Rise of the Amniotes and Biology of the Squamates Flashcards
What are amniotes?
- tetrapod clade defined by possession of an amniotic egg
- key derived character modified in various ways but has never been lost
In addition to the amniotic egg, all crown-group amniotes possess:
- internal fertilisation
- a keratinised integument and integumentary appendages
- a ventilation mechanism involving a negative pressure aspiration pump
- none has been completely lost
amniotic egg innovated after
amphibians
Amniota comprise two great clades:
- Synapsida
- Diapsida
Synapsida
all extant forms are mammals
Diapsida
extant members comprise Archosauria and the various Lepidosauria.
Archosauria
birds & crocodiles and their sister group the turtles,
Lepidosauria
- snakes, “lizards”, amphisbaenians, tuatara
- all extant forms members of the Squamata, except tuatara
Squamata
snakes, “lizards”, and amphisbaenians.
Describe the amniotic egg
- 3 extra-embryonic membranes absent from non-amniotes
- amnion (and amniotic cavity)
- allantois
- chorion (and chorionic cavity)
- yolk sac
extra-embryonic membranes
membranes derived from the embryo, but external to it
amnion
cushions the embryo in the buffered aqueous environment of the amniotic cavity
allantois
- heavily vascularized outgrowth of the gut
- used to store nitrogenous waste, and for
gas exchange - forms part of the umbilical cord in mammals
chorion
- envelopes the egg
- co-joins the allantois in late development to function in gas exchange
- forms the foetal part of the placenta in mammals
yolk sac
- extra-embryonic membrane
- present in non-amniotes too
Describe the adaptive significance of the amniotic egg
- four extra-embryonic membranes
- allantois = key innovation
Describe the innovation of the allantois
- provides a large surface area for exchange of gases and waste that relaxes diametric constraint
- chorion and the surrounding shell makes it possible to seal off the egg in dry environments
- requires a location to safely store nitrogenous waste, which the sac of the allantois provides.
Describe shell formation in the amniotic egg
- following the formation of the extra-embryonic membranes, the mother’s oviduct secretes a protein rich aqueous albumen and lays one or more shell membranes down around this
- eggshell (if present) is then laid down in the oviduct around the shell membrane(s)
Describe the evidence for the evolution of viviparity
- ancestral state for amniotes is to lay leathery eggs without a hard shell (do not fossilise easily)
- earliest (indirect) fossil evidence of amniotic reproduction from evidence of live-bearing of offspring in a Permian mesosaur
mesosaur
marine reptile
Describe the evolution of viviparenty
- evolved multiple times in amniotes
- facilitated by internal fertilisation
- makes use of the extra-embryonic membranes to provide a physiological interface between mother and offspring (typically through the growth of a chorio-allantoic placenta)
Describe the specifics of the Lepidosauria
- almost all of which today are members of the clade Squamata
Squamata
snakes, “lizards”, and amphisbaenians.
Describe the Squamata
- > 9000 extant species
- wide range of body forms
- convergent evolution
Give some convergently evolved characters within the Squamata
- reduction or loss of limbs
- evolution of viviparity
- wings for gliding flight
Descibe extinct Squamates
during the Late Cretaceous, seas were dominated by mosasaurs
mosasaurs
- predatory marine squamates
- thought to be the sister group to snakes
- heterocercal tail fin (convergent with ichthyosaurs and sharks)
- largest were >10 m long.
Give an example of a mosasaur
Prognathodon
Describe endothermy
- high rate of aerobic metabolism provides a significant source of heat, even at rest
- must elevate MR to maintain a constant Tb in the face of high or low Ta
Describe extant Squamate thermoregulation
- all ectotherms
- external environment is main source of body heat
- low metabolic energy demands
Describe ectothermy
- Tb and MR both increase with increasing Ta
- typical of animals with a low resting metabolic rate
Among living squamates, endothermy only occurs…
facultatively in female pythons whilst brooding eggs in their coils
Among extinct squamates, bone isotope evidence suggests that the
marine mosasaurs were obligate endotherms (Harrell et al., 2016).
MR
metabolic rate
Tb
body temperature
Ta
ambient temperature
ability to thermoregulate effectively is therefore expected to be under
strong selection in ectotherms.
Body temperature affects
performance
List some factors regarded as ‘performance;
- sprint
- endurance
- RMB
- aerobic scope
- rate of digestion
- efficiency of digestion
What facilitates behavioural thermoregulation?
many different flows of thermal energy in the environment
Describe thermoregulation in horned lizard, Phrynosoma cornutum
- AM: positive orientation to sun, ribs spread
- PM: negative orientation to sun, ribs compressed
Describe thermoregulation in the Andean lizard, Liolaemus signifer
- emerges from its burrow at dawn
- ambient temperature -5 ̊C
- basking on heaps of vegetation insulates from the ice
What does behavioural thermoregulation rely on?
modifying environmental energy flows related to the solar cycle
What is the consequence of ectothermy, and consequent behavioural thermoregulation?
lifestyles of terrestrial ectotherms are intermittent
Describe the advantages of ectothermy
- cheap energetically
- less constrained than endotherms in shape and size
- allows occupancy of low energy environments such as deserts
What places the hard lower limit on the body size of endoderms?
