Animals: Non-Tetrapod Vertebrates

Question 1

Early chordate evolution

Answer 1

Ancestral chordates likely resembled lancelets, with characteristics of the chordate body plan, such as a notochord, dorsal nerve cord, pharyngeal slits, and a post-anal tail.
- The expression of Hox genes involved in vertebrate brain development is observed during the development of the lancelet’s simple nerve cord tip.

Genome sequencing suggests a whole-genome duplication occurred early in chordate evolution.
- Duplicate genes evolve new functions, contributing to the development of traits and structures in chordates.
- Diversification of gene families, e.g. Hox

Question 2

Clade Vertebrata

Answer 2

During the Cambrian period, a lineage of invertebrate chordates evolved into the first vertebrates.
- Vertebrates are chordates that have a vertebral column (backbone), which provides structural support and protects the nerve cord.
- The evolution of skeletal and complex nervous systems in vertebrates provided advantages in capturing food and avoiding predators, leading to their widespread success and diversification.

Extant vertebrates inhabit a wide range of environments, including marine, freshwater, and terrestrial habitats.
- ~69k extant vertebrate species, including some of the largest animals ever to have lived, such as sauropod dinosaurs and blue whales.

Question 3

Shared Derived Traits of Vertebrates

Name all 6

Answer 3

1. Vertebrates have a vertebral column (backbone).
2. Elaborate braincase enclosing the brain.
3. Vertebrates have two or more sets of Hox genes
4. Neural crest cells develop along the neural tube edge in vertebrate embryos.
5. Dorsal, anal, and tail fins stiffened by fin rays along the centerline.
6. Vertebrates possess a modified, complex circulatory system

Question 4

Vertebral column (backbone)

Vertebrate shared derived traits

Answer 4

Vertebrates have a vertebral column (backbone).

Skeletal elements (cartilage or bone) enclose and protect the nerve cord.

The vertebral column replaces the function of the notochord.
- Enhances rigidity and provides attachment points for muscles and skeletal elements like ribs.
- Facilitates improved food capture and
predator evasion

Question 5

Elaborate braincase enclosing the brain

Vertebrate shared derived traits

Answer 5

Elaborate braincase enclosing the brain.

Endoskeleton development was associated with pronounced cephalization.

The head consists of the brain, sense organs (e.g. paired eyes), and a cranium (skull) without jaws.
- Evolution of the cranium allows for the anterior expansion of the nerve cord into a complex brain and nervous system.
- Enables coordination of more sophisticated movement and feeding behaviours.
- The cranium does not completely encase the brain in early vertebrate lineages

Question 6

Hox genes

Vertebrate shared derived traits

Answer 6

Vertebrates have two or more sets of Hox genes, likely originating from whole-genome duplication.
- Invertebrate chordates (lancelets and tunicates) have only one set of Hox genes.
- Hox genes regulate the embryonic body plan along the head-tail axis, determining segment structures

Question 7

Neural Crest Cells

Vertebrate shared derived traits

Answer 7

Neural crest cells develop along the neural tube edge in vertebrate embryos.
- Neural crest cells migrate within the embryo, contributing to structures such as teeth, some skull bones/cartilage, several types of neurons, and the sensory capsules.

Question 8

Fins and Fin Rays

Vertebrate shared derived traits

Answer 8

Dorsal, anal, and tail fins stiffened by fin rays along the centerline.
- Fins are membrane extensions supported by cartilaginous or bony spines (rays) that provide balance and propulsion during swimming.
- Tetrapods (terrestrial vertebrates) have lost fins

Question 9

Circulatory System

Vertebrate shared derived traits

Answer 9

Vertebrates possess a modified, complex circulatory system.

Closed circulatory systems, including a heart with at least two chambers.
- Invertebrate chordates either have no heart (Cephalochordates) or one-chamber hearts (Urochordates).

O2-transporting hemoglobin in red blood cells is oxygenated via gills or lungs.

Kidneys remove metabolic waste products from the blood.

A complex circulatory system supports higher metabolic rates and more muscularity compared to lancelets and tunicates

Question 10

Basal vertebrates

Answer 10

Fossil evidence shows that the earliest vertebrates lacked jaws.
- Today, only two lineages of jawless vertebrates remain, the hagfishes and lampreys.
- Both hagfishes and lampreys lack jaws and a vertebral column in adulthood.
- The presence of rudimentary vertebrae in embryonic or mature forms, and molecular phylogenetic analyses confirm their vertebrate classification.
- Hagfishes and lampreys are sister taxa in a clade of living jawless vertebrates, clade cyclostomes (“round mouths”).
- Extant cyclostomes look rather like eels: long, flexible, tubular bodies, but no paired lateral fins.

