrates than lampreys are, but there are concerns: