Comparative Anatomy Notes – Set 3
VERTEBRATE CLASSIFICATION
When did animals live?
- Cenozoic
- Mesozoic
- Paleozoic - oldest time - 600 million years ago
Cambrian Period - 550 million
- Ostracoderms - first vertebrates
- Agnatha - jawless fish
- 1st jawless fish were ostracoderms
- 1 ft in length
- various morphologies
- a lot of bone in skins
- no ancestors are living
- from ostracoderms evolve jawed fish
- modified 1st visceral arch
- 1st jaw fishes arose in Silurian period
Placoderms
- presence of bony exoskeleton (in head region)
- consistent with ostracoderms
- Why bone?
- protection from large invertebrates that lived in sea
- 1st jawed fish were quite large
- hypothesis of function of bone = some fish would move to fresh water to breed
= anadromous
= catadromous - move from fresh water to breed
- theory was due to migration up into freshwater of early fish
- bone supplied calcium for muscle contraction
-fossilized jaw fishes gave rise to all other fishes
- took off in Devonian Period = age of fishes
- cartilagenous fish arose here
Fish as a Whole
- common ancestor gave rise to jawed and jawless fishes
Jawed - Placoderms – Chondrichthyes-Chondrichthyes - lost the bone, except in scales
= Placoid Scales
- teeth are modified sclaes
- sharks, rays, skates
Tail Type
Shark - vertebral column goes dorsally into caudal fin
- most primitive type of tail - Heterocercal Tail
- some bony fish, ostracoderms
Homocercal Tail - vertebral column stops, caudal fin with dorsal and ventral portion
similar - most common
Diphycercal - lungfishes, Polypterus, crossopterygians
- spear shaped
- breakage of placoderms to osteichthyes - bony fish
Ray finned and lobed fin fish
Subclass: Actinopterygii - ray finned
Sarcopterygii - fleshy finned
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Actingopterygii - 3 Superorders
1. Chondrostei - most primitive
- heterocercal tail type
- sturgeon, paddlefish, Polypterus (occurs in Africa)
- sturgeon - modified ganoid scales
- paddlefish - long snout
- Polypterus - bichir
2. Holostei - dominant in past
- heterocercal tail type
- gar, bowfin
- 2 living representatives of this group
- ganoid scales in gar
3. Teleostei - dominant today
- homocercal tail type
- 90% of all fish
Sacropterygii
Lungfish - 3 genera, each occur on separate continents (3)
Neoceratodus - Australia
Protopterus - Africa
Lepidosiren - South America
- due to continental drift
- occur in streams that dry up
- burrowing down in mud, secrets cocoon and stays dormant
- it must have an air tube to breath
- Aestivation - burrowing from hot temperature
Order Dipnoi
Order Crossopterygii – true-lobbed finned fishes
- thought to be extinct
- 1939 living relative was found in S. Africa
-Teleosteans - have clycloid and ctenoid scale, more advanced
- Crossopterygii - leads to coelacanth - Latimeria
- living fossil
- found off coast of S. Africa
- rough skin used to sand boards and wood
- ~15-20 have been photographed
- line leads away to amphibians
- can follow this group to give rise to amphibians
- has lobe-fins
- skeletal make up of fins exhibit homologies similar to earliest mphibians
- skulls of crossopterygii, bones lie with in skin
= exoskeleton (dermal skin)
- hold over from earliest fish
- parietal foramen - match up bones of crossopterygii and
earliest amphibians
- tooth structure of cross section. Have indentations
= labyrinthodont tooth
- earliest amphibian tooth are similar - labyrinthodont
- swamps that fish lived in dried up, limbs evolved as an adaptation
- animals were preadapted to live on land
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- Crossopterygiians gave rise to Labyrinthodonts
- took place in Devonian
- Amphibians Diversify during Carboniferous – Age of Reptiles - Mesozoic
- Divided into Pennsylvanian and Mississippian
Pennsylvanian - upper
Mississippian - Lower
- Labyrinthodonts went in several directions
- some toward reptiles, others to Anura, other Caudata, other Apoda
Apoda - represented by caecilians
- long slim with segmented rings along body
- embedded in skin is tiny dermal bones
- hold over from ancestors
Characteristics of Amphibians
- 1st vertebrates to posses cervical vertebrae (only have 1)
- lost scales of ancestral fishes, except apodans
- primitive frogs has embedded in neck regions tiny dermal scales
- lay anamniotic eggs
- with out extraembryonic membranes
- 3 chambered hearts
- go from larval stage to adult = metamorphosis
- 10 pairs of cranial nerves
- 2 occipital condyles
- articulation point with upper cervical and skull
- Reason there are different lines, structure of vertebrae are different
- Primitive amphibians and primitive reptiles are morphologically similar but physiologically different
REPTILES
- took off in Jurassic of Mesozoic Period = Age of Reptiles
- extinction took place at end of Paleozoic - Placoderms
- labyrinthodont amphibians gave rise to stem reptiles - cotylosaurs
- they gave rise to all reptiles and mammals
- stem reptiles have primitive characteristics representative of class Reptilia
Characteristics of Reptiles
- true claws
- amniotic eggs - amniotes
- major evolutionary step
- no metamorphosis
- 12 pairs of cranial nerves
- single occipital condyle
- All reptiles are diversified by skull types
Another feature of reptiles - Tooth Type
- Thecodont - embedded within jaw bone
- crocodiles
- Pleurodont - tooth along medial surface
- lizard
- Acrodont - tooth attached to surface
- many fish; some lizards
- Sphenodon; and snakes
- turtles have no teeth
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4 skull types
-separates reptiles into 4 subclasses
1. Anapsida - turtles
2. Diapsida - lizzards, snakes, croccs.
3. Synapsida
- Anapsids - skull has temporal fossa region
- turtles lack temporal fossa
- Diapsid - has 2 temporal fossa
- one touches parietal
- one touched jugal
- Synapsid - single temporal fossa, lower region touching jugal
- this skull gives rise to mammal-like reptiles which give rise to mammals
- Major reptiles are Diapsids
- lizards and snakes
Subclass: Anapsida
- Order Testudines
- turtles
Diapsida
- Order Squamata
- lizards and snakes
Sphenodon – tuatara - very lizard like
- Order Crocodillia
- Suborder: Serpentes; Sauria
- morphologically different from lizards
- Ruling Reptiles - toward crocodilian
- dinosaurs
Dinosaurs
- divided into 2 groups
1. Saurischians
2. Ornithischians
- morphological character of pelvis makes these tow distinct
- lizard like - Saurischians - have triradiate pelvis
- bird like - Ornithischians - have tetraradia pelvis
- lizard like gave rise to dominant reptiles
- from lizard like pelvis that the birds evolved
parallelism convergence took place
- problem with Euryapsid skull in fossil records was that they were incomplete
Synapsid - temporal fossa lower, touching jugal
- mammal-like reptiles gave rise to ancient mammals
- fossil record for birds began with Archeopteryx
- feathers present
- had teeth
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