Evolution of fishes, colonization of dry land by animals

Origin Of Chordates (including Vertebrates) 

! First appeared in Cambrian Time ~500 million years ago 

! First chordates lacked jaws, like the living hagfish and lampreys 

! Belong to the phylum Chordata, which are distinguished by having: 

! notochord 

! dorsal hollow nerve cord 

! pharyngeal slits (gill slits) 

! post-anal tail 

Chordate Relations 

! Split off early from a common 

ancestor shared with 

echinoderms 

! Adopted bilateral symmetry, 

instead of pentameral symmetry 

! Most primitive chordates are 

very simple animals (tunicates) 

Pikaia 

Tunicates (sea squirts) 

! Have all basic chordate characters at some point in their life cycle 

! Sessile, filter-feeding animals 

! Simple notochord only possessed by larval forms, not adults 

Key Characteristics of Vertebrates 

! The vertebral column, or backbone, which surrounds and protects 

the main nerve cord 

! A relatively well-developed brain 

! paired complex eyes 

! a muscularized mouth and pharynx 

! a well-developed circulatory system with a heart

Characteristics of Vertebrate Skeleton 

There are two types of bone 

! Dermal bone develops in the skin 

! The bony armor of the earliest jawless fish was dermal bone 

! Shark and fish scales, shoulder blades, skull crown segments 

! Dermal bone does not form from cartilage first and then calcify 

! Endoskeletal bone forms from cartilage first and then calcifies 

! Vertebrae, ribs, appendages, and the jaw are endoskeletal bone 

! All vertebrates have cartilage in addition to bone, or instead of bone 

Vertebrate Classification 

CLASS (common name) Living Species 

1. Myxini (hagfishes) 42 

2. Cephalaspidomorphi (Lampreys) 50 

3. Chondricthyes (Sharks & relatives) 550 

4. Sarcopterygii (Coelacanth & lungfishes) 7 

5. Actinopterygii (Ray-finned fishes) 20,000 

6. Amphibia (Amphibians) 4,000 

7. Reptilia (Reptiles) 6,000 

8. Aves (Birds) 8,700 

9. Mammalia (Mammals) 4,100 

Vertebrate Classification 

! Based on mitochondrial DNA (Arneson et al., Lund Univ., Sweden) 

Fossil Record of Fishes 

! Earliest fish to appear in fossil 

record were jawless fish 

! No internal skeleton, exoskeletons 

! Ostracoderms (armored fish) 

! By the Silurian, jawed fishes appear 

! Bony exoskeletons (armor) 

! Cartilaginous fishes appear by the 

late Silurian (sharks, rays) 

! Cartilage skeleton 

! By Devonian, bony fishes appear 

! Most modern fishes are bony fishes

Ostracoderms: early jawless fish 

Agnatha - Jawless Fish 

! Plated Fish 

! An early group of jawless fishes 

! No internal skeleton, but possessed a tough exoskeleton of 

phosphatic plates 

! Calcium phosphate (apatite) - found in modern bones, teeth 

! Belong to the Agnathans, jawless fish 

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Heterostracans ! Possessed flattened headshields 

! Rigid head and stiff, heavily-plated body 

! No lateral fins, simple swimming style 

using only the tail 

Heterostracans 

! Athenaegis chattertoni 

courtesy of Dr. Mark 

Wilson, Univ. of Alberta 

Laboratory of Vertebrate 

Paleontology 

Sacabambaspis 

Pteraspis 

Astrapis

Osteostracans ! Similar to Heterostracans 

! Had paired lateral fins 

! Most had heavy headshields, 

forming a single, solid case of 

bone covering the frontal part of 

the body 

Osteostracan – Cephalaspis sp. 

! Range: Late Silurian(?) to Early Devonian of Europe & Spitsbergen. 

Jawed Fish - Placoderms 

! Heavy bony armor on the head and neck, often with an unusual joint in 

the dorsal armor between the head and neck regions 

! Unlike all other jawed vertebrates, placoderms never had teeth, and did 

not descend from toothed ancestors. Instead, bony plates associated with 

the jaws performed the function of teeth, sometimes forming razor-like, 

literally self-sharpening edges 

Placoderms 

! Botriolepis sp. 

! Middle Devonian, Escumac Bay, Quebec, Canada 

! Replacement 

! Range from Early Silurian to end of Devonian

Placoderms 

! Botriolepis sp. 

! Middle Devonian, Escumac 

Bay, Quebec, Canada 

! Replacement 

Duncleosteus 

! Heavily armored placoderm, with a massive 

headshield 

! Teeth are extensions of the exoskeleton 

! Eye sockets were armored 

! A major predator during the Devonian 

! Did not survive much into the Carboniferous 

Chondrichthyes or "cartilaginous fishes" 

! Internal skeletons are composed of cartilage 

! little or no calcified bone 

! They have small tooth-like scales covered with an enamel-like layer 

! in effect, a tooth-like skin 

! No swim-bladder 

! lift is provided internal oil bodies that are lighter than water 

! also, streamlined shape as it moves through the water 

! Teeth are the most heavily calcified parts 

Chondricthyes – Rays 

! Rhinobatis sp. 

! Senckenberg Museum in Frankfurt, 

Germany, Upper Cretaceous of 

Haqel, Lebanon 

! Dorsal view of a ray-like fish 

! Flattened, partially carbonized and 

replaced on limestone

Chondricthyes – Rays 

Chondricthyes – Shark’s teeth 

! Cyclobatis longicaudatus 

! Upper Cretaceous of Lebanon 

! Flattened, partially carbonized 

and replaced on limestone 

! Cretoxyrhina mantelli 

! Lower Smoky Hill Chalk 

Formation (late Cretaceous) 

Western Kansas 

Chondricthyes – Shark’s teeth 

! Cretoxyrhina mantelli 

! Lower Smoky Hill Chalk 

Formation (late Cretaceous) 

Western Kansas 

Acanthodians – Jawed, bony fish 

! Class of primitive, fossil fish, characterized by the presence of a 

true bony skeleton 

! a hetercercal tail fin 

! Vertebral column continues in the upper tail lobe 

! a persistent notochord 

! ganoid scales 

! Have peg and socket joints, are rhomboid in shape 

! stout spines in front of the fins 

! The acanthodians lived from the Silurian to the Permian Period and 

may be related to ancestors of the modern bony fish

Acanthodian – Climatius sp. 

! Small-bodied (

Sarcopterygii – Lobe-finned fish 

Caridosuctor (Carb.) 

Sarcopterygii – Lobe-finned fish 

! Modern Coelacanth 

! Discovery of live coelacanths off the coast of South Africa in 1938 

! Previously, was thought that all coelacanths went extinct 65 million years ago 

! Lobe-finned fishes gave rise to lungfishes, tetrapods 

Tristichopterus (Dev.) 

Dipterus (Dev.) 

Dipterus (Dev.) 

Classification of modern jawed fish 

Chondrichthyes 

Sharks, Rays 

Jawed Fishes 

(Gnathostoma) 

Sarcopterygii 

Coelacanths 

Lungfish 

Osteichthyes 

Tetrapods 

Actinopterygii 

Ray-finned Fish

Evolution of fishes, colonization of dry land by animals

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