Amniotes and the conquest of the land reptiles Flashcards
What makes the amniotic egg different from other eggs?
Presence of 3 extraembryonic membranes (Chorion, Allantois, Amnion)
What are the two main groups of amniotes?
- Sauropsids
- Synapsids
What group gave rise to the amniotes?
Reptilomorphs
Features of the amniotic egg: Allantois
- Used as a storage for waste (from embryo)
- Vascularised for gas exchange
- Left behind when animal hatches
Features of the amniotic egg: Amnion
- Grows around developing embryo
- Sac filled with amniotic fluid
- Protects embryo
Features of the amniotic egg: Chorion
- Surrounds entire embryo and membranes
- Can become vascularised and used for gas exchange
Features of the amniotic egg: Albumin
- Egg white in chicken egg
- Source of protein and water
- Helps keep the embryo moist
Features of the amniotic egg: Shell
- Can be leathery (ancestral)
- Birds have highly specialised - calcified
- Protection
- Some exchange from outside environment - gas/water
- Eggs can dry out
Features of the amniotic egg: Yolk
Food source / Nutrients
Features of the amniotic egg: Chalaza
- Stringy
- Suspends yolk in albumin
What distinguishes amniotes from non-amniotes?
Amniotic egg
What groups are amniotes?
Birds, Reptiles and Mammals
What groups are non-amniotes?
Fish & Amphibians
What are the advantages of an amniotic egg?
- Increased surface area for gas exchange
- Shell provides support
- Still unsure why the amniotic egg evolved
What type of fertilisation is required?
Reproduction of amniotes
Internal Fertilisation
Reproduction of amniotes
- Most males have intromittent organs / penis (vast majority of birds don’t)
- Females have clitorises/hemiclitores (snakes)
- Link between male and female sex organs
- Environmental sex determination - possibly ancestral.
- No larval stage (direct development inside egg) - has to be laid on land
What are the skeletal characters of amniotes
Derived features of amniotes
- Axis and atlas - head rotation - vertebrae found in the neck.
- Modified in the amniotes (C1 and C2). cervical neck vertebrae
Reduced skin permeability
Derived features of amniotes
- Thicker, keratinised, more lipids
- Skin elaborations: scales, hair feather (all homologous - formed from epidermal placode.)
- Alpha keratin found in all amniotes
- Beta keratin only found in sauropsids
What is Costal (rib) Ventilation?
Derived features of the amniotes
- Use ribs to move air in and out of lungs
- Allows air to be drawn over a further distance - allows development of longer neck
- More complex nerves controlling forelimb (due to longer neck)
What is temporal fenestration?
Derived features of the amniotes
- Holes in the skull
- Synapsid skull (one arch) / one hole
- Diapsid (two arches) / two holes
- Can have modified diapsid skulls - secondary adaptation - arches lost (commonly snakes)
How is the modern mammal skull modified?
- Holes have combines to allow space for muscles
- Muscles attach upper and lower parts of skull behind cheekbones.
How do amniotes and non-amniote skull shapes differ?
- Non-amniotes had flat skulls - buccal pumping, abductor muscles allowed for opening and closing of mouth.
- Amniotes - smaller domes shaped skull - differentiation of abductor muscle into two muscles - allowed for sophisticated movement of jaw.
How did the amniote skull change?
- Muscles moved to attachment on top of skull (not on the inside roof)
- Allowed enlargement of muscles
- Enlargement of hole allowed for larger muscles
Types of skull fenestra
- Anapsid (no hole)
- Synapsid (1 hole)
- Diapsid (2 holes)
Example of an Diapsid
Crocodilian
Example of a synapsid
Mammals
Example of an anapsid
Turtle although they are actually diapsid but have lost the holes
What are the sauropsids?
- Turtles diapsid but look anapsid (sister group to archosaurs)
- Tuatara, snakes and lizards - lepidosaurs
- Crocodiles, dinosaurs, pterosaurs, birds – archosaurs
- Archosaurs diverged from lepidosaurs in Permian
- lizards, snakes, turtles, crocodiles, and birds.
Turtles and tortoises (Chelonia)
Sauropsids
- Very derived group
- Body made up of carapace
- Vertebrae and ribs are fused to carapace
- Keratin scutes on surface
- No teeth - keratin beak-like structure - ancestral forms had teeth
- Flexible neck
- Plastron
How do Chelonia breathe?
- Issues with breathing
- Movement of ribs restricted because they are fused to carapace
- Contraction of muscles causes gut moves down
- Lungs attached to gut via connective tissues
- As gut is pulled down - lungs are moved down - inflated - opposite
- Aquatic turtles use hydrostatic pressure also use pharynx & cloaca for gas exchange
How does the chelonia carapace vary between terrestrial and aquatic forms?
- Most terrestrial chelonia have domed carapace
- Most aquatic forms have flattened / streamlined carapace
The green turtle
- Herbivorous
- Feed and nest far apart
- Massive migrations
- Chemosensory info - light and wave direction / magnetism
- Hatchlings crawl to the brightest light
- Light over sea is brighter than light over land.
- Issues with light pollution.
How did chelonia evolve?
- Debate as to where group developed? Water or land? May have evolved in aquatic environment - debate to this.
- Odontochelys - ancestral type - carapace was not well developed so support for aquatic development.
What are some features of Tortoise mating?
Head bobbing
Fighting
Biting / ramming
Cloaca - waste excretion and reproductive opening
What is environmental sex determination?
- Sex outcome of offspring is temperature dependent
- E.g. females only occur at higher temps in turtles (opposite in lizard example)
- Theory - higher temp sex = larger sized sex
- Conservation implications
What happened to Kemp’s ridley turtle population due to environmental sex determination ?
- Conservationists were trying to re-establish population of turtles
- They didn’t know about environmental sex determination
- Increased incubation temps and thus increased female total
- boosted population with more females to lay eggs
What groups are lepidosaurs?
Snakes and lizards
Tuatara: Sphenodonta adaptations
- New zealand (only found in islands)
- Nocturnal - low body temp (counterintuitive)
- Live in burrows with sea birds - waste produced by birds attracts arthropods that tuatara feed on
- Active at night - possible adaptation for tuatara being nocturnal
What is different about the Tuatara skull?
- Diapsid skull - lower bar is a secondary addition - lost and reformed
- 2 rows of teeth on upper jaw
- 1 set on lower jaw that fits in between two top
what are Squamates?
Lizards and snakes (not including the tuatara)
How are snakes and lizard skulls modified?
- Modified diapsid skull
- Lizards bottom bar is lost
- Snakes both bars are lost
How are squamates skulls kinetic?
- flexible
- Wide gape
- Not a powerful bite
What are Amphisbaenians?
- Clade of lizards that are adapted for burrowing
- Reduction or loss of limbs
- Akinetic skull
- Inflexible skull used for burrowing
- Skull doesn’t move at all
What size are the majority of lizards?
80% are less than 80g
What benefit is being small for lizards diet?
- Can feed on insects
- Large lizards tend to me herbivores
- Exceptions - monitor lizards
Komodo Dragon
- Venom glands
- Lowers prey blood pressure
- Also contains anticoagulant
- Bacteria theory is disproved
How are fast moving lizards adapted to their locomotion and ventilation conflict?
- Same muscles used for running are used for ventilation
- Gular pumping - use of throat as an adaptation to pump air into lungs
What is gular pumping in fast moving lizards?
- An adaptation to the conflict between locomotion and ventilation
- Use throat for lung ventilation when moving fast
- Pump air into lungs
How are Aboreal lizards adapted to this lifestyle ?
- Zygodactylous - opposable toes
- Prehensile tail
- Eyes move independently of each other
- Project tongue
- E.g. Chameleon
How have legless lizards evolved?
- Evolved many times in lizards
- Burrowing (short tails)
- Surface dwellers (long tails)
When did snakes branch off from lizards ?
Thought to have branched in the cretaceous period
What is the Digging Phase Theory of snakes ?
- lizards became subterratean and evolved into snakes
- Early snakes are thought to be subterranean
- Thoguht that they almost lost their eyes when they were subterranean and reformed them when they came back up to the surface
- Differences in snake and lizard eyes
- E.g. lizards tend to have eyelids whereas snakes dont
Structure of a snake skeleton
- Few neck and few tail vertebrae
- Most of the animal is thorax
- Thorax contains ribs
- Lack pectoral and pelvic girdles
Why do snakes have so much thorax?
- Change in gene expression leads to more thoracic vertebrae - Oct4. If this gene is altered in mice they end up with more thoracic vertebrae.
- Leglessness : Protein ‘sonic hedgehog’ (Shh) stimulates limb formation. Controlled ZRS gene
- ZRS mutated in snakes
What does the fossil Eupodophis show
Snake evolution
Fossil shows back but no front legs
What does the fossil Tetrapodophis show?
Snake evolution
- Four small limbs
- Debate as to whether is an actual snake
- Features suggest it burrowed
- Fossil discovered in a collection by accident (named unknown fossil)
How are snakes adapted for feeding?
- Very flexible jaw
- Many points of articulation
- Don’t dislocate jaws (myth)
- Teeth at back on throat pull prey along
- Swallow small prey live
How are snakes adapted for specialised feeding habits?
- Egg feeders have specialised vertebrae for crushing eggs once swallowed whole
- Mollusc eaters can hook out mollusc with teeth
Constriction vs Venom (snakes)
Way of subduing prey
- Venom immobilises prey
- Constriction reduces blood flow to organs of prey.
- Constrictors can detect heartbeat of prey and know when heart has stopped and they can stop squeezing
What groups of animals are Archosaurs?
Alligators, Gharials and crocodiles (crocodylia)
How are crocodilian teeth placed?
Thecodont (teeth fixed in sockets)
What is the secondary palate of crocodilians?
- Separates nasal passages from mouth
- Allows breathing through nostrils when in water whilst mouth is under water
- Gular valve means can open mouth underwater and valve stops water flowing into throat
What are the pressure receptors in crocs called and what are they used for?
- Integumentary sensory organs
- Very sensitive
- Use jaws for manipulation as limbs are very small and can’t use well
- Use jaw for parental car - moving offspring and helping hatch
- Use jaw like we’d use our hands - needs to be sensitive
How do some crocs use lures to catch prey ?
Some use sticks to lure birds
What skull formation do crocodilians have?
Diapsid
How do archosaurs move?
- Swim, crawl, walk & gallop
- Ungulate body for swimming using tail
what do archosaurs use their liver for?
able to move liver to ventilate lungs
Other notes of archosaurs
- Head shape varies
- Use sound in social behaviour (territories, mating, parental care)
- Lay eggs - Environmental sex determination
- Play behaviour - cost energy but has no advantage
