Sphenodon – Morphology, Anatomy, Habitat, Distribution, Affinities

What is Sphenodon?

Sphenodon is the reptile that is commonly known as the tuatara, and it is the only living representative of the order Rhynchocephalia.

It is the reptile that is found naturally in New Zealand, and it is considered a very old lineage that diverged from other reptiles in ancient geological time. The body of Sphenodon is lizard-like with a robust appearance, and the males usually show a well-developed spiny crest on the back which is raised during courtship and for defence.

It is the process in which the jaw shows a unique arrangement where the single row of teeth in the lower jaw fits exactly between the two rows present in the upper jaw, and this is referred to as the acrodont type of dentition.

The tuatara also has a rudimentary parietal eye on the top of the head covered later by scales, and it helps in maintaining the dark-light cycle of the body. It is adapted to low temperature conditions, and the activity of this reptile continues even when the surrounding temperature is very low. The reproduction is slow, and the male does not have an intromittent organ and mating is done by cloacal apposition.

At present these species is restricted mostly to small offshore islands of New Zealand where the natural populations is still surviving.

Habits and Habitat of Sphenodon

The natural habitat of Sphenodon is restricted mostly to small offshore islands of New Zealand where the environment is cold, damp and often covered with stunted vegetation that is tolerant to wind and salt. It is the process in which these islands provide predator-free conditions because the mainland populations was lost after the introduction of mammals. The soil in these islands is enriched by the nesting seabirds, and their burrows are also used by the tuatara for shelter. In many places the burrows are shared with the seabirds, and the nutrient-rich droppings support various invertebrates which act as the major food source.

The habits of Sphenodon show that the animal is mainly solitary and remains active mostly at night. It is adapted to low-temperature conditions, and the activity is possible even at very low body temperature which is generally not found in other reptiles. The males are territorial and the display of the crest is used in defending the territory, and the fighting often results in scars. Feeding is done by a sit-and-wait method, and the diet includes insects, earthworms, spiders, and small lizards or eggs. The young ones show more daytime activity which is helpful in avoiding the adults that may attack or feed on them.

Distribution of Sphenodon

The distribution of Sphenodon is very limited at present, and it is confined only to some small offshore islands of New Zealand. It is mainly found on about 32–35 islands where predators is absent, and these islands are located in the Cook Strait region and also near the northeastern part of North Island. It is the only place where natural populations are surviving.

Earlier, the species was widely distributed on the mainland of New Zealand. It is believed that both the North Island and South Island had large populations inhabiting the dry land regions. This population is thought to be lost after the arrival of humans, as the introduced mammals like the Polynesian rat (Rattus exulans) caused heavy predation on eggs and juveniles. This extinction from the mainland was almost completed by early 1800s.

Among the natural populations, Stephens Island (Takapourewa) has the largest group. It is a very small island of about 1.5 km², but it supports nearly 30,000 individuals. Historically two species were named i.e. S. punctatus and S. guntheri, but later studies show that these are not separate species. It is now considered that all belong to S. punctatus, and the population of Brother Island is only a geographical variant found in North Brother Island of Cook Strait.

The geological distribution also shows that the lineage of tuatara is very old in New Zealand. Fossil evidence from the Manuherikia Group of South Island shows jaw fragments belonging to Sphenodon that date back to the Early Miocene (19–16 Mya). It is the indication that the ancestral form was present long before, possibly from the time when New Zealand separated from Gondwana (82–60 Mya). It is also believed that the order Rhynchocephalia had a wide distribution in the Mesozoic era, but except New Zealand, all other members got extinct by the Paleocene.

At present, conservation programs are working to expand its distribution. It is the process where individuals are translocated to new islands and also into fenced mainland sanctuaries. These areas are protected from predators, so the reintroduced populations can establish and survive.

Morphology of Sphenodon

General Body Form– Sphenodon is a reptile showing a lizard-like external appearance but the internal structures show primitive characters. It is about 60 cm long. It is the process where the body is covered with scaly skin and the tail is long and laterally compressed. This tail helps in balancing and movement on land. The limbs are four in number and each limb is pentadactyl with claws. These are useful for walking and sometimes for burrowing activities.

Skin and Scales– The upper surface of the body is covered with granular scales which gives a rough texture. It acts mainly for protection and also for camouflage in the natural habitat. A median row of spines present from the head up to the tail region but it does not extend on the neck. These spines are more developed in the males. The lower surface of the body is made of squarish plates arranged in transverse rows. These plates protect the internal tissues.

