Sphenodon – Morphology, Habitat, Affinities

What is Sphenodon?

• Sphenodon, commonly known as the tuatara, is a remarkable species that belongs to the order Rhynchocephalia, a lineage of reptiles that evolved during the early Mesozoic Era. Albert Günther, a scientist from the British Museum, named this order in 1867, meaning “beak-headed,” due to the distinct beak-like appearance of the tuatara’s head. Despite their resemblance to lizards, tuataras are not lizards but the last surviving members of their order, which was once widespread and diverse during the Mesozoic Era, about 240 million years ago. The name “tuatara” originates from the Māori language, meaning “spines on the back,” referring to the prominent ridge along the animal’s back, especially in males.
• Tuatara, or Sphenodon punctatus, is the only extant species in the order Rhynchocephalia, which flourished globally during the Triassic and Jurassic periods. At their peak, rhynchocephalians were the dominant small reptiles on Earth. However, they declined significantly during the Cretaceous period, with their most recent fossil records outside of New Zealand dating to the Paleocene epoch. Their closest living relatives are squamates, a group that includes lizards and snakes.
• The tuatara is a strikingly unique reptile, with its greenish-brown or grey body reaching up to 80 cm in length and weighing as much as 1.3 kg. Despite its size and appearance, tuatara’s anatomical features set it apart from most other reptiles. For instance, they possess two rows of teeth in their upper jaw that overlap one row in their lower jaw, a characteristic not found in any other living species today. This dental arrangement is crucial for their diet, which consists mainly of insects, small vertebrates, and occasionally, the eggs of birds.
• A distinctive feature of the tuatara is its lack of external ears, though it can still hear. This ability, combined with its unique skeletal features, such as a well-developed, flexible jaw structure, adds to the interest of scientists studying the evolutionary history of reptiles. Another notable trait is its remarkably large genome, containing between 5 and 6 billion base pairs of DNA, nearly twice as many as humans. This extensive genome is a topic of considerable scientific interest, as it may provide insights into the evolutionary processes that have shaped the tuatara over millions of years.
• Tuatara are often referred to as “living fossils,” a term used to describe species that resemble their ancient ancestors, seemingly unchanged over long periods. However, this term is now less favored by paleontologists, as the tuatara’s evolutionary history does not support a continuous fossil record to demonstrate uninterrupted survival from the Mesozoic. Instead, the tuatara represents a lineage that has maintained ancient features while evolving independently over millions of years.
• Tuatara are endemic to New Zealand, where they have faced numerous threats due to habitat loss and the introduction of predators such as the Polynesian rat. Historically, they became extinct on the mainland, surviving only on offshore islands. Conservation efforts in New Zealand have been pivotal to their survival. Since 1895, tuatara have been legally protected, and the establishment of conservation programs has helped reintroduce them to several islands. For instance, in 2005, tuatara were successfully released in the Karori Wildlife Sanctuary, a highly monitored environment. This marked a significant milestone in their conservation.
• In recent years, there have been encouraging signs of tuatara recovery. In 2008, during routine maintenance at the Zealandia sanctuary, a tuatara nest was discovered, and a hatchling was found the following autumn. This was the first confirmed case of tuatara breeding successfully on New Zealand’s North Island in over 200 years. Such developments demonstrate the ongoing importance of conservation efforts to ensure the survival of this ancient species.

Habits and Habitat of Sphenodon

Sphenodon, commonly known as the tuatara, exhibits a range of distinct behaviors and occupies specific habitats that have enabled it to survive as a relic of ancient reptilian lineages. These habits and habitat preferences are vital to understanding the unique biology and conservation needs of this species. The tuatara’s lifestyle is shaped by its evolutionary history, and its behaviors reflect the adaptation to its environment, which is primarily confined to New Zealand.

