Sphenodon is the most primitive and crocodilians are most advanced of all living Reptiles'. Discuss the statement.

Understanding Primitiveness and Advancement

In evolutionary biology, 'primitive' doesn't imply inferiority but rather the retention of characteristics present in ancestral forms. These traits often represent the earliest stages of a particular evolutionary lineage. Conversely, 'advanced' signifies the development of derived characteristics – novel features that have evolved through natural selection, often enhancing adaptation to specific environments. These advancements are not necessarily 'better' but represent changes from the ancestral state.

*Sphenodon*: The Living Fossil

*Sphenodon punctatus*, commonly known as the Tuatara, is the sole surviving member of the order Rhynchocephalia, a group that flourished during the Mesozoic era. Its primitive characteristics are numerous:

• Skull Morphology: *Sphenodon* possesses a diapsid skull, a characteristic of early reptiles, with two temporal openings. However, unlike most diapsids, its skull is acinetic – meaning the bones are firmly fused, limiting jaw movement. This is a primitive trait.
• Dental Structure: Its teeth are acrodont – fused to the jaw bone rather than implanted in sockets like in most reptiles (thecodont). This is a characteristic of early reptiles.
• Sensory Organ – Parietal Eye: *Sphenodon* retains a parietal eye, a light-sensitive organ on the top of its head, present in ancestral reptiles and used for thermoregulation and circadian rhythm regulation.
• Slow Metabolism: Tuataras have a remarkably slow metabolic rate, even for reptiles, and grow very slowly. This is linked to their ancient physiology.
• Ribs: Possesses gastralia (abdominal ribs) – a feature found in many early reptiles but lost in most modern groups.

Crocodilians: The Apex Reptilian Predators

Crocodilians (order Crocodilia) are considered the most advanced living reptiles due to a combination of derived characteristics that enhance their predatory lifestyle and physiological efficiency:

• Secondary Palate: Crocodilians possess a complete secondary palate, separating the nasal passage from the mouth. This allows them to breathe while having prey in their mouth – a crucial adaptation for ambush predation.
• Four-Chambered Heart: Unlike most reptiles, crocodilians have a four-chambered heart, similar to birds and mammals, providing complete separation of oxygenated and deoxygenated blood, leading to higher metabolic rates.
• Thecodont Dentition: Crocodilians have thecodont teeth – implanted in sockets – allowing for stronger bite force and tooth replacement throughout life.
• Advanced Nervous System: Crocodilians exhibit complex social behaviors and parental care, indicating a more developed nervous system.
• Integumentary Sensory Organs: They possess Integumentary Sensory Organs (ISOs) around their jaws, which detect minute pressure changes in the water, aiding in prey detection.

Comparative Table

Feature	*Sphenodon* (Primitive)	Crocodilians (Advanced)
Skull Type	Diapsid, Acinetic	Diapsid, Kinetic
Dentition	Acrodont	Thecodont
Heart	Three-Chambered	Four-Chambered
Secondary Palate	Absent	Present
Metabolic Rate	Slow	Relatively High
Parietal Eye	Present	Absent

Molecular Evidence

Phylogenetic studies based on molecular data (DNA and RNA sequences) consistently place *Sphenodon* as a basal lineage within Reptilia, diverging early in reptilian evolution. Crocodilians, while still ancient, are shown to be more closely related to birds (the Archosauria clade) and have undergone significant genetic changes compared to *Sphenodon*.