L10&11- Amniotes and Reptiles

2 main groups of non-amniote tetrapods:

1. Batrachomorphs→ gave rise to the amphibians

2. Reptilomophs→ gave rise to the amniotes

The diversification of the tetrapods:

• 1st diversification was to form the reptilomorphs

  • driven by diversification of insects, food source

• 2nd diversification was to form the two main groups of amniotes

  • Synapsids→ gave rise to the mammals

  • Sauropsids→ gave rise to the reptilian groups

  • spilt ~355 mya, end Carboniferous-Permian

Phylogeny:

Early Amniotes varied in size but are linked by common features→ Amniotic Egg:

Distinguishes amniotes (reptiles, birds, mammals) from non-amniotes (fish, amphibians)

• Has 3 extraembryonic membranes→ from the embryo itself

  • Allantois→ waste storage, vascularised (gas exchange), is left

  • Amnion→ grows around embryo, sac has amniotic fluid, protection

  • Chorion- surrounds all contents, can be vascularised too

• Albumin→ surround chorion, source of protein and water

• Shell→ surrounds albumin, added from female tract later on

• Yolk sac→ food source

  • Germinal disc→ where embryo is fertilised

  • Chalaza→ keeps yolk in position

• internal fertilisation

• most males have intromittent organs and females have clitorises/hemiclitores

• the ancestral forms determined sex based on environmental conditions (seen in many groups)

• egg has to be laid on land

The extraembryonic membranes in egg-layers and non-egg layers are homologous:

Theories of why extraembryonic membranes evolved:

• Increases SA for gas exchange

• Having a shell provides support

• Can lead to larger eggs and larger adults

Derived Features of Amniotes:

1. Skeletal characters e.g.

   • Cervical vertebrae (in the neck) have become modified

   • Axis (C2) has become enlarged

   • Is connected to Atlas (C1) that is also connected to the skull

     → are able to rotate head

2. Skin

   • reduced permeability, thicker, keratinised, more lipids

     • all amniotes have alpha keratin, sauropsids have beta keratin (stronger)

     → don’t need to rely on water like amphibians

   • have scales, hair, feathers, formed from epidermal placode

     → all homologous, just different gene expressions

3. Ventilation

   • have costal ventilation→ use rib movements for ventilating lungs

   • allows air to be drawn in a longer distance→ have longer necks

   • longer necks allow more complex nerves to control forelimbs

4. Temporal fenestration

   • presence of holes in the skull

   • anapsid→ no holes

   • synapsid→ have 1 hole and 1 bar (arch) underneath

   • diapsid→ have 2 holes and 2 bars

     • can be modified→ lower arch is lost in lizards, both lost in snakes

   • synapsids had one hole but sauropods did not have holes when they diverged→ has evolved independently in synapsids and sauropsids

   • is highly modified→ orbit has combined with the hole in modern-day mammals
     

Why do amniotes have holes in their skull:

• Non-amniotes→ can only do buccal pumping (mouth sucking to feed) so have flat skulls, has 1 unmodified muscle (adductor muscle)

  → could only open and close mouth

• Amniotes→ can do costal ventilation so have dome-shaped skulls, muscle has differentiated to 2 muscles

  → can open and close mouth and also apply pressure

How did the holes form:

• Non-amniotes→ muscle is attached to the inside of the skull

• Amniotes→ muscles are attached to the top of the outside of the skull through gaps in the bones, allows muscle enlargement and an improved jaw

Holes in the phylogeny:

• Turtles look anapsid but are actually diapsid (holes are just covered)

• Turtles are sister group to the Archosaurs

• Archosaurs are the crocodiles, birds, pterosaurs and dinosaurs

• Lepidosaurs are the tuatara (sphenodon), snakes and lizards

  • these two groups diverged in the Permian

Turtles/Tortoises (Chelonia):

Structure:

• Carapace→ top, Plastron→ bottom

  • made of bone, covered in beta keratin→ can form scutes (pattern)

• The Vertebrate and Ribs are fused to the carapace

• Ancestors had teeth but modern-day have a keratin beak instead

• Have a flexible and retractable neck→ can retract forward or sideways (side-necked/snake-necked turtle)

Ventilation:

• Cannot do costal ventilation as ribs are connected to the carapace

• Instead connective tissue holds lungs and gut together, so:

  • inspire→ contract side muscles, gut moves down, moves lungs down too, increase in volume, take air in

  • expire→ muscles contract, push gut up, lungs move up too, decrease in volume, release air

• Can be modified:

  • aquatic turtles use the hydrostatic pressure of the water too

  • some use a vascularised pharynx or a vascularised cloaca

There is variation in body forms:

• are all egg layers, on land

• most are carnivorous (sometimes mistake plastic as jellyfish)

• green turtles are herbivorous

  • have huge migrations→ feed in one area, nest far away

  • hatchlings crawl to the sky over sea light (brighter than sky over land)→ go out to the ocean

  • use chemosensory info to pick nesting site (light, wave direction, magnetism) and are site-faithful
    

Have many mating behaviours:

