HONR219d
Lecture 8

Vertebrate Origins

I. The phylum problem: The Linnean system has conditioned us to regard phyla as equivalent units representing fundamentally different body plans. Prior to the development of phylogenetic systematics, the question of how different phyla were related to one another was simply not addressed in a detailed way. Where animals were concerned, however, two major contrasts were evident:

  1. Radially, vs. bilaterally symmetrical animals.

  2. Radially symmetrical animals, such as sea anemones, are much simpler, with only two layers of body tissue, and a single opening to the gut that functions both as a mouth and as an anus.

  3. Bilaterally symmetrical animals have the three germ layers that we are familiar with, and a one-way gut tube with mouth and anus.

Among bilaterally symmetrical animals, there is a fundamental division between Protostomes and Deuterostomes:

  1. In protostomes:
    1. the blastopore becomes the mouth
    2. development is determinate - i.e. cells have unalterable fates, regardless of what their neighbors do
    3. Cleavage is spiral

  2. In deuterostomes:
    1. the blastopore becomes the anus
    2. development is indeterminate
    3. Cleavage is radial.

Every analysis to date indicates that the deuterostomes are a monophyletic group, called Deuterostomia. Who are they? In addition to Vertebrates and their close relatives, they include:

  1. Echinodermata

  2. Hemichordata

  3. Urochordata

  4. Cephalochordata

II. Let's explore their diversity:

  1. Echinodermata: General living diversity: starfish, brittle stars, sea urchins, sea cucumbers, crinoids. In addition many ancient stalked, suspension feeding echinoderms. Very weird.
    1. Calcium carbonate internal skeleton
    2. Water vascular system with tube feet
    3. five part symmetry
    4. Ancestrally suspension feeders, using macroscopic tube feet to capture food particles and transport them to the mouth.

  2. Hemichordata: Like echinoderms, these are suspension feeders, however food capture is by means of ciliary action.
    1. Bodies divided into proboscis, collar, and trunk:
      1. Cilia on proboscis move food particles to mouth. Proboscis may be simple or developed into a complex set of tentacles.
      2. Collar encircles mouth and proboscis.
      3. Trunk contains a large pharynx with pharyngeal slits (gill slits)
      4. Open circulatory system present, with at least some members having a heart.
    2. Dorsal nerve cord, hollow in some parts of collar.

      Note, hemichordates share the pharynx and gill slits with vertebrates - these characters are synapomorphies of a monophyletic group containing them but excluding echinoderms.

      Diversity:

      1. Enteropneusta: "Acorn worms" - largish (up to 10 cm.) worm like burrowing suspension feeders.
      2. Pterobranchia: Little (1-5 mm.) colonial critters that secrete branching colonial structures. Each individual lives in a cylindrical compartment, but is able to move around on the outside of the colony. Individuals attached to colony by a strand of contractile tissue. When the colony is alarmed, individuals are quickly "reeled in." Cute. BTW, for fossil buffs, it appears that pterobranchs are closely related to the graptolites of the Paleozoic.

    3. Urochordata: Also known as "tunicates" or "sea squirts." Adults are attached to some hard surface.
      1. Anatomy:
        1. Their bodies consist of a large basket-like pharynx perforated by numerous slits. On one side, a gland secretes mucus which is transported across the inner surface of the pharynx by cilia, capturing food items as it goes.
        2. 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.
        3. Heart and open circulatory system present. Blood flow reverses periodically.
      2. 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.

      Because the derived characters of the:

      • Notochord
      • Hollow dorsal nerve chord
      • Myomeres
      are shared with vertebrates, they are synapomorphies of the monophyletic group Chordata, containing both urochordates and vertebrates, but not hemichordates or echinoderms.

    4. Cephalochordata: Represented by the living Branchiostoma.
      1. Nomenclature: Earlier names have priority in scientific nomenclature. Unfortunately, the publication in which this animal was originally named (Branchiostoma) was largely ignored, and it was named a second time (Amphioxus). Additionally, there had been a common name (Lancelet). When the dust settled, Branchiostoma was acknowledged the proper scientific name, and amphioxus and lancelet are both considered common names. Live with it.
      2. Anatomy: In many ways, Branchiostomais like a large sleek version of a urochordate larva. Similarities include:
        1. Food is captured on a film of mucus during filtration of water through a large pharynx.
        2. Both water and mucus moved through pharynx by cilia.
        3. After filtration, water enters atrium, then expelled through atriopore.
        4. Body propelled by lateral undulations, and supported by a notochord.
        5. Like adult urochordates, Branchiostoma just sits and filters most of the time. Instead of attaching to a hard surface, it buries itself in mud, with only mouth and tip of shout protruding.
      3. Differences:
        1. Rather than being a perforated basket, the pharynx has long vertical slits separated by branchial bars, on either side.
        2. Water enters pharynx through opening in a membrane called the velum.
        3. Propulsion is by muscles that are organized in chevron-shaped myomeres.
        4. The notochord extends to the tip of the snout.
        5. When disturbed, the animal swims around, fish-like, before settling down.
        6. Although there is no heart, the circulatory system is closed. Blood flows through a ventral aorta to the branchial bars, up through them, and into a dorsal aorta through which it is taken to the body.
        7. The dorsal nerve cord is hollow and there is a slight anterior enlargement.
        8. A diverticulum of the gut occupies place of liver.

        Because the derived characters of the:
        • Retention of the notochord, hollow dorsal nerve cord, and myomeres throughout life
        • Closed circulatory system
        are shared with vertebrates, they are synapomorphies of a monophyletic group containing both cephalochordates and vertebrates, but not urochordates, hemichordates, or echinoderms.

        Cephalochordata has a sparse fossil record, including the Cambrian period Pikaia from British Columbia and Yunnanozoon from Yunnan in China.

        IV. Now add craniates in form of ammocoetes - larval lamprey
        Anatomy: Generally similar to Branchiostoma, but:

        1. Distinct brain with capsules for special senses.
        2. Notochord ends posterior to brain rather than extending to tip of snout
        3. Water pumped through pharynx by muscular action of velum rather than by cilia.