BI62– Vertebrate Zoology Origins and Classification of Early Fishes

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

Agnatha Class: Myxini (hagfishes) 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

Agnatha Myxiniformes – a hagfish Petromyzontiformes – a lamprey

Agnathan Phylogeny

Hagfishes

Ammocoete Larvae

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

Lamprey Ichthyomyzon castenatus - chestnut lamprey Petromyzon marinus - sea lamprey attached to lake trout

Lamprey Life Cycle

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

Conodonts

Oldest “Fishes” Fossil finds in China have added to record of early fish-like “vertebrates” Myllokunmingia Date to over 500 MYA

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

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

Ostracoderms Pteraspis sp. Hemicyclaspis sp. Zenaspis sp.

Fish Clades Gnathostomata Vertebrata “Agnathans” Chondrichthes Osteichthyes “Cyclostomes” Cephalochordata Elasmobranchii Actinopterygii Sarcopterygii Conodonts Placoderms Holocephali Hagfishes Lampreys Acanthodii “Ostracoderms”

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)

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

Placoderms Coccosteus Arctolepis Lunaspis Phyllolepis Rhamphodopsis Gemuendina Coccosteus Lunaspis Rhamphodopsis Bothriolepis

Placoderms Dinichthys chasing Cladoselache

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

Evolution of Jaws

Mandibular arch - 1st branchial arch Jaws chondrocranium Mandibular arch - 1st branchial arch modified as jaws Remaining branchial arches support gills

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

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

Fish Phylogeny

Class: Chondrichthyes 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 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

Chondrichthyes

Chondrichthyes A great site for information on sharks . . . and other fishes

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

Devonian fishes

More primitive fishes - Palaeoniscids

Bony Fish Phylogeny

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

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”

Bony Fish Phylogeny

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

Holostei and Chondrostei Lepisosteus osseus Polyodon spatula Amia calva

Class: Osteichthyes

There’s more to fishes than phylogeny Etheostoma spectabile Pteronotropis welaka