Chapter  2 - The Origin of Vertebrates

Download an ADOBE version of the lecture outline for introduction to Chordata

Download an ADOBE version of the lecture outline for introduction to Vertebrata

return to vertebrate zoology syllabus

  Vertebrates in the Kingdom Animalia (Figure 2-1) from Dr. Bill Tietjen, Bellarmine College

KINGDOM ANIMALIA

29 phyla: includes invertebrates (97% of animal species) and vertebrates (3%)

occur in marine (place of origin), freshwater, and terrestrial habitats

Characteristics

1. Multicellular eukaryotes: cells are specialized (division of labor) to form tissues and organs (except for the most primitive)
2. collagen: a structural protein
3. homeobox genes
4. heterotrophic: ingest food; digest internally, usually in a digestive system
5. locomotion - during at least some part of the life cycle (many have sessile adult stages) and motion: by means of contracting fibers
6. sexual reproduction producing sperm and eggs; some asexual
7. embryo undergoes stages of development including a blastula stage

Mesozoa: mesozoans

minute ciliated, wormlike parasites (mainly on cephalopods); typically 20-30 cells in two layers; origins and relationships to other animals uncertain

Parazoa: sponges

Eumetazoa: tissues and organs present; nervous system with neurons

radiata: radial symmetry, diploblastic - Cnidaria and Ctenophora

bilateria:

bilateral symmetry (right and left halves)

movement as an adult

triploblastic:  three embryonic tissues (germ layers:)

ectoderm: outer layer - skin and nervous tissue

endoderm: inner layer - lines digestive tract

mesoderm: middle layer - muscles, bones, circulatory system, organs

1. acoelomates: no body cavity; body double-walled sac surrounding digestive cavity; single opening to outside; characteristic of flatworms.
2. pseudocoelomates: body cavity between mesoderm and endoderm (roundworms)
3. COELOMATA
   1. coelom: tube within a tube; body cavity
   2. body cavity completely lined with mesoderm
   3. protostomes and DEUTEROSTOMES

Embryological Difference 

Protostomes

blastopore becomes mouth; spiral and determinate cleavage; schizocoely - coelom forms from splitting of cavity in mesoderm: trochophore larva

annelids, arthropods, mollusks; lophophorates

Deuterostomes

blastopore becomes anus, develops new mouth; radial and indeterminate cleavage; enterocoely - coelom forms from outpocketing of wall of gut;

echinoderms, chordates

 

THE RELATIONSHIPS AMONG DEUTEROSTOMES

DEUTEROSTOMIA

PHLUM ECHINODERMATA

• Characteristics
1. water vascular system and stereom
2. includes asteroids, ophiuroids, echinoids, crinoids, holothuroids, and carpoids:
3. The extinct Carpoids (Subphylum Homalozoa) are traditionally regarded as primitive echinoderms without pentameral symmetry.  They have been reinterpreted as calcichordates by Jefferies, which include sister taxa to the cephalochordates, urochordates and craniates.   Carpoids may have had gill slits.
4. Ancestor to the vertebrates may have been an extinct, free-swimming echinoderm.

• Characteristics
  1. possess a stomochord (which may or may not be homologous with the notochord)
  2. gill slits in the pharynx
  3. tubular dorsal nerve chord in the collar zone (perhaps only in enteropneusts)
  4. characters that support hemichordate monophyly include a unique excretory structure and a preoral gut diverticulum
• Web Sites with more information
  1. UCMP
  2. Biomedia section of the Biological Sciences Website; Biological Sciences, University of Paisley, Glasgow University Zoology Museum

PTEROBRANCHIA

sessile, colonial life style; (e.g. Cephalodiscus, Rhabdopleura); no definitive chordate characters; filter feed using tentacles; a single pair of pharyngeal slits (for elimination of excess water ingested with food) is present in Cephalodiscus, absent in others; closely related to ?extinct graptolites

ENTEROPNEUSTA

acorn worms (e.g. Sacoglossus) burrowing life style; pharyngeal slits are present

