Evolution of Vertebrates

Darwin's Dilemma

  • Darwin questioned the lack of fossils older than the Cambrian period (540 million years ago), when a significant diversity of animals with skeletons suddenly appeared.

Animal Body Plans and Evolution

  • Metazoa: Animals or multicellular, heterotrophic eukaryotes.

  • Three Animal Body Plans:

    • Poriferans: No/Single Tissue, No Symmetry (e.g., Sponges)

    • Cnidarians: Two Tissues “Diploblastic”; Radial Symmetry (e.g., Jellyfish, Corals, Anemones)

    • Bilaterians: Three Tissues; “Triploblastic”; Bilateral Symmetry (e.g., Vertebrates, Molluscs, Arthropods, etc…)

Tissue Development: Gastrulation

  • During embryogenesis, cells differentiate into various tissues (germ layers).

  • The split between protostomes and deuterostomes occurred early in animal evolution.

  • Both groups form monophyletic groups, which are proper clades.

Hox Genes

  • Genes that tell other genes where to “build” specific body parts.

  • They directly reflect evolutionary relationships, with more derived animals having more Hox genes.

Ediacaran Period

  • Animal evolution begins in the Ediacaran period.

  • This is understood through Fossil Lagerstätten (exceptional preservation of soft-bodied organisms).

  • It marks the:

    • Beginnings of mobility

    • Clear bilaterians

    • Complex ecosystem (though no predators yet)

    • As Darwin predicted!

    • Includes:

      • Mobility

      • Sexual reproduction

      • Early skeletons

      • Bilaterians

      • Very diverse

The Cambrian "Explosion"

  • Also known as the Cambrian Radiation, this was a major overall diversification of animals (Metazoans) over a relatively short period of time (Geologic Time, that is!).

    • Increase in trace fossil diversity

    • Increase in taxonomic diversity (number of types of animals)

    • Increase in skeletonization of animals

    • Predation

    • Increase in body size

Precambrian-Cambrian Transition

  • Stratigraphy aims to place boundaries.

  • Originally at the point of the fossil record.

  • Global reference point: Trichophycus pedum.

Trace Fossils

  • Increase in diversity

  • Increase in complexity

  • The Cambrian Boundary shows a trace fossil complexity increase demonstrating an increase in ecological diversity (Rusophycus, Cruziana).

The Cambrian "Explosion"

  • Is when all major animal phyla in existence today appeared.

Major Phyla of Fossil Invertebrates

  • Porifera or sponges

  • Cnidaria

  • Bryozoa

  • Brachiopoda

  • Mollusca

  • Arthropoda

  • Echinodermata

  • Hemichordata

  • Chordata (including vertebrates)

  • This period involved evolutionary experimentation.

Burgess Shale Faunas (Fossil Lagerstätten)

  • Diverse, occur globally, soft-bodied, and abundant.

Examples include

  • Marella splendens

  • Hallucigenia sparsa

  • Anomalocaris canadensis

  • Opabinia regalis

  • Wiwaxia corrugata

  • Cloudina

Neoproterozoic Small Shellies

*Small Shelly Fauna
* Occurs simultaneously in many groups
* Different kinds of organisms use different minerals
* Start off as little mineral deposits and build up into complete skeletons

Widespread Skeletonization

  • Functions of Skeletons

    • Support muscles

    • Aid in locomotion

    • Protection *Origin of Skeletonization

      • Oxygen?

      • Predation?

Precambrian-Cambrian Transition: Major Boundary of Life??

  • Lots of things going on in a geologically short period of time

  • Lots of questions:

    • Advent of skeletonization?

    • Effect of bioturbation?

The Cambrian "Explosion"

  • Broad-scale diversification (radiation) of skeletonized and soft-bodied animals

  • Widespread skeletonization

  • Increase in trace fossil diversity and complexity

  • Soft bodied fossil record

Life and Times of the Paleozoic

  • Geologic Time Scale

    • Animals diversify in the Cambrian.

    • There was a Great Ordovician Biodiversification Event, as well as Mass Extinctions.