Coloration and Vision– The coloration of Sphenodon is usually dull olive-green with white or yellow speckles. This coloration helps in blending with the surrounding. The eyes are large and dark brown. The pupil is vertical. It is the structure that helps in judging distance especially in its nocturnal conditions. The vision is monocular which means both eyes work independently.

Unique Morphological Features– One special feature is the presence of a vestigial third eye on the top of the head. This is referred to as the pineal eye. It is not used for normal vision but it helps in circadian rhythm regulation and hormonal control. The anal opening is transverse which is a character seen in many reptiles.

Teeth and Jaw Structure– The teeth of Sphenodon are acrodont type. These are fixed and not replaced during its lifetime. Two rows of teeth present on the upper jaw and one row on the lower jaw. The arrangement helps in a shearing movement during feeding. The jaw movements is backward-forward together with up-down motion which helps in proper cutting of the prey.

Skeletal and Internal Anatomy– The vertebrae are amphicoelous which means both ends are concave. Remnants of notochord is present between the centra showing primitive nature. The skull contains a median bone called proatlas situated in the occipital region. It is a rare feature among reptiles. The ribs are single-headed and some ribs show uncinate processes. These help in strengthening the rib cage. The sternum is median with coracoid and interclavicle bones attached anteriorly for support of the thoracic region.

Limbs and Appendicular Skeleton– The pectoral girdle has a T-shaped interclavicle and a clavicle. These are important for limb movement. The forelimbs contain eleven carpal bones arranged in two rows. Four bones in proximal row including pisiform, two centralia and five bones in distal row. This arrangement helps in walking on uneven surfaces and sometimes in digging.

Pelvic Structure– The pelvis shows pubes united in front to form pubic symphysis. In front of the pubes a cartilaginous epipubis is present. A cartilaginous hypoischium attached behind the ischia. These structures support the internal organs and help during locomotion.

Anatomical Features of Sphenodon

Endoskeleton
The vertebrae of Sphenodon are amphicoelous. It is the condition where both ends of each vertebra are concave. This gives limited flexibility to the spine. Between the centra the remnants of notochord is present which shows primitive nature. Intercentra sometimes present as ossified ventral elements. In the tail region chevron bones are present. These are supporting the caudal region. A small median bone called proatlas is present between the atlas and the occipital region. This bone is rarely found in modern reptiles. The ribs are single-headed in the pre-sacral region. The caudal ribs is fused with vertebrae. Some ribs show uncinate process which help in strengthening the rib cage.

Skull
The skull is lizard-like but the lower temporal arch is complete. The quadrate bone is fixed and wedged firmly by quadrato-jugal, squamosal and pterygoid bones. The premaxillae remain separate and not fused which allow some flexibility. The palate is broad and it is formed by the vomer, palatine and pterygoid bones. These plate-like bones help in holding and processing food.

Limbs and Pelvic Girdle
The humerus contains two foramina. One is above the outer condyle and another above the inner condyle. These may help in muscle attachment. The wrist region has eleven carpal bones. Four bones in the proximal row including pisiform, two centralia and five bones in the distal row. The pelvic girdle shows pubes united in front forming symphysis. An epipubis is present in front which is cartilaginous. Behind the pubes a cartilaginous hypoischium is attached to the ischia. In the ankle the tibial and fibular elements are distinct but firmly joined. The intermedium and centrale are attached to the tibiale and three distal tarsal bones support the foot.

Alimentary Canal
The teeth of Sphenodon are pointed, triangular and laterally compressed. These teeth are present in two rows on the maxilla and palatine. The lower jaw has single row which fits between the two upper rows. This arrangement helps in shearing the food. In adults the teeth get worn and form continuous ridges. In young ones a tooth is present on each vomer which is a primitive condition.

Respiratory System
The trachea is long and its anterior end expands to form larynx. The walls of trachea have cricoid and arytenoid cartilages for support. Sphenodon produces a soft croaking sound like a frog. The lungs are similar to lizards and adapted for terrestrial respiration.

Heart
The sinus venosus is present but not easily seen externally. The heart otherwise resemble that of lizards.

Jacobson’s Organ
Jacobson’s organ is present but in primitive condition. It takes part in smell detection though less developed than in modern reptiles.

