chordates II

sharks

• Vertebrates 

  • Vertebrate 

  • Eye 

  • Cranium 

  • Brain 

  • Tongue 

  • Gill arches 

• Gnathostomes 

  • Mineralised teeth 

  • First gill arch forms jaws 

  • Paired fins 

  • Osteichthyes (bony fish) and Chondrichthyes (sharks, rays and chimaeras) 

• Vertebrates and gnathostomes are united by adaptations for predation (or to resist predation) 

parental investment

• Some bony fish can lay millions of eggs 

• Sharks lay a few, large eggs with big yolks 

  • Fewer, larger, well developed young 

• In many sharks, eggs are retained and hatch internally 

  • They feed via the placenta or on other eggs 

• Different reproductive strategies 

  • Oviparity (eggs enclosed in protective cases) 

  • Ovoviviparity (eggs hatch inside the mother) 

  • Viviparity (embryos receive nutrients directly from the mother) 

intermittent organ and internal fertilisation

• Bony fish use spray and pray approach to external fertilisation 

  • This is messy and risky 

  • Many eggs are not fertilised 

• Sharks have claspers 

  • Extensions of pelvic fins 

  • Used to transfer sperm into the female 

    • Inserted into the female cloaca 

  • Male sharks fertilise eggs inside a female 

  • High chance of successful fertilisation 

  • Convergent on intromittent organ (specialised reproductive structure used to transfer sperm directly into the female reproductive tract) of amniotes 

buoyancy

• Sharks have a big oily liver 

  • Helps them float 

  • Still denser than water 

  • Do not have a swim bladder 

  • To stay up in water, sharks must swim 

• Heterocercal tail 

  • Uneven tail (larger upper lobe) produces upward thrust 

• Lightweight skeleton 

• Ram-breathers 

  • Some specialised pelagic species  

  • Ventilate gills by forcing water through an open mouth 

  • Means they must swim or they suffocate 

reduced skeleton

• Advanced jawless fish (placoderms) were armoured 

• Bony fish retain the bony cranium 

• Sharks lack bone internally (never evolved bony vertebrae) 

• Neutral buoyancy reduces the need to produce lift (lift-production creates drag, thus there is less hydrodynamic drag) 

• External bony plates of skull, jaws lost 

• No marrow filled bones 

• Bony scales reduced to tiny denticles 

  • Enamel and dentine – like teeth 

  • Reduce drag by around 10% 

  • Parallel riblets control vortice formation 

high speed

• Lamnidae specialised for high speeds 

• Teardrop shaped body 

• Lunate tail 

• Elongate fins to reduce drag 

• Like marlin, dolphins and extinct ichthyosaurs 

• Strong myotomal muscles produce powerful swimming strokes 

spines

• Many sharks have fin spines 

• Lost in certain species  

  • e.g. great white 

• Made of dentine like material 

  • Covered by a thin enamel-like layer  

• Used for defence and deterrence 

fast tooth replacement

• Not unique to sharks 

  • e.g. humans replace baby teeth 

• Have a high rate of replacement 

  • Every 9-36 days 

• Important as fish bones/scales rapidly dull teeth 

jaws

• Jaw cartilage is not connected to the skull 

• Means jaws are flexible and can extend from the face to grab prey 

• Multiple rows of teeth 

• Wide gape 

• Can rapidly extend their jaws 

Eyeshine (reflective layer in retina) tapetum lucidum 

• Photons that go through the retina without hitting photoreceptors are bounced back by a shiny layer of guanine 

  • Gives photoreceptors a second chance to detect light 

  • Improves vision in low-light conditions 

• Widespread in mammals and some fish (coelacanth) 

sharks are electroreceptive

• Pitlike organs on the snout detect electric fields 

• Some fish (coelacanth, paddlefish) are also electroreceptive, suggesting that its an ancient feature 

• How does it work 

  • Living organisms produce weak electrical fields from muscle and nerve activity 

  • These are conducted through seawater 

  • Ampullae of lorenzi on the head and snout detect tiny voltage differences 

• Uses 

  • Locate hidden prey 

  • Navigation 

  • Hunting in low visibility 

smell in sharks

• Water goes over the folds of chemosensitive tissue 

  • Can detect scents of 1 part per billion 

  • Useful for long-distance prey detection 

• Bilateral arrival time differences can give useful directional information 

  • Can compare input from left or right nares 

relationships

chimaeras

• e.g. ratfish, rabbirtfish, elephantfish, spookfish 

• Deepwater (200 m to over 8 km) 