Phanerozoic Marine Family Biodiversity Curve

  • Characterized by:

    • Periods of rapid increase: Radiations

    • Periods of rapid decrease: Extinctions

    • Diversity Plateaus

    • Overall Increase in Diversity

Evolutionary Faunas: Patterns of Origination, Dominance, Extinction

  • Cambrian: Trilobites & Archaeocyaths

  • Paleozoic: Brachiopods & Echinoderms

  • Modern: Decapods & Bivalves

The Paleozoic Era

  • Cambrian radiation (beginning of an era)

  • Ordovician radiation

  • End Ordovician mass extinction

  • End Devonian mass extinction

  • Advent of life on land

  • Origin of vertebrates

  • Late Permian mass extinction (end of era)

Paleozoic: The Ordovician Radiation

  • Period of Rapid Diversification; Diversity Tripled

  • Rise to dominance of the Paleozoic Fauna

  • Increase in Ecological Complexity

  • New Body Plans

The Ediacaran is the time of new body plans

  • the Cambrian is the time of new phyla

  • the Ordovician is about lower taxonomic levels and ecological restructuring.

Ecosystem Restructuring

  • Cambrian Fauna is different than the Paleozoic Fauna. Diversity is the same, but relative abundance matters.

Reef Builders Change

  • Cambrian: Archaeocyathids

  • Ordovician: Stromatoporoids, rugose & tabulate corals

  • The shift from short-lived calcified sponges to enduring reefs with other kinds of calcifying sponges and corals.

Other Ecological Changes

  • Organisms living in close association formed tightly packed communities (e.g., Shell Beds).

  • There were more animals living in more places and in crowded ecosystems.

Ecospace Utilization: Tiering

  • Greatest Concentration of Nutrients typically on/at Ocean floor, however, there is less competition in the water column.

Paleozoic: The Ordovician Extinction

  • Extinctions

Background Extinctions

  • Normal Rate at which individual species go extinct

Mass Extinctions

  • Period with numerous species extinctions which are significantly greater than normal “Background” rates.

  • Normal rules don’t apply.

The Ordovician Extinction

  • First of big 5

  • 2nd biggest (~60% genera; 80% species)

  • Two-Punch:

    • Glaciations - Lowered sea level, Destroyed shallow marine habitats

    • Followed by quick sea level rise of low oxygen waters and stagnant deep-sea waters

  • Stromatoporoids, Rugose, & Tabulate Corals survive, but are hit hard.

  • It was not the end of the Paleozoic fauna!

The Silurian and Devonian

  • The Devonian is known as the “Age of Fishes.”

Paleozoic: The Late Devonian Extinction

  • More about depressed origination rates rather than major extinction

  • Poorly Understood

  • Huge effects on reefs: major ecological restructuring

    • Possible Causes

      • Glaciation, sea level & climatic changes

      • Asteroid impacts

      • Global marine anoxia (widespread dissolved oxygen shortages)

      • Volcanism

      • Devonian Plant Hypothesis (reduce CO2CO_2 and get cooling)

      • Any combination of these

    • Severely Affected

      • Brachiopods

      • Trilobites

      • Ammonoids

      • Placoderms

      • Reef Communities

      • Geographic extent shrank severely

      • Tabulate corals lost 70-80%, did not recover

      • Stromatoporoids did not recover - extinct

      • Diversity Recovery Looks OK… Nothing disappears, however, the ecologic structure was completely different.

      • There was a transition from Brachiopod Dominated Beds to Crinoid Dominated.

Not All Extinctions Are Equal

  • Ordovician 60% Genera Loss

    • Similar Diversity Loss

    • Good recovery rate at large scale

    • Stromatoporids, Rugose & Tabulate Reefs dominant before & after

    • Little Ecologic Effect

  • Devonian 55% Genera Loss

    • Similar Diversity Loss

    • Rugose & Tabulates corals nearly wiped out, Stromatoporids extinct

    • Brachiopods lose dominance to Crinoids

    • Severe Ecological Effect

The Late Ordovician & Late Devonian extinctions had similar losses in diversity

  • BUT the Late Ordovician had few long-term effects on the biosphere, whereas the Late Devonian caused major ecological restructuring and thus had a more dramatic effect on the biosphere.

  • This graph depicts a major extinction event. After the event, Echinoderms became much more common, while Brachiopods decreased significantly.

Origin of Vertebrates

  • Three Body Plans

    • Poriferans: Sponges (also a phylum) asymmetrical, No tissues

    • Cnidarians: Corals, Jellies, & Anemones (also a Phylum,) Radial Symmetry, 2 Tissues: “Diploblastic”

    • Bilaterians: NOT A “PHYLUM” but a clade Bilateral Symmetry, 3Tissues “Triploblastic”

Seas of the Paleozoic

  • The Protostomes: “MOUTH FIRST”

  • The Deuterostomes: “ANUS FIRST”

  • Cambrian Bilaterians

Two Groups of Bilaterians

  • Protostomes:

    • Arthropoda

    • Brachiopoda

    • Mollusca

    • Annelida

  • Deuterostomes:

