The Vertebrate Pattern, continued


Deuterostomes are distinguished from other bilaterians by having deuterostomous development (later). Major groups include:

Cyprinius carpio
Chordata: (Cambrian - Recent) Including

Leopard sea cucumber Bohadschia argus
Echinodermata: (Cambrian - Recent)

Acorn worm from The Telegraph
Hemichordata: (Cambrian - Recent) Including

From Clark University - Ecology of Atlantic Shores
Chordate diversity:
Craniata: (Cambrian - Recent) The last common ancestor of hagfish and vertebrates and all of its descendants.

Synapomorphies of craniata:

Hagfish - from Joseph Jameson-Gould, Real Monstrosities Blog

A modest example

For now, we will represent Craniata with its most primitive living group - Hypertreti - the hagfish (Carboniferous - Recent).

Synapomorphies of Hyperotreti:

Hagfish display interesting behaviors and are endearing in a grotesque sort of way. All synapomorphies of Craniata are present in hagfish.

Other characteristics:

But hagfish lack many features we see in other craniates:

As such, they are a decent stand-in for the ancestral craniate.

How are these deuterostome groups related?

Simply scoring a matrix of their obvious characters can lead to naive results. (Try it.)

To make it more interesting, consider some fossil deuterostomes:

Vetulicolians by Skelefrog from Deviant Art
Vetulicolia: Known from the classic fossil localities of Chengjiang (Yunnan Province, China) and Sirius Passet (Greenland). These animals encompass a range of shapes but have these characteristics in common:

The consensus interpretation is that:

Pikaia graciliens from Smithsonian Institution

Pikaia - Almost a chordate:

The Burgess Shale fossil Pikaia graciliens, known from hundreds of specimens, is often cited in popular literature as the "earliest chordate." In fact:

Conway-Morris and Caron plausibly regard it as a "stem-chordate," closer to Chordata than to Ambulacraria, but just barely. What Pikaia does have is: Potential synapomorphies with proper chordates. <

What a difference twenty years make.

When your instructors completed their PhDs, the consensus of deuterostome phylogenies held that:

Discoveries that have changed that view:

Craniate diversity:

University of California Berkeley
Sarcopterygian diversity:

Tetrapoda: the last common ancestor of Lissamphibia (salamanders, frogs, etc.) and Amniota (mammals, reptiles) and all of its descendants.

Tetrapod diversity:

Amniota: the last common ancestor of Mammalia and Reptilia and all of its descendants.

Amniota issues:

Amniote diversity: Two major groups