rapid increase in mass-specific BMR with decreasing body mass
What allows ectotherms to achieve smaller body size?
lower mass-specific BMR
Describe the feeding physiology and behaviour of Squamates
- lower mass-specific BMR of ectotherms makes the conversion of biomass >10 times more efficient than in endotherms
- high biomass conversion efficiency and low absolute energy requirements allows squamates to thrive in habitats with very low primary production, such as deserts
- eat much less food than a similarly sized endotherm
- can do so much less frequently on account of their intermittent lifestyle
Compare and contrast ectothermic with endothermic biomass conversion
- Panamanian anole Anolis limifrons: 23-28% efficiency
- Field mouse Peromyscus polionotus: 1.8% efficiency
Which Squamate characteristics facilitate dry lifestyles
- amniotic egg
- keratinised skin
Describe ambush predation in snakes
- sit-and-wait and ambush modes of predation are common both on land and in water
- associated with camouflage
- use of lures
List some ambush predators
- Spider-tailed horned viper, Pseudocerastes urarachnoides
- yellow sea snake Hydrophis platurus xanthos
Describe Feeding behaviours in snakes
- all snakes are predatory
- ancestral mode of predation: constriction
Describe Anaconda Eunectes murinus
- extant
- 250kg
- takes prey as large as pigs, capybara and tapirs
Describe giant Titanoboa
- extinct
- > 1000 kg
- 13m in body length
- facilitated by high equatorial temperatures in the Palaeocene, requiring an estimated 30-34 ̊C.
- fed on crocodyliforms
Describe snake venoms
- proteins are extremely complex
- neurotoxins, myotoxins, haemorrhagins and haemolysins
- toxins evolved by gene duplication of digestive enzymes
- begin the digestive process
List some venomous snakes
- Fer-de-lance, Bothrops asper
- Russell’s viper, Daboia russelii
Describe snakes and humans
- snakebite 100,000 deaths each year
- 300,000 amputations and permanent disabilities (WHO, 2019).
Describe the vomeronasal organs
- specialised sensory systems
- forked tongue used to transfer chemicals from outside to the paired vomeronasal organs in roof of mouth for olfactory chemosensation
What is the advantage of strike-and-release predation?
- reduces the risk of injury
- exploits chemical tagging of envenomated prey for subsequent tracking using specific venom proteins (Saviola et al., 2013).
Give a strike-and-hold clade
elapids
Give a strike-and-release clade
pit vipers
Describe bimodal snake predation
strike-and- hold or strike-and-release
Describe pit organs and analogous organs
- specialised sensing systems
- detecting prey
- specialised infrared sensors
- heat-sensitive ion channels
- an orthologue of TRPA1 “wasabi receptor” of humans expressed at unusually high levels in the TG, innervating the pit organs
- evolved several times independently
List some pit organned clades
- pit vipers
- boids and pythons
TG
trigeminal ganglia
Describe tentacles
- specialised sensory system
Describe Erpeton tentaculatum
- tentacled snake
- appendages operate as mechanosensors, detecting water movement
- when hunting, the snake forms a cryptic J-shape, striking when the fish is between its head and body
- feint of the body before the strike causes the fish to C-start straight into the snake’s jaws
Describe goo-eaters
specialised ingestion mechanism
Snakes that eat occasional, large or difficult meals, often display
specific adaptations for ingestion and digestion.
Describe Dipsas albifrons
- snail-eating snake
- specialises on molluscs and annelids
- uses protein secretions to assist in swallowing viscous prey
Describe Dasypeltis fasciata
- egg-eating snake
- swallows bird eggs four times its normal head diameter
- cranial kinesis
Describe cranial kinesis
- relative movement of skull parts, exclusive of the jaw
- ancestral character of tetrapods (lost in mammals, taken to extreme in snakes)
Describe cranial kinesis in snakes
- reduction of the skull frees squamosal-quadrate articulation
- allows lower jaw to swing forward
- movable palatopterygoid erects fang-bearing maxilla
- L and R sides of lower jaw only joined by a ligament
- allows them to separate and ratchet over food
Describe upregulation
- specialised digestion mechanism
- some larger snakes feed so infrequently that they must enlarge their digestive system to accommodate large meals
Describe Burmese pythons Python bivittatus
- small intestine mass increases by 40% in day or two after feeding
- size reduced again later
- ventricular muscle mass may also increase up to 40% within 2 days of feeding
Describe enlargement
- specialised digestion mechanism
- temporary enlargement of the small intestine accommodated by temporary enlargement of cells that make up the epithelial tissue
- intracellular lipid droplet deposition
Give an enlarging snake
garter snakes, Thamnophis sirtali
Describe Squamate metabolism
- by increasing their anaerobic metabolism, squamates achieve a similar total metabolism to mammals
- only in short bursts
- only with long recovery time between bursts
Describe the Basilisk lizard, Basiliscus
- can run on water
- foot slaps water
- upper surface of foot at air pressure; lower surface at pressure of water at depth
- foot withdrawn before
air cavity collapses to avoid too much drag - tail provides thrust
