Chapter 32 An Overview of Animal Diversity

Reading Questions Notes Summary

Animal Kingdom Characteristics

• Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers

• Animals are multicellular eukaryotes• Their cells lack cell walls• Their bodies are held together by structural proteins

such as collagen• Nervous tissue and muscle tissue are unique, defining

characteristics of animals• Tissues are groups of cells that have a common

structure, function, or both

1. Embryological Development Stages

• Most animals reproduce sexually, with the diploid stage usually dominating the life cycle

• After a sperm fertilizes an egg, the zygote undergoes rapid cell division called cleavage

• Cleavage leads to formation of a multicellular, hollow blastula

• The blastula undergoes gastrulation, forming a gastrula with different layers of embryonic tissues

• The gastrual has a blastopore (the opening) and the archenteron ( the blind pouch)

• Figure 32.2

• Many animals have at least one larval stage• A larva is sexually immature and

morphologically distinct from the adult; it eventually undergoes metamorphosis

• A juvenile resembles an adult, but is not yet sexually mature

2. What are Hox genes?

• Most animals, and only animals, have Hox genes that regulate the development of body form

• Although the Hox family of genes has been highly conserved, it can produce a wide diversity of animal morphology

• Homeotic genes are regulatory genes that control the placement and spatical organization of body parts in animals, plants and fungi by controlling the developmental fate of a group of cells.. in animals were named Hox genes.

• Hox genes (homeobox containing genes) regulate the formation of anterior – posterior axis of the animal body. (Head, thorax, abdomen,)

• Homeoboxes are 180 nucleotide sequence of DNA which specifies for a 60-amino acid homeodomain in proteins These proteins in turn affect development of cells into certain body parts.

• Hox genes in more complex animals arose through duplication of earlier homeobox genes in the genetic code.

• Certain homeoboxes and homeotic genes are conserved and show up in the genetic code between different animals. (Example: drosophila, mice and humans)

• This implies that homeoboxes arose very early in the evolution of animals and are conserved in the animal DNA.

• Duplication and variation of these Hox genes allow for a versatile potential of diversity to arise in the morphology in animal evolution.

3. Dominant life forms during the eras

• Neoproterozoic Era (1 Billion–542 Million Years Ago)• Early members of the animal fossil record include the

Ediacaran biota, which dates from 565 to 550 million years ago

• Animals include sponges and cnidarians (jellyfish, coral, sea anemone)

• Simple body plans• Radial symmetry• Some bilateral symmetrical animals with repeating

body segments

• Paleozoic Era (542–251 Million Years Ago)• The Cambrian explosion (535 to 525 million years ago)

marks the earliest fossil appearance of many major groups of living animals

• Animals included arthropods, chordates, and echinoderms• There are several hypotheses regarding the cause of the

Cambrian explosion and decline of Ediacaran biota (Figure 32.5)– New predator-prey relationships– A rise in atmospheric oxygen– The evolution of the Hox gene complex

• Animal diversity continued to increase through the Paleozoic, but was punctuated by mass extinctions

• Fishes become the top predators in the marine food web

• Animals began to make an impact on land by 460 million years ago

• Arthropods adapted to terrestrial environments (leading to insects)

• Vertebrates made the transition to land around 360 million years ago

• Some animals have adaptations that allow them to start living on land and diversified into numerous terrestrial groups which include the amphibians (frogs and salamanders and reptiles

• Mesozoic Era (251–65.5 Million Years Ago)• Coral reefs emerged, becoming important

marine ecological niches for other organisms• The ancestors of plesiosaurs were reptiles that

returned to the water• During the Mesozoic era, dinosaurs were the

dominant terrestrial vertebrates• The first mammals emerged• Flowering plants and insects diversified

• Cenozoic Era (65.5 Million Years Ago to the Present)

• The beginning of the Cenozoic era followed mass extinctions of both terrestrial and marine animals

• These extinctions included the large, nonflying dinosaurs and the marine reptiles

• Mammals increased in size and exploited vacated ecological niches

• The global climate cooled

• The Burgess Shale located in BC is famous for its fossils which document the diversity of Cambrian animals. The fossils give us a picture of the types strange animals that existed during the Cambrian explosion.

