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BI360 – Vertebrate Zoology
Origins and Classification of Early Fishes
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Superclass: Agnatha (jawless fishes)
70 living species
Jawless
Gills supported by pharyngeal bars of cartilage
No paired appendages
Single, median nostril
With blind, sac-like olfactory pouch
Notochord persists in the adult
Predators or scavengers
Fresh- and salt-water species
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Agnatha
Hagfishes and lampreys sometimes referred to as "cyclostomes"
Class: Myxini (hagfishes)
Order: Myxiniformes
Strictly marine
Class: Cephalaspidomorphi (Monorhina)
Order: Petromyzontiformes (lampreys)
Marine and fresh-water species
Parasitic forms and non-feeding adults
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Agnatha
Myxiniformes – a hagfish
Petromyzontiformes – a lamprey
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Agnathan Phylogeny
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Hagfishes
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Ammocoete Larvae
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Ammocoete larvae (filter feeders) have:
7 gill slits (respiratory in function)
Notochord extending from head region into tail (no vertebral column)
Dorsal, hollow nerve tube; typical embryonic vertebrate brain with three primary vesicles
A stratified epidermis
Typical embryonic vertebrate kidney
Typical closed, embryonic vertebrate circulatory system with two-chambered heart and red blood cells
Tadpole-like morphology typical of vertebrate larvae (such as frogs)
Sense organs more numerous than lancelets - median naris that leads to an olfactory sac, two mid-dorsal eyes on head, otic vesicles that develop into inner ears
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Lamprey
Petromyzon marinus - sea lamprey attached to lake trout
Ichthyomyzon castenatus - chestnut lamprey
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Lamprey Life Cycle
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Conodonts
Phylogenetic placement is uncertain
Soft-bodied
Series of tooth-like plates that form pharyngeal apparatus
Notochord
Cranium
Myomeres
Small tail fin with fin rays
Large eyes
Small size – only about 40 mm long
Occur in fossil record from late Cambrian to late Triassic period
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Conodonts
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Oldest “Fishes”
Fossil finds in China have added to record of early fish-like “vertebrates”
Myllokunmingia
Date to over 500 MYA
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Ostracoderms
Extinct jawless “fishes”
Most are known from fossils of the Ordivician, Silurian, and Devonian periods
Paraphyletic assemblage of groups
Osteostraci, Anapsida, Heterostraci and Coelolepida
“Shelled skin”
Had armour-plated body
Flourished during the Paleozoic Era
Died out by end of Devonian
Small
Most < 10 cm
Bottom-dwelling, filter feeders
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Ostracoderms
Heterocercal tail
No fins
Single, median nostril (may have lived buried in substrate)
Pineal eye
Photoreceptive structure on the dorsal part of the head
Notochord present throughout life
Bony skeleton
Most likely preyed upon by eurypterids (large, scorpion-like, aquatic arthropods)
Included: Hemicyclapsis, Cephalaspis, and Pharyngolepis
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Ostracoderms
Zenaspis sp.
Hemicyclaspis sp.
Pteraspis sp.
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Fish Clades
Cephalochordata
Hagfishes
Lampreys
Conodonts
Holocephali
Placoderms
“Ostracoderms”
Elasmobranchii
Acanthodii
Actinopterygii
Sarcopterygii
“Cyclostomes”
Gnathostomata
“Agnathans”
Chondrichthes
Osteichthyes
Vertebrata
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Superclass: Gnathostomata (jawed vertebrates)
Class: Acanthodii
Extinct spiny fishes
Class: Placodermi
All extinct
The first jawed vertebrates
Jaws arose as a derivative of the anterior pharyngeal skeleton
Remainder of pharynx supports gills
Probably not ancestors of sharks
Heavy bony dermal armor (sub-epidermal)
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Placodermi
“Plate skinned” fishes
With much armor plating, but reduced compared to the Ostracoderms
Bony skeleton
Paired pelvic and pectoral fins
Fins allow for greater swimming ability
In order to capture prey 
In order to escape being preyed upon
Prevention of roll, pitch, and yaw
More stream-lined body shape
Better swimming ability
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Placoderms
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Placoderms
Dinichthys chasing Cladoselache
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The Evolution of Jaws
From pharyngeal support elements
Jaws and branchial arches are derived from embryonic neural crest cells
Enlargement of the first branchial arch (mandibular arch) was probably in association with the need for greater/stronger pumping of water over gills
Later, became modified as jaws
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Evolution of Jaws
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Jaws
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Advantages of Jaws
Ability to grasp prey
Teeth bring the ability to chew and grind/tear prey
Pick up objects for nest building
Grasp a mate during courtship
Grab young during parental care
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Placoderm “improvements”
Possessed “lungs”
Some sort of accessory structure off the esophagus
Lived in freshwater
Modified dermal bones which served as “teeth” 
Could be replaced if broken
May have been an evolutionary “dead-end”
May have led to the Chondrichthyes
May have led to the Osteichthyes
Taxonomic affinities are uncertain
Extinct by the middle of Carboniferous period
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Fish Phylogeny
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About 700 species
Sharks, skates, rays, etc.
Includes the largest fish (whale shark )
Paired appendages
No bony skeleton
Support from cartilage
No swim bladder
Internal fertilization
May lay eggs	oviparous
May have “live birth”	viviparous
Represent an independent evolutionary lineage
Not closely related to other fishes
Class: Chondrichthyes
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Class: Chondrichthyes
Subclass: Elasmobranchii (naked gill slits)
Order: Squaliformes (sharks and sawfishes)
Order: Rajiformes (rays and skates)
Subclass: Holocephali (gill slits covered by operculum)
Includes the Chimaera: ratfishes
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Chondrichthyes
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Chondrichthyes
A great site for information on sharks . . . and other fishes
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Devonian Period as the “Age of Fishes”
Warm, humid environment with many fresh-water habitats to exploit
High primary productivity, much food available in aquatic environments, but low terrestrial diversity
Only mosses and simple vascular plants on land
By mid-Devonian, there were horsetails, indicating moist terrestrial habitats
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Devonian fishes
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More primitive fishes - Palaeoniscids
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Bony Fish Phylogeny
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Class: Osteichthyes
The bony fishes
30,000+ living species
Internal skeleton of bone
Remnants of placoderm armor remain as dermal scales in some species
Pharyngeal elements covered by operculum
Most successful vertebrate invaders of aquatic environment
Great diversity of form and function
Swim bladder usually present for buoyancy
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Fish Phylogeny
Class: Osteichthyes - bony fishes
Subclass: Sarcopterygii - fleshy-finned fishes
Subclass: Actinopterygii - ray-finned fishes
Chondrostei - bichirs (11), paddlefish (2), and sturgeons (23)
“cartilage and bone”
Holostei - Amia (1) and gar (7)
“entirely bone”
Teleostei - all other fishes (~22,000)
“perfect bone”
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Bony Fish Phylogeny
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Subclass: Actinopterygii (ray-finned fishes)
Majority of bony fish species
1st Great Chordate Radiation
Paired fins supported by fin rays
Not ancestral to “higher” vertebrates
Superorder: Chondrostei
Polypterus, sturgeons, paddlefishes
Superorder: Neopterygii
Includes Teleosts
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Holostei and Chondrostei
Amia calva
Lepisosteus osseus
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Class: Osteichthyes
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There’s more to fishes than phylogeny
Etheostoma spectabile
Pteronotropis welaka
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