Adaptive radiation in reptiles
As competition for food and living space prevails, a single ancestral species evolves into different forms which occupy different habitats. This is called adaptive radiations or divergent evolution. Reptiles have shown the greatest evolutionary diversity and adaptive radiation of all vertebrates. Their adaptive radiation took place twice, first in the Paleozoic and secondly in the Mesozoic.
Paleozoic radiation
During Paleozoic, with no competitors on land, the ancestral reptiles or cotylosaurs multiplied rapidly occupying all ecological niches available to them. Their radiation involved adaptations to different methods of locomotion and feeding. Distinct anapsid and synapsid forms dominated.
Mesozoic radiation
By the end of Paleozoic , the ancestral colylosaurs had disappeared. Their descendants produced a second and bigger radiation during Mesozoic. They dominated not only land but also sea and the air. This Mesozoic era is called as Age of Reptiles. This lasted over a great span of time about 130 million years. By comparison man is no older than 2 to 4 million years. The Mesozoic reptiles are represented by as many as 16 Orders. Of these one led to birds, one led to mammals and four to the modern reptiles. The rest disappeared. We will discuss only notable lines based on the morphology of the skull such as anapsid, synapsid, euryapsid, parapsid and diapsid .
Anapsid line
The modern Chelonia (turtles and turtoises) represent a direct and an early offshoot of cotylosaurs retaining anapsid skull. They have remained unchanged since Triassic some 160 million years ago.
Synapsid line
The mammal like reptiles or Synapsid had a single temporal cavity in skull ventral to postorbital and squamosal. Early Pelycosauria or Theromorpha were similar to cotylosaurs. Later Therapsida with differentiated dentition and improved locomotion were more mammal like. Before disappearing in Jurassic they gave rise to ancestral mammals.
Euryapsid line
The euryapsid or plesiosaurs had a single temporal fossa in skull above the joint of postorbital and squamosal. They were large marine turtle like heavy bodied and logn necked creatures. They were obviously fish eaters. All became extinct towards end of Cretaceous.
Parapsid line
There was another marine blind alley like Euryapsida represented by fish like or purpoise like ichthyosaurs. They also became extinct.
Diapsid line
Most of the today’s reptiles are diapsid with tow temporal openings on either side of skull separated by squamosal and postorbital bones. The earliest diapsids divide into two branches like Lepidosauria and Achosauria. The Lepidosauria were probably the ancestral to modern Squamata ( snakes and lizards) and Sphenodon. The Archosauria were the ruling reptiles dominating the Mesozoic Era. They represented the extinct Pterosauria, the extinct Dinosaurs and modern Crocodilia. They also gave rise to the modern birds.
A) Pterosauria
The extinct flying reptiles called Pterosauria or Pterodactyla were of light built. Their fore limbs evolved into membranous wings or patagia. Rhamphorhynchus of late Jurassic was a primitive pterosaur with 1 meter wingspan, a long balancing tail and toothed jaw. Pteranodon of Cretaceous had a 9 meter wingspan but no tail .
B) Dinosaur
At the end of Triassic, Thecodontia, the early descendents of Archosauria gave rise to the most fantastic Mesozoic reptiles the dinosaurs ( Gr. Dinos, terrible+saurus , lizard). They sub divided early into two orders Saurischia and Ornithischia.
Saurischia means “reptiles hip” they possessed a tri radiate pelvis with pubis entirely separate and anterior ischium. Sub order Thorapoda included flesh eating and bipedal carnivores. Tyrannosaurus rex from Cretaceous was 15 meters long and stood 6 meters high. Branchiosaurus was more than 25 m long and weighed over 50 tons.
Ornithischia means “ bird hip” they had a typical tetraradiate bird like pelvic girdle with pubis directed backwards parallel to ischium. They were all herbivores. Stegosaurus measures 8 meters and weight 10 tons. Fore limbs were much shorter than hind limbs.
Causes of extinction
After thriving and dominating the earth for 130 million years, the great dinosaurs and their contemporaries became suddenly extinct by the end of Cretaceous period. Various factors have been suggested for their total extinction like catastrophism, epidemic, food poisoning, racial senescence, climate change, overspecialization, intra specific warfare. Probably a combination of several factors was responsible for their extinction.