• Large eyes 

• Single gill opening 

• Teeth fuse into toothplates 

  • Inscisor-like structures for crushing prey 

• Modern species differ from fossils and each other 

batoidea

• Flat body glides over seafloor 

• Big pectoral fins go to head 

• Gills under head 

• Spiracles on head 

  • To draw in water while resting on the seabed 

• Blunt teeth to crush hard food 

• Evolve in mesozoic 

  • A few stingray spines known from late cretaceous 

pristiformes

• Swordfish  

• Denticles 

• Flattened body 

• Gill slits on underside 

• Large pectoral fins 

torpendinformes

• Flattened body 

• Short, thick tail 

• Large pectoral fins 

• Gill slits on underside 

• Electric rays 

  • Modified muscle cells discharge electric pulses to stun prey (or predators) 

rajidae

• Skates 

• Long, slender tail 

• Gill slits on underside 

• Flattened body 

• Instead of swimming, they walk 

  • Stumpy, fingerlike nubs on pelvic fins push against the seafloor 

myliobatiformes

• Rays 

• Fin spine modified into a long, serrated barb for defence 

  • Often venomous 

sharks

• Squalomorphi (dogfish and kin) 

• Galeomorphi (requiem shark, mackerel sharks, catsharks) 

squalomorphi (dogfish and kin)

squalomorphi

• Lantern shark 

  • Bioluminescent photophores on stomach 

  • Used for counterillumination and communication 

  • Large eyes 

  • Dark colouration 

• Sleeper shark 

  • Slow moving 

  • Dark colouration 

  • Slow moving 

  • Soft and flabby body 

• Cookie cutter shark 

  • Small 

  • Cylindrical 

  • Bioluminescent photophores for camouflage via counterillumination 

galeomorphs

heterodontiformes

• Horn sharks 

• Egg-case is spiral shaped 

  • Screws into sediment 

• Molar-like teeth crush molluscs and crustaceans 

• Sharp front teeth for grasping 

• 2 dorsal fins with spines 

orectolobiformes

• Carpet sharks 

• Small bodied 

• Benthic (bottom dwelling) 

• Cryptic (well hidden) 

• Often have camouflage markings 

• Whale sharks 

  • 12-18 m 

  • Filter feeder 

  • White spots and stripes for camouflage 

carcharhiniformes

• Ground sharks 

• Hammerhead 

• Tiger shark 

• Galapagos shark 

• Chain catshark 

• Nicitating membrane 

  • Third eyelid 

• Anal fin 

• Five gill slits 

• Two dorsal fins 

lamniformes

• Large 

• Conical snout and large jaws 

• Two dorsal fins 

• Large gill slits 

  • High oxygen demand 

• Goblin shark 

• Thresher shark 

• Megamouth shark 

  • Thousands of tiny teeth 

• Mackerel sharks 

  • Mako 

  • Porbeagle 

mackerel sharks

• Streamlined, torpedo shaped body 

• Powerful tail 

• Large gill slits 

• No nictitating membrane 

• Regional endotherms 

  • Heat is generated by active swimming muscles during movement 

  • Uses countercurrent heat exchange 

    • Warm blood leaving muscles runs next to cold blood returning from gills 

    • Recycles heat and prevents it being lost at the gills 

great white shark

• Mackerel shark 

• To maintain high activity in cold water, eats marine mammals 

  • Have rich fat reserves 

• Humans often mistaken for seals or sea lions 

shark evolution

• Oldest relative thought to be in late Devonian (360 million years old) 

  • Had cartilage skeleton and no skull bones 

  • Cladoselache 

• Acanthodians are recognised as shark relatives 

  • Have dorsal fin spines 

  • Scales resemble denticles 

  • Retain bony skill 

• Paleozoic diversity 

  • After evolution of sharks in the devonian, they continued to evolve 

  • Most lineages no longer exist 

    • e.g. helicoprion 

    • Falcatus 

are sharks living fossils

• Implications 

  • Physical limits are being hit 

  • As long as it stays in the same predatory niche, it remains similar  

  • No as suggests no evolution 

• Rapid evolution slows dramatically 

  • Evolution eventually stops because you can't improve endlessly 

  • Body plan has remained relatively similar over time 

• Are still innovations 

  • Evolution of filter-feeder sharks in early Cenozoic, after dinosaur extinction 

  • Follows extinction of giant filter feeder fish in cretaceous 

    • Adapted as a new niche was available 

  • Competitors prevent animals from expanding into new niches 

    • If removed, rate of evolution would increase again 

chordates

Osteichthyes

• Bony fish 

• Distinguishing characteristics from cartilaginous fish 

  • Swim bladder/lung 

  • Highly mobile fins 

  • Reduced number of gill arches with gill cover 

  • Bony skeleton 

lung/swim bladder

• Many bony fish have air-filled chambers in the body 

• In lungfish and tetrapods, this chamber gets oxygen from air (lung) 

