Affinities of Sphenodon (Tuatara)

 

Affinities of Sphenodon (Tuatara)

Introduction

Sphenodon punctatus, commonly known as the Tuatara, is a primitive reptile found only in New Zealand. It belongs to the order Rhynchocephalia, a group that flourished during the Mesozoic era but is now represented by only one living species. Because of its primitive characteristics and resemblance to several vertebrate groups, Sphenodon is often called a “living fossil.”

Systematic Position

Category	Classification
Phylum-	Chordata
Subphylum-	Vertebrata
Class-	Reptilia
Subclass-	Diapsida
Order-	Rhynchocephalia
Genus-	Sphenodon

General Characters of Sphenodon

• Medium-sized lizard-like reptile
• Body divided into head, trunk, and tail
• Skin dry and covered with scales
• No external ear opening
• Presence of a parietal or “third eye”
• Teeth fused with jaw bones (acrodont dentition)
• Cloaca present
• Oviparous (egg-laying)
• Cold-blooded animal

Affinities of Sphenodon

1. Affinities with Amphibians

Sphenodon shows several primitive amphibian-like features.

Similarities

1. Presence of a parietal eye, similar to primitive amphibians.
2. Vertebrae are amphicoelous (concave on both sides), a primitive condition.
3. Heart is incompletely divided.
4. Slow metabolism and primitive circulation resemble amphibians.
5. Persistence of some primitive skull features.

Significance

These features indicate that reptiles evolved from amphibian ancestors.

2. Affinities with Lizards (Order Squamata)

Externally, Sphenodon resembles lizards.

Similarities

1. Elongated body with long tail
2. Presence of scales
3. Pentadactyl limbs with claws
4. Similar mode of locomotion
5. Internal fertilization
6. Cloaca present

Differences from Lizards

1. No copulatory organ in males
2. Teeth are acrodont and fused to jaw
3. Skull is more primitive
4. Presence of complete temporal bars

Significance

These features show that Sphenodon and lizards share a common reptilian ancestry.

3. Affinities with Crocodiles

Similarities

1. Presence of diapsid skull
2. Ribs with uncinate processes
3. Well-developed abdominal ribs (gastralia)
4. Cloacal aperture is transverse

Significance

These characters indicate relationship with primitive diapsid reptiles.

4. Affinities with Birds

Similarities

1. Presence of uncinate processes on ribs
2. Diapsid skull condition
3. Similar embryonic membranes (amnion and allantois)

Significance

These similarities support the idea that birds evolved from reptilian ancestors.

5. Primitive Reptilian Characters

Sphenodon retains many ancestral reptilian traits.

Primitive Features

1. Amphicoelous vertebrae
2. Presence of abdominal ribs
3. Pineal eye
4. Fixed quadrate bone
5. Simple copulatory structure absent
6. Primitive skull pattern

Because of these primitive characters, it is regarded as a living fossil.

Evolutionary Importance of Sphenodon

1. Represents the only surviving member of Rhynchocephalia
2. Helps in understanding reptilian evolution
3. Connects primitive reptiles with modern reptiles
4. Provides evidence for vertebrate evolution
5. Important in comparative anatomy and phylogeny

Conclusion

Sphenodon occupies a unique position among vertebrates. It possesses characteristics of amphibians, lizards, crocodiles, and birds, while also retaining primitive reptilian features. Therefore, it is considered an important connecting link in vertebrate evolution and is popularly known as a “living fossil.”

Multiple Choice Questions (MCQs)

MCQs with Answers

1. Sphenodon belongs to the order:

A. Squamata
B. Crocodilia
C. Rhynchocephalia
D. Chelonia

Answer: C. Rhynchocephalia

2. Sphenodon is commonly known as:

A. Iguana
B. Tuatara
C. Gecko
D. Skink

Answer: B. Tuatara

3. Sphenodon is called a living fossil because:

A. It is extinct
B. It has feathers
C. It retains primitive characters
D. It lives in caves

Answer: C. It retains primitive characters

4. The vertebrae of Sphenodon are:

A. Procoelous
B. Amphicoelous
C. Heterocoelous
D. Acoelous

Answer: B. Amphicoelous

5. The “third eye” of Sphenodon is:

A. Compound eye
B. Lateral eye
C. Parietal eye
D. Median eye spot

Answer: C. Parietal eye

6. The teeth of Sphenodon are:

A. Thecodont
B. Pleurodont
C. Acrodont
D. Diphyodont

Answer: C. Acrodont

7. Sphenodon is found mainly in:

A. Australia
B. India
C. Africa
D. New Zealand

Answer: D. New Zealand

8. Presence of uncinate processes in Sphenodon shows affinity with:

A. Amphibians
B. Birds
C. Mammals
D. Fishes

Answer: B. Birds

9. Sphenodon lacks:

A. Scales
B. Limbs
C. Copulatory organ
D. Tail

Answer: C. Copulatory organ

10. The skin of Sphenodon is:

A. Moist
B. Slimy
C. Dry and scaly
D. Naked

Answer: C. Dry and scaly

Exam-Oriented Questions

Very Short Questions (1 Mark)

1. What is the common name of Sphenodon?
2. Name the order to which Sphenodon belongs.
3. Why is Sphenodon called a living fossil?
4. What is the nature of vertebrae in Sphenodon?
5. What is the parietal eye?

Short Questions (5 Marks)

1. Write a short note on the parietal eye of Sphenodon.
2. Mention any four primitive characters of Sphenodon.
3. State the affinities of Sphenodon with lizards.
4. Why is Sphenodon considered evolutionarily important?
5. Differentiate between Sphenodon and lizards.

Long Essay Questions (10 Marks)

1. Describe the affinities of Sphenodon with different vertebrate groups.
2. Discuss the primitive characters and evolutionary significance of Sphenodon.
3. Explain why Sphenodon is regarded as a living fossil.
4. Give a detailed account of the affinities of Sphenodon with reptiles and birds.

References

1. Kotpal, R.L. – Modern Textbook of Zoology: Vertebrates
2. P.S. Verma & V.K. Agarwal – Chordate Zoology
3. Young, J.Z. – The Life of Vertebrates
4. Jordan & Verma – Chordate Zoology and Comparative Anatomy
5. Kardong, K.V. – Vertebrates: Comparative Anatomy, Function and Evolution
6. Romer, A.S. – The Vertebrate Body