• Habitat Preferences:
  • Sphenodon inhabits burrows, which are often made in soft soil or in rocky areas. These burrows offer protection from predators and environmental conditions.
  • In some cases, tuatara burrows are shared with other species, notably petrels. This association provides mutual benefits, with tuatara taking advantage of the petrels’ burrowing habits.
  • The tuatara is primarily found on offshore islands of New Zealand, where they have been reintroduced and established under conservation efforts. Historically, they have been confined to these islands due to habitat loss on the mainland.
• Behavioral Traits:
  • Tuatara are nocturnal, meaning they are most active during the night. This behavior likely helps them avoid daytime predators and take advantage of cooler nighttime temperatures, which are more suitable for their ectothermic metabolism.
  • During the day, they remain inactive, typically staying in their burrows to avoid heat and conserve energy.
  • Sphenodon is known for its slow movements when not disturbed. However, when necessary, the tuatara can run quickly, especially when escaping threats or when hunting prey.
• Feeding Habits:
  • The tuatara is a carnivorous reptile. Its diet primarily consists of invertebrates such as insects and spiders, but it will also consume small fish.
  • Tuatara feed opportunistically and are skilled hunters of small, nocturnal creatures. Their low metabolic rate allows them to survive with relatively infrequent meals, which is a trait adapted to their slow and steady lifestyle.
• Reproductive Traits:
  • Tuatara breed during the warmer months, with egg-laying occurring between November and February. This timing aligns with the warmer season, likely to ensure that eggs can develop properly.
  • The eggs of Sphenodon have a notably long incubation period, taking approximately 13 months (over a year) to hatch. This extended incubation is a unique reproductive characteristic of the species.
  • Once the eggs hatch, the young tuatara are left to fend for themselves, starting life in the same burrows as their parents.
• Longevity and Survival in Captivity:
  • In captivity, tuatara have demonstrated an impressive lifespan, with some individuals living up to 28 years. This extended longevity suggests that with controlled conditions, tuatara can thrive for several decades, which is a key factor in the success of conservation programs aimed at protecting them.

Distribution of Sphenodon

Sphenodon punctatus, commonly referred to as the tuatara, holds a significant place in the biological landscape of New Zealand. It is the sole surviving member of the order Rhynchocephalia, a group that thrived approximately 200 million years ago. The distribution and habitat preferences of the tuatara reflect its evolutionary history and the environmental pressures it has faced.

• Historical Range:
  • Historically, Sphenodon punctatus was widespread throughout New Zealand. This extensive range indicates that tuataras once thrived in various environments across the islands.
  • However, with the introduction of terrestrial mammals, including humans, the distribution of tuatara became severely restricted. The arrival of these mammals led to habitat destruction and predation pressures that significantly impacted tuatara populations.
• Current Distribution:
  • Today, Sphenodon is predominantly found on coastal islands of New Zealand. This shift in distribution is a direct consequence of conservation efforts and the necessity to escape the threats posed by terrestrial predators.
  • The tuatara prefers to inhabit areas with soft soil, where it can dig burrows for shelter and protection. These burrows are essential for the tuatara’s survival, providing a safe space to retreat during the day and during adverse weather conditions.
• Behavioral Adaptations:
  • The nocturnal behavior of Sphenodon punctatus is a crucial adaptation that facilitates its survival in the wild. By being active at night, tuataras reduce their exposure to potential predators while foraging for food.
  • Emerging primarily at night, the tuatara exploits the cooler temperatures and the cover of darkness to hunt for invertebrates and small fish, which constitute its diet.
• Legal Protection:
  • Sphenodon is rigorously protected by law in New Zealand, reflecting its status as a species at risk due to its limited distribution and historical decline. This legal framework aims to conserve the remaining populations and their habitats, ensuring that tuataras continue to thrive on the islands where they reside.
• Conservation Status:
  • The tuatara’s status as a “living fossil” underscores its evolutionary significance. It has remained relatively unchanged for 200 million years, providing valuable insights into the characteristics of diapsids from the late Permian period.
  • Conservation programs focusing on habitat restoration and predator control have been implemented to facilitate the tuatara’s recovery and to promote the establishment of populations on various offshore islands.

Morphology of Sphenodon

The morphology of Sphenodon, commonly known as the tuatara, is characterized by several unique and primitive features that distinguish it from other reptiles, particularly modern lizards. Although it may appear lizard-like externally, its internal structure and certain physical traits reflect a deep evolutionary lineage, with some features that have remained unchanged for millions of years.