Parietal or Pineal Eye
A median pineal eye is present in the parietal foramen. It is not used for vision but helps in regulating biological rhythm and hormone release. In young ones it has a lens and retina and covered by a transparent scale. A nerve from brain supplies this eye which later degenerates.

Tympanic Membrane
The tympanic membrane is not visible from outside. It is seen only after removing skin from the ear region. The tympanic cavity is represented by a large pharyngeal recess which shows primitive character.

Systematic Position of Sphenodon

Ancient Origin
Sphenodon is an ancient reptile which originated in the Permian period. It is the process where the early reptiles were diversifying into different groups. Fossil forms of Sphenodon itself is not found but the order Rhynchocephalia is well known from fossil records. This absence of direct fossils makes the present species more significant.

Living Fossil
Sphenodon is referred to as a living fossil. It is because many primitive characters are retained. These characters show close resemblance to the early stem reptiles. The anatomy and behaviour remain almost unchanged for millions of years. These features indicate slow evolutionary changes.

Evolutionary Relationships
Sphenodon shares some external resemblance with lizards. It also shows some similarity with crocodiles, turtles and even with certain dinosaur groups. But the overall body plan is different from all these groups. Hence it cannot be placed in Squamata or any other reptile order. This is referred to as the main reason why it is grouped separately.

Order Rhynchocephalia

Sphenodon is the only surviving member of Rhynchocephalia. This order was once widespread but now only Sphenodon remains. The characters of skull, teeth and reproductive organs supports this separate position. These are primitive traits not seen in modern lizards.

Isolation and Significance

The species survives only in some island regions of New Zealand. It is believed that this isolation helped in retaining primitive characters. The absence of predators and competition allowed the ancestral features to continue. Thus Sphenodon is important for understanding reptile evolution.

Systematic Position

Kingdom – Animalia
Phylum – Chordata
Subphylum – Vertebrata
Class – Reptilia
Order – Rhynchocephalia
Genus – Sphenodon
Species – Sphenodon punctatus

Species of Sphenodon

Sphenodon punctatus

Sphenodon punctatus is known as the Northern Tuatara. It is the more common species. The body colour is olive-green or grey and sometimes it may show dark pink or brick-red shade. The skin usually has white spots which gives a mottled appearance. The males are larger and may reach about 61 cm in length and around 1 kg in weight. The females are smaller and usually about 45 cm long. It is the species found on many islands of New Zealand mainly in coastal regions. The size and coloration help this species to adjust in these environments.

Sphenodon guntheri

Sphenodon guntheri is called the Brothers Island Tuatara. It is mainly restricted to North Brother Island in Cook Strait. The population is small, around 400 individuals. This species is smaller than Sphenodon punctatus. The body weight is about 660 g. The colour is olive-brown with yellowish patches. These features separate it from the Northern Tuatara. It is the species showing adaptation to a limited island habitat.

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– Sphenodon survived for a long period mainly due to the absence of natural predators in its habitat. It is the condition where New Zealand had no placental mammals for millions of years. There was no competition for food and no major predatory pressure. This allowed the species to continue in a stable environment. The ecosystem remained unchanged for long time which helped Sphenodon to retain many primitive characters.
• Slow Metabolism– The metabolic rate of Sphenodon is very slow. This is referred to as an important factor for its survival. Slow metabolism helps in conserving energy. It can survive on limited food supply because the energy demand is low. This feature is useful when food is scarce. It also helps the animal to live in cool climates where metabolic activities remain slow.
• Lethargic Habit– Sphenodon shows a lethargic lifestyle. The movements are slow and the activity level remains low. This reduces the chances of danger. It is the behaviour that avoids unnecessary exposure to predators or environmental risks. The slow habit also reduces energy expenditure.
• Long Incubation Period– The eggs of Sphenodon have a long incubation period. It takes several months for hatching. This long developmental period gives slow but steady growth of embryo. The conditions remain stable and young ones get better chances of survival. It is considered one of the important reasons behind the continuity of this species.

Why called a living fossil?

Sphenodon is referred to as a living fossil because it is the only surviving member of the ancient reptilian order Rhynchocephalia. It is the group that was widely distributed in the Permian period. Most of the closely related forms is extinct, but this animal is still present with many primitive features.

It is the organism that shows several characters which resemble the early reptiles. These characters remain almost unchanged for millions of years. This is referred to as the reason why it is considered the living representative of the past.