• In most fish this device is a float used for buoyancy 

• Negatively buoyant sharks swim to stay up in water 

• Ray-finned fish float 

• It's been long suspected lung is homologous with the swim bladder 

Feature	Lungs	Swim Bladder
Main function	Gas exchange	Buoyancy
Location	Connected to respiratory system	Dorsal to gut
Organism type	Terrestrial vertebrates, some fish	Bony fish
Evolution	Likely ancestral	Derived from lungs

polypterus

• Most primitive living ray-finned fish 

• Have no swim bladder 

• Has lungs and breathes air 

  • Useful for a freshwater tropical fish  

  • Warm, stagnant water has little oxygen 

• Implies lungs first evolved for low oxygen in water, not life on land 

• Then lungs evolved into the swim bladder 

mobile fins

• In sharks, fins are used for steering and stabilisation, tail for propulsion or sometimes swimming 

• In bony fish, fins are highly mobile 

• Sarcopterygians and polypterus have a mobile fin base 

  • Lets them move pectoral and pelvic fins, shoulder and hip joints 

• The fin itself is modified, bony struts supporting flexible membrane 

• Bony fish can fold and unfold fins, and change fin shape, using them for propulsion and maneuvering 

• Advanced ray-finned fish can tightly fold fins against the body to reduce drag 

• Mobile fin bases (along with lungs) let polypterus crawl up onto land 

• Mobility of pectoral and pelvic fins taken to the extreme in tetrapods 

modified respiratory system

• Sharks and lamprey have 5-7 pairs of gills. Hagfish have more! 

• Bony fish have 3 pair of gills, movable operculum (gill cover) pumps water. Lost in amniotes, retained in larval amphibians 

• Jawless fish have simple pumping mechanisms, many sharks are ram-breathers, swim to force water over gills, or have simple pumps 

• High-volume opercular pump of bony fish lets them oxygenate blood while stationary. May explain success in low O2 environments 

bones

• Skeleton supported by mineralised tissue 

  • Skull 

  • Jaws 

  • Vertebrae 

  • Fin rays 

• Shark ancestors had bony skulls and lost them 

  • Bony fish extended mineralisation from the head to the rest of the skeleton 

• A bony internal skeleton allows complex bones and joints to be made 

  • e.g. between vertebrae, limb bones and fingers 

  • Complex body shapes 

  • Complex movements 

movement

• Complex articulations & rigid struts make possible elaborate movement 

• Sharks have limited number of ways of moving 

• Bony fish have far more sophisticated movement of spine, fins, jaws, skull 

actinopterygia (ray-finned fish)

teleost tree

geologically recent

• Sharks have been here forever- many modern groups go back to the Cretaceous 

• Teleosts first stage initial radiation in mid-Cretaceous. 

• Modern families appear in Paleocene and Eocene after K-Pg boundary when dinosaurs go extinct 

  • e.g. flatfish, tuna, billfish, pufferfish, surgeonfish, moray eels- suggesting origin is driven by mass extinction of fish & marine reptiles end of Cretaceous 

• Explains weird relationships: adaptive radiations 

sarcopterygia (lobe-finned fish)

actinistia (coelacanth)

• Two living species: Latimeria chalumnae from Indian Ocean, L. menadoensis from Indonesia 

• Deep-water predators, live in caves 100-500 m deep, emerge at night to feed along coral reefs. 