• males have to mount the females, can be kicked off

• bobbing→ signal to work out who is who

• male hooks tail underneath female

• both have a cloaca→ reproductive opening that also excretes waste

Environmental Sex Determination:

• temperature determines sex

• females are formed when temperature is high in turtles

• females are formed when temperature is low in lizards

  → theory is that the sex at the higher temperature is the larger sex

• can have implications for conservation, e.g. Kemp’s Ridley turtles:

  • environmental sex determination was discovered in 1985

  • tried to establish another population before this but had few females

  • realised needed to increase the temperature→ got more females

Lepidosaurs:

• Tuataras→ Sphenodonta

• Lizards and Snakes→ Squamates

Tuatara:

• are found on islands, have spines on their back

• are nocturnal but have a low body temp and are still active

• live in burrows with seabirds

  • mess of seabird nest attracts arthropods→ feed on these

  • arthropods are active at night→ may be why tuatara are nocturnal

• have a diapsid skull

• have 2 rows of teeth on the upper jaw and 1 set of teeth on the lower jaw that fits in between→ manipulate food

Jaws of Lepidosaurs:

• Tuatara→ classically diapsid skull

  • the lower bar was lost and then reformed again (secondary addition)

• Lizards→ bottom bar has been lost and not reformed

  • have an enlarged hole but still 2 holes

• Snakes→ top and bottom bar have been lost and both have not reformed

  • have 1 big hole

    
    

• Snake and lizard skulls are kinetic

  • allows a very flexible jaw, not as strong bite force though

  • an exception is the Amphisbaenian→ does not move jaw at all as it uses its head for burrowing into the ground

Lizards:

• 80% are <20g→ are very small to feed on insects

• Larger lizards are mostly herbivores

  • Exception→ Monitor Lizards e.g. Komodo Dragon/Monitor:

    • feeds on large prey

    • has venom glands→ lower blood pressure + anticoagulant

    • move fast using gular pumping

• Lizards move by undulating body but use the same muscles for costal ventilation→ creates a conflict for locomotion and ventilation

  • some have adapted gular pumping to counter this→ use throat to pump air into lungs instead of ribs

• Many are arboreal (live in trees)

  • e.g. Chameleon, have adaptations for this:

    • Zygodachtylous→ join digits together to form an opposable toe for grip

    • Prehensile tail for grip

    • Eyes can move independently and also focus together too

    • Have a fast tongue projection

• Have evolved limb reduction >60 times

  • is an adaptation for moving

  • surface dwellers (in high vegetation) have long tails

  • burrowers have short tails

Snakes:

• Are very specialised legless lizards

  • branched from lizards in the Cretaceous

  • main theory is that a group of lizards started digging, almost lost eyes, came back up, reformed eyes→ snakes have no eyelids

• Have few neck and tail vertebrae, most of animal is thorax

  • caused by changes in gene expression in developmental genes:

    • Oct4 genes→ get lots of thoracic vertebrae and less limbs

    • Shh protein stimulates limb formation, is controlled by ZRS protein→ ZRS is mutated so there is less ZRS, less Shh, less limb formation

    → all leads to elongation of the body and a reduction in limbs

• lack pectoral and pelvic girdles

  • some species have vestigial (tiny) ones e.g. pythons and boas, used for mating (not locomotion)

• fossil forms had back legs but no front legs→ lost front legs first

• another fossil form had 4 small limbs and has features suggesting burrowing→ is debated if this was a snake or not though

• Elongation of body lead to a reduction in diameter but have a very flexible jaw with lots of points of articulation to eat a wide variety of prey

• Have backward facing teeth→ push prey back into oesophagus

• Feed live

  • Have specialised methods→ some swallow whole, some have teeth to feed

  • Subdue prey by either constriction (reduce blood flow to organs) or venom (use fang to immobilise prey)

Archosaurs→ Crocodylia:

1. Alligators

2. Gharials

3. Crocodiles

• Have a classically diapsid skull→ 2 holes

• Have thecodont teeth→ teeth in sockets

• Have a secondary palate (like mammals):

  • separates nasal passage from the mouth

  • allows breathing through nostrils when swimming

  • also have a gular valve→ stops water going down the trachea

• Are semi-aquatic and primarily aquatic predators

• Have integumentary sensory organs around jaw/face/body

  • are very sensitive pressure receptors

  • use jaw instead of forelimbs to detect prey and parental care

• Can use lures to attract prey e.g. putting sticks on top of themselves to attract birds and then catch them

• There are 26 living species but there used to be a lot more, e.g.:

Features of Crocodilians:

1. Varied head shape

   • believed to be associated with diet→ Gharial feeds on fish and False Gharial feeds on lots of prey but both have slender snout

     → is not associated with diet

2. Can swim (tail), crawl (drag body), walk (lift body) and gallop (fast)

3. Use liver movement to ventilate lung

   • adaptation to moving and ventilation conflict

4. Use sound for social behaviour (like birds)

   • e.g. territories, mating, parental care

5. Lay eggs

   • using environmental sex determination

6. Show extensive parental care

   • e.g. guarding nest, transferring in mouth, looking after in crest

7. Show play behaviour

   • costs energy but has no advantage