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Hilary Term 1 > Rise of the Amniotes and Biology of the Squamates > Flashcards

Rise of the Amniotes and Biology of the Squamates Flashcards

1
Q

What are amniotes?

A
  • tetrapod clade defined by possession of an amniotic egg
  • key derived character modified in various ways but has never been lost
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2
Q

In addition to the amniotic egg, all crown-group amniotes possess:

A
  • internal fertilisation
  • a keratinised integument and integumentary appendages
  • a ventilation mechanism involving a negative pressure aspiration pump
  • none has been completely lost
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3
Q

amniotic egg innovated after

A

amphibians

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4
Q

Amniota comprise two great clades:

A
  • Synapsida
  • Diapsida
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5
Q

Synapsida

A

all extant forms are mammals

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6
Q

Diapsida

A

extant members comprise Archosauria and the various Lepidosauria.

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7
Q

Archosauria

A

birds & crocodiles and their sister group the turtles,

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8
Q

Lepidosauria

A
  • snakes, “lizards”, amphisbaenians, tuatara
  • all extant forms members of the Squamata, except tuatara
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9
Q

Squamata

A

snakes, “lizards”, and amphisbaenians.

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10
Q

Describe the amniotic egg

A
  • 3 extra-embryonic membranes absent from non-amniotes
  • amnion (and amniotic cavity)
  • allantois
  • chorion (and chorionic cavity)
  • yolk sac
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11
Q

extra-embryonic membranes

A

membranes derived from the embryo, but external to it

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12
Q

amnion

A

cushions the embryo in the buffered aqueous environment of the amniotic cavity

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13
Q

allantois

A
  • heavily vascularized outgrowth of the gut
  • used to store nitrogenous waste, and for
    gas exchange
  • forms part of the umbilical cord in mammals
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14
Q

chorion

A
  • envelopes the egg
  • co-joins the allantois in late development to function in gas exchange
  • forms the foetal part of the placenta in mammals
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15
Q

yolk sac

A
  • extra-embryonic membrane
  • present in non-amniotes too
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16
Q

Describe the adaptive significance of the amniotic egg

A
  • four extra-embryonic membranes
  • allantois = key innovation
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17
Q

Describe the innovation of the allantois

A
  • 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.
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18
Q

Describe shell formation in the amniotic egg

A
  • 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)
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19
Q

Describe the evidence for the evolution of viviparity

A
  • 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
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20
Q

mesosaur

A

marine reptile

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21
Q

Describe the evolution of viviparenty

A
  • 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)
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22
Q

Describe the specifics of the Lepidosauria

A
  • almost all of which today are members of the clade Squamata
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23
Q

Squamata

A

snakes, “lizards”, and amphisbaenians.

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24
Q

Describe the Squamata

A
  • > 9000 extant species
  • wide range of body forms
  • convergent evolution
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25
Q

Give some convergently evolved characters within the Squamata

A
  • reduction or loss of limbs
  • evolution of viviparity
  • wings for gliding flight
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26
Q

Descibe extinct Squamates

A

during the Late Cretaceous, seas were dominated by mosasaurs

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27
Q

mosasaurs

A
  • predatory marine squamates
  • thought to be the sister group to snakes
  • heterocercal tail fin (convergent with ichthyosaurs and sharks)
  • largest were >10 m long.
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28
Q

Give an example of a mosasaur

A

Prognathodon

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29
Q

Describe endothermy

A
  • 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
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30
Q

Describe extant Squamate thermoregulation

A
  • all ectotherms
  • external environment is main source of body heat
  • low metabolic energy demands
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31
Q

Describe ectothermy

A
  • Tb and MR both increase with increasing Ta
  • typical of animals with a low resting metabolic rate
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32
Q

Among living squamates, endothermy only occurs…

A

facultatively in female pythons whilst brooding eggs in their coils

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33
Q

Among extinct squamates, bone isotope evidence suggests that the

A

marine mosasaurs were obligate endotherms (Harrell et al., 2016).

34
Q

MR

A

metabolic rate

35
Q

Tb

A

body temperature

36
Q

Ta

A

ambient temperature

37
Q

ability to thermoregulate effectively is therefore expected to be under

A

strong selection in ectotherms.

38
Q

Body temperature affects

A

performance

39
Q

List some factors regarded as ‘performance;

A
  • sprint
  • endurance
  • RMB
  • aerobic scope
  • rate of digestion
  • efficiency of digestion
40
Q

What facilitates behavioural thermoregulation?

A

many different flows of thermal energy in the environment

41
Q

Describe thermoregulation in horned lizard, Phrynosoma cornutum

A
  • AM: positive orientation to sun, ribs spread
  • PM: negative orientation to sun, ribs compressed
42
Q

Describe thermoregulation in the Andean lizard, Liolaemus signifer

A
  • emerges from its burrow at dawn
  • ambient temperature -5 ̊C
  • basking on heaps of vegetation insulates from the ice
43
Q

What does behavioural thermoregulation rely on?

A

modifying environmental energy flows related to the solar cycle

44
Q

What is the consequence of ectothermy, and consequent behavioural thermoregulation?

A

lifestyles of terrestrial ectotherms are intermittent

45
Q

Describe the advantages of ectothermy

A
  • cheap energetically
  • less constrained than endotherms in shape and size
  • allows occupancy of low energy environments such as deserts
46
Q

What places the hard lower limit on the body size of endoderms?

A

rapid increase in mass-specific BMR with decreasing body mass

47
Q

What allows ectotherms to achieve smaller body size?

A

lower mass-specific BMR

48
Q

Describe the feeding physiology and behaviour of Squamates

A
  • 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
49
Q

Compare and contrast ectothermic with endothermic biomass conversion

A
  • Panamanian anole Anolis limifrons: 23-28% efficiency
  • Field mouse Peromyscus polionotus: 1.8% efficiency
50
Q

Which Squamate characteristics facilitate dry lifestyles

A
  • amniotic egg
  • keratinised skin
51
Q

Describe ambush predation in snakes

A
  • sit-and-wait and ambush modes of predation are common both on land and in water
  • associated with camouflage
  • use of lures
52
Q

List some ambush predators

A
  • Spider-tailed horned viper, Pseudocerastes urarachnoides
  • yellow sea snake Hydrophis platurus xanthos
53
Q

Describe Feeding behaviours in snakes

A
  • all snakes are predatory
  • ancestral mode of predation: constriction
54
Q

Describe Anaconda Eunectes murinus

A
  • extant
  • 250kg
  • takes prey as large as pigs, capybara and tapirs
55
Q

Describe giant Titanoboa

A
  • extinct
  • > 1000 kg
  • 13m in body length
  • facilitated by high equatorial temperatures in the Palaeocene, requiring an estimated 30-34 ̊C.
  • fed on crocodyliforms
56
Q

Describe snake venoms

A
  • proteins are extremely complex
  • neurotoxins, myotoxins, haemorrhagins and haemolysins
  • toxins evolved by gene duplication of digestive enzymes
  • begin the digestive process
57
Q

List some venomous snakes

A
  • Fer-de-lance, Bothrops asper
  • Russell’s viper, Daboia russelii
58
Q

Describe snakes and humans

A
  • snakebite 100,000 deaths each year
  • 300,000 amputations and permanent disabilities (WHO, 2019).
59
Q

Describe the vomeronasal organs

A
  • specialised sensory systems
  • forked tongue used to transfer chemicals from outside to the paired vomeronasal organs in roof of mouth for olfactory chemosensation
60
Q

What is the advantage of strike-and-release predation?

A
  • reduces the risk of injury
  • exploits chemical tagging of envenomated prey for subsequent tracking using specific venom proteins (Saviola et al., 2013).
61
Q

Give a strike-and-hold clade

A

elapids

62
Q

Give a strike-and-release clade

A

pit vipers

63
Q

Describe bimodal snake predation

A

strike-and- hold or strike-and-release

64
Q

Describe pit organs and analogous organs

A
  • 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
65
Q

List some pit organned clades

A
  • pit vipers
  • boids and pythons
66
Q

TG

A

trigeminal ganglia

67
Q

Describe tentacles

A
  • specialised sensory system
68
Q

Describe Erpeton tentaculatum

A
  • 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
69
Q

Describe goo-eaters

A

specialised ingestion mechanism

70
Q

Snakes that eat occasional, large or difficult meals, often display

A

specific adaptations for ingestion and digestion.

71
Q

Describe Dipsas albifrons

A
  • snail-eating snake
  • specialises on molluscs and annelids
  • uses protein secretions to assist in swallowing viscous prey
72
Q

Describe Dasypeltis fasciata

A
  • egg-eating snake
  • swallows bird eggs four times its normal head diameter
  • cranial kinesis
73
Q

Describe cranial kinesis

A
  • relative movement of skull parts, exclusive of the jaw
  • ancestral character of tetrapods (lost in mammals, taken to extreme in snakes)
74
Q

Describe cranial kinesis in snakes

A
  • 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
75
Q

Describe upregulation

A
  • specialised digestion mechanism
  • some larger snakes feed so infrequently that they must enlarge their digestive system to accommodate large meals
76
Q

Describe Burmese pythons Python bivittatus

A
  • 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
77
Q

Describe enlargement

A
  • 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
78
Q

Give an enlarging snake

A

garter snakes, Thamnophis sirtali

79
Q

Describe Squamate metabolism

A
  • by increasing their anaerobic metabolism, squamates achieve a similar total metabolism to mammals
  • only in short bursts
  • only with long recovery time between bursts
80
Q

Describe the Basilisk lizard, Basiliscus

A
  • 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

Hilary Term 1 (61 decks)

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