• Have electroreceptors (like sharks) to detect prey 

• Unusually for fish, have internal fertilization, give birth to live young 

limb-like fins

• Like tetrapods, coelacanths have mobile, limb-like finbases 

• Unlike in more primitive fish, one bone connects it to the 

• shoulder- a shoulder joint. A feature shared by lobe-finned fish incl. Tetrapods 

living fossils

• Fossil coelacanths found 1839, thought to go extinct with dinosaurs. Only known from fossils until 1938, when living coelacanth caught off of South Africa 

• Coelacanths once diverse, successful, but over time failed to compete against other fish 

• Resemble forms from 100s of millions of years ago- in some ways ‘living fossils’, but ecology (deepwater marine) is specialized, not like ancient coelacanths (evolution slows, doesn’t stop) 

• Deepwater, nocturnal ecology probably meant impact winter caused by Chicxulub didn’t affect them much 

dipnoi (lungfish)

• Like amphibians, larvae breathe through feathery external gills; adults breathe air 

• South American & African species obligate air-breathers 

• South American & African species aestivate: burrow into mud in dry season and enter dormancy 

• Evolve in the Devonian, once far more diverse, found on many continents, freshwater + marine environments 

tetrapod evolution

• Origin from Lobe-Finned Fish 

  • Tetrapods evolved from lobe-finned fishes (Sarcopterygii) 

  • Fins had bone structures similar to limbs 

• Transition to Land 

  • Early forms lived in shallow water/swampy environments 

  • Gradual shift from aquatic → semi-terrestrial → fully terrestrial life 

• Key Transitional Fossils 

  • Tiktaalik – fish with limb-like fins and a neck 

  • Acanthostega – had limbs but still aquatic 

  • Ichthyostega – more adapted for land movement 

• Major Adaptations 

  • Limbs with digits (replacing fins) 

  • Stronger skeleton to support body weight 

  • Lungs for air breathing 

  • Development of a neck (head movement independent of body) 

• Later Diversification 

  • Tetrapods gave rise to: 

    • Amphibians 

    • Reptiles 

    • Birds 

    • Mammals 

amphibians

• Dual life  

  • Land and water (for reproduction) 

• Highly diverse 

  • 9000 species 

• External ferilisation 

  • Female lays eggs in water, male releases sperm which swim to eggs 

• Young develop in water and have tail fins 

  • Some have external gills 

• Adults are terrestrial and grow limbs to walk or hop on land, hunt for insects and other prey 

how did we get onto land

• Fossils help understand the transition 

• Eusthenopteron foordi (lobe finned fish) 

• Panderichthys 

• flatheads

• Tiktaalik roseae 

• Acanthastega gunnari 

Eusthenopteron foordi (lobe finned fish) 

• Late devonian 

• Large, predatory fish 

• Recognised as a tetrapod releative, based on a well-developed fin skeleton 

• Evolves too late to be directly ancestral to tetrapods 

Panderichthys 

• Loss of midline fins (dorsal and anal fins) 

  • Shared with tetrapods 

• Borad, flattened head 

• Eyes on top of the head 

• Still marine 

flatheads

• Flattened body plan 

  • Typical of a bottom dwelling fish 

• Eyes on top of head 

  • Typical of bottom dwellers 

• Benthic adaptation 

  • Ancestors were bottom feeders and muck-dwellers 

Tiktaalik roseae 

• Late devonian 

• Well developed limb bones 

• Encased in fin rays 

• Fin rays reduced 

• Functionality between limb/fin 

  • Used for bottom crawling 

Acanthastega gunnari 

• Late devonian 

• 8 fingers and toes 

  • Hands and feet 

• Elongate limbs, weak wrist (not ossified) 

• Retains long, oarlike tail with fin rays for underwater propulsion 

• Retains gill arches for breathing underwater 

• Retains fish-like lateral-line system for detecting water movements 

fins and limbs

• Some fish use fins to walk 

• Limbs evolved for walking underwater 

  • Limbs useful for many things, not just terrestrial walking 

  • Walking along bottom may be easier than swimming against a stream or tidal current 

  • May be useful for ambush predators- lets you move extremely slowly 

  • At any rate, limbs (like lungs) don’t evolve for life on land, but to make life in water easier 

  • Only later these adaptations become useful on land 

why did aquatic animals come onto land

• The end-Devonian mass extinction 

  • Devastation of sarcopterygian fish and tetrapodomorph fish 

  • Tetrapods evolve in the Devonian. 

  • But only after extinctions at end of Devonian do they go onto land. 