PHYLUM CHORDATA

Learn  more about chordates from the UCMP  (chordate morphology) or from the Tree of Life

Characteristics

1. gill slits in the pharynx
2. tubular (hollow) dorsal nerve cord
3. notochord (usually replaced by vertebral column)
4. endostyle or thyroid
5. muscular postanal tail
6. tadpole larva
7. loss of mesodermal tentacles
8. Segmentation of muscles (in cephalochordates and craniates)
9. Closed circulatory system containing blood
10. Kidney are important for waste excretion & water balance
11. Sexes are usually separate and reproduction is sexual

• notochord: arises embryologically from the dorsal median portion of the mesodermal tissue; composed of soft, gelatinous, thick walled, closely compressed cells surrounded by an impervious sheath of connective tissue. Replaced by backbone in "higher" vertebrates; forms intervertebral discs.
• myotomes: segmented muscle blocks that are anchored to the notochord; swimming by waves of muscular contraction
•  endostyle secretes mucus & traps food;  can also bind proteins with iodine, may be homologous with thyroid

Taxa

• besides the vertebrates, the chordates include tunicates (Urochordata) and lancelets (Cephalochordata), chordates that, among other characteristics, lack a backbone.

SUBPHYLUM UROCHORDATA (Figure 2-3)

• tunicates or sea squirts
•  free swimming (using muscles) larvae, sessile adults 
• enclosed in a tunic of cellulose
• filter feeding
• learn more from the UCMP

EUCHORDATA

• axial skeleton retained through life; muscle somites present

SUBPHYLUM CEPHALOCHORDATA (Figure 2-3)

• lancelets
• Pikaia (Figure 2-2) from the middle Cambrian Burgess Shale is one of the earliest known chordates. Vertebrates are now known from the early Cambrian of China (Fig. 3-1).
• small, free swimming marine filter feeders
• numerous ( 50) pharyngeal slits, use cilia to pump water
• V-shaped myotomes, adults are poor swimmers
• mainly live half buried in sediment with mouth protruding
• learn more from the UCMP
• learn more from Dr. Bill Tietjen, Bellarmine College

SUBPHYLUM CRANIATA

• Pisces: (the fishes) are covered in chapters 3 , 5, and 6
• Tetrapods (the terrestrial vertebrates) are covered in chapters 8-24.

  Vertebrates Origins (Figure 2-1)

• Comparison of nonvertebrate chordates and primitive vertebrates (Table 2.1)
  • Origin of the Chordates: PDF file from David Cannatella, Comparative Anatomy, Biology 478L
  • Vertebrate Phylogeny: PDF file from David Cannatella, Comparative Anatomy, Biology 478L
• protochordates and vertebrates. see Section F.  Garstang theory of vertebrate evolution; possible steps in the evolution of vertebrates.  from UT Arlington.  Dr. Jonathan Campbell

HETEROCHRONY: genetic shifts in the timing of development.  Leads to evolutionary changes.  more on heterochrony

1. paedomorphosis: retention of ancestral juvenile characteristics by their descendants throughout life
   1. progenesis: precocious (accelerated) sexual maturation in a morphological juvenile.  Explains origin of vertebrates from a sessile tunicate ancestor.
   2. neoteny: slowing down of somatic development (reduced rate of growth) relative to reproductive development (common in salamanders)
2. peramorphosis: addition of extra growth phases to bring about changes in bodily form
   1. acceleration: a particular structure makes an early appearance and undergoes accelerated development (possibly Irish Elk antlers)
   2. hypermorphosis (recapitulation): structure continues development past ordinary time of cessation at maturity (common in invertebrates, unknown in vertebrates)

David Cannatella Herpetology--Taxonomy and  phylogeny had a graphic representation of some developmental trajectories.  Also a good introduction to tetrapod phylogeny.

PHYLOGENY OF THE VERTEBRATES

Dr. David Cannatella's Comparative Vertebrate Anatomy course has an excellent Adobe Acrobat pdf file you can download on Vertebrate Phylogeny

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