• General Body Form:
  • Sphenodon is a reptile that typically reaches a length of about 60 cm (approximately 2 feet). It has a distinctly lizard-like appearance but exhibits several features that set it apart from true lizards.
  • Its body is covered in scaly skin, and it has a long, laterally compressed tail, which aids in balance and locomotion.
  • The tuatara has four pentadactyl limbs, each with claws adapted for walking, reflecting a generalized reptilian form suitable for terrestrial movement.
• Skin and Scales:
  • The skin of Sphenodon is covered with granular scales on its upper surface, which gives it a rough texture. This helps in protection and camouflage within its natural environment.
  • A median row of spines runs from the head to the tail, though this does not extend to the neck. This row of spines gives the tuatara a distinctive appearance, particularly noticeable in males.
  • The lower surface of the body consists of large, squarish plates arranged in transverse rows, which aid in protecting the underlying tissues and provide some structural support.
• Coloration and Vision:
  • The tuatara’s coloration is typically dull olive-green, with white and yellow speckling scattered across its body. This coloration likely serves as camouflage, helping it blend into its environment.
  • Its eyes are large and dark brown, with a vertical pupil similar to that of many lizards. This trait enhances its ability to judge distance in its nocturnal habitat, although its vision is monocular, meaning each eye functions independently.
• Unique Morphological Features:
  • The tuatara also possesses a vestigial third eye located on the top of its head. This third eye is not functional for vision but is thought to play a role in regulating circadian rhythms and hormone production, reflecting an ancient feature from its diapsid ancestors.
  • The anal opening of Sphenodon is transverse, a trait typical of many reptiles, and differs from the longitudinal anal opening seen in some other groups.
• Teeth and Jaw Structure:
  • One of the most distinctive aspects of Sphenodon‘s morphology is its teeth. The tuatara has acrodont teeth, which are fixed in place and not replaced throughout its life. These teeth are found in two rows in the upper jaw, with a single row in the lower jaw, allowing for a shearing motion when chewing.
  • The jaw movements are unique, combining backward-and-forward movements with an up-and-down shearing action. This enables the tuatara to process its prey efficiently.
• Skeletal and Internal Anatomy:
  • Sphenodon has amphicoelous vertebrae, meaning both ends of its vertebrae are concave. There are remnants of the notochord present in the cavities between the centra, a primitive feature indicative of its ancient lineage.
  • The skull includes an additional small median bone called the proatlas, located in the occipital region, a feature rarely seen in modern reptiles.
  • The rib structure of the tuatara consists of single-headed ribs, some of which possess uncinate processes. These processes may aid in reinforcing the rib cage and improving respiratory efficiency.
  • A median sternum is present, with coracoid and interclavicle bones attached at the anterior end, which provide structural support for the thoracic region.
• Limbs and Appendicular Skeleton:
  • The limb structure of Sphenodon includes a T-shaped interclavicle and a clavicle, which are important for limb movement and stabilization.
  • The forelimbs have eleven carpal bones arranged in two rows: four in the proximal row (including a pisiform), two centrals, and five in the distal row. This complex arrangement reflects the adaptability of the limbs for walking and burrowing.
• Pelvic Structure:
  • The pelvis of Sphenodon shows some unique adaptations, including pubes that are united in symphysis at the front of the pelvis. In front of the pubes, there is a cartilaginous epipubis, while a cartilaginous hypoischium is attached to the ischia behind. These structures are part of the support system for locomotion and internal organ protection.

Anatomical Features of Sphenodon

The anatomical structure of Sphenodon, or the tuatara, reflects its ancient evolutionary lineage, as well as its adaptation to a variety of environmental challenges. This lizard-like reptile has a number of distinctive features that set it apart from more modern lizards, and these include aspects of its endoskeleton, alimentary system, respiratory system, heart, and sensory organs. Each of these structures serves a specific function and is critical to understanding the tuatara’s biology.