  • Colonization of land may be opportunistic response to extinctions 

• Terrestrial tetrapods appear 

  • Have eyes facing to the side 

  • To peer out over the land, not up from sea bottom 

  • Powerful legs 

  • Five strong toes 

pentadactyl limb

• Indicates common ancestry among tetrapods 

  • Modified for different functions: 

    • Running 

    • Flying 

    • Swimming 

    • Grasping 

• Same underlying structure is found in many vertebrates → homologous structure 

• Common bone pattern: 

  • Humerus (upper limb bone) 

  • Radius and ulna (forearm) 

  • Carpals (wrist bones) 

  • Metacarpals 

  • Five digits (phalanges) 

adaptive radiation

• Following move onto land, a major radiation 

• Partly driven by diversification of amphibians- including many extinct groups (plus one lineage that becomes modern amphibians) other group is amniotes- tetrapods that lay eggs on land. They are ancestors of reptiles, birds, mammals 

• First amniotes are tiny, but soon became larger and more specialized, dominate the land from end of the Carboniferous 

tetrapods and amniotes

• Tetrapods evolved for life on land: 

  • Air breathing 

  • Limbs with digits 

  • Walking ability 

• Amniotes include: 

  • Mammals 

  • Birds 

  • Reptiles 

• Total tetrapod diversity: >35,000 species 

• Amniotes are adapted to: 

  • Live and reproduce on land 

key amniote adaptations

• Amniotic egg 

  • Contains membranes: 

    • Amnion, chorion, allantois 

  • Functions: 

    • Prevent drying out 

    • Allow gas exchange (oxygen) 

• Internal fertilisation 

  • Required because eggs are laid on land 

  • Involves contact between male and female 

  • Structures: 

    • Penis (mammals, turtles, crocodiles) 

    • Hemipenes (lizards & snakes) 

    • May have evolved in water first (also seen in fish) 

• Direct development 

  • No larval stage 

  • Hatch as miniature adults 

  • Already have: 

    • Lungs 

    • Limbs 

• Skin adaptations 

  • Covered in keratinous skin 

  • Waterproof → prevents water loss 

  • Shed regularly

first amniotes

• Appeared ~300 million years ago (Carboniferous) 

• Small (~1 foot long) 

• Split into: 

  • Synapsids → mammals 

  • Sauropsids → reptiles & birds 

• Example: Hylonomus 

major evolutionary events

• Permo-Triassic extinction (~252 MYA) 

  • Massive volcanic eruptions 

  • Wiped out many species (especially synapsids) 

• Mesozoic Era (252–66 MYA) 

  • Known as the “Age of Reptiles” 

  • Rise of: 

    • Dinosaurs 

    • Modern reptile groups 

    • Mammals remained small and nocturnal 

reptile classification (sauropsida)

• Includes: 

  • Lizards & snakes (Lepidosaurs) ~11,770 species 

  • Turtles ~365 species 

  • Birds ~10,000 species 

  • Crocodilians ~27 species 

• “Reptiles” are not a natural group 

• Some reptiles are more closely related to birds than other reptiles 

tuatara

• Found in New Zealand 

• Nocturnal, prefers cool environments 

• Teeth fused to jaw 

• Only 1 living species 

• Diverged from lizards ~240 MYA 

squamata (lizards and snakes)

• Most diverse tetrapods (~11,700 species) 

• Includes: 

  • Lizards (~7,751) 

  • Snakes (~4,108) 

• Wide diversity: 

  • Diets: insectivores, carnivores, herbivores 

  • Habitats: terrestrial, arboreal, aquatic, burrowing 

  • Activity: nocturnal & diurnal 

geckos

• Mostly nocturnal (some exceptions) 

• Excellent climbers: 

  • Use Van der Waals forces to stick to surfaces 

• Some species: 

  • Lost legs → snake-like bodies 

• Appear ~100 MYA (possibly earlier) 

• autonomy

  • Ability to drop tail to escape predators 

  • Tail can regenerate 

  • Ancestral trait in lizards 

• flying geckos

  • Have skin flaps 

  • Can glide/parachute 

iguanidae (iguanas)

• Found mainly in the New World 

• Diet: 

  • Insectivores, herbivores, omnivores 

• Example: 

  • Marine iguana: 

  • Basks to warm up 

  • Feeds on algae in ocean 

acrodonts (e.g. chameleons)

• Found in Old World (Africa, Asia, Australia) 

• Include: 

  • Chameleons 

  • Frilled lizards 

  • Flying dragons 

• Chameleon adaptations 

  • Grasping: 

  • Opposable digits 

  • Prehensile tails 

• Vision: 

  • Independent eye movement 

• Feeding: 

  • Long sticky tongue 

• Camouflage: 

  • Colour change 

worm lizards (amphisbaenia)

• Burrowing reptiles 

• Worm-like bodies 

• Move using accordion motion 

• Reduced: 

  • Eyes 

  • External ears 

• Detect prey via vibrations 

anguimorpha

• Includes: 

  • Komodo dragon 

  • Gila monster (venomous) 

