Vertebrate Zoology Exam 1

Rank the following taxonomic group based on biodiversity (most to least): amphibians, birds, fishes, mammals, reptiles

Fishes, birds, reptiles, amphibians, mammals

Systematic biology

Field of biology that deals with the diversity of life

Divided into phylogenetic systematics and taxonomy

Provides the foundation for all of comparative biology

Phylogenetic systematics

The study of the relationships between organisms

Taxonomy

The science of naming and classifying organisms

Carolus Linnaeus

In 1748, Carolus Linnaeus published a system of taxonomy based on resemblances

Two key features of his system remain useful today

• Two-part name for species (genus name and specific epithet)

• Hierarchical classification

Speciation

Allopatric: speciation by geographic separation

• Vicariance (environment changes to separate a population into two groups)

  • Always the null hypothesis

• Dispersal with large new population: Some individuals move to a new habitat, and the populations diverge genetically over time

Peripatric: ancestral population seeds a small peripheral population

• Dispersal with a small new population. Divergence occurs over time, but most of the change occurs in the small population

Phylogenies

Hypotheses of evolutionary relationships

Sister groups/sister taxa

Two groups that are more closely related to each other than either is to anything else on the tree

Paraphyletic group

Includes a common ancestor and some, but not all, of its descendants

E.g. fishes

Polyphyletic group

A group that does not include the common ancestor

E.g. winged tetrapods (i.e. bats and birds do not share a common ancestor)

Monophyletic group

Includes a common ancestor and all its descendants

E.g. mammals

Only monophyletic groups reflect evolutionary relationships and show the evolutionary path a group has taken since its origin

Explanation/hypothesis

A way to make sense of individual observations

10th Edition Systema Naturae

1758

Introduced hierarchical classification system in use today

Taxa (sing. taxon) are categories, smaller nest in larger

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Specific Epithet

• Each level distinguished by specific traits

Binomial nomenclature

• Genus species

Intended as an information retrieval system

• Evolution gave this system more meaning

• Organisms share traits because they are more closely related to each other

Taxa in order from largest to smallest

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Specific Epithet

Cladogram definition and terminology

Depicts ancestor-descendant relationships

Node: ancestor

Branch: connect descendant nodes and terminal taxa

Branching pattern determined based upon “special homology”

• Synapomorphy: shared, derived homologies

  • Define monophyletic groups

• Apomorphy: derived character

• Plesiomorphy: ancestral trait (e.g. when studying different bear species, a vertebral column would be a plesiomorphy)

• Symplesiomorphy: shared ancestral character

• Autapomorphy: unique derived character

Clade: ancestor and all descendants; monophyletic as opposed to paraphyletic

We want our taxa to be natural: monophyletic and diagnosed by synapomorphies

• Thus, “fishes” is an artificial group

Homoplasy: convergent evolution (two lineages independently evolve the same trait)

The hypothesis that minimizes homoplasy is favored (called the most parsimonious, greatest consistency among characters)

Homologous characters

Present in groups that descended from a common ancestor

Analogous characters

Arose because of similar selective environments (i.e., convergent evolution)

Also called homoplasy

Parallel evolution

Species that have recently diverged develop similar specializations/characters

Crown group

All taxa descended from a major cladogenesis event, recognized by possessing the clade's synapomorphic character(s)

Stem group

All the taxa in a clade preceding a major cladogenesis event. They are often difficult to recognize because they may not possess synapomorphies found in the crown group

How are homologous structures recognized?

Position

• System location

Structure

• Common features

Transition

• Historical change

Gene tree discordance

Genes from the same individual may suggest different evolutionary patterns

Precambrian or Vendian Era

Long thought to be devoid of life

Prokaryotic and Eukaryotic life arose

Stromatolites (fossilized bacteria layers)

Vendian (Ediacaran) fauna (650 to 543 mya): first appearance of a group of large fossils

First evidence of animals (sponges)

Paleozoic Era

Cambrian

Ordovician

Silurian

Devonian

Carboniferous

Permian

Mesozoic Era

Triassic

Jurassic

Cretaceous

Cenozoic Era

Tertiary (66-2.6 mya)

Quaternary

Cambrian

Continental mass breaks up

Atmospheric oxygen approaches current levels

Explosive radiation of animal species (“Cambrian Explosion”), including first vertebrates

Ordovician

First jawless fishes

Fragmentary evidence of jawed fishes

First vertebrates → Conodonts

• First vertebrates with mineralized skeletal elements in their mouth and pharynx