• Endoskeleton:
  • The vertebrae of Sphenodon are amphicoelous, meaning they are concave at both ends, a primitive feature that allows for limited flexibility in the spine. Between the centra (the main body of the vertebrae), remnants of the notochord are present, an ancient structure that provides support during early development.
  • The intercentra, which are bony elements formed by ossification of the ventral portions of the intervertebral discs, may also be present in the tuatara’s vertebral column.
  • In the caudal (tail) region, chevron bones are found, which provide additional support and reinforcement for the tail.
  • A median bone known as the proatlas is located between the atlas (the first vertebra) and the occipital region of the skull. This bone is rare and contributes to the tuatara’s unique anatomy.
  • The rib structure of Sphenodon includes single-headed ribs that are present with all vertebrae in the pre-sacral region (the part of the spine before the sacrum). In addition, the caudal ribs are fused with the vertebrae, which is unusual for reptiles.
  • The ribs also possess an uncinate process, a bony projection that helps reinforce the rib cage and improve breathing efficiency.
• Skull:
  • The skull of Sphenodon is lizard-like in appearance but features a complete lower temporal arch, which is a bony structure around the eye socket. The quadrate bone, a key element of the jaw, is fixed and wedged in place by the quadrato-jugal, squamosal, and pterygoid bones.
  • Unlike most modern reptiles, the premaxillae (the front part of the upper jaw) are not fused together but remain separated by a suture. This provides flexibility in jaw movement.
  • The palate of Sphenodon is broad and is formed by the plate-like vomer, palatine, and pterygoid bones, which aid in feeding and processing food.
• Limbs and Pelvic Girdle:
  • The humerus (upper arm bone) of Sphenodon contains two foramen (holes), one above the outer condyle and one above the inner condyle, which may play a role in muscle attachment and limb movement.
  • The carpal bones (wrist bones) are composed of eleven elements: four in the proximal row (including the pisciform), two central bones, and five in the distal row.
  • The pelvic girdle features pubes that are united in a symphysis, a cartilaginous joint. An epipubis, a cartilaginous element, is present in front of the pubes. Behind the pubes, a hypoischium, another cartilaginous structure, is attached to the ischia.
  • The tarsus (ankle region) has distinct tibial and fibular elements that are firmly united. These include the intermedium and centrale, which are securely fixed to the tibiale. Additionally, there are three distal tarsal bones that provide structural support.
• Alimentary Canal:
  • The tuatara’s teeth are distinctive, being pointed, triangular, and laterally compressed. These teeth are arranged in two parallel rows, one along the maxilla (upper jaw) and one along the palatine (roof of the mouth). The lower jaw contains similar teeth, with the lower row fitting in between the upper rows, allowing for efficient shearing during feeding.
  • Over time, these teeth become worn down in adults, forming continuous ridges that aid in processing food. In young Sphenodon, a tooth has been found on each vomer, which is an unusual trait among reptiles.
• Respiratory System:
  • The trachea of Sphenodon is elongated, with its anterior portion dilating to form the larynx. The walls of the trachea are supported by cricoid and arytenoid cartilages, which help maintain the structure and function of the air passage.
  • The tuatara emits a soft croak, which resembles the sound of a frog. This croak is a distinguishing feature of its vocalization, and the lungs are lizard-like, with a structure adapted to terrestrial respiration.
• Heart:
  • In Sphenodon, the sinus venosus, a part of the heart that receives deoxygenated blood from the body, is not easily distinguished externally, unlike in many other reptiles. The heart structure, otherwise, is similar to that of other lizards.
• Jacobson’s Organ:
  • Sphenodon possesses Jacobson’s organs, which are involved in the sense of smell. These organs are present in a primitive form, playing a role in olfactory detection, although they are less developed compared to some modern reptiles.
• Parietal or Pineal Eye:
  • A striking feature of Sphenodon is the presence of a median pineal eye, located in the parietal foramen of the cranial roof. This eye is not functional for vision but is related to regulating biological rhythms and hormone release.
  • It is covered by a transparent scale in young individuals, and it possesses a lens and retina. A nerve from the brain supplies this eye, although in most reptiles, the nerve degenerates as the animal matures.
• Tympanic Membrane:
  • The tympanic membrane (eardrum) of Sphenodon is not visible externally, but it can be seen after removing the skin from the aural region. The tympanic cavity, where sound waves are processed, is represented by a large pharyngeal recess, another primitive feature of its auditory system.

Systematic Position of Sphenodon

The systematic position of Sphenodon, often referred to as the tuatara, places it in a unique and pivotal role within the evolutionary history of reptiles. This lizard-like reptile is the only extant member of the order Rhynchocephalia, a lineage that dates back to the Permian period, making it one of the oldest known reptiles. The systematic classification of Sphenodon is of great interest due to its retention of many primitive features, suggesting it is a “living fossil” that has undergone relatively little evolutionary change over millions of years.