  • Slow worms 

• Some evolved limbless forms 

• Large extinct species: 

  • Megalania (~6m) 

cretaceous diversity

• Very high reptile diversity 

• More lizard species than dinosaurs in some areas 

• Wide ecological roles 

• K-Pg extinction

  • around 83% of lizard species wiped out

snakes

• General features 

  • ~4,000 species 

  • Highly successful group 

  • Lost limbs (despite being tetrapods) 

• Locomotion 

  • Crawling 

  • Burrowing 

  • Swimming 

  • Climbing 

  • Some can glide 

• Feeding 

  • All are predators 

  • Eat: 

    • Insects → mammals 

  • Can consume large prey 

  • Adaptations: 

    • Flexible skull (multiple hinges) 

    • Expandable rib cage 

• Prey capture 

  • Methods: 

    • Constriction (cut blood flow) 

    • Venom (toxic chemicals) 

  • Have hooked teeth 

origin of snakes

• Marine hypothesis 

  • Snakes evolved from marine reptiles (mosasaurs) 

  • Evidence: 

    • Early snakes with small legs 

    • Swimming adaptations 

• Burrowing hypothesis 

  • Snakes evolved from burrowing lizards 

  • Evidence: 

    • Reduced eyes 

    • No external ears 

    • Long, narrow body 

fossil evidence

• Early snakes 

  • Najash – had legs, burrowing traits 

  • Coniophis – Cretaceous proto-snake 

• Tetrapodophis (possible “missing link”) 

  • Had four legs 

  • Snake-like features: 

    • Skull 

    • Spine (>150 vertebrae) 

    • Scales 

  • Feeding behaviour 

    • Controversial: 

    • Some scientists disagree it is a snake 

evolution insights

• Long bodies + reduced limbs evolved multiple times 

• Snakes are essentially: 

  • Highly specialised lizards 

tetrapods

sauropsida

Chelonia (turtles and tortoises)

• Freshwater, marine or terrestrial 

• Plant-eaters, carnivores or omnivores 

• 356 species 

• 14 families 

  • Most evolved in the age of the dinosaurs 

• Armour 

  • Top of shell (carapace) composed of rib cage, with additional bones around the edge 

  • Big, tough scales cover bone 

  • Bottom of shell composed of belly ribs (gastralia) 

  • Arms, legs, tail, head can retract inside shell

• Evolution 

  • Initially terrestrial, evolving after the Permian-Triassic extinction 

  • Turtles probably peak in diversity in the late cretaceous 

  • Marine and terrestrial species hit hard but freshwater lineages survived the K-Pg impact (massive asteroid collision with earth around 66 million years ago between Cretaceous and Paleogene periods) 

crocodila

• crocodiles, alligators, caimans, gharials 

• 27 living species 

• Large 

  • Living are relatively large bodies 

  • Small species are around 6 ft (50 pounds) 

  • Saltwater crocodile is around 21 ft (4000 pounds) 

• Semiaquatic 

  • Specialised for aquatic lifestyle 

  • Flattened, oarlike tail for sculling 

  • Webbed feet for swimming 

  • Massive bones act like a diver's weight belt, helping them sink 

  • Eyes, nostrils sit atop head, lets crocodiles breathe and look for prey while the body is submerged 

  • Dense bones, allowing them to sink 

• Apex predators 

  • Wide range of prey (insects, crustaceans, fish, etc) 

  • Large, heavily armoured and armed 

  • Have few predators of their own 

  • Long lifespans 

• Cannibalism 

  • Biggest threat is another crocodilian 

  • Primary cause of mortality for young is the elders 

  • Cannibalism serves multiple purposes 

    • Source of protein 

    • Eliminates competitors for food 

    • Eliminates offspring of others from gene pool 

crocodila p2

• Parental care 

  • Females make nests of mud and vegetation. It rots to make heat to incubate eggs 

  • Females guard the nest and when eggs hatch, they dig up hatchlings and carry them to water 

  • Females guard offspring 

  • Parental care seen in closest living relatives – birds 

    • Suggests parental care inherited from a common ancestor 

• Used to be more diverse 

  • Grew larger in the past 

  • For example 40 ft Purrusaurus 4-20 million years ago 

  • The warmer climate allowed them to grow to a larger size 

• Marine crocodilians 

  • In Jurassic and early Cretaceous, marine Geosauridae-crocodilians with flippers and shark-like tail- were marine predators 

