DAY 14 ANIMAL DIVERSITY

Most animals share

mobility, specialized muscle and nerve cells, and a complete digestive tract

Animals likely evolved

from single-celled eukaryotes similar to today’s choanoflagellates

The Ediacaran fossils

(about 635–541 mya) are the first clear large animal body fossils — soft-bodied, and strange enough that some are still debated

535–525 million years ago

large forms of many present-day animal phyla appear suddenly in the fossil record

The oldest fossils of many living phyla date to this window

roughly 10 million years, which is a blink geologically

Cambrian fossils include the first hard, mineralized skeletons

which is also why they preserve so well

A body plan

a set of morphological and developmental traits integrated into a functional whole

Symmetry

radial (cnidarians) vs. bilateral (most animals)

Tissue layers

diploblastic — two layers — vs. triploblastic, which adds mesoderm and makes muscles and organs possible

Body cavity

present or absent, and how it forms

Sponges

no true tissues; filter feeders

Cnidarians

radial, diploblastic, stinging cells; jellies, corals, anemones

Molluscs

soft bodies, often shelled; snails, clams, octopuses

Annelids

segmented worms

Arthropods

segmented body, exoskeleton, jointed appendages. Trilobites dominated early; arthropods are still the most species-rich animals alive.

Four derived chordate characters

a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a

muscular post-anal tail

Some chordates never develop a backbone

lancelets and tunicates

Vertebrates

named for vertebrae, the series of bones forming the backbone

The first vertebrates were jawless

they filtered or sucked. Lampreys and hagfish are the living remnants.

Gnathostomes

jawed vertebrates — appear around 440 million years ago. Jaws probably evolved from modified pharyngeal skeletal supports

Jaws plus paired fins

made active, large-bodied predation possible

vertebrates have been the ocean’s dominant predators ever since

more than 400 million years

Land offered opportunity

plants and fungi had arrived first, so there was food and cover and almost no competition

arthropods got there first

the exoskeleton already resisted drying and supported weight out of water, and insects later added flight

tetrapods

evolved from lobe-finned fishes; limbs with digits were modified fins, and the fossil sequence shows the steps

Amphibians were the first tetrapods on land

most still need water to reproduce.

Amniotes

are tetrapods whose living members are reptiles — including birds — and mammals

The amniotic egg

encloses the embryo in fluid inside a shell, with membranes handling gas exchange, waste storage and nutrition

this is the animal equivalent of the seed:

reproduction no longer requires standing water

Early oceans

cloudy with microorganisms and suspended organic matter, low in oxygen, and dominated by cyanobacteria for over a billion years

Filter-feeding and burrowing animals cleared the water and stirred the sediments

which changed the chemistry of the seafloor itself

On land

animals became pollinators, seed dispersers, herbivores and predators — reshaping plant evolution in turn

Animals arose

from choanoflagellate-like ancestors more than 700 mya

The Cambrian explosion

produced most living phyla in about 10 million years

Aquatic radiations

gave us the invertebrate groups, then chordates, then jawed vertebrates

Arthropods and tetrapods colonized land

the amniotic egg finished the job