Vertebrates with jaws make up a much larger clade, the gnathostomes.

Question 11

What are the two groups of jawless vertebrates (cyclostomes)?

Answer 11

the hagfishes and lampreys

Question 12

Hagfish Structure

Cyclostomes

Answer 12

Hagfishes (class Myxini) are jawless vertebrates that have a cartilaginous cranium, greatly reduced vertebrae, and a flexible rod of cartilage derived from the embryonic notochord.
- Eel-shaped, slime-producing marine fish (20-30 extant species).
- Hagfish have a small brain, eyes, and a mouth composed of tooth-like keratin projections that protract and retract to pull in food.

Question 13

Hagfish ecological role and behaviour

Cyclostomes

Answer 13

Hagfishes are marine; most are bottom-dwelling scavengers.
- Feed on decaying corpses of larger animals, using the rasping action of tooth-like projections to strip flesh from carrion.
- Bury themselves in a corpse and absorb the carcass’s nutrients through their skin.

Hagfish are notable for exuding copious fibrous slime as a defense mechanism.
- Slime absorbs water and rapidly swells to choke predators:

Question 14

What is a Cyclostome?

Answer 14

A jawless vertebate
- Hagfishes
- Lampreys

Question 15

Lampreys

Cyclostomes

Answer 15

Lampreys (class Petromyzontida) have a cartilaginous skeleton.

The lamprey’s cartilaginous skeleton includes a more elaborate cranium (without jaws), a gill basket, and rudimentary vertebral elements.
- Adults have a notochord surrounded by cartilaginous segments that partially arch over the nerve cord forming primitive vertebral elements. nerve cord

Lampreys have no paired lateral fins notochord but do have dorsal and tail fins. cartilaginous tube segment with dorsal spines

Larval lampreys exhibit lancelet-like behaviours (cephalochordates).
- Suspension-feeders that bury themselves in stream substrate

Question 16

Lamprey ecological role

Cyclostomes

Answer 16

Lampreys inhabit marine and freshwater habitats (~38 species).

Many lamprey species parasitize fish by clamping onto them with a funnel-like sucking mouth.
- They scrape skin using a rasping tongue (keratin spines) and suck body fluids.
- Lampreys can hold onto rocks using their oral sucker when not feeding (petro = rock, myzo = suck).
- Sea lampreys have become a significant invasive parasite in the North American Great Lakes.

Question 17

Conodonts

Early Vertebrates

Answer 17

Conodonts were among the earliest vertebrates in the fossil record, dating from 200–500 mya.
- Conodonts had cartilaginous cranium and vertebral column.
- Jawless, but possessed mineralized skeletal elements in their mouth and pharynx.
- These fossilized dental elements are common in the fossil record.

Question 18

Clade Gnathostomes

Answer 18

Gnathostomes are a clade of vertebrates that have jaws.
- gnath = jaw, stoma = mouth
- ~99% of all living vertebrates are gnathostomes.
- Living gnathostomes are a diverse group that includes sharks and their relatives, ray-finned fishes, lobe-finned fishes, amphibians, reptiles (incl. birds), and mammals

Question 19

Shared Derived Traits of Gnathostomes

Name all 6

Answer 19

1. Opposing jaws that open/close the mouth forcefully to capture and process diverse foods.
2. Mineralization of skeleton
3. Two pairs of lateral appendages, such as fins or legs
4. Genome Duplication, including duplication of Hox genes
5. Enlarged forebrain
6. Lateral line system

Question 20

Opposing jaws

Clade Gnathostomes Shared derived traits

Answer 20

Opposing jaws that open/close the mouth forcefully to capture and process diverse foods.

Gnathostome jaws are hypothesized to have evolved from skeletal supports of the pharyngeal slits.
- Two pairs of branchial elements (hinged skeletal rods supporting the gill arches) evolved to open and close the mouth to more effectively pump water over the gills.
- Natural selection likely favoured larger and wider mouths, enabling the capture of larger prey.

The posterior branchial elements, no longer required for suspension feeding, evolved as specialized supports for gas exchange (gill slits).

Question 21

Lateral Line System

Clade Gnathostomes: Shared derived traits

Answer 21

Lateral line system.
- Aquatic gnathostomes possess a lateral line system that detects vibrations in the water.
- The lateral line system is lost in terrestrial gnathostomes.

Question 22

Origin of jaws and mineralization of the skeleton

Answer 22

The earliest vertebrates likely had skeletons made of cartilage.