  • Another group-Dyrosauridae- moderately successful in Late 

  • Cretaceous, diversifies in early Cenozoic to exploit niches left vacant by extinction of mosasaurs 

  • Marine crocodiles decline as ice ages begin 

• Terrestrial crocodilians 

  • Aquatic crocs are one branch of a once diverse tree 

  • In Late Cretaceous, cat-sized crocs prowled sands of the Gobi in Mongolia 

  • As recently as the Eocene, 35 MYA, terrestrial, predatory crocs hunted Pampas of South America 

  • Many were carnivores, others omnivores- even herbivores 

  • Pug-snouted Simosuchus from Late Cretaceous had leaf-shaped teeth like iguanas- to eat leaves 

  • Modern crocs a specialized offshoot of this diverse radiation 

• Triassic crocodilians 

  • Crocodilians peak in the Triassic, 252-201 MYA, when they radiate to replace mammal-like reptiles 

  • Evolve armored herbivores, bipedal, ostrich-like plant eaters, tyrannosaur-like meat eaters, agile, greyhound-like pursuit predators 

  • Rivaled dinosaurs in diversity! 

  • Massive eruptions 200 MYA- breakup of Pangaea and formation of the Atlantic- eliminate all but the pursuit predators 

  • Dinosaurs take over terrestrial ecosystem in Jurassic-Cretaceous, while crocodilians launched a major radiation in water 

  • Remaining terrestrial crocs largely wiped out by the asteroid 

aves (birds)

Pellets 

• Some birds (including hawks and owls) regurgitate foods they cannot digest. 

• These ‘pellets’ may include bones, feathers, teeth, claws etc. 

• Pellets have historically been used to determine prey variation over time. 

• However, DNA-based approaches are more effective. 

key features of birds

• Insulatory feathers 

• Flight feathers 

• Warm blood (41 C) 

• No teeth 

• Sophisticated respiratory system: air sacs, air-filled bones 

• Upright limbs, bipedal, digitigrade 

• Intense parental care – brood eggs, guard young, many feed young 

hindlimb

• Uproght limbs, vs sprawling limbs of crocs and lizards 

• Short thigh hidden by feathers 

• Ankle held clear of the ground, run on tiptoes (digitigrade) 

flight

• Fast. Speeds up to 100 kph 

• Energetically efficient. Power intensive, but less energy to fly than to walk a given distance: high fuel/minute but low fuel/km 

• Direct. ‘As the crow flies.’ Can simply fly over obstacles 

• Access any environment. Treetops, cliffs, oceans, the air 

• Safe. Once airborne, invulnerable to most predators. Not coincidentally, birds have much longer lifespans than similarly-sized mammals

separate hindlimbs and wings

• Bats and pterosaurs have hindlimbs bound into wings. Legs and arms work concert in flight, and in other modes of locomotion (climbing/walking) 

• Specialization of limbs for terrestrial/climbing/swimming locomotion tends to conflict with specialization for flight 

• Bird wings and legs are separate – 2 ways of getting around 

evolutionary relationships of birds

• Waterbirds like penguins, tubenoses and pelicans sit within a broader assemblage 

• Earliest landbirds may have been predatory 

• Origin from Dinosaurs 

  • Birds evolved from theropod dinosaurs 

  • Closely related to carnivorous dinosaurs like Velociraptor 

• Link to Reptiles 

  • Birds are part of the reptile lineage (amniotes) 

  • Share a common ancestor with modern reptiles like Crocodilia 

  • Crocodiles are their closest living relatives 

• Transitional Fossils 

  • Key fossil: Archaeopteryx 

  • Shows both: 

    • Reptilian traits (teeth, long tail) 

    • Avian traits (feathers, wings) 

• Shared Characteristics with Theropods 

  • Hollow bones 

  • Three-toed limbs 

  • Wishbone (furcula) 

  • Feathers (present in some dinosaurs) 

• Placement in Evolution 

  • Birds belong to: 

    • Clade Aves 

    • Within Dinosauria (they are technically living dinosaurs) 

• Evolutionary Significance 

  • Represent a transition from: 

    • Ground-dwelling dinosaurs → flying vertebrates 

where are birds classified

• Domain: Eukarya 

• Kingdom: Animalia 

• Phylum: Chordata 

• Subphylum: Vertebrata 

• Clade: Tetrapoda 

• Clade: Amniota 

• Clade: Diapsida (reptile group) 

