Post on 23-Dec-2015
transcript
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