Some vertebrate groups evolved mineralized skeletons, replacing or supplementing cartilage with bone.
- The evolution of opposing jaws and mineralized skeletal elements likely occurred simultaneously.
- Early changes in jaw structure coincided with the mineralization of jaw elements, improving their strength and durability.
- These adaptations provided selective advantages, enhancing predatory efficiency.
- Conodonts were among the earliest vertebrates to evolve mineralized skeletal elements in their mouth and pharynx.
- Mineralization of the axial skeleton (e.g. vertebral column and cranium) evolved later, providing better body support and improved locomotion.

Question 23

Cartilage

Answer 23

Cartilage is a flexible connective tissue composed of collagen fibers embedded in a protein-sugar polymer (proteoglycan); while cartilage provides support, it lacks the rigidity of bone

Question 24

Bone Mineralization

Answer 24

Bone mineralization involves the deposition of calcium phosphate, which offers several advantages such as increased structural support, enhanced protection for internal organs, and the ability to support larger body sizes.

Question 25

What are the two types of vertebrate skeletal systmes?

Answer 25

**Endoskeleton** derived from preformed cartilage. **Dermal skeletal structures** are derived from intramembranous ossification.

Question 26

Endoskeleton derived from preformed cartilage | Vertebrate skeletal systems

Answer 26

Endoskeleton derived from preformed cartilage. - The **endoskeleton** is an internal framework of bone and/or cartilage that supports the body, protects internal organs, and facilitates movement. - In gnathostomes, the **cartilage-based endoskeleton** undergoes **ossification** during development, where bone tissue is formed through calcium phosphate deposition. - In humans, ossification is incomplete at birth and continues until ~25 years of age

Question 27

Dermal skeletal structures | vertebrate skeletal systems

Answer 27

Dermal skeletal structures are derived from intramembranous ossification. - Dermal skeletal bones **develop directly within the dermal (skin) layers without a cartilage template**. - Dermal skeletal elements evolved independently in early vertebrates, forming external armour in early armoured fishes. These structures persist in living vertebrates as: - bony fin rays in fish - facial and pectoral bones in tetrapods - teeth, which are specialized dermal structures.

Question 28

Early gnathostome fossils

Answer 28

Fossils from the Silurian and Devonian periods reveal a diverse array of extinct jawed vertebrate fish, including **placoderms** (“plate-skinned fish”) and **acanthodians** (“spiny sharks”). - **Placoderms** evolved ~450 mya and included giant predators (e.g. Dunkleosteus), and smaller (<1 m), widespread detritivores (e.g. Bothriolepis). - Placoderms were **among the first jawed vertebrates**, the first fish with true (bony) teeth, and the first fish to develop pelvic fins. - **Acanthodians** retained a primarily cartilaginous skeleton, but with fins supported by a wide, bony base and an anterior bony spine. - These groups thrived during the Paleozoic era but were mostly extinct by the end of the Devonian period (~360 mya).

Question 29

What are the three surviving lineages of gnathostomes

Answer 29

- **chondrichthyans** (Includes sharks) - **ray-finned fishes** (Your typical fish) - **lobe-fins** (this includes humans)

Question 30

Chondrichthyans

Answer 30

Chondrichthyans (class Chondrichthyes) - ~1000 extant species, mostly marine, some freshwater. - The largest and most diverse chondrichthyans include **sharks, rays, and skates**. - A second subclass consists of ~50 species of chimaeras (ghost sharks, ratfish). Chondrichthyans have **cartilaginous skeletons**. - chondro = cartilage, ichthys = fish. - **Endoskeleton of cartilage**, but have bony teeth and **bony placoid** (pointed, tooth-shaped) **scales**. - The cartilaginous skeleton is not a primitive skeletal condition: **fossil evidence suggests Chondrichthyans descended from ancestors with bony skeletal elements**.

Question 31

Shark Feeding | Chondrichthyans

Answer 31

While the largest sharks are **filter-feeders**, consuming zooplankton and small fish, **the majority are carnivores**

Question 32

# Why do sharks need to constantly swim? Shark Body Structure

Answer 32

Sharks have streamlined bodies and are swift swimmers. - Swimming propulsion comes from the tail fin; pectoral fins create lift. - Sharks primarily rely on **ram ventilation for respiration**: sharks swim with their mouths open, allowing water to pass over their gills as they move forward, facilitating oxygenation. - Some shark species ventilate their gills by **buccal pumping**, where they force water over their gills by alternately opening and closing their mouths. - Since shark bodies are denser than water, **they must swim continually to maintain buoyancy**. - Sharks do not have a gas-filled swim bladder.

