Geologic Time Scale see Page 39 text Precambrian

Geologic Time Scale see Page 39, text Precambrian

Phylogeny of the Fishes • The first vertebrates to appear in the fossil record occurred in the late Cambrian Period (~500 mya). These were jawless armored fishes known as Ostracoderms. • All jawless fishes were formerly placed in the Class Agnatha, with 4 separate Orders of Ostracoderms + 2 living Orders (1 each for hagfishes and lampreys).

Jawless Fishes • Current scenario breaks Agnathans (now a Superclass) into 3 separate classes. • Class Myxini – includes hagfishes. – entirely marine – cartilaginous skeleton without bony armor, no hint of vertebral development – feed by boring into dead or dying fish with raspy tongue – evidence suggests that hagfishes are the most primitive living vertebrate • Some authors separate out hagfish from vertebrates because they lack any trace of vertebral formation around the notochord. In this scenario, hagfish and vertebrates are sister taxa and comprise the Subphylum Craniata. • However, recent evidence suggests a secondary loss of vertebral elements in Hagfish, so they appear to belong to Vertebrata.

Jawless Fishes • Class Pteraspidomorphi – Diplorhina (two nostrils) – – oldest documented vertebrate fossils appear in late Cambrian-Devonian paired nasal openings most have bony head shields • Class Cephalaspidomorpha – Monorhina (one nostril) – – varied body shapes and varied lifestyles single nasal opening fossil forms heavily armored (Silurian-late Devonian) includes living lamprey (Order Petromyzontiformes) • freshwater and marine groups • cartilaginous skeleton, bony armor completely lost (secondary condition) • feed by attaching sucker-like mouth to other fishes and suck blood • larval form (Ammocoetes) very different from adult – burrow into stream bottom and filter feed (similar to Amphioxus)

Hagfish Lamprey

Jawless Fishes • Ostracoderms are now included in Classes Pteraspidomorphi and Cephalaspidomorpha, rather than their own separate Orders within Agnatha. • Other general characteristics of Ostracoderms: – mostly small to medium-sized fishes, protected by a heavy, bony dermal (derived from skin) armor – bottom-dwellers; filter-feeders or grazers – no paired fins, but many with stabilizing paired flaps on either side of head – internal head skeleton bony in some forms, but postcranial skeleton was cartilaginous

Examples of Ostracoderms see Figs 3. 9 and 3. 10

Jawed Fishes (Gnathostomes) • A major feature in fish evolution was the development of jaws from the 1 st gill arch. This opened new niches into which jawed fishes radiated. • 3 Classes of jawed fishes in Superclass Gnathostoma. – Class Placodermi – placoderms – Class Chondrichthyes – cartilaginous fish – Class Osteichthyes – bony fish • Some authors lump placoderms and Chondrichthyes into Elasmobranchiomorphi; some split Osteichthyes into Acanthodii and Osteichthyes.

Class Placodermi • Appear in the fossil record in late Silurian (400+ mya), extinct by end of Permian (~230 mya). • Well developed jaws and paired fins (dramatically increased mobility); bony plates served as teeth • Mainly marine, but some FW forms • Anterior region of body covered with heavy bony armor in most forms • Many were active predators, ranging in size from a few inches to 30 feet • Present evidence suggests that they are unrelated to any living fishes, although formerly thought to be potential ancestors to Chondrichthyes

Dinicthyes Placoderms see Fig 3. 12

Class Chondrichthyes • Includes sharks, skates, rays, ratfish • Cartilaginous skeleton, secondary condition with loss of bony armor and internal bone • First appear during Devonian (~380 mya), peaked during Carboniferous and Permian (350 -230 mya) • Possess well developed jaws and paired fins; jaws with true teeth • Mostly marine, but some FW representatives

Ratfish Rays Sharks Class Chondrichthyes see Fig 3. 13

Class Osteichthyes (Bony Fish) • Represents the most numerous and diverse fish class • Bony skeleton retained from ancestral vertebrates • 3 Subclasses – all with well developed jaws and paired fins • Subclass Acanthodii – “spiny-sharks” – odd, minnow-sized fishes occurring from Silurian (~425 mya) into Permian (~230 mya) – covered by peculiar diamond-shaped scales – heterocercal tail = upper lobe longer than lower lobe (like sharks) – well developed paired fins supported by a strong spine; include the normal two pairs, plus up to 5 additional pairs – possess true teeth and bony plate (operculum) covering gill region

Climatius (top left; Lower Devonian), Diplacanthus (top right; Middle Devonian) and Acanthodes (center; Lower Permian) Acanthodians see Fig 3. 14

Class Osteichthyes (Bony Fish) • Subclass Actinopterygii = ray-finned fishes; includes vast majority of living fishes • Fins supported by bony rays • First appear in early Devonian (~400 mya) • Formerly divided into 3 Superorders (primitive to advanced): – Chondrosteans = most primitive group – Holosteans = succeed Chondrosteans in Jurassic and Cretaceous Periods (180 -65 mya); Not a monophyletic group. – Teleosts = advanced ray-finned fishes, form the vast majority of living fishes; became dominant by the end of the Cretaceous (65 mya); show reduction in bones of skull and scales; adapted in many cases for efficient and rapid locomotion (e. g. , placement of paired fins)

Subclass Actinopterygii • Now divided into 2 Superorders: • Superorder Palaeonisciformes = primitive rayfinned fishes – Probably the earliest bony fishes. – Includes living paddlefish, sturgeon and bichir (Africa) – Marine and FW forms – Characterized by… • heterocercal tail • ganoine scales (ganoine scale = layers of ganoine [like enamel] on scale surface with successive layers of compact bone underlying pulp) • primitive forms with lungs to gulp air in oxygen-poor Devonian FW habitats

Subclass Actinopterygii • Superorder Neopterygii = advanced ray-finned fishes • Replaced Palaeonisciformes as dominant fish group in early Mesozoic • Great range of morphologies and inhabit variety of habitats worldwide; trend toward invasion of SW habitats • Loss of ganoine scales and shortening of tail (homocercal tail) • Primitive living Neopterygians include gars and bowfins (former Holosteans) • Most recent group = Teleosts – 20, 000 species; represent vast majority of living fishes

Bichir Paddlefish Sturgeon Gar Palaeonisciformes (Chondrosteans) & “Holosteans” see Fig 3. 17 Bowfin

Teleosts see Figs 3. 16 & 3. 17

Class Osteichthyes (Bony Fish) • Subclass Sarcopterygii = lobe-finned fishes; appear in early Devonian • Fins supported by a fleshy lobe • Possessed lungs and internal nares (like land vertebrates) • Possesed unique cosmoid scales (cosmine = similar to dentin; becomes covered with enamel to form scale)

Sarcopterygii with 2 Orders • Order Dipnoi = lungfishes; possess lungs and may breathe air; – 3 living members, 1 each in Africa, Australia, and South America. – South American and African forms can survive out of water for months or years. – African form is obligate air-breather. – Fleshy fins supported by bones shaped like a fern frond, therefore not ancestral to land vertebrates. • Order Crossopterygii – lobes supporting fins with bones similar to tetrapod limb. – Skull and teeth similar to those in early amphibians; early (Devonian) forms with lungs and internal nares; one branch (Rhipidistians) thought to be ancestors to Tetrapods. – A second branch evolved during Mesozoic Era in oceans (Coelacanths); thought to be extinct at end of Cretaceous until one was caught in 1939 off the coast of southern Africa. This is the only living Crossopterygian.