As competition for food and living space prevails, a single ancestral species evolves into different forms which occupy different habitats. This is called adaptive radiations or divergent evolution. Reptiles have shown the greatest evolutionary diversity and adaptive radiation of all vertebrates. Their adaptive radiation took place twice, first in the Paleozoic and secondly in the Mesozoic.
Paleozoic radiation
During Paleozoic, with no competitors on land, the ancestral reptiles or cotylosaurs multiplied rapidly occupying all ecological niches available to them. Their radiation involved adaptations to different methods of locomotion and feeding. Distinct anapsid and synapsid forms dominated.
Mesozoic radiation
By the end of Paleozoic , the ancestral colylosaurs had disappeared. Their descendants produced a second and bigger radiation during Mesozoic. They dominated not only land but also sea and the air. This Mesozoic era is called as Age of Reptiles. This lasted over a great span of time about 130 million years. By comparison man is no older than 2 to 4 million years. The Mesozoic reptiles are represented by as many as 16 Orders. Of these one led to birds, one led to mammals and four to the modern reptiles. The rest disappeared. We will discuss only notable lines based on the morphology of the skull such as anapsid, synapsid, euryapsid, parapsid and diapsid .
Anapsid line
The modern Chelonia (turtles and turtoises) represent a direct and an early offshoot of cotylosaurs retaining anapsid skull. They have remained unchanged since Triassic some 160 million years ago.
Synapsid line
The mammal like reptiles or Synapsid had a single temporal cavity in skull ventral to postorbital and squamosal. Early Pelycosauria or Theromorpha were similar to cotylosaurs. Later Therapsida with differentiated dentition and improved locomotion were more mammal like. Before disappearing in Jurassic they gave rise to ancestral mammals.
Euryapsid line
The euryapsid or plesiosaurs had a single temporal fossa in skull above the joint of postorbital and squamosal. They were large marine turtle like heavy bodied and logn necked creatures. They were obviously fish eaters. All became extinct towards end of Cretaceous.
Parapsid line
There was another marine blind alley like Euryapsida represented by fish like or purpoise like ichthyosaurs. They also became extinct.
Diapsid line
Most of the today’s reptiles are diapsid with tow temporal openings on either side of skull separated by squamosal and postorbital bones. The earliest diapsids divide into two branches like Lepidosauria and Achosauria. The Lepidosauria were probably the ancestral to modern Squamata ( snakes and lizards) and Sphenodon. The Archosauria were the ruling reptiles dominating the Mesozoic Era. They represented the extinct Pterosauria, the extinct Dinosaurs and modern Crocodilia. They also gave rise to the modern birds.
A) Pterosauria
The extinct flying reptiles called Pterosauria or Pterodactyla were of light built. Their fore limbs evolved into membranous wings or patagia. Rhamphorhynchus of late Jurassic was a primitive pterosaur with 1 meter wingspan, a long balancing tail and toothed jaw. Pteranodon of Cretaceous had a 9 meter wingspan but no tail .
B) Dinosaur
At the end of Triassic, Thecodontia, the early descendents of Archosauria gave rise to the most fantastic Mesozoic reptiles the dinosaurs ( Gr. Dinos, terrible+saurus , lizard). They sub divided early into two orders Saurischia and Ornithischia.
Saurischia means “reptiles hip” they possessed a tri radiate pelvis with pubis entirely separate and anterior ischium. Sub order Thorapoda included flesh eating and bipedal carnivores. Tyrannosaurus rex from Cretaceous was 15 meters long and stood 6 meters high. Branchiosaurus was more than 25 m long and weighed over 50 tons.
Ornithischia means “ bird hip” they had a typical tetraradiate bird like pelvic girdle with pubis directed backwards parallel to ischium. They were all herbivores. Stegosaurus measures 8 meters and weight 10 tons. Fore limbs were much shorter than hind limbs.
Causes of extinction
After thriving and dominating the earth for 130 million years, the great dinosaurs and their contemporaries became suddenly extinct by the end of Cretaceous period. Various factors have been suggested for their total extinction like catastrophism, epidemic, food poisoning, racial senescence, climate change, overspecialization, intra specific warfare. Probably a combination of several factors was responsible for their extinction.
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