Question 33

# True or False Sharks have a cloaca

Answer 33

**True!** Sharks and most vertebrates have a cloaca, a shared posterior opening for the digestive, urinary, and reproductive tracts

Question 34

Shark Sensory Systems | Chondrichthyans

Answer 34

Sharks have acute sensory systems. - **Nostrils function as olfactory receptors** (smell), but not for gas exchange (also true for most bony fish). - Sharks can **detect electrical fields** from the muscular movement of nearby animals via pores (electroreceptors) around the head. - Sharks can **detect water vibrations through a lateral line system**. - The lateral line is also found in bony fish.

Question 35

# Name and explain all three types Shark Reproduction | Chondrichthyans

Answer 35

Sharks reproduce sexually through **copulation**. - Male sharks have **modified pelvic fins** known as **claspers**, which serve as copulatory organs, delivering sperm into the female’s cloaca. Shark eggs are fertilized internally, with embryos developing through various methods: - **Oviparous reproduction** - laying of fertilized eggs outside the mother's body, often encased in protective leathery egg cases. - **Ovoviviparous reproduction** - embryo development within the uterus - nourished by the egg yolk. - **Viviparous reproduction** - embryo development within the uterus - nourished through a yolk-sac placenta connected to the mother’s bloodstream.

Question 36

Clade Osteichthyans

Answer 36

Most vertebrates belong to a clade of gnathostomes called **Osteichthyans** (“bony fish”). - Nearly all extant osteichthyans have a **bony (ossified) endoskeleton**. - cf. extant Chondrichthyes with cartilaginous endoskeletons. - Clade Osteichthyans **include bony fish and tetrapods** (amphibians, avian and non-avian reptiles, and mammals). - **Bony fish refers specifically to aquatic osteichthyans**

Question 37

Bony fish

Answer 37

Earlier taxonomic classifications placed bony fish in class Osteichthyes (osteo = bone, ichthys = fish) - Class Osteichthyes is **paraphyletic if restricted to fish**. - The common ancestor of Osteichthyes includes tetrapods as descendants. - Bony fish are incredibly diverse with >28k extant species of bony fish; ~half of all extant vertebrates. - Bony fish inhabit marine and freshwater environments, ranging greatly in size (1 cm up to 6 m in length)

Question 38

Shared Derived Traits of Clade Osteichthyans | Name all three

Answer 38

1. **Lungs or lung derivatives** 2. **The operculum** - a plate of dermal bone that covers and protects the gill arches and gills. 3. **Maneuverable fins**

Question 39

# Structure and Function Swim Bladder | Bony Fish

Answer 39

Most extant bony fish have a swim bladder. - The swim bladder is **an air sac that maintains the neutral buoyancy of fish** by adjusting its inflation through gas exchange with the blood. - Fish can ‘float’ at the depth they desire; cf. Chondrichthyes which sink when not swimming. **Perform different functions**: - The swim bladder (single) regulates buoyancy. - The lungs (paired) facilitate gas exchange. In “primitive” ray-finned fish, the swim bladder retains a connection to the gut via a pneumatic duct. In most ray-finned fish, this connection is lost, resulting in a closed swim bladder **A few bony fish, such as lungfish, continued to use simple lungs for their original purpose**: supplementary organs for gas exchange. - In contrast, one Osteichthyan lineage (tetrapods) transitioned to a fully terrestrial lifestyle, completely dispensing with gills.

Question 40

# Origins Swim Bladder | Bony Fish

Answer 40

Swim bladders **evolved from simple lungs** found in the extinct ancestors of bony fish. - Lungs and swim bladders are both gas- filled structures. - Ancestral bony fish living in brackish water possessed **both simple lungs and gills**. - A simple lung is a dorsal out-pocketing of the gut filled with gas, which augments gas exchange by gills; gulp air in oxygen-poor waters. - Over time, this original simple lung underwent modification, leading to the development of the swim bladder observed in most extant bony fish The **swim bladder** of bony fish is **homologous** to the lungs of tetrapods, i.e. same evolutionary origin. - Both structures arise during embryonic development as out- pockets from the gut.

Question 41

The Operculum | Clade Osteichthyans: Shared derived traits

Answer 41

The **operculum** is a plate of dermal bone that **covers and protects the gill arches and gills**. - Osteichthyans have a single gill opening covered by an operculum. - cf. chondrichthyan multiple gill slits. - Most bony fish “breathe” by drawing water over operculum-protected gills. - **The mouth and operculum function as valves in an efficient buccal pump**, allowing bony fish to ventilate their gills **even while stationary**. - cf. Chondrichthyans must swim to ventilate gills, or “gulp” water if stationary.

