In this article we will discuss about Extinct Dinosaurs:- 1. Types of Extinct Dinosaurs 2. Causes of Dinosaur Extinction 3. Biological Views.
Types of Extinct Dinosaurs:
Reptile-like Dinosaurs (Saurischia):
The earlier members of this order were small, bipedal and carnivorous and resembled the ancestral form—the Thecodont.
Later from them arose two divergent groups:
Theropoda and Sauropoda.
I. Suborder 1. Theropoda:
(i) They were long-legged forms of moderate size.
(ii) The illustrious members of this group are the ‘Ostrich dinosaurs’ and the great carnivorous dinosaurs.
(iii) The ostrich dinosaurs resembled an ostrich in construction and size.
(iv) The foot was three-toed and the leg was bird-like.
(v) Forelimbs were well-developed.
(vi) They had no teeth and they had horny beak.
(vii) They were egg-eaters and they used their forelimbs to grasp the stolen eggs.
The carnivorous dinosaurs first appeared in the upper Triassic. They became progressively larger, powerful and reached to giant forms in Cretaceous. These giant forms are exemplified by Ornitholestes, Ornithomimus, Tyrannosaurus, Coelophysis, Megatosaurus, Dinonychus etc.
The group is called the “earliest dinosaurs”. They appeared in the Triassic Period.
a. Ornitholestes [Gk. Bird catcher]:
(i) They were about 3.4 meters (11 feet) in length.
(ii) They were bipedal with elongated neck and tail.
(iii) The short forelimbs were used for food-collection.
(iv) They were very agile, carnivorous dinosaur, preying on lizards and young dinosaurs.
b. Ornithomimus [Gk. Bird imitator]:
(i) Ornithominus was a saurischian dinosaur.
(ii) It was very similar to that of an Ostrich and often called “Ostrich dinosaur”.
(iii) The average length was between 3.5 and 4 metres.
(iv) They were strongly bipedal with slender, long hind limbs and short forelimbs.
(v) They walked on three toes and had 3 opposable clawed fingers on hand.
(vi) The head was small and neck was long and flexible.
(vii) Jaws were without teeth, replaced by horny beak.
(viii) The lower leg bone was longer than the thigh bone, suggesting that they were quick runners.
(ix) Their diet includes small lizards and mammals, fruits and eggs. Ornithomimus lived in the upper Cretaceous in different habitats, ranging from swamps, lakes to open forest areas.
c. Tyrannosaurus [Gk. Tyrant lizard (Fig. 8.73)]:
Of all dinosaurs, Tyrannosaurus rex is called the “King of tyrant dinosaurs”.
(i) They were heavily built and were over 5 metres tall, 13 m from nose to the tip of the tail, and estimated weight is about 8 tonnes.
(ii) The hind limbs were pillar-like, massive but forelimbs were minute structures.
(iii) Each foot was provided with three powerful toes ending in curved claws and hands had two clawed fingers.
(iv) The head was enormous. The jaws were over 1 metre in length bearing numerous dagger-like, 15 cm teeth.
(v) They were bipedal predator, and were adapted for hunting and killing other dinosaurs that roamed in the plains in this time.
It was the largest terrestrial dinosaur that lived in the upper Cretaceous period of North America and Mongolia, about 75-70 million years ago.
d. Coelophysis [Gk. Hollow form]:
(i) Coelophysis was an early theropod dinosaur and an ancestor of “Ostrich dinosaurs”.
(ii) They were small and bipedal and were about 3 metres in length and 20 kgs in weight.
(iii) The short forelimbs with two long middle fingers and two shorter outer ones were used for grasping food.
(iv) The hind limbs were strong and long, used for walking.
(v) Neck was long.
(vi) The head was long and pointed, and armed with numerous serrated teeth.
(vii) The tail was long and slender, and the length was more than half of the whole animal.
(viii) Diet included reptiles and small mammals and sometimes became cannibalistic when food was short.
They lived in dried up land areas during the Upper Triassic times and fossils were unearthed near northern New Mexico.
e. Megalosaurus [Gk. Big lizard]:
(i) Megalosaurus, a Jurassic and Lower Cretaceous form, was 6 to 7 metres in length, and weight was about 2.5 tonnes.
(ii) They were bipedal and the hind limbs were several times larger than the forelimbs.
(iii) The head was large and the jaws were armed with large, sharp teeth.
(iv) Hind limbs had three large, clawed toes, used in the prey.
(v) It existed for over 90 million years and was one of the most successful vertebrates ever known.
Megalosaurus was the first dinosaur to be discovered by man.
f. Deinonychus [Gk. Terrible claw]:
(i) Deinonychus was a bipedal, lightly built, advanced coelurosaur.
(ii) It was about 3 metres in length.
(iii) They had large head and comparatively large eyes.
(iv) The forelimbs possessed three powerful clawed fingers, measured about over 12 cm each.
(v) The hind-limbs were slender, strong and also bore three clawed toes.
(vi) Deinonychus was discovered in 1964 in the Lower Cretaceous of Montana, U.S.A., whilst from the Upper Cretaceous of Mongolia in 1965. They were the most fearsome creature in its period for their “terrible claw”.