Animal Body Plans & Symmetry

• Animals can be categorized according to the symmetry of their bodies, or lack of it

• Figure 32.7• Some animals have radial symmetry, with no

front and back, or left and right• Radial animals are often sessile or planktonic

(drifting or weakly swimming)

• Two-sided symmetry is called bilateral symmetry• Bilaterally symmetrical animals have

– A dorsal (top) side and a ventral (bottom) side– A right and left side– Anterior (head) and posterior (tail) ends– Cephalization, the development of a head

• Bilateral animals often move actively and have a central nervous system

Tissues

• Animal body plans also vary according to the organization of the animal’s tissues

• Tissues are collections of specialized cells isolated from other tissues by membranous layers

• During development, three germ layers give rise to the tissues and organs of the animal embryo

• Sponges and a few other groups lack true tissues• Diploblastic animals have ectoderm and endoderm

– These include cnidarians and comb jellies• Triploblastic animals also have an intervening mesoderm layer; these

include all bilaterians– These include flatworms, arthropods, vertebrates, and others

Body Cavities

• Most triploblastic animals possess a body cavity• A true body cavity is called a coelom and is derived

from mesoderm• Coelomates are animals that possess a true coelom• A pseudocoelom is a body cavity derived from the

mesoderm and endoderm• Triploblastic animals that possess a pseudocoelom are

called pseudocoelomates• Triploblastic animals that lack a body cavity are called

acoelomates

• Figure 32.8

• A coelom or body cavity has many functions:• Its fluid cushions internal organs and prevents injury• Some coelomates (such as earthworms) have

coeloms that contains noncompressible fluid that acts like a skeleton against which muscles can work

• A body cavity allows for internal orgnas to grow and move independently of the outer body wall (ex. Movement of heart beating, lungs expanding, rippling of intestines)

• Coelomates and pseudocoelomates belong to the same grade because of similar body plans

• Phylogenetic studies indicate that true coeloms and pseudocoeloms have been independently gained and lost multiple times over the course of animal evolution

• A grade is a group whose members share key biological features

• A grade is not necessarily a clade, an ancestor and all of its descendants

• So these terms are useful in describing body structure but must be interpreted with caution when referring to evolutionary history of animals

Protostomes and Deuterostomes

• Based on early development, many animals can be categorized as having protostome development or deuterostome development

• Figure 32.9

Protostome vs Deuterostome : Cleavage

• In protostome development, cleavage is spiral and determinate

• In deuterostome development, cleavage is radial and indeterminate

• With indeterminate cleavage, each cell in the early stages of cleavage retains the capacity to develop into a complete embryo

• Indeterminate cleavage makes possible identical twins, and embryonic stem cells

Protostome vs Deuterostome : Coelom Formation

• In protostome development, the splitting of solid masses of mesoderm forms the coelom

• In deuterostome development, the mesoderm buds from the wall of the archenteron to form the coelom

Protostome vs Deuterostome : Fate of the Blastopore

• The blastopore forms during gastrulation and connects the archenteron to the exterior of the gastrula

• In protostome development, the blastopore becomes the mouth

• In deuterostome development, the blastopore becomes the anus

Animal Phylogeny

• Zoologists recognize about three dozen animal phyla

• Phylogenies now combine morphological, molecular, and fossil data

• Current debate in animal systematics has led to the development of multiple hypotheses about the relationships among animal groups

• One hypothesis of animal phylogeny is based mainly on morphological and developmental comparisons

• Figure 32.10

• One hypothesis of animal phylogeny is based mainly on molecular data

• Figure 32.11

• Most scientists think the data and research support the second view of animal phylogeny tree based mainly on molecular data

• As new information emerges, our understanding of evolutionary relationships in these animal phylogeny tree models may also change

• Over time models in science always change to fit new data from research in order to find a better understanding of our world.

Points of Agreement

1. All animals share a common ancestor2. Sponges are basal animals3. Eumetazoa is a clade of animals

(eumetazoans) with true tissues4. Most animal phyla belong to the clade

Bilateria, and are called bilaterians5. Chordates and some other phyla belong to

the clade Deuterostomia

Progress in Resolving Bilaterian Relationships

• The morphology-based tree divides bilaterians into two clades: deuterostomes and protostomes

• In contrast, recent molecular studies indicate three bilaterian clades: Deuterostomia, Ecdysozoa, and Lophotrochozoa

• Ecdysozoans shed their exoskeletons through a process called ecdysis

• Figure 32.12

• Some lophotrochozoans have a feeding structure called a lophophore

• Others go through a distinct developmental stage called the trochophore larva

• Figure 32.13