Question 42

Maneuverable fins. | Clade Osteichthyans: Shared derived traits

Answer 42

Osteichthyans have **slender, flexible bony rays or spines** protruding from their bodies that are **covered by skin to form maneuverable fins**. Pectoral and pelvic girdles support **paired maneuverable fins**. - Flexible fin rays articulate from basal, bony elements that link to the internal pectoral or pelvic girdles. - cf. stiff fins of most chondrichthyans. Apart from the tail fin, fins have **no direct connection to the vertebral column** and are solely supported by muscles. The fins serve various functions including propulsion, maneuvering, and defence through the presence of spines.

Question 43

Characteristics of extant bony fish | Aquatic osteichthyans

Answer 43

Bony fish have a **fully ossified (bony) endoskeleton**. Bony fish have a **lateral line system**, a sensory organ that detects vibrations and changes in water pressure, aiding in navigation and prey detection. Bony fish typically have dermal armour of **thin, plate-like bony scales**, offering protection while maintaining flexibility. - cf. thick, tooth-like scales of chondrichthyans. Most bony fish reproduce via **external fertilization**, where male and female fish release gametes into the water. - Broadcast spawning - Fertilized eggs may receive parental care, with some species also caring for their young. - A few species exhibit internal fertilization and ovoviviparity, as seen in seahorses:

Question 44

What are the two extant groups of bony fish?

Answer 44

**Ray-finned fish** - **Class Actinopterygii** (actin = ray; pteryx = fin). - Monophyletic. - **Fins** are **supported** by elongated, flexible **bony rays** connected to basal, bony elements. - Includes the vast majority (>99%) of bony fish species, inhabiting both marine and freshwater environments. - All commercially significant bony fish belong to Class Actinopterygii. **Lobe-finned fish** - **Class Sarcopterygii** (sarx = flesh; pteryx = fin). - Paraphyletic. - Lobe-finned fish have **fleshy, lobed fins, distinct from the ray-like fins of Actinopterygii.**

Question 45

Clade lobe-fins

Answer 45

**Clade lobe-fins** have distinctive **fleshy, muscular pectoral and pelvic fins**, which are supported by bony elements. - **Lobed fins** are a **shared derived trait** of clade lobe-fins. - Lobed fins consist of a single basal skeletal bone, homologous to the humerus or femur, and central supporting bones. - Some skeletal elements of lobed fins are homologous with bones found in the limbs of tetrapods. - Pectoral lobed fins evolve into forelegs; pelvic lobed fins evolve into hind legs. Three surviving lineages of clade lobe-fins: **coelacanths, lungfishes**, and **tetrapods**.

Question 46

What are the three surviving lieages of clade lobe-fins?

Answer 46

Three surviving lineages of clade lobe-fins: - **coelacanths** - **lungfishes** - **tetrapods**

Question 47

Coelacanths | Lobe-finned fish

Answer 47

Class Sarcopterygii, **sub-class Actinistia** (coelacanths) - Coelacanths are diverse in the fossil record. - Used muscular pectoral and pelvic fins to swim and “walk” underwater across the substrate. - Coelacanths probably inhabited shallow water and had lungs and gills. - One extant genus of Coelacanths. - “Living fossil”; entire sub-class thought extinct until 1938. - Restricted to deep marine waters - Retain a vestigial lung.

Question 48

Lungfish | Lobe-finned fish

Answer 48

Class Sarcopterygii, **sub-class Dipnoi (lungfish)** Two modes of gas exchange: **gills and lungs**. - di = two, pneu = air/breath - Lungfish use a buccal pump mechanism to gulp air from the water’s surface into their lungs by lowering and raising the floor of the mouth cavity. Three extant genera are confined to freshwater habitats in the southern hemisphere. - During dry periods, lungfish can survive by burrowing themselves into mud Most **lungfish** species have **two lungs**, which are **homologous with the lungs of tetrapods**. - Tetrapods are 1 of 3 surviving lineages of clade lobe-fins.

Question 49

Economic importance of Bony Fish

Answer 49

Bony fish are crucial to the fishing economy. - But... fishing is the most significant cause of declines in ocean fish populations. - Increasing demand for fish and modern fishing equipment as dramatically increased fish catches. - **Overfishing** has led to the **collapse of many fish stocks**. - More than 90% of global marine fish stocks are now fully exploited or overfished - Deep-sea fish are especially vulnerable to overfishing due to their slow growth rates and delayed maturation. Solutions to overfishing? - Establishment of no-catch marine reserves may allow replenishment of fish stocks outside of the reserve. - Fish farming: aquaculture can alleviate pressure on natural fisheries, but may create new problems: pollution, parasites, invasive species, etc.