• Clade: Dinosauria 

• Class: Aves 

origins of birds

• Archaeopteryx 

• Discovered 1861 

• From late Jurassic – 150 million years ago 

• Distinctively avian features, but also more typical reptile features 

• Advanced bird features 

  • Feathers (central shaft, barbs, barbules) 

  • Wings 

  • Wishbone (furcula) 

• Primitive (reptile) features 

  • Expected in early birds (if they come from reptiles) 

  • Clawed hands 

  • Teeth 

  • Long, bony tail 

• Thereopod dinosaurs 

  • Bipedal 

  • Digitigrade 

  • Birdlike toe arrangement 

  • Large number of hip vertebrae 

  • Long, birdlike, S-shaped neck 

  • Similarities were interpreted as representing convergence for bipedal lifestyle 

  • Dinosaurs are seen as too specialised to be bird ancestors 

dromeosaurs

• Wrist 

  • Identical wrist construction in dromaeosaurs and Archaeopteryx 

  • This spurs scientists to revisit dinosaur-bird connection 

  • More features place birds inside Dinosauria 

    • Shape of hip bones 

    • Shape of ankle 

    • Tail structure 

    • Three fingered hand 

    • Foot structure (3 weight bearing toes and a small inner toe) 

    • Wishbone (found in dinosaurs such as Velociraptor and tyrannosaurus) 

    • Upright digitigrade posture 

• Nesting 

  • Dinosaur nests provide more evidence for dinosaurian ancestry 

  • Skeletons from Gobi Desert show dinosaurs sat on nests, like brooding hens 

  • Suggests body heat is warmed eggs- warm blooded? 

• Physiology 

  • Rapid growth rates shared with birds and dinosaurs, but not crocodilians 

  • Implies elevated body temperature 

• Scales 

  • Most dinosaurs were scaly 

  • Birds have scales on their legs and feet, made of keratin 

• Feathers 

  • Feathers have been discovered in carnivorous dinosaurs 

  • Feathers start as down, later modified into airfoils 

• Wings 

  • Microraptor had 4 wings 

birds and dinosaurs

• Birds are just carnivorous dinosaurs, adapted to flight 

• How did flight evolve 

  • trees down hypothesis

  • ground up hypothesis

• Tree climbing 

  • Hand 

    • Long, slender digits like in bats 

    • Claws curved and flattened side-to-side, as in tree-climbing birds and mammals 

  • Foot 

    • First toe turns to oppose other toes 

    • Toe claws moderately curved, as in birds that both walk and perch with the feet 

Trees-down (arboreal) hypothesis 

• Early bird ancestors lived in trees and: 

• Climbed and jumped between branches 

• Began gliding using feathered limbs 

• Gradually evolved powered flight 

• Presence of claws and grasping limbs in early fossils like Archaeopteryx 

• Feathers could have first evolved for insulation/display, later used for gliding 

Ground-up (cursorial) hypothesis 

• Early theropod dinosaurs ran quickly and: 

• Used their forelimbs for balance and lift 

• Began flapping while running 

• Eventually developed powered flight 

• Early theropod dinosaurs ran quickly and: 

• Used their forelimbs for balance and lift 

• Began flapping while running 

• Eventually developed powered flight 

implications for the origins of flight

• Multi-winged design like that of parachuting animals, like flying frogs and flying geckos 

• Suggests birds originated from tree-dwelling animals that extended fore and hindlimbs while parachuting through trees 

• Four winged stage 

  • Tandem biplane, fore and aft airfoils 

  • Doesn't make sense being an ancestral condition for birds as Microraptor was the only animal that had this 

  • Or was it? 

  • There were well developed feather impressions around the Archaeopteryx legs 

  • Suggests that 4 winged design was probably ancestral, and lost in later birds 

birds and the K-Pg extinction

• Birds diversify in cretaceous, but they aren't modern birds 

• They are primitive birds, not closely related to anything living 

• e.g. Enantiorithes 

• No archaic birds after the K-Pg boundary 

• Suggests a mass extinction, but the fossil records are poor, so it is difficult to tell if it is a gradual or catastrophic transition 

• How to test this 

  • A cladistic analysis based on anatomical characters to see if these things have anything to do with modern birds 

• Molecular clocks show rapid radiation 

  • Shows modern birds emerged rapidly after the K-Pg boundary 

  • Around 66-55 million years ago, the lineages leading up to modern bird groups – parrots, songbirds, hawks, doves, ducks, fowl etc appear 

  • Possibly as few as 3 species gave rise to all living birds, suggesting aroung 99.9 % extinction