First evidence of plants on dry land

Widespread radiation of Echinoderms and other marine invertebrates

Glaciations toward end of the period

Silurian

Sea levels rise

Large land masses appear

Vascular plants and arthropods colonize land

Definitive appearance of jawed fishes

Jawless fishes radiate

Age of Fishes

• Will be on the exam

Devonian

Extensive terrestrial forests (forms coal seams)

Diversification of terrestrial arthropods

Diversification of fishes, but major extinctions in jawed and jawless fishes at the end of the period, including disappearance of ostracoderms (armored jawless fishes)

Freshwater basins form in equatorial regions

First tetrapod vertebrates appear at the end of the period

Carboniferous

Low CO₂, high O₂

• Cools the climate

Radiation of insects

Glaciers form in the southern hemisphere

Coal swamps in equatorial regions of northern landmasses

Radiation of non-amniote vertebrates

First amniote vertebrates toward the end of the period

Permian

Glacial sheets reach their greatest extent, then start to retreat

Amniote vertebrates diversify and radiate

O2 levels decline rapidly toward the end of the period

• Runaway greenhouse gas effect

Pangea forms toward the end of the period

Largest extinction in history

• Around 95% of marine species and more than 75% of terrestrial species

Triassic

Hot and humid climate

Some arid areas due to mountains

No glaciation

Many new vertebrates

• Mammals

• Pterosaurs

• Crocodilians

• Teleost fishes

Pangea begins to break up

Conifers become dominant

New groups of insects (including more beetles)

Major extinction event at the end of the period

Jurassic

Warm climate continues

Pangea continues to separate into Gondwana and Laurasia

Diversification of marine predators

• Sharks and rays appear

• Marine reptiles diversify

Mammals continue to evolve but remain small and relatively inconspicuous

Modern birds, lizards, and salamanders emerge

Cretaceous

High levels of greenhouse gases → warm

Angiosperms (flowering plants) first appear on land

• Insects diversify (pollinators)

First evidence of snakes

Teleost fishes radiate and marine reptiles flourish

Biotic diversity on land overtakes that in the oceans (Cretaceous Terrestrial Revolution)

• Dinosaurs remain dominant land tetrapod

Birds and Pterosaurs coexist

Mass extinction at the end of this period, in part the result of a meteorite strike near Mexico’s Yucatan Peninsula

• Extinguishes the dinosaurs, pterosaurs, marine reptiles, and many marine invertebrate species

Marginocephalia (fringed dinosaurs)

• Found only in western North America, Asia, and Europe

• Found only in Cretaceous age strata

Tertiary

Climate cools

Antarctic ice cap forms early on

Angiosperms radiate, as do pollinators

Mammals, birds, snakes, and teleost fishes radiate

Marsupials arrive on Australia and begin to radiate in isolation

Continents approach their modern configuration

Arctic ice cap forms near end of period

Hominins arise near end of period

Fauna extinctions later in the period

Quaternary

Cyclical, extensive glaciations in the Northern Hemisphere

Drying of terrestrial land due to water locked in glaciers

Biotic interchange due to Panamanian land bridge in the Americas

Many large land mammals become extinct toward the end of the period

Humans appear

Evidence for Continental Drift

Continental Fit

Stratigraphy

• Alignment of stratigraphic features on continents hypothesized to be previously connected

Paleoclimatic evidence

• Matching glacier moraines

Marine geology

• Sea mounts (guyots): formed by volcanic action above the surface, later truncated by wave action, finally sunk to 1-2 km

Differential in age of continents (older) versus ocean basins (younger)

• Seafloor spreading: mid-oceanic ridges as spreading centers

• Trenches: location of crust “recycling”

Paleomagnetism

• Basalt rocks have iron crystals that align themselves to the Earth’s changing magnetic field

Synapomorphies of the Deuterostomes

Blastopore fate

• Blastopore: blastula → hollow ball of cells early in development. Pouch forms in blastula called the blastopore

• In protostomes, this initial opening develops into the mouth, and an opening that develops later becomes the anus

• In deuterostomes, it develops into the anus, and an opening that develops later becomes the mouth

Cleavage type

• Early in development, cleavage is the name for cell division without growth

• Spiral cleavage is characteristic of protostomes

• Radial cleavage is characteristic of deuterostomes

Schizocoelous vs. enterocoelous development

• In protostome development, the splitting of the initially solid masses of mesoderm to form the coelomic cavity is called schizocoelous development

• In deuterostome development, formation of the body cavity (coelum) is described as enterocoelous development.