Some of the main primitive features are–

• Skull structure– The skull bones of Sphenodon are arranged in a typical ancient pattern. It is similar to the skull of extinct reptiles. The skull is completely diapsid, and the temporal region is not modified like modern lizards.
• Teeth fused with jaw bones– The teeth are acrodont and remain fused with the jaw bones. It is the primitive condition, and the replacement of teeth does not occur in adult stages.
• Presence of pineal foramen– It has a well-developed pineal foramen on the skull roof. This is the place where the pineal eye or third eye is present. It is a characteristic of early reptilian forms.
• Resemblance with dinosaurs and extinct reptiles– The general body structure, the nature of vertebrae, ribs with uncinate process, and the gait show similarities with ancient reptiles that lived with dinosaurs.
• Similarity with ancient homaeosaurs– It shows close similarity with homaeosaurs, which is an extinct group. The main difference is the presence of uncinate process in Sphenodon ribs, but many characters are same as those ancient reptiles.

Justification as a living fossil

Because it has survived from the ancient geological period without much evolutionary change, Sphenodon is considered a living fossil. It is the animal that still carries the primitive reptilian features which were present in its extinct relatives. It is also called the “Voice of the Past” as it represents the early history of reptilian evolution.

Affinities of Sphenodon

Sphenodon shows primitive characters that link it with different vertebrate groups. It is the animal that holds a separate position in reptiles, but many features show that it shares some relation with amphibians, chelonians, crocodilians and lizards. These affinities arise because it has retained several ancestral features.

Affinities with Amphibia

The circulatory system shows some primitive characters which resemble amphibians, especially the Urodela group.

• The aortic arches arise from a short common trunk which is similar to the condition of conus arteriosus in amphibians.
• Ductus arteriosus and ductus caroticus is present, and this is the primitive arrangement.
• The general pattern of blood vessel distribution is also similar with amphibians.

These characters show that some of the primitive circulatory features is retained, but it does not indicate a close relationship.

Chelonian Affinities

Some important features show resemblance with turtles.

• The quadrate bone is fixed, and this resembles chelonians.
• The caudal ribs are fused with vertebrae, and this condition is also present in turtles.
• A urinary bladder is present.
• The pecten of the eye is absent.

Some differences are also observed.

• Sphenodon has true teeth, but chelonians have a beak-like structure.
• The vomer is unpaired.
• Sternum is absent.
• Chelonians generally show aquatic habitat while Sphenodon is terrestrial.

Crocodilian Affinities

Some characters link Sphenodon with crocodiles.

• The skull shows the diapsid condition.
• The quadrate is fixed.
• Pro-atlas is present in both groups.
• Abdominal ribs are present.
• Caudal ribs are fused with vertebrae.
• Ribs show an uncinate process.
• Chevron bones are present.
• The cochlear process is tubular.

Some differences separate them.

• Crocodiles show thecodont dentition, but Sphenodon teeth is acrodont and fused with the jaw.
• Crocodiles have single external nostril, but two are present in Sphenodon.
• Vertebrae are procoelous in crocodiles, but amphicoelous in Sphenodon.
• Clavicles are present in Sphenodon but not in crocodiles.
• Pecten is present in crocodiles but absent in Sphenodon.
• A distinct penis is present in crocodile males, but absent in Sphenodon.

Affinities with Lacertilia (Lizards)

Sphenodon shares general body plan with some lizards.

• Pro-atlas is present in both.
• Single-headed ribs occur.
• Chevron bones are seen.
• Respiratory organs are similar.
• Parietal organ is present.
• Cloacal glands are present.
• Some geckos show amphicoelous vertebrae like Sphenodon.

Differences show that Sphenodon is not a true lizard.

• Quadrate is fixed in Sphenodon, but movable in most lizards.
• Most lizards have procoelous vertebrae but Sphenodon has amphicoelous vertebrae.
• The rami of jaw are united by symphysis in lizards but not in Sphenodon.
• The ilium is erect in Sphenodon.
• Clavicles and interclavicles are present in Sphenodon but many lizards lack these.
• Conus arteriosus is present in lizards but absent in Sphenodon.
• Pecten in the eye is present in Lacertilia but absent in Sphenodon.
• Uncinate process is absent in lizards.
• Copulatory organs are present in Lacertilia but Sphenodon lacks them.
• Lower temporal arch is present in lizards but not in Sphenodon.