• Ancient Origins:
  • Sphenodon first appeared during the Permian period, making it one of the earliest known reptiles. Its evolutionary history is significant because it offers a glimpse into the characteristics of early reptiles.
  • Despite its ancient lineage, no fossil specimens of Sphenodon have been discovered. This absence of fossils adds to the mystery surrounding its ancient origins and underscores the unique nature of this species in modern times.
• Living Fossil:
  • Due to its retention of many primitive characteristics, Sphenodon is considered a “living fossil.” It has preserved features that are reminiscent of stem reptiles from the Permian period, providing important insights into the early evolutionary stages of reptiles.
  • The tuatara has undergone less evolutionary change than many other reptile species. Its anatomy, morphology, and behavior are closer to ancient forms than to modern reptile groups.
• Evolutionary Relationships:
  • Sphenodon shares similarities with several groups of reptiles, such as lizards (Squamata), crocodiles (Crocodilia), dinosaurs (Dinosauria), and turtles (Chelonia). However, it also exhibits distinct differences from these groups, making its classification challenging.
  • While it exhibits some resemblances to these groups, Sphenodon does not fit neatly into any of them. This distinctiveness has led to its classification in a separate order, Rhynchocephalia.
  • Rhynchocephalia is considered a distinct and ancient order, and Sphenodon represents the sole surviving member. Its evolutionary position reflects the divergence of reptile lineages, with Sphenodon occupying a unique niche within this broader evolutionary tree.
• Isolation and Evolutionary Significance:
  • The fact that Sphenodon is restricted to isolated island regions in New Zealand may have contributed to its preservation in a more primitive form. This geographic isolation likely reduced the influence of environmental pressures and competition, allowing the tuatara to remain relatively unchanged over time.
  • The island habitat may have played a role in limiting the evolutionary pressures that typically drive more significant changes in species over time. As a result, Sphenodon offers a unique opportunity to study the evolutionary history of reptiles that spans hundreds of millions of years.
• Position in Reptile Phylogeny:
  • While Sphenodon has features in common with modern lizards (Squamata) and other reptiles, it retains a number of primitive traits that set it apart. For this reason, it is placed in its own order, Rhynchocephalia, distinct from other reptile groups.
  • The tuatara’s anatomical features, such as its unique skull structure, teeth, and reproductive anatomy, provide key evidence for its classification in this separate order. These primitive traits highlight the evolutionary gap between Sphenodon and other modern reptiles.

Species of Sphenodon

There are two recognized species of Sphenodon, each with distinct characteristics and geographical distributions. Both species are part of the same genus, Sphenodon, and are commonly referred to as tuataras. However, their physical appearances and habitats vary, providing insight into their evolutionary adaptations.

• Sphenodon punctatus (Northern Tuatara):
  • Sphenodon punctatus, known as the Northern Tuatara, is the more widespread species. It is characterized by its distinctive coloration, which can range from olive green to grey, and in some cases, dark pink or brick red. The coloration is often mottled, and the skin is marked with white spots.
  • The males of this species are generally larger, reaching up to 61 cm in length and weighing approximately 1 kg. Females are comparatively smaller, measuring around 45 cm in length.
  • This species is commonly found on the islands of New Zealand, where it inhabits a variety of coastal regions. Its physical appearance and size allow it to adapt well to the specific environmental conditions in these areas.
• Sphenodon guntheri (Brothers Island Tuatara):
  • Sphenodon guntheri, also known as the Brothers Island Tuatara, is confined to the North Brother Island in Cook Strait. The population of this species is relatively small, with only about 400 individuals remaining.
  • The Brothers Island Tuatara is smaller compared to its northern counterpart. It weighs up to 660 grams and has a unique appearance, characterized by olive brown skin with yellowish patches.
  • This species is more restricted in its range, inhabiting a specific island location. Its smaller size and distinctive coloration set it apart from Sphenodon punctatus, reflecting adaptations to its more localized environment.

Causes of its Long Survival

The long survival of Sphenodon can be attributed to several key factors that have allowed this ancient reptile to persist for millions of years with relatively little change. These factors contributed to its ability to survive in a rapidly changing world, where many other species have come and gone.