    • Echinodermata

    • Hemichordata

    • Cephalochordata

    • Chordates

Deuterostomes

  • Triploblastic

  • Have a coelom

  • Deuterostomes

  • Bilaterally symmetrical

  • Also pentaradial

  • Phylum Echinodermata

    • Seastars, sea urchins, sea cucumbers, crinoids, feather stars, brittle stars

      • Have a central nervous system

      • Have a water vascular system

  • Phylum Hemichordata

    • Acorn worms

      • Triploblastic

      • Have a coelom

      • Bilaterally symmetrical

      • Deuterostomes

      • No backbone

  • Phylum Chordata

    • By the end of the Cambrian period, 540 million years ago, an astonishing variety of animals inhabited Earth’s oceans. One of these types of animals gave rise to vertebrates, one of the most successful groups of animals.

Phylum Chordata

  • Chordates are bilaterian animals that belong to the clade of animals known as Deuterostomia.

  • The two phyla of invertebrate deuterostomes, Phylum Hemichordata and Phylum Echinodermata, are more closely related to vertebrates (within Phylum Chordata) than to invertebrates.

Phylum Chordata

  • Chordates Have:

    • Bilateral symmetry

    • A coelom

    • Deuterostome development

    • Metamerism (segmentation)

    • Cephalization (heads)

  • Two Protochordate Subphyla (no true backbone)

    • Subphylum Urochordata

    • Subphylum Cephalochordata

  • One Subphylum with Backbones

    • Subphylum Vertebrata

Phylum Chordata

  • All are found at least at some embryonic stage in all chordates, although they may later be lost.

  • Five distinctive characteristics define the chordates:

    • Notochord: The notochord is a flexible, rod-like structure derived from mesoderm. Place for muscle attachment. In vertebrates, the notochord is replaced by the vertebrae.

    • Dorsal Tubular Nerve Cord: In chordates, the nerve cord is dorsal (on top) to the notochord and is a hollow tube. The anterior end becomes enlarged to form the brain. The hollow cord is produced by the infolding of ectodermal cells that are in contact with the mesoderm in the embryo; Only in triploblasts. Protected by the vertebral column in vertebrates.

    • Pharyngeal Pouches (Gill Slits): Pharyngeal slits are openings from the pharyngeal cavity to the outside. The perforated pharynx evolved as a filter feeding apparatus. Later, they were modified into internal gills used for respiration. The pharyngeal pouches give rise to the middle ear cavity and tonsils among other structures in later vertebrates.

    • Endostyle: Endostyle secretes mucus that traps food particles. Only found in Urochordates, Cephalochordates, and the larvae of most basal Chordates. Lost in more derived Chordates. Evolved for filter feeding.

    • Postanal Tail: The postanal tail, along with the notochord, provides motility in larval tunicates and cephalochordates. Evolved for propulsion in water. Reduced to the coccyx (tail bone) in humans.

Subphylum Urochordata

  • Aka sea squirts!

  • Tunicates (subphylum Urochordata) are found in all seas and were present by the Cambrian Period. Most are sessile and highly specialized as adults. In most species, only the larvae show all of the chordate hallmarks. The larvae are tadpole-like.

  • Tunicates filter feed using the pharyngeal slits and a mucous net secreted by the endostyle. Adults lack a notochord!

  • Some tunicates are colonial, and some are pelagic.

  • They are very diverse and found all over Earth’s oceans today!

Subphylum Cephalochordata

  • Cephalochordates are the lancelets, also called amphioxus. “Cephalo” means head in Latin. All five chordate characters are present in a simple form.

  • Filter feeding is accomplished using pharyngeal slits and a mucous net secreted by the endostyle (same as tunicates). The dorsal, hollow nerve cord lies just above the notochord. A post-anal tail is clearly present.

Subphylum Vertebrata

  • Subphylum Vertebrata is a monophyletic group that shares the basic chordate characteristics with the urochordates and cephalochordates. The animals called vertebrates get their name from vertebrae, the series of bones that make up the backbone.

  • Sometimes called Subphylum Craniata for the presence of brains!

  • There are approximately 52,000 species of vertebrates which include the largest organisms ever to live on the Earth, like Fishes, Amphibians, Reptiles, Birds, and Mammals.

Craniates

  • Craniates are chordates that have a head with a brain (all vertebrates).

  • The origin of a head opened up a completely new way of feeding for chordates: active predation.

  • Craniates share some common characteristics, such as a skull, brain, eyes, and other sensory organs.

Endoskeleton

  • Vertebrates have an endoskeleton made of cartilage or bone.

