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Vertebrate Evolution & Diversity. Trends in Animal Evolution. Is there evidence that any of these trends have reversed over time?. Symmetry none  radial  bilateral Pattern of gastrulation no blastopore  protostome  deuterostome Digestive system

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trends in animal evolution
Trends in Animal Evolution

Is there evidence that any of these trends have reversed over time?

  • Symmetry

none  radial  bilateral

  • Pattern of gastrulation

no blastopore  protostome  deuterostome

  • Digestive system

None  gastrovascular cavity  complete digestive system

  • Body cavities

acoelomate  pseudocoelomate  eucoelomate

  • Segmentation

none  segmented  fused segments

  • Skeletons

none  hydrostatic  exoskeleton  endoskeleton

What are exoskeletons made of? …endoskeletons?

Consider animals with endoskeletons…Do they demonstrate evidence of segmentation? Describe the relationship.

  • Phylum Echinodermata
    • “spiny skinned”
    • water vascular system

What is this vascular system used for?

  • Phylum Chordata
    • named for the notochord

What features of embryonic development are shared by members of these two groups?

Do all chordates have a vertebral column?

phylum chordata chordates evidence from anatomical comparisons
Phylum Chordata (chordates)-evidence from anatomical comparisons

Segmentation: muscles arranged in segmented blocks

Most with an internal and jointed skeleton, differing from that of echinoderms





  • Where is the notochord located, relative to the digestive tube and nerve cord? What is its function?
  • Where is the nerve cord located in most non-chordate animals?
  • What is the pharynx? How are these slits specialized in aquatic organisms?
  • Describe the advantage of a muscular, post-anal tail.
subphylum urochordata tunicates
Subphylum Urochordata – tunicates

Sessile marine animals, some are colonial

Filter-feed, using pharyngeal slits

Animal encased in a tunic of cellulose-like CHO

Larvae (c) presents all four chordate characteristics (“tail chordates”)

How have the slits been adapted for filter feeding?

Do you find evidence of segmentation in the larval form?

Fig 34.3

What is the term used to describe the change in body form here?

subphylum cephalochordata lancelets
Subphylum Cephalochordata – “lancelets”

Small, burrowing animals, marine

Adults possess all four chordate traits (“head chordates”)

Suspension feeders

Muscles arranged in “chevron” (<<<<<) around notochord

Sensory tentacles around mouth

How is the mouth modified to support this method of feeding?

2 cm

Fig 34.4

the origin of vertebrates
The Origin of Vertebrates

Cephalochordates are the closest living relatives of modern vertebrates.

Larval urochordate exhibiting paedogenesis*cephalochordate


* Early sexual maturity. Recall that genes that control development have played a major role in evolution (pg. 478)

The fossil record suggests an intermediate stage between cephalochordates and vertebrates, that lacks a cranium, but has eyes.


Some fossils from the same period exhibit all of the vertebrate characteristics.


? ancestral vertebrates, @ 530 mya


Fig 34.1

Fig 34.6

Neural crest: Unique group of embryonic cells that develop into various structures, including skeletal elements like the skull.

Note that not all craniates have a vertebral column.

phylogeny of craniates

Fig 34.7

Phylogeny of Craniates
  • Which adaptations enabled:
  • better maneuvering in the environment?
  • more active lifestyle?
  • larger size?
  • terrestrial lifestyle?
  • reproduction independent of water?
  • Describe the blood circulatory adaptations that also support a more vigorous lifestyle.

The endoskeletons of craniates are made of cartilage or a combination of cartilage and bone (mineralized tissue). How are these tissues related in embryonic development?


Jawless fishes



yellow = fibrous c.t.

blue = bone

green = notochord



Adapted from Tree of Life web site, available at

class myxini hagfish
Class Myxini – hagfish

All marine. Mostly bottom-dwelling scavengers

Slime glands along the sides for defense

Cartilaginous skeleton; no jaws, no teeth, no appendages, no spine

Mostly blind, well-developed sense of smell

Why are hagfishes considered the most primitive of the craniates?

Fig 34.8


Fig 34.1

Rigid spine of cartilage or bone gives support and anchors muscles better than the notochord.

c cephalaspidomorphi lampreys
C. Cephalaspidomorphi – lampreys

Marine and freshwater environments

Clamp round mouth onto flank of live fish, use rasping tongue to penetrate skin and ingest blood

Cartilaginous skeleton, including spine

No paired appendages, no jaws

Fig 34.9

jaws appendicular skeleton
Jaws & Appendicular skeleton

Jaws and mineralized teeth: firmly grip and slice food items, eat prey that had been inaccessible

Paired appendages (fins): accurate maneuvering in aquatic environments

evolution of vertebrate jaws

Fig 34.10

Evolution of vertebrate jaws

Class Chondrichthyes: sharks and rays

  • Lungs or lung derivatives:
  • in most fishes developed into swim bladder (buoyancy)
  • in other organisms, lungs function in
  • gas exchange

What are the respiratory organs in fish?

c actinopterygii ray finned fishes
C. Actinopterygii - ray-finned fishes

These diverse fish have a swim bladder; it permits neutral buoyancy

Fig 34.12a


better locomotion in terrestrial and shallow water environments

What does “tetrapod” mean?

Does this adaptation mean a completely terrestrial lifestyle?

  • extraembryonic membranes (including the amnion) bring the aquatic environment onto land!
  • enables completion of their life cycle on land
  • first appeared in mammal-like reptile

Do these animals demonstrate other adaptations to life on land?

amniotic egg
Amniotic egg

Fig 34.19


Fig 34.24

  • Keratinized skin; lungs; internal fertilization

an adaptation for thermal insulation and flight

class aves birds
Class Aves – birds


Light and hollow skeleton; - other flight adaptations

Legs and wings, most species move by flying

Amniote egg with a shell

Mouth developed into a beak

A variety of feeding mechanisms

bird flight
Bird flight

Fig 34.26


provides the ability to adequately nourish offspring

mammalia mammals
Mammalia – mammals


Mammary glands in the females to provide milk to young

Legs lost in some (marine mammals)

Amniote embryo, but does not develop a shell

Variety of feeding mechanisms


Which vertebrate characteristic is most responsible for their success in relatively dry environments?

Lungs and feathers were most important. That makes me best at taking advantage of the “dry” environment!

The amniote egg was the most important adaptation to life on land.

Nobody would have gotten anywhere without my cranium!

Wait a minute! I can fly…I think it’s the ability to provide milk to offspring.


General traits of reviewed phyla, subphyla and classes

Important evolutionary trends in body plan:

Endoskeleton: Echinoderms

Notochord, nerve cord, tail, pharyngeal slits: Chordates

Cranium, brain development, neural crest cells: Craniates

Vertebral column: Vertebrates

Jaws, 2 sets of paired appendages, mineralized skeleton and teeth: Chondrichthyes (sharks and rays)

Lungs or lung-derivatives: Osteichthyes (bony fishes)

Legs: Amphibians

Amniote egg: Mammals, turtles, snakes and lizards, birds

Feathers: Birds

Milk: Mammals

These trends helped animals adapt to different environments or exploit the same environment in a different manner