An Overview of Animal Phylogeny

How the Animal Kingdom Evolved

Chapter 32

An Overview of Animal Phylogeny

Animals diversified rapidly during the late Precambrian and early Cambrian periods.

It is difficult to determine the exact sequence of branching from the fossil record.

Phylogenetic trees are also based on comparative anatomy, embryology of living animals, and molecular systematics.

Most zoologists agree that the ancestral organism was probably a choanoflagellate ( a Protist with flagella)

The Parazoa - Eumetazoa Split The first split in the animal kingdom

probably occurred when true tissues evolved in some animals.

Parazoa – includes only the phylum Porifera (sponges) simple anatomy unlike other animals because they lack true

tissues Sponges are an evolutionary “dead end”

Eumetazoa - animals with true tissues tissues allow for complex anatomy and functions

The Radiata-Bilateria Split The second major split came when

some animals with true tissues developed bilateral symmetry. This split the Eumetazoa into two groups

Radial Animals Bilateral Animals

Asymmetry These animals

have no real body symmetry

The only asymmetrical animals are the phylum Porifera ( the sponges)

Radial Symmetry Radial animals are organized around a central axis Body structures radiate from the center They have 2 Body directions

oral –toward mouth aboral – away from the mouth They have no right or left

They are Diploblastic – have 2 embryonic germ tissues Endoderm Ectoderm No mesoderm ( the layer that gives rise to muscles)

lacking well developed muscles, they are usually sessile; some swim weakly or drift with currents.

Oral

Aboral

Bilateral Symmetry Bilateral animals are divided into right

and left sides that are approximately equal.

They have 6 body directions dorsal(top) ventral (bottom) anterior(towards head) posterior(towards tail) right & left

dorsal

ventralanterior

posterior

Bilateral SymmetryBilateral animals are triploblastic ( form all 3 embryonic germ

tissue layers) exhibit cephalization are associated with directional movement Bilateral symmetry and cephalization develop

as a result of directional movement. The front of the animal faces the new environment

and is where sensory structures accumulate The right and left side face about the same

environment and become very similar

Symmetry Symmetry is based on body plan & embryonic

development of the germ tissue layers During the embryonic stage called gastrulation

the embryo forms the germ tissue layers Radial animals are diploblastic

develop 2 germ layers Endoderm & ectoderm

Bilateral animals are triploblastic develop 3 germ layers endoderm, ectoderm, & mesoderm

Germ Layers of the Early Embryo

Ectoderm - covers surfaces; forms the outer covering(epidermis) & central nervous system

Endoderm – the innermost layer; forms lining of digestive tract, the liver & lungs of vertebrates

Mesoderm – the middle layer; forms muscles & most organs between the digestive tract & the outer covering

The Acoelomate-Coelomate Split The next major split among animals

occurred when the bilateral animals began to develop different types of body cavities.

These animals are grouped according to whether they have a body cavity and how it develops

Types of Body Cavities Acoelomates – have no body

cavity between digestive tract & outer body wall

Pseudocoelomates – have a fluid-filled body cavity between the digestive tract & the body wall, but it is not lined with peritoneum (mesodermal tissue)

Coelomates – have a true body cavity between digestive tract & outer body wall which is lined with mesodermal tissue

The Coelum - Functions The coelum was a significant

evolutionary advance Functions

The fluid cushions the organs thus preventing injury.

Internal organs can grow and move independently of the outer body wall.

It can serve as a hydrostatic skeleton in soft bodied coelomates such as earthworms.

The Protostome-Deuterostome Split The coelomates can be divided

into two distinct evolutionary lines: protostomes and deuterostomes.

This is based on differences that occur during embryonic development such as: cleavage patterns the fate of the blastopore coelom formation

Protostomes vs. Deuterostomes

Protostomes Spiral cleavage Determinate cleavage Schizocoelus coelum

formation – the coelum develops from splits in mesoderm

The blastopore becomes the mouth

Deuterostomes Radial cleavage Indeterminate

cleavage Enterocoelus coelum

formation – the coelum develops from outpocketings of the mesoderm

The blastopore becomes the anus

Comparing Protostomes & Deuterostomes

Spiral cleavage Cell divisions are

unequal Determinate cleavage

The developmental fate of each embryonic cell is determined very early – the cell can only form a particular structure

Radial Cleavage The plane of cell

division produces equal divisions

Indeterminate cleavage Each early embryonic

cell retains the ability to develop into a complete embryo. This is why humans can form identical twins, and embryonic stem cells can develop into any type of mature cell

Comparing Protostomes & Deuterostomes

Protostomes and deuterosomes also differ in the process by which the coelum is formed

Protostomes

Deuterostomes

Comparing Protostomes & Deuterostomes

The blastopore is the first indentation that develops in the the hollow ball of cells.

In protostomes, this opening becomes the mouth.

In deuterostomes, the blastopore becomes the anus, and the mouth develops from a second opening.

Protostomes

Deuterostomes

Compare Protostomes and Deuterostomes

The Cambrian Explosion

Origins of Animal Diversity The diversification that produced

modern animals occurred rapidly during the late Precambrian and early Cambrian period.

This “brief” evolutionary episode lasted about 40 million years ( and occurred about 565 to 525 million years ago)

Precambrian Animals The earliest animal fossils date to

about 570 million years ago Molecular evidence suggests that

animals might have originated about 1.1 billion years ago

Precambrian animals were soft-bodied and resembled Cnidarians, mollusks and worms

The Cambrian Explosion A “rapid” burst of diversification that took

place during the first 40 million years of the Cambrian period

Nearly all the major animal body plans appear in Cambrian rocks ( about 543-525 m.y.a.)

By the end of this episode, all the major body palns for all the major animal phyla were established.

NO ADDITIONAL PHYLA EVOLVED AFTER THIS PERIOD

Evidence of Evolution The fossils of the Burgess Shale

in western Canada indicate a tremendous radiation of animals at the beginning of the Cambrian period.

Modern-day animals are descendants of the animals found in the Burgess Shale.

Two other fossil sites ( Greenland and the Yunnan region of China) support this.

These fossil beds show the first animals with hard, mineralized skeletons

What Sparked the Cambrian Explosion?

Probably a combination of factors Ecological Causes

Predator-prey relationships were changing protective shells and modes of locomotion affected

these relationships Geologic Causes

The buildup of atmospheric oxygen to a level that would support animals with active metabolism & activities

Genetic Cause The evolution of Hox genes drastically changed

embryonic development