Chapter 28

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

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

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 There are several hypotheses regarding the cause of the Cambrian explosion and decline of Ediacaran biota New predator-prey relationships A rise in atmospheric oxygen The evolution of the Hox gene complex

Radial symmetry

- Most are either sessile or planktonic (drifting) - Symmetry allows them to meet environment well from all sides

Stephen R. Fairclough, Mark J. Dayel and Nicole King (2010),

-> Studying colony formation in choanoflagellates may provide a context for reconstructing the evolution of animal multi-cellularity. -> Here, we find that the transition from single cells to multicellular colonies in the choanoflagellate Salpingoeca rosetta (previously known as Proterospongia sp.) occurs by cell division, with sister cells remaining stably attached.

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

Homeotic genes

All animals (and only animals) have homeobox-containing family of genes called Homeotic or Hox genes that regulate the development of body form Because all animals share Hox genes, this gene family may have arisen in the eukaryotic lineage that gave rise to animals Although the Hox family of genes has been highly conserved, it can produce a wide diversity of animal morphology

Animals

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

Cell Structure and Specialization

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

Symmetry

Animals can be categorized according to the symmetry of their bodies (or lack of it) In general - symmetry fits lifestyle Animals either have radial symmetry or bilateral symmetry - both present in fossil record for 550 million years

Grade

Based on body plans & embryonic development Each major branch is a grade of body-plan features shared by the taxa belonging to that branch

Burgess Shale

Cambrian Diversity captured in the Burgess Shale Burgess Shale in Canadian Rockies holds the remains of an ancient sea from 530 mya

Interconnected causes

Earth system, developmental, and ecological processes have been hypothesized as isolated, singular causes of the major diversification of marine taxa early in the Cambrian.

Causes of Animal Diversification

Ecological Causes: Emergence of predator-prey relationships led to diversity of evolutionary adaptations, such as various kinds of protective shells and diverse modes of locomotion Geological Causes: Atmospheric oxygen may have finally reached high enough concentrations to support more active metabolism Genetic causes: Diversity in body form is associated with duplication and variation in the spatial and temporal expression of Hox genes

Most animals reproduce sexually

In most species, a small flagellated haploid sperm fertilizes a larger, haploid, non-motile, polarized egg Mitotic cell divisions called cleavage, lead to the formation of the blastula

Hox genes control morphology

Lewis et al. found a Cluster of homeotic (Hox) genes in the chromosome Colinearity in time and space - The gene order in the cluster mimics the order of expression of genes and their function along the anterior-posterior (A-P) body axis Exhibit temporal colinearity - anterior genes expressed first during development and posterior later

Basic Body Plans Stable Since Cambrian

Many Cambrian animals that are now extinct represented "experiments" in animal form In last half-billion years, animal evolution has mainly generated new variations of old "designs," one of the main themes of evolution New animal Phyla have evolved since the Cambrian radiation Variation in developmental patterns continues, allowing subtle changes in body structures and functions, leading to speciation and the origin of taxa below the level of Phylum

Complex life cycles of animal

Many animals have distinct larval stages The larva is sexually immature & morphologically and ecologically distinct from the adult Larvae eventually undergo metamorphosis, transforming into an adult

choanoflagellates

Modern choanoflagellates are tiny, stalked organisms inhabiting shallow ponds, lakes, and marine environments.

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

Edward B. Lewis, Christiane Nüsslein-Volhard and Eric Wieschaus

Nobel prize for medicine 1995 discoveries concerning "the genetic control of early embryonic development

Evidence that Choanoflagellates are related to animals

Sequence data indicates that Choanoflagellates and animals are sister groups Collar cells only found in animals and not in protists Choanoflagellate cells resemble sponge collar cells

The history of animals

The animal kingdom includes a great diversity of living species and an even greater diversity of extinct ones The common ancestor of living animals may have lived between 675 and 800 million years ago This ancestor may have resembled modern choanoflagellates, protists that are the closest living relatives of animals

Time of changes

Times of change.The major diversification of marine taxa at high taxonomic levels between 635 and 443 million years ago.

body plans

Zoologists sometimes categorize animals according to a body plan, a set of morphological and developmental traits Some developmental characteristics are conservative For example, the molecular control of gastrulation is conserved among diverse animal groups A grade is a group whose members share key biological features A grade is not necessarily a clade (a group that includes an ancestral species and all of its descendants)

Gastrulation

produces the gastrula, endoderm, and ectoderm

Ediacara fauna

unique assemblage of soft-bodied organisms preserved worldwide as fossil impressions in sandstone from the Proterozoic Eon at the close of Precambrian time. represent an important landmark in the evolution of life on Earth: they immediately predate the explosion of life-forms at the beginning of the Cambrian Period 541 million years ago. named for the Ediacara Hills of South Australia in which they were discovered in 1946, were the first metazoans (animals made up of more than one type of cell) that required atmospheric oxygen for their growth.