According to the textbook version of the Triassic/Jurassic Extinction, not much survived into the Jurassic:
"The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 201.3 million years ago,[1] and is one of the major extinction events of the Phanerozoiceon, profoundly affecting life on land and in the oceans. In the seas, a whole class (conodonts)[2] and 34% of marine genera disappeared.[3] On land, all archosaurs other than crocodylomorphs(Sphenosuchia and Crocodyliformes) and Avemetatarsalia (pterosaurs and dinosaurs), some remaining therapsids, and many of the large amphibians became extinct."
However, the above statement contradicts what we supposedly know of dinosaurs. The various possible ancestors of dinosaurs arose around 240 million years ago, including Dinosauromorpha. A few possible ancestors or Saurichian dinosaurs evolved from this basalk group and continued through the Triassic/Jurassic Boundary. There's a clear evolutionary transition from the Triassic Dinosauromorpha to the Saurischian dinosaurs. Both groups were mostly bipedal, and this explains the difference in hips between Saurischia and other reptiles. But there's nothing to connect these early Dinosauromorpha to Ornithischians. In fact, the true common ancestor of Ornithischia and Saurischia is unknown. We don't even know its probable age. And there's no reason why ornithischians need to have had a bipedal ancestor.
No matter how big quadrupedal reptiles get, they maintain a sprawling posture. A recent study on Varanid lizards confirmed this observation.
http://jeb.biologists.org/content/214/18/3013
http://www.biology.ucr.edu/people/faculty/Garland/ChriGa96.pdf
There is no clear path from Dinosauromorpha, Eoraptor, or Herrerasaurus to non-Saurischian dinosaurs, the large herbivores known as Ornithischians. Most Ornithischians walk on all four legs, though a few species divergent from the basal quadrupedal form are known. Quadrupedal ornithischians walked totally differently from quadrupedal Saurischia, and bipedal ornithischians walked totally differently than bipedal Saurischia. The difference in their hips gave them their names.
Scientists believe ornithischians evolved from bipedal archosaurs, but no explanation exists as to why they should have evolved separate walking styles and features from Saurischians.
A bipedal ancestor for Ornithischians is nothing more than an assumption. And other archosaurs share more traits in common with Ornithischians than Dinosauromorpha.
Take Aetosaurs for instance. Superficially, they resemble the first ornithischians, quadruped dinosaurs with armor plates that later gave rise to stegosaurs and ankylosaurs. The diet and morphology of the two groups is the same, and Aetosaur and ankylosaur teeth are nearly identical. Their clade, Suchia, occupies a sister group position to Dinosauromorpha on the phylogenic tree.
No Dinosauromorpha, Eoraptor, Herrerasaur or Saurischia possessed teeth similar to these.
In order to explain ornithischian armor, scientists suppose that an unknown species of bipedal archosaur rapidly evolved an exoskeleton made of intricately formed bone and scutes (primitive scales). They have no transitional fossils of anything like this occurring. There's no evidence that any bipedal ancestor of dinosaurs even possessed scutes. For aetosaurs, a common ancestry with crocodylomorphs is assumed since they were both protected by the same osteoderm covering.
Aetosaur fossils end exactly at the Triassic/Jurassic Extinction boundary, but soon afterward Thyreophora species begin to appear....at the same time fossils begin to appear again for other animal lineages.
After each major extinction event in Earth's history, fossils become scarce. This isn't only because species died out, it's also because surviving species experienced bottlenecks. Bones only fossilize by rare chance, and when a species is reduced to a small population fossils may not be left at all. Imagining ornithischians and Thyreophora evolved from a species of Dinosauromorpha requires a ghost lineage of over 30 million years, whereas imagining an Aetosaur ancestor requires less than half that time.
Superficially, anatomically, and behaviorally, Ornithischians resemble Aetosaurs more than Dinosauromorpha.
Is this just a coincidence? Should we discard a simple explanation of Ornithischian armor for a complex and unsupported one? Should we ignore the lack of proof for a bipedal ancestor of Ornithischians, and forgo an explanation as to why Ornithischians and Saurischia evolved completely different hips and walking styles?
Should we dismiss the teeth as a coincidence too?
Ankylosaurs survived and diversified through the Jurassic and Cretaceous, only going extinct 66 million years ago at the time of the KTg extinction.
After the KTg extinction, fossils don't appear again with frequency until around 55 million years ago. Major groups in South America don't appear until around 46 million years ago. This is about the same time as the Antarctic/South American exchange, so many of these animals may have migrated from S. America as the continents reconnected.
Since Antarctica is covered with ice, we find few fossils there. However, because of the few fossils we have found there and because of what we know of continental drift we can deduce that many S. American animals lived in Antarctica during the Late Cretaceous and early Paleogene.
One of the animal groups that appears at the time of the first exchange is Xenarthra, considered the most primitive of the four major groups of placental mammals. The earliest Xenarthrans possess exoskeletons made of the same intricate osteoderms, but again no transitional fossils show how or why the osteoderm formed.
