Rhynchocephalia, an Order of Class – Reptilia is considered as an aberrant group of reptiles which appeared first in Triassic and passed smoothly through the Mesozoic. It is a true diapsid reptile now represented today by one very conservative living species Sphenodon punctata. It is called by the natives of New Zealand as ‘tuatara’ (tua = back, tara = spine).
It is a lizard like reptile which grows to about 2 feet long and has well developed pentadactyle limbs adapted for walking. Gadow refers it as “the last living witness of the by-gone ages”. This primitive reptile is called the “living fossil”. It is a very lazy creature. It lives in the holes on the slopes of the sand hills of the shore with the petrels on amicable returns. This animal is rigidly protected by law. It emits a soft frog like croak. It feeds on insects, crustaceans and worms.
Sphenodon was formerly known on the main islands of New Zealand, but is now restricted to some small islands in the Bay of Plenty and appears to be on the verge of extinction.
General anatomical features
(1) The lizard like body of Sphenodon is about 2 feet long and the tail measures about 1 /3 of the whole length of the body- The animal is nocturnal.
(2) The body and tail are laterally compressed and a series of spines are dorsally present (along the middle line).
(3) Limbs are pentadactyle, legs are short and primitive in nature. Sometimes 10-11 carpels are present.
(4) Hind limbs are typically plantigrade.
(5) The upper surface of the body is covered with small granular scales and the lower surface is covered with transverse rows of large squarish scales.
(6) The head is large and there is prominent ridge over the eye.
(7) There is a foramen above the outer and one above the inner condyle of the humerus.
(8) There are eleven carpal elements in all, four in the proximal row, two centrals and five in the distal row.
(9) In the tarsus the tibial and fibular elements are quite distinct, though firmly united. The intermedium and the centrale are firmly fixed to the tibiale. There are three tarsal bones.
(10) Teeth are pleurodont and homodont like reptiles but they are monophyodont. The teeth are pointed, triangular, laterally compressed and arranged in two parallel rows, one along the maxilla, and the other along the palatine. The teeth of the lower jaw bite in between the two upper rows, all the rows becoming worn down in such a way as to form continuous ridges. Each premaxilla bears a prominent, chisel-shaped incisor, represented in the young animal by two pointed teeth. In the young Sphenodont a tooth has been found on each vomer — a condition unusual among reptiles.
(11) The pubes are united in a Symphysis, infront of which is a cartilaginous epipubis. A large oval foramen intervenes between the ischium, and the pubis. A cartilaginous hypoischium is attached to the ischia behind.
(12) In the skull, there is a complete lower temporal arch. The quadrate is immovably fixed, wedged in by the quadrate- jugal, squamosal, and pterygoid. The premaxilla are not fused together but separated by a suture. There is a broad palate, formed by the plate-like vomer, palatine and pterygoids. The pterygoids meet in the middle line and extend forwards to the vomers. There is an epipterygoid extending from the parietal to the pterygoid and quadrate.
(13) Sphenodon retains the large parietal foramen, in which, is the non-functional median pineal eye.
(14) There is no antorbital foramen.
(15) The anal opening of Sphenodon is transverse.
(16) There is no copulatory organ (Penis) in the male.
(17) The vertebrae are amphicoelous and fully ossified and intercentra are present.
(18) The ribs are single headed and have uncinate processes. There is a sternum and abdominal ribs.
(19) Chevron bones occur in the caudal region of the vertebrae.
(20) There is a median bone, the Pro-atlas, intercalated between the atlas and the occipital region of the skull.
(21) Caudal ribs become fused with the vertebrae.
(22) The lateral temporal fossa is bounded below, by an inferior temporal arch composed of jugal and quadrato-jugal (Palato-quadrate).
(23) Jacobson’s organ is present in a rather primitive form.
(24) A urinary bladder is present.
(25) There is a T-shaped interclavicle.
(26) Coracoid is without fenestra.
(27) It lays 10-13, leathery shelled eggs which are buried few inches below the surface of the soil. The young ones are hatched 13 months later.
Causes of its long survival
There are three important causes of its long survival. These are —
a) It was free from its enemies because the placentals and other large terrestrial animals were introduced in New- Zealand quite lately.
b) Slow rate of metabolism and conservation of sufficient energy.
c) Lethargic habit and long period of incubation of the fertilized eggs.