Ambulacraria

Tripartite coelom, similar larval forms, and an axial complex (specialized metanephridium)

Echinoderms

Hemichordates

• Hemichordates share characteristics with echinoderms

  • Early embryogenesis

  • Similar larvae

Tripartite body plan as larvae

• Proboscis, collar, trunk

Glomerulus - a unique excretory organ in Hemichordates; a combined primitive kidney and heart.

Buccal diverticulum (stomachord) - an anterior extension of the pharynx forms within the collar; possibly serves to support the proboscis

• Resembles a notochord, but the structures are not homologous

Chordate Synapomorphies

Notochord

• Elongate, rod-like, skeletal structure dorsal to the gut tube and ventral to the nerve chord

• In vertebrate chordates, but not cephalochordates or tunicates, it is replaced by a vertebral column long before maturity

Dorsal hollow nerve chord

• Differentiates during development into the brain anteriorly and spinal cord that runs through the trunk and tail.

Pharynx and pharyngeal pouches

Endostyle

• Ciliated, glandular groove on the floor of the pharynx that secretes mucus for trapping food particles while filter feeding

• Homologous with the thyroid gland of vertebrates

Muscular postanal tail

• Used for movement

Cephalochordates

Known as lancelets or Amphioxus

Cephalochordates are small, eel-like, unprepossessing animals that spend much of their time buried in the sand

Exchange gases through skin (dermal gas exchange)

Filter-feed through gills

Anatomical features shared with vertebrates

• Myomeres

• Vertebrate-like tail fin

• Closed circulatory system

• Podocytes: specialized excretory cells

Cephalochordates lack features found in most or all Craniates (i.e. organisms with a skull)

• No skull

• Brain is very small and poorly developed

• Sense organs are also poorly developed

• No true vertebrae

Because cephalochordates have no hard parts, their fossil record is extremely sparse

Urochordata

Urochordate larvae have a notochord that extends from just behind the head to the tail (rather than from head to tail)

Adult body covered with an opaque tunic composed of a material secreted by the epidermis (includes cellulose). Inside the tunic is a thin, transparent layer called the mantle

• Only animal group that can synthesize cellulose

Water is propelled by gills and circulates in the oral siphon, through the pharynx, through the gills, into the atrium, and out the atrial siphon.

Filter feed with an endostyle.

• Unicellular green algae (including diatoms) are the main food source; mucous on the pharynx wall is secreted by glands in the endostyle.

Can use asexual or sexual reproduction

Tunicates most resemble chordates during their larval stage, which may last only a few minutes

Fertilized eggs develop in 24 hours into free-swimming larvae in which a tail, notochord, and dorsal nerve chord are well developed

The larva undergoes metamorphosis after a few hours or days and transforms into the highly modified adult

• Loss of notochord

• Loss of post-anal tail

• Sessile

3 classes: Ascidiacea, Thaliacea, Larvacea

Ascidiacea

Most are hermaphrodites (produce both egg and sperm)

All are filter feeders

Commonly called “sea squirts”

Thaliacea

Odd, barrel-shaped things commonly known as “salps”

Planktonic and free-swimming near the surface of the open sea

Their bodies are surrounded by circular muscular bands and both ends of their cylindrical body are open

Larvacea

Larvacians superficially resemble ascidian larvae

Most specialized of the urochordates.

They build a peculiar "mobile home" within which they travel throughout their lives

The similarity between ascidian larvae and adult larvacians suggests that larvacians may be neotenous urochordates (i.e., larval forms have attained sexual maturity without loss of larval structures, such as the tail).

Unique embryotic features of vertebrata

Hox Genes

• Hox genes assign particular positions to differentiating tissues to form organs, limbs, etc.

• All animals that have been examined have at least one Hox cluster.

• Lampreys and vertebrates have quadrupled their HOX gene cluster (that is, vertebrates have 4 clusters of HOX genes located on 4 different chromosomes)

• Changes in the interactions between regulatory genes and their target loci may be one of the most important causes of morphological evolution

Neural Crest Cells

• Neural crest cells give rise to a variety of structures, including some of the bones and cartilage of the skull

• Parts of the peripheral nervous system (nerve ganglia, Schwann Cells = myelin sheath)

• Endocrine glands

• Pigment cells

Sense organs

• Three types of sensory organs derived in ontogeny from ectodermal placodes (i.e. thickened patches of embryonic skin that sink inward toward the brain where they develop into sensory chambers)

  • Olfactory organ

  • Eyes

  • Paired acoustic organs or inner ears

MicroRNAs

• Non-coding RNA sequences 22 bases long; regulate protein synthesis by binding to complementary base sequences of mRNA

Tripartite brain

• Fore-, Mid-, Hind-brain

Embryogenesis

The development and growth of an embryo

Vertebrate zygote = fertilized egg

• A zygote is a typical eukaryotic cell except that:

  • It contains yolk (inert nutritive material)

  • It has the internal capacity to undergo differentiation and division

Zygotes are classified on the basis of yolk deposition

• Oligolecithal = little yolk (cephalochordates, marsupials, placental mammals)

• Mesolecithal = medium yolk (lampreys, lungfish, amphibians, sturgeon)

• Macrolecithal = large yolk (hagfish, sharks/rays, teleost, other tetrapods)

Zygotes are also classified based upon the distribution of yolk

• Oligolecithal eggs also = isolecithal (yolk thin and evenly distributed)

• Macro, Mesolecithal also = telolecithal (yolk thick and more concentrated at one end)

Cleavage

Development begins after fertilization with a proliferation of cells through division, but with no increase in the size of the embryo (i.e. cells get smaller and smaller and more numerous).

Blastula

A hollow ball of cells one layer thick.

All cells can be removed up to the blastula stage and will yield a whole embryo. Therefore they are said to have indeterminate cleavage

Following this stage in development, each cell is said to have determinate development depending upon their location in the organism.

• The developmental fate of each cell is determined

The blastula will have a hollow opening in the ball of cells or between the blastodisc and the yolk = blastocoel

• Becomes body cavity, not GI system

Blastula regions:

• Animal pole

  • Epidermis (skin)

  • Neural plate (nervous system)

  • Mesoderm (muscle, kidney, bones, gonads)

• Vegetal pole

  • Endoderm (digestive organs, lungs)

Gastrulation

Formation of germ layers.

The blastocoel is important in the gastrulation stage when the gut and the germ layers are formed

Chordamesoderm = mesodermal cells that aggregate to form the Notochord

Neurulation

Process leading to the formation of the nervous system.

Accomplishes three major things in vertebrates

• Creates the neural tube, which gives rise the central nervous system

• Creates the neural crest cells, which migrate away from the dorsal surface of the neural tube, and gives rise to a diverse set of cell types

• Creates the epidermis, which covers over the neural tube once it is created

What does the mesoderm on either side of the notochord become?

Myotomes = muscle blocks

Coelom = body cavity

Dermatome = deep portions of skin

Sclerotomes = vertebrae

Nephric ridges = kidneys

Genital ridges = gonad

Mesenchyme (wandering) cells

Some cells are not part of the organized system of differentiation of tissues. These cells break loose from the germ layer at some part of development and migrate through the embryo to other locations of the body to differentiate. They come from different locations in the body of the embryo.

• Primitive sex cells

• Skeletogenous septa

• Neural crest cells

Placodes

In the head region there are placodes that are neurogenic epidermal thickenings that evolved at the same time as NCC. However, their association is unclear. Cells of placodes contribute to developing sense organs.

Olfactory placode - nose

Optic placode - eyes

Otic placode - ears

Lateralis placode - lateral line mechanism and electroreceptors

• Taste buds and cranial sensory ganglia also come from lateralis placodes.

• The entire lateralis system of fishes and amphibians is derived from these placodes. These cells migrate over the body of the fish or amphibian to help form the lateralis system.

Pharyngulation

Development of pharyngeal pouches

Four types of tissue

Epithelial

Connective (includes blood)

Muscular

Nervous

Integument System

Largest organ system of the body; it constitutes roughly 15-20% of the body weight of vertebrates; much more in armored species.

Includes the skin and a series of derivatives like hair, feathers, scales, hoofs, horns, glands, etc.

The integument system includes the layers known as:

• Epidermis

• Dermis

• Hypodermis

Mineralized Tissues

Hydroxyapatite, a complex compound of calcium and phosphorus.

Four major types of tissues can become mineralized in vertebrates:

• Enamel, dentine and bone are found only in the mineralized condition in the adult.

• Cartilage is usually unmineralized in tetrapods, but is the main mineralized internal skeletal tissue in sharks. Cartilaginous fishes have lost true bone.

Osteocytes - bone cells; osteoblasts - cells that actually make the bone; osteoclasts - cells that dissolve bone and allow it to be remodeled

Chondrocytes - cells that form cartilage.

• These cells are derived from mesoderm; dentine comes from neural crest; enamel is from ectoderm.

