Diapsids: the serpentes

Question 1

Approximately how many species of snake are there?

Answer 1

3000.

Question 2

2 main clades of snake are recognised. What are they?

Answer 2

1. Scolecophidia

2. Alethinophidia

Question 3

What are the scolecophids like?

Answer 3

Small, microphagous burrowers. Microphagous means they feed on minute particles and microorganisms.

Question 4

The alethinophidia is split into 2 groups. What are they?

Answer 4

1. Aniloids

2. Macrostomata

Question 5

What are the aniloids like?

Answer 5

Small, primitive species.

Question 6

What does ‘macrostomata’ mean?

Answer 6

‘Big mouthed’.

Question 7

What is characteristic about the macrostomes?

Answer 7

They have highly kinetic skulls. The upper and lower jaw bones plus the palatal bones can all move independently of each other.

Question 8

List 3 unique movements the jaw of a macrostome can perform due to high levels of cranial kinesis.

Answer 8

1. The left and right sides can move independently
2. The tips of the lower jaws can separate anteriorly
3. The jaw suspension can swing out

Question 9

What does the extreme cranial kinesis of macrostomes allow them to do?

Answer 9

Eat prey that is larger than the diameter of their own head.

Question 10

What characteristic groups does macrostomata include?

Answer 10

The constrictors and higher snakes.

Question 11

Which groups are classed as higher snakes?

Answer 11

Colubrids, viperids and elapids.

Question 12

Which groups are classed as constrictors?

Answer 12

Boas and pythons.

Question 13

Venomous snakes such as vipers and cobras are derived. How do they deliver their venom?

Answer 13

Through hypodermic needle fangs: the teeth are long and hollow with a venom cavity inside.

Question 14

What sense organs do snakes have to track their prey?

Answer 14

1. Heat sensors

2. Vomeronasal or ‘Jacobson’s’ organ

Question 15

How does the vomeronasal organ work?

Answer 15

The snake flicks its tongue in and out collecting scent particles. These are then transferred to the VNO at the base of the nostrils. Chemicals in the scent particles bind to receptors which fire neurons, sending information to the brain.

Question 16

Limblessness requires 2 morphological conditions. What are they?

Answer 16

1. Elongation of the body

2. Reduction of the limbs and girdles

Question 17

How is elongation of the body achieved? Give 2 methods.

Answer 17

1. Multiplication of the segments

2. The elongation of individual segments

Question 18

With method of body elongation is more common?

Answer 18

Multiplication of the segments, thus this is probably more effective.

Question 19

Elongation of the body and reduction/loss of limbs are two completely separate processes. True or false?

Answer 19

False: developmentally they are separate however they are inherently linked. There are no short-bodied, limbless squamates or long-bodied, normal-limbed squamates.

Question 20

How might body elongation and limb loss be linked?

Answer 20

Axial elongation would affect limb patterning and formation.

Question 21

There is dispute as to the ancestry of snakes. What are the 2 possible lifestyles they may have originated from?

Answer 21

1. Marine swimmers

2. Terrestrial burrowers

Question 22

If snakes originated from a swimming marine ancestor, which groups are they most related to?

Answer 22

Snakes are a sistergroup to the marine mosasauroids. This makes varanids their living relatives.

Question 23

If snakes originated from a terrestrial burrowing ancestor, which groups are they most related to?

Answer 23

Snakes are a sistergroup to the burrowing amphisbaenids. Varanids are a related group.

Question 24

Snake fossils from Bosnia, Palestine, Lebanon, North Africa, Portugal and France serve as support for the hypothesis that snakes arose from marine ancestors. List 4 things they all have in common.

Answer 24

1. They are all from the late Cretaceous
2. They have more than 120 vertebrae and small, complete hindlimbs
3. They were found in shallow, coastal marine deposits
4. They shared this habitat with small, fully limbed mosasauroid reptiles

Question 25

Give an example of a small, fully limbed mosasauroid reptile.

Answer 25

Dolichosaurs.

Question 26

Snake fossils from America and North Africa serve as support for the hypothesis that snakes arose from fossorial ancestors. Give 3 reasons why.