Often when school children notice the striking similarity between Ankylosaurs and early Xenarthrans, they are told the resemblance is only superficial and it's just a case of "convergent evolution." But the resemblances are anything but superficial. The skeletons of the creatures show the same fused bones, the same number of vertebrae and teeth, the same bizarre cheek flanges, and the same pair of tiny horns in the supraorbital position of the skull. The youngest Ankylosaurs even show the same number of temporal fenestra as early mammals, which is one of the main ways we tell reptiles and mammals apart. The skulls of Ankylosaurus and Megatherium, another early Xenarthran, are so close that one could be a subspecies of the other.
Ankylosaur
Sloth
Xenarthran
Other than crocodylomorphs and turtles, Aetosaurs were the only animals with osteoderms in the Late Triassic.
Interestingly, Ankylosaurs and their close kin were the only animals with full body osteoderm armor in the Jurassic and Cretaceous. Today. only Xenarthrans have full body osteoderms. The pangolin's armor is not mode of true osteoderms, as it does not contain bone matter. However, pangolins were first placed within Xenarthra, before DNA tests linked them with Carnivora.
So, according to mainstream science, crocodylomorphs and turtles remained mostly unchanged for 300 million years or more, retaining the same osteoderm covering. Meanwhile, three unrelated animal groups evolved similar osteoderms in succession, one right after the other. As soon as Aetosaurs died out, Ankylosaurs rapidly evolved osteoderms and Aetosaur teeth. As soon as Ankylosaurs died out, Xenarthra rapidly evolved the same osteoderm and skull.
Since this happens so quickly in such different animals, with no transitional fossils or explanation for the change, we would think the formation of an intricate full-body osteoderm suit should be quite common. Yet it only seems to happen to one animal group per time period, at the very beginning of the time period directly following a global disaster. During all of the 100 million years of the Jurassic and Cretaceous, for instance, not a single other group of animals with a full osteoderm covering arose.
Since exoskeletons are present in ancient fish and even amphibians, it's not hard to imagine why crocs, turtles, and Aetosaurs may have had an osteoderm. Perhaps the condition was present in the first terrestrial amniote, and Aetosaurs and crocs retained the trait. The osteoderms could have evolved in the ancestor of Aetosaurs and crocs during the Permian and Triassic.
However, there is no explanation at all for the similar osteoderms of Ankylosaurs and Xenarthrans.
Is this just a big coincidence? Does nature keep recreating the same type of creature from scratch, but only once per 100 million years, directly after the disaster which ended the previous age and the previous armored herbivore?
Seems a bit hard to fathom. But there's another possibility few have considered. If you'd like to learn of a more plausible and probable answer for this highly unlikely sequence of events, you can find my full report on the matter here:
"The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 201.3 million years ago,[1] and is one of the major extinction events of the Phanerozoiceon, profoundly affecting life on land and in the oceans. In the seas, a whole class (conodonts)[2] and 34% of marine genera disappeared.[3] On land, all archosaurs other than crocodylomorphs(Sphenosuchia and Crocodyliformes) and Avemetatarsalia (pterosaurs and dinosaurs), some remaining therapsids, and many of the large amphibians became extinct."
However, the above statement contradicts what we supposedly know of dinosaurs. The various possible ancestors of dinosaurs arose around 240 million years ago, including Dinosauromorpha. A few possible ancestors or Saurichian dinosaurs evolved from this basalk group and continued through the Triassic/Jurassic Boundary. There's a clear evolutionary transition from the Triassic Dinosauromorpha to the Saurischian dinosaurs. Both groups were mostly bipedal, and this explains the difference in hips between Saurischia and other reptiles. But there's nothing to connect these early Dinosauromorpha to Ornithischians. In fact, the true common ancestor of Ornithischia and Saurischia is unknown. We don't even know its probable age. And there's no reason why ornithischians need to have had a bipedal ancestor.
No matter how big quadrupedal reptiles get, they maintain a sprawling posture. A recent study on Varanid lizards confirmed this observation.
http://jeb.biologists.org/content/214/18/3013
http://www.biology.ucr.edu/people/faculty/Garland/ChriGa96.pdf
There is no clear path from Dinosauromorpha, Eoraptor, or Herrerasaurus to non-Saurischian dinosaurs, the large herbivores known as Ornithischians. Most Ornithischians walk on all four legs, though a few species divergent from the basal quadrupedal form are known. Quadrupedal ornithischians walked totally differently from quadrupedal Saurischia, and bipedal ornithischians walked totally differently than bipedal Saurischia. The difference in their hips gave them their names.
Scientists believe ornithischians evolved from bipedal archosaurs, but no explanation exists as to why they should have evolved separate walking styles and features from Saurischians.
A bipedal ancestor for Ornithischians is nothing more than an assumption. And other archosaurs share more traits in common with Ornithischians than Dinosauromorpha.
Take Aetosaurs for instance. Superficially, they resemble the first ornithischians, quadruped dinosaurs with armor plates that later gave rise to stegosaurs and ankylosaurs. The diet and morphology of the two groups is the same, and Aetosaur and ankylosaur teeth are nearly identical. Their clade, Suchia, occupies a sister group position to Dinosauromorpha on the phylogenic tree.
