Introducing Vertebrata

The Vertebrate Body:



Radinsky, Leonard, 1987 Evolution of Vertebrate Design
Chordates are the major taxon (a "phylum" in traditional taxonomy) that includes vertebrates, plus a few other odd-balls. In 1987, Leonard Radinsky published The Evolution of Vertebrate Design - a textbook that was actually creative - that featured schematic illustrations distilling the essence of vertebrate anatomy. From it, we take Radinsky's image of the basic vertebrate. Fundamentally, this body consists of:



Radinsky, Leonard, 1987 Evolution of Vertebrate Design

Focusing on the head: In addition to touch, the brain received information from three special senses. Each was mediated by a pair of special outpouchings of the brain:



Haikouichthys ercaicunensis from Feinberg and Mallatt 2013

Glimpses of Origins

Haikouichthys: Our closest actual glimpse of the ancestral vertebrate was the free-swimming Cambrian suspension feeder Haikouichthys ercaicunensis (right) described in 1999. It is a remarkable approximation of Radinsky's schematic in the (former) flesh, feeding by straining particulate matter from the water, and capable of swimming around. (Note: Haikouichthys resembles, and may be the same as Myllokunmingia fengjiaoa. Who cares?)


From Wikipedia
Ammocoetes: Arguably the best living approximation of the ancestral vertebrate is the ammocoetes larva of the lamprey. The living creature grows to roughly 4 cm length before undergoing metamorphosis into specialized adult parasitic lamprey. If we imagine the ammocoetes phase as the final life stage, we have something resembling the earliest fossil vertebrates. Interestingly, the ammocoetes larva only swims when disturbed. Typically it rests on the bottom and suspension-feeds.


Vertebrates in context:

Animals in the broadest sense:

The phylogeny of major animal groups appears at right. Included are:


"Porifera," - the sponges, the simplest animals have distinct cell types but don't really have proper tissues and certainly not organs. One cell type is indistinguishable from choanoflagellates, the closest single-celled relatives of animals. Have amazing powers of regeneration. Organizationally, they are living sieves, filtering microscopic food particles from water.

  • Cnidaria and Ctenophora: represent the next simplest grade of organization, with distinct tissues (ectoderm and endoderm only) and organs. Still the gut is a simple pocket with an opening that can function as mouth or anus depending. Some examples:


    From Erectus
  • Bilateria: Animals with:
  • Among such animals, we see two major branches identified by distinct patterns of embryology. (Later)


    Protostomia briefly noted:

    Protostomia contains the vast majority of animal diversity, including many familiar animals. Among them are two major groups, Ecdysozoa and Spiralia.

  • There is a trend in these active animals toward the evolution of special sense organs (eyes, antennae) and toward concentrating them at the front end along with an expansion of the nervous system to deal with the information they provide (heads).
  • And yet, these structures originate independently. Consider some close relatives of the major groups: Tardigrades for ecdysozoans, and phoronids for Spiralians.


    Deuterostomia

    Deuterostome survey: The deuterostomes contain Chordata (us), Echinodermata, and some minor groups. Of all of the major natural groups of animals, echinoderms are our closest relatives, and yet they are our weird relatives.



    From Wikipedia
    Chordate diversity:

    Fossil near-chordates:


    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: