Chapter 34: Vertebrates

Vertebrate characteristics

Includes:

• Skull and backbones composed of vertebrae

• Jaws and a mineralized skeleton

• Lobed fins

• Limbs with digits

• Amniotic eggs

Vertebrates

Animals with a backbone; includes over 60,000 species

• Include the largest and heaviest animals to have ever lived

  • Smallest is less than a centimeter long

Chordates

Animals belonging to phylum Chordata, made of all vertebrates, urochordates, and cephalochordates

All exhibit, at some point in their life, a:

• Notochord

• Dorsal, hollow nerve chord

• Pharyngeal slits or clefts

• Muscular tail

Chordate characteristics

Includes, at some point in their life, a:

• Notochord

• Dorsal, hollow nerve chord

• Pharyngeal slits or clefts

• Muscular tail

These may appear only during developmental periods

Notochord

A longitudinal, flexible rod between the digestive tube and nerve cord

• Provides flexible skeletal support; may be replaced by a jointed skeleton in adulthood

Nerve cord

Develops from a plate of ectoderm rolled into a neural tube dorsal to the notochord

• After embryonic period, this develops into the central nervous system (brain and spinal cord)

Central nervous system

The nervous system comprising the brain and spinal cord, developed from the fundamental nerve cord

Pharyngeal clefts

Grooves along the outer surface of the pharynx in chordate embryos, later developing into pharyngeal slits opening into the pharynx

Pharyngeal slits

Slits that develop from the pharyngeal clefts that allow water to pass from the mouth to the outside of the body, bypassing the digestive tract

Can function as:

• Suspension-feeding structures in invertebrates

• Gills for gas exchange

• Parts of the ear, head, and neck when developed as arches

Pharyngeal slit functions

Includes:

• Suspension-feeding structures in invertebrates

• Gills for gas exchange

• Parts of the ear, head, and neck when developed as arches

Muscular tail

This is contained posterior to the chordate with skeletal elements and muscles

• Provides propelling force in many species, but can be greatly reduced during embryonic development

Lancelets (Cephalochordates)

Marine suspension feeders named for their bladelike shape, swimming using a simplified fish mechanism

• Retain many of the key chordate characteristics

• Ancestral chordates likely resembled these

Tunicates (Urochordates)

Organisms more closely related to other chordates than lancelets

• Chordate characteristics are most apparent during larval stage, lasting only a few minutes

• Use tail muscles and notochord to swim until settling for metamorphosis

• Adults (sea squirts) are sessile, filtering in and shooting out water for feeding and defense

Sea squirts

Adult tunicates; these draw in and shoot out water to filter out food particles and repel attackers

• These have fewer Hox genes compared to other vertebrates

Vertebrates

Chordates that have a backbone, skeletal system, and complex nervous system that enables efficiency in capturing food and evading predators

• These have two or more sets of Hox genes while other chordates only have one

  • Enables nervous system evolution and skeleton innovations

Neural crest

Vertebrate characteristics appearing along the edges of the closing neural tube, dispersing through the embryo and giving rise to:

• Teeth

• Some bones and cartilage of the skull

• Several types of neurons

• Sensory capsules

Cyclostomes

A clade including hagfishes and vertebrates with the only living vertebrates lacking jaws and a backbone; however, these do have rudimentary vertebrae

Hagfishes

Jawless vertebrates with reduced vertebrae and a cartilaginous skull

• These retain the notochord and have eyes, ears, a small brain, and nasal opening connected to the pharynx

• Mouth contains tooth-like keratin formations

• All species of these are marine and most are bottom-dwelling scavengers

• Rows of slime glands produce a slime to repel competitors and predators

Lampreys

Jawless vertebrates with reduced vertebrae, a notochord, and cartilaginous skeleton

• Includes about 38 species in marine and freshwater environments

• Larvae live and feed in streams for several years

• Parasitic species migrate to oceans or lakes, clamping with mouths onto host fish to feed

Haikouella

The most primitive fossil found documenting the transition to vertebrates

• Were 3-cm-long suspension feeders resembling lancelets

• Had a well-formed brain, eyes, and muscular segments, but lacked a skull and ear organs

Head

A part of vertebrates consisting of a brain, eyes, other sensory organs, and a skull, enabling more complex movement and feeding behaviors

• First seen in Myllokunmingia with eye and ear capsules but lacking vertebrae

Conodonts

The earliest vertebrates in the fossil record, dating to 500 mya, capturing prey with large eyes and mineralized barbed hooks in the pharynx

Gnathostomes

Jawed vertebrates with teeth to grip and slice food; these outnumber jawless vertebrates and include:

• Sharks and their relatives

• Ray-finned fishes

• Lobe-finned fishes

• Amphibians

• Reptiles (including birds)

• Mammals

Jaws

Hinged structures in gnathostomes with teeth used to grip and slice food

• May have evolved by modification of the skeletal rods supporting the pharyngeal (gill) slits

Gnathostome characteristics

Includes:

• Jaws

• Genome duplication, including four sets of Hox genes

• Enlarged forebrain

• Enhanced senses of smell and vision

• Lateral line system, or rows of organs sensitive to vibrations along body sides

Lateral line system

Rows of organs in aquatic gnathostomes sensitive to vibrations located along body sides

440 mya

Time where gnathostomes first appeared in the fossil records, exhibiting adaptations like paired fins, a tail, and a more efficient gas exchange system in the gills for efficiency and control

Placoderms

Early gnathostomes; these are extinct armored vertebrates ranging in size from less than a meter to more than 10 m

Gnathostome lineages

Includes:

• Chondrichthyans (sharks, rays, and their relatives)

• Ray-finned fishes

• Lobe-fins

Chondrichthyans

One of the three lineages of gnathostomes including the 1,000 species of sharks, rays, and their relatives; these have skeletons composed primarily of cartilage and some calcium

• Traces of bone are found in scales, at the base of teeth, and coating vertebrae in some sharks

• Bone likely evolved in these before diverging

Shark

One example of a chondrichthyan, with a streamlined body for swift but less maneuverable swimming

• Dorsal fins function as stabilizers, and paired pectoral and pelvic fins are used for maneuvering

• Oil is stored in the liver for buoyancy

• Continual swimming is necessary to avoid sinking, also keeping water flowing for gas exchange

  • During rests, muscles of the jaw and pharynx are used to pump water over gills

• Eggs are fertilized internally, but can undergo oviparous, ovoviviparous, or viviparous development

Oviparous development

Development of the embryo where eggs hatch outside the shark mother’s body after internal fertilization

Ovoviviparous development

Development of the embryo where eggs are retained within the oviduct and young are born after hatching within the uterus

Viviparous development

Development of the embryo within the uterus, nourished by:

• A yolk sac placenta,

• Absorption of nutrient-rich fluid, or

• Eating other eggs

Cloaca

A common chamber with a single opening to the outside of the body, with the reproductive tract, excretory system, and digestive tract all emptying into this

Osteichthyans

The vast majority of vertebrates, with a majority having bony endoskeletons

• Includes bony fishes and tetrapods; aquatic species of these are informally called fishes

Operculum

A bony flap protecting the gills of fishes, helping draw water into the mouth and over the gills via muscle contractions

Swim bladder

An air sac in fishes filled to maintain buoyancy

Ray-finned fishes

A group of gnathostomes including over 27,000 species, including most familiar osteichthyans

• Have modifications in body form and fin structure affecting maneuvering, defense, and other functions

Lobe-fins

A group of gnathostomes with rod-shaped bones surrounded by a thick layer of muscle

• Used to maneuver across the substrate of aquatic habitats

Lungfishes

Types of lobe fins initially arising in the ocean, but today living in stagnant ponds and swamps with gas exchange in water using gills or gulping air into lungs attached to the pharynx

Tetrapods

Gnathostomes that have limbs via the evolution of a lineage of lobe-fins 365 mya, leading to great diversification

• Have four limbs and feet with digits

• A neck, enabling independent head movement

• Fusion of the pelvic girdle to backbone

• The absence of gills

• Ears for detecting airborne sounds

Tiktaalik

A fossil species with characteristics of both fish and tetrapods

• Fish traits include fins, gills, lungs, and scales

• Tetrapod traits include ribs, a neck and shoulders, front fins with similar bone patterns, and a pelvis and rear fin more robust than those found in fish

• Likely could walk in water, but is unlikely that it walked in land

Amphibians

A class with 6,150 species in three clades:

• Salamanders (Urodela or “tailed ones”)

• Frogs (Anura or “tail-less ones”)

• Caecilians (Apoda or “legless ones”)

Name means “both ways of life,” referring to the life stages of many frog species first in water and then on land

• Usually found in damp environments; these have skin that function in gas exchange

• Eggs lack a shell and must be laid in water or moist environments on land

Salamanders (Urodela)