In order to explain ornithischian armor, scientists suppose that an unknown species of bipedal archosaur rapidly evolved an exoskeleton made of intricately formed bone and scutes (primitive scales). They have no transitional fossils of anything like this occurring. There's no evidence that any bipedal ancestor of dinosaurs even possessed scutes. For aetosaurs, a common ancestry with crocodylomorphs is assumed since they were both protected by the same osteoderm covering.
Aetosaur fossils end exactly at the Triassic/Jurassic Extinction boundary, but soon afterward Thyreophora species begin to appear....at the same time fossils begin to appear again for other animal lineages.
After each major extinction event in Earth's history, fossils become scarce. This isn't only because species died out, it's also because surviving species experienced bottlenecks. Bones only fossilize by rare chance, and when a species is reduced to a small population fossils may not be left at all. Imagining ornithischians and Thyreophora evolved from a species of Dinosauromorpha requires a ghost lineage of over 30 million years, whereas imagining an Aetosaur ancestor requires less than half that time.
Superficially, anatomically, and behaviorally, Ornithischians resemble Aetosaurs more than Dinosauromorpha.
Is this just a coincidence? Should we discard a simple explanation of Ornithischian armor for a complex and unsupported one? Should we ignore the lack of proof for a bipedal ancestor of Ornithischians, and forgo an explanation as to why Ornithischians and Saurischia evolved completely different hips and walking styles?
Should we dismiss the teeth as a coincidence too?
Ankylosaurs survived and diversified through the Jurassic and Cretaceous, only going extinct 66 million years ago at the time of the KTg extinction.
After the KTg extinction, fossils don't appear again with frequency until around 55 million years ago. Major groups in South America don't appear until around 46 million years ago. This is about the same time as the Antarctic/South American exchange, so many of these animals may have migrated from S. America as the continents reconnected.
A paleoisthmus linking southern South America with the Antarctic Peninsula during Late Cretaceous and Early Tertiary
Chapter 6 Paleogene
Since Antarctica is covered with ice, we find few fossils there. However, because of the few fossils we have found there and because of what we know of continental drift we can deduce that many S. American animals lived in Antarctica during the Late Cretaceous and early Paleogene.
One of the animal groups that appears at the time of the first exchange is Xenarthra, considered the most primitive of the four major groups of placental mammals. The earliest Xenarthrans possess exoskeletons made of the same intricate osteoderms, but again no transitional fossils show how or why the osteoderm formed.
Often when school children notice the striking similarity between Ankylosaurs and early Xenarthrans, they are told the resemblance is only superficial and it's just a case of "convergent evolution." But the resemblances are anything but superficial. The skeletons of the creatures show the same fused bones, the same number of vertebrae and teeth, the same bizarre cheek flanges, and the same pair of tiny horns in the supraorbital position of the skull. The youngest Ankylosaurs even show the same number of temporal fenestra as early mammals, which is one of the main ways we tell reptiles and mammals apart. The skulls of Ankylosaurus and Megatherium, another early Xenarthran, are so close that one could be a subspecies of the other.
Ankylosaur
Other than crocodylomorphs and turtles, Aetosaurs were the only animals with osteoderms in the Late Triassic.
Interestingly, Ankylosaurs and their close kin were the only animals with full body osteoderm armor in the Jurassic and Cretaceous. Today. only Xenarthrans have full body osteoderms. The pangolin's armor is not mode of true osteoderms, as it does not contain bone matter. However, pangolins were first placed within Xenarthra, before DNA tests linked them with Carnivora.
So, according to mainstream science, crocodylomorphs and turtles remained mostly unchanged for 300 million years or more, retaining the same osteoderm covering. Meanwhile, three unrelated animal groups evolved similar osteoderms in succession, one right after the other. As soon as Aetosaurs died out, Ankylosaurs rapidly evolved osteoderms and Aetosaur teeth. As soon as Ankylosaurs died out, Xenarthra rapidly evolved the same osteoderm and skull.
Since this happens so quickly in such different animals, with no transitional fossils or explanation for the change, we would think the formation of an intricate full-body osteoderm suit should be quite common. Yet it only seems to happen to one animal group per time period, at the very beginning of the time period directly following a global disaster. During all of the 100 million years of the Jurassic and Cretaceous, for instance, not a single other group of animals with a full osteoderm covering arose.
Since exoskeletons are present in ancient fish and even amphibians, it's not hard to imagine why crocs, turtles, and Aetosaurs may have had an osteoderm. Perhaps the condition was present in the first terrestrial amniote, and Aetosaurs and crocs retained the trait. The osteoderms could have evolved in the ancestor of Aetosaurs and crocs during the Permian and Triassic.
However, there is no explanation at all for the similar osteoderms of Ankylosaurs and Xenarthrans.
Is this just a big coincidence? Does nature keep recreating the same type of creature from scratch, but only once per 100 million years, directly after the disaster which ended the previous age and the previous armored herbivore?
Seems a bit hard to fathom. But there's another possibility few have considered. If you'd like to learn of a more plausible and probable answer for this highly unlikely sequence of events, you can find my full report on the matter here:
The Mother of All Placental Mammals Discovered: Identification of the first fossil Placental Mammal from the Cretaceous Period
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