The Vertebrate Pattern, continued

Vertebrata:


The living diversity of vertebrates boils down to three groups:

Let's examine these in turn.



Hagfish - from Joseph Jameson-Gould, Real Monstrosities Blog

Hyperotreti:

Hagfish - (Carboniferous - Recent) We presume their ancestry goes back farther but their fossilization potential is low.

Synapomorphies of Hyperotreti:

Hagfish display interesting behaviors and are endearing in a grotesque sort of way.

Other characteristics:

But hagfish lack many features we see in other craniates:



Pacific lamprey - from Brendan Maher, Natureblog

Hyperoartia:

(Lampreys) (Devonian - Recent) Larvae resemble ancient suspension-feeding craniates. These metamorphose into highly specialized and disgusting parasitic adults with many specialized features.




Mako shark - from Sam Cahir, Mail Online

Gnathostomata:

(Jawed vertebrates) (Silurian - Recent) The last common ancestor of all jawed vertebrates and all of its descendants. Characterized by jaws:

(Silurian - Quaternary) Living gnathostomes are distinct from living either Hyperotreti or Hyperoartia in many respects. Conspicuously, they have:

The lifestyles of craniates were revolutionized by the ability to process large food items.


Vertebrate Relationships

"All I can say is that if cyclostomes form a clade, either hagfishes are the most extraordinary example of reversion among vertebrates, or lampreys and gnathostomes are the most extraordinary example of evolutionary convergence."
Philippe Janvier - 2007.

Illuminating the relationship between Hyperotreti, Hyperoartia, and Gnathostomata is difficult and contentious. Two major hypotheses predominate that take their names from the positions they imply for lampreys:

Cyclostomata:

(Devonian - Quaternary) Support for a close relationship of lampreys and hagfish is mostly molecular (E.G. Kuraku et al., 2009), however this support is widely replicated and very strong. Clear morphological synapomorphies are not obvious, but possible candidates include:


At least the early evolutionary stages of vertebrae:
Above - lamprey; below - shark

Vertabrata:

(Cambrian - Quaternary) The majority of morphological evidence (and at least one molecular analysis, Gürsoy et al., 2000) supports the monophyly of Vertebrata - lampreys and jawed vertebrates. Characterized by the presence of extensive internal cartilage ("chondrifications")



Craniata?

If we accept the Vertebrata hypothesis, then Vertebrata and Hyperotreti (hagfish) form a larger group - Craniata - chordates with heads.

This is a huge problem in vertebrate systematics that cries out for resolution. Being a morphologist, your humble instructor is more comfortable with the Vertebrata hypothesis, but not really very comfortable. So, in this course, we will show the base of Vertebrata as an unresolved polytomy.


Vertebrate synapomorphies:


But enough! Gnathostome diversity awaits!

The lifestyles of vertebrates were revolutionized by the synapomorphies of Gnathostomata:

"Finny tribes"

Before we proceed, know the names of the fins of a proper fish. Each tells an interesting evolutionary story:

Two major living groups:


Cladoselache, a Devonian stem chondrichthyan from Wikipedia

Osteichthyan survey:

The diversity of "fish" breaks down into two groups:


Fin rays of the Swordfish pectoral fin

Actinopterygii:

(Devonian - Recent) Ray-finned fish. Most living fish. A huge diversity from:

A starting synapomorphy:


Sarcopterygii:

the last common ancestor of Actinistia (coelacanths) and land vertebrates, and all of its descendants.

A starting synapomorphy:



University of California Berkeley
Sarcopterygian diversity:

Tetrapoda:

(Carboniferous - Recent): The last common ancestor of Lissamphibia (salamanders, frogs, etc.) and Amniota (mammals, reptiles) and all of its descendants.


Tetrapod diversity:

Tetrapod synapomorphies:

Amniota:

(Carboniferous - Recent) the last common ancestor of Synapsida and Sauropsida and all of its descendants.

Beginning synapomorphies of Amniota:

Amniota gave rise to the primary diversity of land vertebrates.
Amniote diversity: Two major groups:


Brown bandicoot Isoodon macrourus - a synapsid.

Synapsida:

(Carboniferous-Recent) Mammals and their fossil relatives.

Dimetrodon from Paleofile

Beginning synapomorphy of Synapsida:


Sauropsida:

(Carboniferous - Recent) Reptiles.