Why called a living fossil
The living fossil is an animal that has survived beyond its era. It is the one species or such a group of species that has continued existence when the other closely related contemporary species have become extinct. Sphenodon is the sole survival of the order Rhynchocephalia. Its anatomical features indicate several primitive characters such as —
1) The skull bones are shaped and disposed in the manner of extinct group of reptiles.
2) Teeth are fused to the jaw bones.
3) Presence of pineal foramen.
4) There are several characters (discussed in affinities) which resemble that of Dinosaurs and other extinct reptiles.
5) It resembles with the ancient Homaeosaurs in several features except the presence of uncinate process of the ribs in Sphenodon.
Thus, the term ‘living fossil’ is justified for Sphenodon. This animal which has been called the “Voice of the Past” has been given absolute protection by law in New Zealand.
Sphenodon (Order – Rhynchocephalia) possesses several characters which are present in different groups of animals. So, actual position of Sphenodon in the animal kingdom was somewhat controversial for some time. Its affinities with different groups have been discussed below:
Affinities with amphibia
The circulator)’ system of Sphenodon has some resemblances with Caudata or Urodela. These resemblances are —
1) Aortic arches arise from a short common stalk which is comparable to conus arteriosus of the amphibians.
2) Presence of ductus arteriosus and ductus caroticus
3) The pattern of distribution of blood vessels almost similar but, these resemblances are mainly because of the reason that Sphenodon is an animal of primitive nature. Because of the presence of several reptilian features which have been discussed below, amphibians affinities can not be accepted.
Affinities with Dinosaurs
There are some resemblances with the dinosaurs. These are—
1) Diapsid condition of the skull i.e. the skull has two vacuities (fossae).
2) Fixed quadrate bone
3) Abdominal ribs.
4) Uncinate process of the ribs.
But, there are some differences also — e.g.
1) In Dinosaurs, double headed ribs are present – not single headed like Sphenodon.
2) Dentition is thecodont in dinosaurs
3) Absence of clavicle and interclavicle
4) Absence of Parietal foramen
5) Absence of third eye elements
So, Sphenodon can not be linked with the dinosaurs, however, its reptilian ancestry is evident.
The following are the important resemblances with the chelonians:
1) Fixed quadrate bone
2) Ribs of caudal region fused with the vertebrae
3) Pecten not present in the eye
4) Urinary bladder present
The differences with chelonians include the following —
1) Sphenodon is terrestrial but chelonians are aquatic.
2) Teeth absent in the chelonians, only horny beaks present
3) Parietal foramen in chelonians
4) Unpaired vomer
5) No sternum in chelonia
6) Cloacal opening longitudinal in chelonia but transverse in Sphenodon
7) Males have penis in chelonia
8) Opening of oviduct on ventral side
Affinities with Crocodilia
The following are the important resemblances.
1) Diapsid condition of the skull
2) Fixed quadrate
3) Presence of Pro-atlas
4) Presence of abdominal ribs
5) Fusion of caudal ribs with vertebrae
6) Uncinate process of ribs
7) Presence of chevron bones
8) Cochlear process tubular
The differences between the two are:
1) Thecodont dention in crocodiles
2) Single nostril in Crocodile but double in Sphenodon
3) Procoelons vertebrae in Crocodile and amphicoelous in Sphenodon
4) Clavides not present in Crocodilia.
5) Pecten present in Crocodiles
6) Penis present in male Crocodile
Affinities with Lacertilia
The important resemblances are the following —
1) The body plan is similar
2) Pro-atlas present
3) Amplucoelous vertebrae in certain geckos
4) Single headed ribs
5) Chevron bone present in both
6) Structure of the respiratory organs
7) Parietal organ common
8) Cloacal glands present
However, there are some differences also, which are —
1) Quadrate fixed in Sphenodon
2) Most lizards have procoelous vertebrae
3) Rami of the jaw united by symphysis in lizards
4) Erect ilium in Sphenodon
5) Clavicles and interclavicles present in Sphenodon
6) Absence of conus arteriosus in Lacertilia
7) Presence of Pecten
8) Uncinate process of tire ribs absent in Lacertilia
9) Copulatory organs present in Lacertilia
10) Presence of lower temporal arch
Thus, we see that Rhynchocephalia retains many primitive characters and resembles chelonians, crocodilia and dinosaurs in many features. But it is convincing that Sphenodon is more closely allied to the lacertilia than any other group discussed above.
Because of the occurrence of certain peculiar features it seems justified that the Rhynchocephalia should be placed as a separate order of class-Reptilia as suggested by Romer.