• Lack of Natural Enemies:
  • One major cause of the Sphenodon‘s prolonged survival is its absence of natural predators for a significant portion of its evolutionary history. Before the introduction of placental mammals and other large terrestrial animals in New Zealand, the tuatara had little to no competition or threat from predators. The ecosystem was relatively stable and allowed Sphenodon to thrive without the pressures of predation that would typically lead to evolutionary changes in many species.
• Slow Metabolism and Energy Conservation:
  • Another critical factor is the Sphenodon‘s slow metabolic rate. This characteristic enables it to conserve energy over long periods, which is essential for survival, particularly in environments where food may be scarce or inconsistent. A slow metabolism reduces the need for frequent feeding and allows the tuatara to endure periods of low food availability, a survival trait that has proven vital in the face of environmental changes over millennia.
• Lethargic Habit and Long Incubation Period:
  • The Sphenodon also exhibits a lethargic lifestyle, which may have played a role in its survival. Its slow movements and sedentary nature reduce its exposure to dangers, allowing it to avoid the risks associated with being overly active or exposed in the environment. Additionally, the long incubation period for its fertilized eggs contributes to its survival strategy. This extended period allows for a more controlled and stable development of offspring, which may result in higher survival rates for the young.

Why called a living fossil?

The term “living fossil” refers to an organism that has survived well beyond its era, continuing to exist when its closely related species or groups have become extinct. Sphenodon, the tuatara, is often referred to as a living fossil because it represents the sole surviving member of the order Rhynchocephalia, an ancient group of reptiles that dates back to the Permian period. This title is supported by several characteristics of Sphenodon that highlight its primitive features, which resemble those of reptiles that are no longer in existence.

• Anatomical Features:
  • Sphenodon exhibits several primitive anatomical traits that link it to extinct reptiles. These characteristics have remained largely unchanged for millions of years, underscoring its ancient lineage.
  1. Skull Structure:
     • The bones of the Sphenodon skull are arranged and shaped in a manner that is consistent with extinct groups of reptiles. This primitive configuration sets it apart from modern reptiles, whose skulls have evolved into different forms.
  2. Teeth Fused to Jaw Bones:
     • In Sphenodon, the teeth are fused directly to the jaw bones, a feature that is rare in most modern reptiles. This trait is considered a primitive characteristic that was more common in earlier reptilian forms.
  3. Presence of Pineal Foramen:
     • Sphenodon possesses a pineal foramen, a small opening in the skull that houses the “third eye,” or pineal eye. This feature is an ancient trait that is rarely found in modern reptiles, further linking the tuatara to early reptilian ancestors.
  4. Resemblance to Dinosaurs and Extinct Reptiles:
     • Several features of Sphenodon are similar to those seen in dinosaurs and other extinct reptilian species. These resemblances indicate that Sphenodon retains traits from a much earlier time, making it a living link to ancient reptiles.
  5. Similarity to Ancient Homaeosaurs:
     • While Sphenodon shares several characteristics with the extinct Homaeosaurs, it differs in one key aspect: the presence of an uncinate process on the ribs. This feature further exemplifies how Sphenodon bridges the gap between ancient reptiles and modern species.
• Justification as a Living Fossil:
  • Based on these primitive features, the term “living fossil” is justified for Sphenodon. This species has survived for millions of years with little evolutionary change, making it a rare and valuable representation of ancient reptilian forms. Sphenodon has earned the title of the “Voice of the Past,” highlighting its unique place in the natural world as a living remnant of a distant era.

Affinities of Sphenodon

Sphenodon, commonly known as the tuatara, belongs to the order Rhynchocephalia and exhibits numerous anatomical and physiological characteristics that connect it to various groups within the animal kingdom. Due to these traits, its systematic position has been subject to debate. The following outlines the affinities of Sphenodon with different animal groups, emphasizing both the similarities and differences that define its unique position in the evolutionary tree.