  • All have a cranium to protect the brain, and all have a vertebral column/backbone to protect the spinal cord.

  • This is important for muscle attachment.

The Origin of Vertebrates

  • Vertebrates evolved at least 530 million years ago, during the Cambrian explosion.

  • Pikaia was an early chordate discovered in the Burgess Shale, and it may be a Cephalochordate.

  • The oldest fossil primitive to vertebrata fossils are those of the 3-cm-long Haikouella, which had eyes and a brain present, but no skull. It is transitional in morphology between cephalochordates and vertebrates. Some hypothesize Haikouella is the sister taxon of vertebrates.

  • In Cambrian rocks, paleontologists have found fossils of even more advanced chordates, such as Haikouichthys, which had a skull present and is considered a true vertebrate.

Origins of the Vertebrates

  • Vertebrates are divided into 2 groups:

    • Agantha (Jawless Vertebrates)

    • Gnathostomata (Vertebrates with Jaws)

Origins of the Vertebrates

  • Many Extinct Examples:

    • Ostracoderms

    • Heteostracans

    • Osteostracans

Aganthans (Jawless Vertebrates)

  • Most Primitive (least derived) of the Vertebrates.

  • Only a Few Extant (living) Examples:

    • Hagfish

    • Lampreys

Hagfish

  • Most Primitive Living Agnathan… No Cartilage, bone, or scales. Secondarily lost true vertebrae and now have cartilaginous skeletons but still belong in Vertebrata! (Same as sharks).

  • Lives in deep waters, and it secrets sticky slime as defense.

  • Fun Fact – these are the reason the subphylum Vertebrata was originally named subphylum Craniata!

Lampreys

  • Possess a true backbone and cranium.

  • Mouth full of sharp teeth, and it is parasitic (sucks blood from Host).

Gnathostomes (Jawed Vertebrates)

  • Where do Jaws Come From?

    • Modified Gill Arches Initially evolved for wider gape to help with Respiration.

    • The evolution of jawed fish led to the decline of Agnathans (jawless fish).

Four Major Groups within fishes

  • Extinct Groups:

    • Acanthodians

    • Placoderms (Devonian armored fish)

  • Extant Groups

    • Chondrichthyes (Sharks & Rays)

    • Osteichthyes (Ray-Finned & Lobe-Finned Fish)

  • Devonian is considered the “Age of the Fish”.

  • A group of early jawed vertebrates, the Acanthodians, with paired fins and large spines, may have given rise to the bony fishes.

  • Placoderms were among the first jawed vertebrates that existed in the Silurian period and went extinct in the Devonian.

Acanthodians

  • Lower Silurian 1st Jawed Fish Potentially Contained true teeth

Placoderms

  • Major Devonian Predators

  • Massive Head Shields with Cartilaginous Bodies (probably couldn’t move well)

  • No TRUE teeth

  • Dominant for ~50Ma, then extinct at end-Devonian

    • An example is Dunkleosteus, which could reach 10 meters in length!!

Chondrichthyes (Sharks and Rays)

  • Evolved in the Devonian period.

  • Have been abundant from the Devonian to the present.

  • Characterized by cartilaginous skeletons, yet still classified as vertebrates.

  • A very successful and diverse group.

Osteichthyes ("Bony Fish")

  • Represented by two main groups:

    • Ray-finned fishes (Actinopterygians)

    • Lobe-finned fishes (Sarcopterygians)

Ray-Finned Fish (Actinopterygians)

  • Dominant from the Devonian period to the present.

  • Feature a skeleton of light bone.

  • Known for being very agile.

  • Their fins are full of many long, ray-like bones.

Lobe-Finned Fish (Sarcopterygians)

  • Possess a fin with few, thick bones.

  • Present from the Devonian to the Recent period but less abundant than ray-finned shes.

  • Not as agile as ray-finned fish.

  • Some have lungs and gulp air to breathe.

  • Ancestor to the Tetrapods, which include humans.

  • Osteolepiforms

Osteolepiforms

  • A group of lobe-finned fishes that appeared during the Devonian period.

  • Considered the ancestral taxa to all Tetrapods, including humans.

Review

  • Agnathans (Jawless Vertebrates) The most primitive (least derived) of the vertebrates.

    • Extinct Examples:

      • Ostracoderms

      • Heteostracans

      • Osteostracans

    • Extant Examples:

      • Hagfish

      • Lampreys

  • Gnathostomes (Jawed Vertebrates)

    • Osteichthyes "Bony Fish"

      • Ray-fins (Actinopterygians)

      • Lobe-fins (Sarcopterygians)