Their ability and speed indicate that they had the high metabolic rate and were worm- blooded creatures.
II. Suborder 2. Sauropoda:
The Sauropoda started their career as small bipedal forms but progressively they became large and massive and became quadruped secondarily. In almost all sauropods the hind legs were disproportionately large.
Sauropods were herbivorous quadrupeds. They are mostly of large size and included largest land living animals. The best known genera of the Sauropoda are Diplodocus, Apatosaurus ( = Brontosaurus), Camarosaurus and Brachiosaurus.
Diplodocus [Fig. 8.74]:
(i) Diplodocus was a giant, herbivorous, quadrupedal sauropod.
(ii) It was over 30 metres long and weight was about 50800 kg.
(iii) The skull was short and broad.
(iv) The weak jaws and small-sized teeth in the front part of the mouth indicate that they were herbivorous, because the small teeth served the purpose of cropping.
(v) They were swamp dwellers, feeding on aquatic vegetation.
The fossils of Diplodocus carnegii first discovered by E. Douglass (1909) from the Vernal site (U.S.A.).
a. Apatosaurus [Gk. Deceptive lizard] (= Brontosaurus) (Fig. 8.75A):
(i) Apatosaurus was a giant sauropod that lived during Jurassic period. The fossils were described under two names until recently—Apatosaurus and Brontosaurus, of which the last one is well known.
(ii) The creature was over 22 metres (72 feet) in length and estimated weight was about 30 tonnes.
(iii) They were quadrupedal and the forelimbs were slightly smaller than the hind limbs. Both the limbs were pillar-like and bulky. The animal was heavier than Diplodocus for its heavier limb bones and more bulky body.
(iv) It had a large head but small brain, and thick elongated neck and tail.
(v) The vertebral column was made up numerous large vertebrae, which had strong spines, that provided centres for muscle attachment.
(vi) Teeth were leaf-shaped, used for the feeding of soft vegetation.
(vii) Formerly it was thought that Apatosaurus were swamp dwellers but according to Moody (1977), “they were plain and forest dwellers and used its long neck to browse off tall vegetation”. However, they became extinct during Cretaceous period by the carnivorous land-dwelling carnosaurs.
The remains of Apatasaurus louisiae also first collected by E. Douglass (1909) from the Vernal site of U.S.A.
b. Brachiosaurus [Gk. Arm lizard]:
(i) Brachiosaurus was the largest animal ever to have lived on land.
(ii) They are known from Upper Jurassic deposits of North America and East Africa.
(iii) It was over 26 metres (85 feet) in length and estimated weight was about 75 tonnes, and little smaller than Brontosaurus.
(iv) The forelimbs were larger than the hind limbs, so the back sloped down towards the tail.
(v) The head was small and the brain was also excessively small.
(vi) They had elongated necks and tails.
(vii) The limb bones were non-pneumatic.
(viii) Small skull and small-sized teeth are found on the front part of the jaw.
(ix) They were herbivorous, slow-mover and lived almost submerged in water.
c. Bird-like Dinosaurs (Ornithischia):
The bird-like Dinosaurs have been grouped into four suborders:
(1) Ornithopoda,
(2) Stegosauria,
(3) Ankylosauria and
(4) Ceratopsia.
1. Ornithopoda:
(i) They were the most primitive of all bird-like Dinosaurs and probably they descended from some thecodont stalk. Most of them were bipedal.
(ii) The illustrious members are Camptosaurus and Iguanodon (Fig. 8.75D).
(iii) Iguanodon was about 4.5 metres in length.
(iv) The peculiar feature of Iguanodon was that its thumb was like that of sharp dagger.
(v) As they were herbivorous it is believed that the dagger-like thumb was used for defence.
2. Stegosauria:
(i) They possessed a peculiar armature in the form of heavy plates and spines (Fig. 8.75C).
(ii) In size they were like present-day elephant.
(iii) Their head was reduced and the brain was very small.
(iv) The spinal cord at the base of the tail was enormously enlarged and according to some it formed the real brain at that unusual site.
(v) At their back they had two rows of large plates and the powerful tail had two or more pairs of ‘foot-long’ spines (Fig. 8.75C).
3. Ankylosaurs:
They too were armed Dinosaurs and took the place of the Stegosaurs during the Cretaceous. They are also called ‘Reptilian tanks’ for the completeness of their armature.
4. Ceratopsia:
(i) They are also known as horned Dinosaurs. They stand as end products of Dinosaurian evolution.
(ii) They were moderate in size, like that of present-day Rhinoceros.
(iii) They had enormously large head armed with long horns.
(iv) The striking feature was its large and bony collar frill. This frill extended sideways and backwards forming a shield over the shoulder.
(v) The front part of the upper jaw formed a small and hooked beak.
(vi) They were herbivorous.
Triceratops is a well-known example (Fig. 8.75B).