• Affinities with Amphibia:
  • The circulatory system of Sphenodon shares certain resemblances with that of amphibians, particularly those in the order Caudata (or Urodela). Notable similarities include:
    1. Aortic Arches: The aortic arches in Sphenodon arise from a short common stalk, akin to the conus arteriosus seen in amphibians.
    2. Presence of Ductus Arteriosus and Ductus Caroticus: These structures are also found in amphibians, indicating a primitive circulatory design.
    3. Blood Vessel Distribution: The pattern of blood vessel distribution is comparable to that of amphibians; however, these similarities primarily arise from Sphenodon‘s primitive characteristics rather than indicating a close evolutionary relationship.
• Chelonian Affinities:
  • Sphenodon shows several similarities with chelonians (turtles), including:
    1. Fixed Quadrate Bone: The quadrate bone in Sphenodon is fixed, resembling that of turtles.
    2. Fused Ribs and Vertebrae: The ribs in the caudal region are fused with the vertebrae, a feature seen in chelonians.
    3. Absence of Pecten in the Eye: Unlike some reptiles, Sphenodon lacks a pecten in its eye.
    4. Presence of Urinary Bladder: The urinary bladder is present, similar to chelonians.
  • Differences include:
    1. Habitat: Sphenodon is terrestrial, whereas chelonians are primarily aquatic.
    2. Dental Structure: Chelonians have a beak-like structure instead of teeth, while Sphenodon has true teeth.
    3. Jaw Structure: Sphenodon possesses an unpaired vomer and lacks a sternum, distinguishing it from chelonians.
• Crocodilian Affinities:
  • Certain anatomical features of Sphenodon resemble those found in crocodilians:
    1. Diapsid Skull Condition: Both groups exhibit a diapsid condition of the skull.
    2. Fixed Quadrate: The fixed quadrate bone is a shared trait.
    3. Presence of Pro-atlas: This feature is also found in both groups.
    4. Abdominal Ribs: The presence of abdominal ribs adds to the similarities.
    5. Fusion of Caudal Ribs with Vertebrae: This fusion is common in both Sphenodon and crocodilians.
    6. Uncinate Process of Ribs: Both groups have this feature.
    7. Presence of Chevron Bones: The chevron bones are present in both groups.
    8. Cochlear Process: The cochlear process is tubular in both.
  • However, distinctions also exist:
    1. Dentition: Crocodilians exhibit thecodont dentition, whereas Sphenodon has a different dental structure.
    2. Nostrils: Crocodiles possess a single nostril, while Sphenodon has two.
    3. Vertebrae: Crocodiles have procoelous vertebrae, while Sphenodon possesses amphicoelous vertebrae.
    4. Clavicles: Sphenodon has clavicles, which are absent in crocodilians.
    5. Presence of Pecten: Crocodilians have a pecten in their eyes, which Sphenodon lacks.
    6. Reproductive Structures: Males of crocodiles have a distinct penis, a feature absent in Sphenodon.
• Affinities with Lacertilia (Lizards):
  • Similarities between Sphenodon and certain lizards include:
    1. Body Plan: The general body plan is similar between Sphenodon and some lizards.
    2. Pro-atlas: This anatomical structure is present in both groups.
    3. Vertebrae: Some geckos share amplucoelous vertebrae with Sphenodon.
    4. Single-headed Ribs: Both groups have single-headed ribs.
    5. Presence of Chevron Bones: Chevron bones are found in both.
    6. Respiratory Organs: The respiratory structures share similarities.
    7. Parietal Organ: Both groups possess a parietal organ.
    8. Cloacal Glands: Cloacal glands are also present in both.
  • Differences include:
    1. Quadrate Bone: The quadrate bone is fixed in Sphenodon, while it is not in most lizards.
    2. Vertebral Structure: Most lizards exhibit procoelous vertebrae, differing from the amphicoelous vertebrae in Sphenodon.
    3. Jaw Structure: In lizards, the rami of the jaw are united by a symphysis, which is not the case in Sphenodon.
    4. Ilium Structure: The ilium is erect in Sphenodon, a characteristic that differs from most lizards.
    5. Presence of Clavicles: Clavicles and interclavicles are present in Sphenodon but absent in many lizards.
    6. Presence of Conus Arteriosus: Sphenodon lacks a conus arteriosus, which is present in lizards.
    7. Pecten: Unlike Sphenodon, lizards typically have a pecten in their eyes.
    8. Uncinate Process of Ribs: This structure is absent in Lacertilia.
    9. Copulatory Organs: Lacertilia generally have copulatory organs, a feature that Sphenodon does not possess.
    10. Temporal Arch: A lower temporal arch is present in Lacertilia, which is not found in Sphenodon.