A clade under amphibians with about 550 species; these have tails

• Some are aquatic

• Exhibit paedomorphosis, or the retention of larval features when mature

• Typically carniverous

Paedomorphosis

The retention of larval features when mature enough to reproduce; this is common in aquatic salamanders

Frogs (Anura)

A clade under amphibians with about 5,420 species; these lack tails but have powerful hind legs for locomotion on land

• These with leathery skin are called “toads”

• Larvae are herbivoeres with gills, a lateral line system, and long, finned tails; this eventually is grown out of and a carnivorous diet persists

Caecilians (Apoda)

A clade under amphibians with about 170 species; these are legless and nearly blind, resembling earthworms

• Legs were lost as a secondary adapation

• These inhabit moist forest soils in tropical areas

Amniotes

Tetrapods whose living members are the reptiles (including birds) and mammals

• Named for their amniotic egg, containing four embryo-protecting membranes and reducing dependence on water for reproduction

Amniotic egg

An egg type in amniotes reducing water dependence for reproduction and has four membranes of the amnion, chorion, yolk sac, and allantois

• These often have shells when produced by reptiles to slow dehydration

• Many mammals have lost the eggshell, developing within the mother’s body instead

Amnion

A fluid-filled sac that surrounds, bathes, and cushions the embryo as the other membranes function in gas exchange, nutrient transfer, and waste storage

Rib cage

A body part in amniotes that improved breathing efficiency through the ventilation of the lungs

• This allowed for less dependency on gas exchange through the skin, causing lower permeability and enabling better water conservation

Reptiles

A group a member of the amniotes including tuataras, lizards, snakes, turtles, crocodilians, and birds; many share some characteristics:

• Scales containing keratin to prevent desiccation and abrasion

• Shelled eggs to prevent drying out

• Internal fertilization before the secretion of the shell

• Being ectothermic, except for birds which are endothermic

Ectothermism

Being able to absorb external heat as the main source of body heat; seen in most reptiles except birds

• Temperature is regulated through behavioral adaptations, like basking in the sun or shade

Endothermism

Being able to maintain body temperature through metabolic activity

• Seen in birds as reptiles that do not exhibit ectothermism

Pterosaurs

The first tetrapods to exhibit flapping flight, originating in the late Triassic

• Filled ecological roles that would later be played by birds after their extinction, about 66 mya

Dinosaurs

Land-based tetrapods that diversified into a vast range of shapes and sizes

• Herbivorous members evolved diverse adaptations for defense

• Theropods were bipedal carnivores including Tyrannosaurus rex

• Many were agile and fast moving; and some lived in groups, protected eggs, or were endothermic

• Went extinct about 66 mya potentially from an asteroid or comet impact

Theropods

Bipedal carnivorous dinosaurs including Tyrannosaurus rex and the ancestors of birds

Turtles

Group of tetrapods containing 351 species, most closely related to crocodilians and birds

• Their boxlike shell was acquired in stages over millions of years, made of upper and lower shields fused to the vertebrae, clavicles and ribs

• The earliest could not retract their head into their shell

• They have adapted to a variety of environments, like deserts, ponds, rivers, or the sea, but can be threatened by human development

Lepidosaurs

Tetrapod group represented by a single lizard-like species called the tuatara, restricted to 30 islands off the coast of New Zeealand and threatened by rats which consume their eggs

Squamates

Group of amniotes that includes all ~10,425 species of lizards and snakes, ranging in size from 16 mm to 3 m long

Snakes

Descended from lizards with legs; these move on land by producing waves of lateral bending from head to toe or via belly scales to grip and pull from the ground

• These are carnivorous and have adaptations to aid in preying, such as chemical sensors, vibration sensitivity, heat-detecting organs, tongue flicking, toxic venom, and loosely articulated jawbones and skin for swallowing

Crocodilians

Tetrapod group that includes all alligators and crocodiles; early species were small terrestrial quadrupeds before becoming larger and aquatic

• The 24 species of these are restricted to warm regions of the globe

Birds

Archosaurs with extensive modifications in their adaptation to flight; this group has about 10,000 species

• Adaptations include having no urinary bladder, only one ovary, small gonads, toothless mouths, and air-filled bones with honeycombed internal structures

• They also have color vision, acute eyesight, and fine muscle control to support flight for hunting, escape, and migration

• Large amounts of energy are dedicated to flight; feathers and some fat can insulate against heat loss while efficient respiratory and circulatory systems supply oxygen to tissues

Ratites

An order of flightless birds including the ostrich, rhea, kiwi, cassowary, and emu