Answer 26

1. Many are earlier than the marine fossils as they are from the early Cretaceous
2. The US snake remains are comparable to primitive terrestrial aniloids
3. Terrestrial remains have also been recorded from the same time period as the marine species. For example in Argentina scientists discovered a limbed, burrowing snake that is apparently more primitive than the marine species.

Question 27

Modern snake ecology serves as support for the burrowing hypothesis in the origin of snakes. Refer to the scolecophinidians to explain this.

Answer 27

The scolecophinidians are basal snakes and are burrowers.

Question 28

What are the 2 morphotypes of long-bodied, non-snake squamates? What lifestyles are these 2 morphotypes suited to?

Answer 28

1. Long body-short tail: burrowing

2. Long body-long tail: surface or loose-litter dwellers.

Question 29

Do modern snakes have long or short tails?

Answer 29

Short tails.

Question 30

Did the marine snake specimens have long or short tails?

Answer 30

Short tails.

Question 31

Marine mosasauroids have long tails for undulatory swimming. How does this serve as support for the burrowing hypothesis?

Answer 31

There appears to be no selective advantage to shortening the tail in a marine environment. However if snakes reduced their tails early on for burrowing, secondarily marine snakes would retain this ancestral condition.

Question 32

How does the morphology of snake eyes and ears serve as support for the burrowing hypothesis?

Answer 32

They are degenerate compared to those of lizards. This is common in burrowing clades.

Question 33

How does the cranial kinesis of snakes serve as support for the burrowing hypothesis. Give 3 reasons.

Answer 33

1. Amphisbaenids have very rigid skulls for burrowing with no kinesis. Snakes have an extremely rigid braincase, despite all the kinesis of the facial bones.
2. Scolecophidians and aniloids are both burrowers and display (reduced) kinesis
3. It is though kinesis may be a redevelopment: the kinesis shown in lizards differs in several ways. Macrostomatan snakes are derived.

Question 34

The marine snakes have hindlimbs, leading some to believe they are primitive and had not yet lost their limbs. Why is this contested?

Answer 34

The marine, limbed snakes had macrostomatan skulls, which is a derived state and thus more recent.

Question 35

If limbed, marine snakes are more recent this could mean 2 possibilities terms of limb loss. What are they?

Answer 35

1. Limbs were lost multiple times in snakes

2. Limbs were re-acquired in marine, limbed snakes

Question 36

Why is it more likely that limbs have been lost multiple times in snakes? Give 2 reasons.

Answer 36

1. It is more parsimonious than the other hypothesis.

2. Limblessness has occurred within squamata at least 60 times independently

Question 37

Define Dollo’s Law.

Answer 37

An evolutionary trajectory cannot be reversed.

Question 38

Why is the idea that marine, limbed snakes re-acquired limbs controversial?

Answer 38

In contradicts Dollo’s Law and is parsimonious.

Question 39

New molecular evidence suggests that actually, the reacquisition of limbs may not be so far-fetched. Why?

Answer 39

Genes may be retained and evolution relies on changes in expression. Essentially snakes may still have limb genes and they have evolved to switch them off, in which case marine, limbed-snakes may have simply switched them back on.

Question 40

How is the potential reacquisition of limbs in marine, limbed snakes supported by modern snakes? Give 2 reasons.

Answer 40

1. Scolecophindians and aniloids have vestigial hindlimbs

2. Pythons (macrostomes) retain genes for limb growth/patterning.

Question 41

Define the neck.

Answer 41

The region of the vertebral column anterior to the first vertebra bearing a rib that meets the sternum.

Question 42

Why might neck length be important in determining the origin of snakes?

Answer 42

Amphisbaenids and other burrowers have very short necks. Aquatic species such as the mosasauroids have very long necks.

Question 43

What is the problem with snakes that means we cannot use the neck to help classify them?

Answer 43

The definition of a neck states that it ends at a rib that meets the sternum. Snakes do not have sternums.

Question 44

In order to use to neck to aid snake classification we must find other characters that define it. True or false?

Answer 44

True.