Bone

• Dermal bone - formed in the skin; primitive type of vertebrate bone, first seen in jawless vertebrates like ostracoderms.

• Endochondral bone - formed inside cartilage

Types of Teeth

The basic structure of the teeth of gnathostomes is like that of the odontodes, the original tooth-like components of primitive vertebrate dermal armor.

Only mammals and some reptiles (archosaurs) have truly rooted teeth set in sockets (thecodont teeth).

Acrodont teeth - fused to the jaw bone

Pleurodont teeth - set in a shelf on the inner side of the jawbone

Cranial Skeleton

Chondrocranium - surrounding the brain; if ossified then neurocranium

Splanchnocranium - forming the gill supports

Dermatocranium - forming in the skin as an outer cover

• Dermal bones

Endoskeleton

Includes the axial and appendicular skeletons.

Appendicular Skeleton = paired fins, or limbs, and girdles

Axial Skeleton = includes the cranium, pharyngeal skeleton, notochord, vertebrae, ribs.

The notochord is obliterated in some species; segmented into small segments in others, and complete in primitive vertebrates.

Number of chambers in vertebrate hearts

Fish have 2 chambers

Amphibians have 3 chambers (blood mixes in ventricle)

Birds and Mammals have 4 chambers

Vertebrate Kidneys

Function in homeostatic control of water balance and waste control

Metabolism produces waste products that are poisonous and these must be eliminated from the body.

Both functions operate with kidneys but the ancestral function of this organ system was probably for the removal of nitrogenous wastes and other metabolic waste products.

In the evolution of vertebrates we will see that most groups have gone from aquatic to terrestrial habits. Associated with this change has been an increase in the efficiency of the kidney system.

Nephron - structural unit of the kidney

• Renal corpuscle

  • Glomerulus surrounded by Bowman’s capsule

    • Glomerulus receives and filters blood; wastes collect in Bowman’s capsule and are to moved to mesonephric tubules; some water resorption occurs

• Collecting tubule - leads to ducts that convey urine to the exterior

Some species are aglomerular, especially species in the deserts

• Conserves water

Types of Kidneys

Holonephros/Archinephros, Pronephros, Opisthonephros, Metanephros

Holonephros/Archinephros kidneys

Lacks glomeruli

Kidney extends full length of the body

Kidney drained by the holonephric duct

Kidney is diffuse

Present in embryonic hagfish (agnatha) & presumably in ostracoderms and protochordates

Pronephros kidneys

Primitive kidney

Nephrostomes - anterior funnels that empty into body cavity by way of pronephric tubules

Forms functioning kidney in frog tadpoles

A pair of pronephric ducts develop and carry wastes from kidney to hindgut.

This kidney is the earliest embryonic kidney of all vertebrates

External glomeruli

Open peritoneal funnels

Kidney located in extreme anterior end of the nephrogenic mesoderm

Retains the old holonephric duct, now termed the pronephric duct

Functional kidney in embryonic fish and amphibians, but not in most amniotes where the tissue develops but remains nonfunctional

Opisthonephros kidneys (functional mesonephros)

Adult kidney in some bony fishes, amphibians

Internal glomeruli

Peritoneal funnel vestigial & narrow

Numerous tubules

Kidney extends full length of the abdomen

Duct is the opisthonephric duct; also called the Wolfian duct; the duct is derived from the holonephric duct and includes all but the anterior end.

Embryos of fish and amphibians have a pronephric kidney which is nonfunctional in the adult

Metanephros kidneys

Kidney type of all adult amniotes

Internal glomeruli lacking funnels

Millions of tubules

Arcualia

Partial vertebrae elements

Arcualia + centrum = vertebra

Cambrian Vertebrates

Chordates: notochord, myomeres

Vertebrates: cranium, paired sensory organs

• Both are formed from neural crest cells

• Dorsal fin; ventral fin; 6-7 gill pouches; w-shaped myomeres

• No evidence of bone or mineralized scales

Ordovician Vertebrates

Ostracoderms: “shell skinned”

Conodonts

• Minute, comb-shaped or claw-shaped denticles

• Complete fossils display an elongated body, with a cranium, imprints of chevron-shaped muscles (i.e. myomeres), a trace of the notochord, large paired eyes, and a caudal fin strengthened by radials

Silurian Vertebrates

Freshwater Ostracoderms (implies glomerular kidneys)

Early vertebrates had a marine origin

Myxiniformes, Petromyzontiformes, Ostracoderms, Gnathostomes (including Placoderms and Acanthodii)