Causes of Dinosaur Extinction:
The dinosaurs which roamed the earth about 150 million of years, ruled majestically in all environments, such as land, water and air, over 140 million years, suddenly disappeared in the end of Mesozoic era. No fossils are recorded after Cretaceous period. Why they failed to survive after the Cretaceous period (the K-T boundary), no satisfactory explanation prevails among the palaeontologists or zoologists.
The extinction of dinosaurs (K-T extinction) from the surface of earth as well as from the water and air, on the whole, is dramatic and mysterious. Many views have put forward from time to time to explain the dramatic disappearance of the dinosaurs and some of them are bizarre and even incredulous.
Biological Views on Extinction of Dinosaurs:
a. Competition:
The extinction of dinosaurs is due to the competition with the small, predaceous agile mammals. The predaceous mammals stole the eggs from the nests of dinosaurs, causing their extinction of their races.
Remarks:
According to Moody (1977), “at the end of Cretaceous period, the mammals were small and mostly insectivorous, hardly any mammal could challenge and replace the dinosaurs. The real radiation of mammals took place after a few million of years of extinction of dinosaurs”. So this view demands a little support.
b. Racial senescence:
It is believed that the groups of individuals became old and followed extinction due to senile degeneration.
c. Diseases:
It is suggested that most of the dinosaurs became extinct due to numerous illness and diseases with slipped discs. Some others again put forward the theory that new kinds of insects evolved which spread deadly diseases.
d. Size:
Some of the dinosaurs were to gigantic in size. With this enormous size they were too clumsy to survive. These gigantic forms possessed very small brains and they were not brainy enough to make rational decisions in adverse situations.
e. Vegetation:
In the Cretaceous period the changes of vegetation had begun and new flowering plants appeared, that was the direct cause of extinction of numerous Ornithopod families which had adapted in feeding on them. The appearance of new floras containing a high percentage of alkaloids, made many plants un-consumable to the dinosaurs and the dinosaurs fell in death by starvation.
The taste buds could have been poorly developed in herbivorous dinosaurs and the dinosaurs could have died by the accumulation of poison in the body for the consumption of these poisonous vegetation. The new floras could have affected the production of hormones of the dinosaurs, resulting the production of infertile eggs.
f. Changes in Climatic Condition:
By the end of Cretaceous period, the different continents occupied the present day shore outlines and with the change of the position of the land masses, the seasonal changes rather extreme cold highly affected the many regions.
The sudden drop of the temperatures of the living area of dinosaurs affected the metabolic rate, by which the temperature of the body decreased and they were unable to feed. Even warmblooded animals suffered in the climatic extremes in various regions. Dismemberment of various land masses and isolation from each other placed the dinosaurs at the mercy of the localized climatic conditions.
One school of palaeontologists presumes that wide spread draught in the Upper Cretaceous Period, was the result of extinction of dinosaurs.
Drought is related in the deposition of calcium in the shell that a thin-shelled eggs would have affected the development of the embryonic dinosaur.
g. Extraterrestrial Impact Theory:
This theory was put forward in 1980 by Luis Alvarez, a Noble Laureate nuclear Physicist and his son Walter Alvarez, a geologist, both professors at Barkeley, University of California U.S.A. They concluded that at the near end of Cretaceous period, a huge crater of about 180 kilometers was formed when a colossal meteorite or comet plunged out of the sky, striking the earth at a velocity of more than 10 kilometers per second.
It smashed into the Yucatan Peninsula in Mexico about 64.98 million years ago, plus or minus 50000 years. The enormous liberated energy, created environmental disasters like storms, cold, darkness, acid rains and global fires.
This collision might also have created a nuclear winter due to the dust canopy preventing sunlight entry to the earth for years. The darkness and extreme cold weather eliminated most of the marine organisms, plants, animals including dinosaurs.
The blanket of dust cloud of an immense area of the earth created the canopy over the earth with its extraterrestrial cargo of iridium, eventually settling down on the surface of the earth. When the dust of the canopy settled, sunshine reached the land and sea again.
But the moisture still over the earth might have trapped the sun’s heat near the ground, increased in the temperature higher and higher over the surface. Even if heat did not kill the dinosaurs, affected on the eggs to be males. Dinosaurs without females gradually became extinct without copulation.
Cold blooded creatures like turtles, and crocodiles could have survived hiding in the burrows and low metabolism would permit long fast without any much ill-effects. But the dinosaurs with their large body and high metabolism made them vulnerable in this disaster.
The theory developed by the occurrence of abundant iridium in the Cretaceous clay strata that occurred in the village Gubbio, about half way from Florence to Rome. Iridium is a noble metal, extremely rare on the earth’s surface but occurs much in asteroids, meteors and dead stars. It is similar to platinum but denser and like platinum, does not form compounds with other elements.
h. Eruption Theory:
Recent palaeomagnetic work by Vincent E. Courtillot of the Institute of Physics in Paris considers that volcanism may be the prime factor for mass extinction of dinosaurs. He indicates that volcanoes in Deccan Traps, enormous lava in India that occurred about 65 million years ago. Most of the Deccan Traps erupted during a single period of reversed geomagnetic polarity and KT extinction occurred during this period.