Myxiniformes

No traces of vertebrae; skeleton is composed of cartilage

• Craniata

• Hagfish sister clade to Vertebrata (lampreys plus Gnathostomata)

Synapomorphies

• Three accessory hearts in caudal region

• Pair of keratin tooth plates

• Oral tentacles

• Slime glands

Unique in many respects

• High body fluid content (more than 10%)

• Isosmotic (same solute content as environment) osmoregulation

• Low oxygen affinity of their blood cells

• No cerebrum or cerebellum

Petromyzontiformes

Single nasal opening on top of head, combined with a duct leading to the hypophysis (pituitary)

• Nasohypohyseal opening

A large sucker surrounding the mouth, strengthened by an annular cartilage

When not stuck to anything, lampreys respire through the mouth

• Flow-through ventilation

When stuck to something, lampreys draw water through gill pouches with a pumping action

• Tidal ventilation

Lampreys always spawn and lay eggs in freshwater

Lampreys undergo a larval development which can last up to seven years.

The larval lamprey, or ammocoete, has no sucker and poorly developed eyes.

Between the mouth and the pharynx, the ammocoete has a two-valved pumping and anti-reflux device, the velum, which in the adult plays no role in the respiration

What is a larval Lamprey called?

Ammocoete

Ostracoderms

Not a monophyletic clade

Characterized by

• Covering of dermal bone, usually an extensive armored shell (carapace)

• Jawless, but some had movable mouth plates that are not analogous to a similar character in living vertebrates

Gnathostome Synapomorphies

1. Jaws formed from the mandibular gill arch

2. Hypobranchial musculature allows strong suction in inhalation and suction feeding

3. Two distinct olfactory bulbs, leading to two distinct nostrils

4. Three semicircular canals in the inner ear

5. Addition of a conus arteriosus to the heart between the ventricle and the ventral aorta

Fins

Fins control and stabilize movement

Dorsal and anal fins control the tendency to roll or yaw

Paired fins (pelvic and pectoral) control pitch, facilitated acceleration, coasting speed, and deceleration

Also useful for defense or conspecific signaling (sexual selection)

Semicircular Canals

Allow detection of movement/acceleration

Analogous to inner ear in humans

Placoderms

Heavy bony armor on the head and neck; endochondral bone

Craniovertebral joint: ball-and-socket joint that allowed increased gape

Never had teeth —> self-sharpening plates instead

Some species show evidence of paired “lungs”

Acanthodii

“Spiny-sharks”

Characters

• Two or more pairs of pelvic fins, none with girdle

• Bony opercle

• Branchiostegal rays (bony structures associated with the gills)

Oldest fossil record to late Ordovician. Extinct during Permian

Theories of how jaws evolved

Serial Theory: first ancient branchial arch gave rise exclusively to the mandibular arch, the next branchial arch exclusively to the hyoid arch, and the rest of the arches to the branchial arches of gnathostomes

• Issues

  • Gill positioning

  • No indication the mandibular arch ever had gills

Monorhiny vs. Diplorhiny

• Nasal sac + adenohypophysis (anterior pituitary) limits expansion of forebrain and prevents development of the upper jaw

• Diplorhiny allows neural crest cells to migrate through and form the jaw

Theories of why jaws evolved

“Improved feeding” hypothesis

• Began with filter-feeding where cilia around the mouth drew food in and strained it through basket-like filter bars

Mallatt’s hypothesis: improved gill ventilation

• Initial enlargement of mandibular arch into proto-jaws was for improved gill ventilation

• Facilitated buccal pumping of water over gills for increased respiration to meet increased metabolic demands of increased activity (relative to agnathans)

Associated jaw modifications

Branchiomeric muscles: close the jaw and contract the gills

Hypobranchial muscles: open the jaw and expand the gills

Types of jaw attachments

Paleostyly (e.g. agnatha): none of the arches attach themselves directly to the skull

Eustyly (e.g. placoderms): mandibular arch is suspended from the skull by itself, without help from the hyoid arch.

Amphistyly (e.g. early sharks, some bony fishes): jaws are attached to the braincase through two primary articulations

Hyostyly (e.g. most bony fishes): mandibular arch is attached to the braincase primarily through the hyomandibula

Metautostyly (e.g. most amphibians, reptiles and birds): jaws are attached to the braincase directly through the quadrate; hyomandibula has no part in supporting the jaw.

Craniostyly (e.g. mammals): entire upper jaw is incorporated into the braincase, but the lower jaw is suspended from the dermal squamosal bone of the braincase