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What species of Cerapoda lived during the Jurassic?

What species of Cerapoda lived during the Jurassic?


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So far, I've found eleven species of Cerapoda that lived during the Jurassic period. Are there any other known species that I've missed?

The eleven that I've found out about are:

  • Agilisaurus
  • Albalophosaurus
  • Camptosaurus
  • Chaoyangsaurus
  • Dryosaurus
  • Heterodontosaurus
  • Othnielosaurus
  • Uteodon
  • Xuanhuaceratops
  • Yandusaurus
  • Yinlong

I am a game board designer working on a game related to dinosaurs. The "only" research i did is Wikipedia + the DK book "Prehistoric" + Dino Directory web site + several more lesser sites. I would appreciate any information on other species if my list isn't exhaustive, or literature suggestions that might help me.


Here is a cladogram from "The phylogeny of the ornithischian dinosaurs", Butler, Upchurch, & Norman, Journal of Systematic Palaeontology (2008):

Of these, :

  • Ferganocephale (Pachycephalosauridae)
  • Chaoyangsaurus
  • Camptosaurus (Ankylopollexia)
  • Uteodon (Ankylopollexia)
  • Callovosaurus (Dryosauridae)
  • Dryosaurus (Dryosauridae)
  • Dysalotosaurus (Dryosauridae)

lived during the Jurassic.


The synopsis explains well why this classification is subject to adjustment, and why you may have seen contradictory models in your research:

SYNOPSIS Ornithischia is a familiar and diverse clade of dinosaurs whose global phylogeny has remained largely unaltered since early cladistic analyses in the mid 1980s. Current understanding of ornithischian evolution is hampered by a paucity of explicitly numerical phylogenetic analyses that consider the entire clade. As a result, it is difficult to assess the robustness of current phylogenetic hypotheses for Ornithischia and the effect that the addition of new taxa or characters is likely to have on the overall topology of the clade.

The new phylogenetic analysis presented here incorporates a range of new basal taxa and characters in an attempt to rigorously test global ornithischian phylogeny. Parsimony analysis is carried out with 46 taxa and 221 characters. Although the strict component consensus tree shows poor resolution in a number of areas, application of reduced consensus methods provides a well-resolved picture of ornithischian interrelationships. Surprisingly, Heterodontosauridae is placed as the most basal group of all well-known ornithischians, phylogenetically distant from a stem-defined Ornithopoda, creating a topology that is more congruent with the known ornithischian stratigraphical record. There is no evidence for a monophyletic 'Fabrosauridae', and Lesothosaurus (the best-known 'fabrosaur') occupies an unusual position as the most basal member of Thyreophora. Other relationships within Thyreophora remain largely stable. The primitive thyreophoran Scelidosaurus is the sister taxon of Eurypoda (stegosaurs and ankylosaurs), rather than a basal ankylosaur as implied by some previous studies.

The taxonomic content of Ornithopoda differs significantly from previous analyses and basal relationships within the clade are weakly supported, requiring further investigation. 'Hypsilophodontidae' is paraphyletic, with some taxa (Agilisaurus, Hexinlusaurus, Othnielia) placed outside of Ornithopoda as non-cerapodans. Ceratopsia and Pachycephalosauria are monophyletic and are united as Marginocephalia; however, the stability of these clades is reduced by a number of poorly preserved basal taxa.

This analysis reaffirms much of the currently accepted ornithischian topology. Nevertheless, instability in the position and content of several clades (notably Heterodontosauridae and Ornithopoda) indicates that considerable future work on ornithischian phylogeny is required and causes problems for several current phylogenetic definitions.


Thyreophora

Thyreophora ("shield bearers", often known simply as "armored dinosaurs") is a group of armored ornithischian dinosaurs that lived from the early Jurassic Period until the end of the Cretaceous.

Thyreophorans are characterized by the presence of body armor lined up in longitudinal rows along the body. Primitive forms had simple, low, keeled scutes or osteoderms, whereas more derived forms developed more elaborate structures including spikes and plates. Most thyreophorans were herbivorous and had relatively small brains for their body size.

Thyreophora includes various subgroups, including the suborders Ankylosauria and Stegosauria. In both the suborders, the forelimbs were much shorter than the hindlimbs, particularly in stegosaurs. The clade has been defined as the group consisting of all species more closely related to Ankylosaurus than to Triceratops. Thyreophora is the sister group of Cerapoda within Genasauria.


Contents

Nanosaurus is known from material from all parts of the body, including two good skeletons, although the skull is still poorly known. [1] It was a small animal, 2 meters (6.6 ft) or less in length and 10 kilograms (22 lb) or less in weight. [2]

It was a bipedal dinosaur with short forelimbs and long hindlimbs with large processes for muscle attachments. [3] The hands were short and broad with short fingers. The head was small. It had small leaf-shaped cheek teeth (triangular and with small ridges and denticles lining the front and back edges), and premaxillary teeth with less ornamentation. [4]

Like several other neornithischian dinosaurs, such as Hypsilophodon, Thescelosaurus, and Talenkauen, Nanosaurus had thin plates lying along the ribs. Called intercostal plates, these structures were cartilaginous in origin. [5]

Marsh's original groundwork Edit

In 1877, Marsh named two species of Nanosaurus in separate publications, based on partial remains from the Morrison Formation of Garden Park, Colorado. One paper described N. agilis, based on YPM 1913, with remains including impressions of a dentary, and postcranial bits including an ilium, thigh bones, shin bones, and a fibula. [6] The other paper named N. rex, a second species which Marsh based on YPM 1915 (also called 1925 in Galton, 2007), a complete thigh bone. [4] [7] He regarded both species as small ("fox-sized") animals. [7] A third species, N. victor, was named, which he soon recognized to be something completely different, and is now known as the small, bipedal crocodylomorph Hallopus. [6] [8]

The next year, he named the new genus Laosaurus on material collected by Samuel Wendell Williston from Como Bluff, Wyoming. Two species were named: the type species L. celer, based on parts of eleven vertebrae (YPM 1875) [9] and the "smaller" L. gracilis, originally based on a back vertebra's centrum, a caudal centrum, and part of an ulna (review by Peter Galton in 1983 finds the specimen to now consist of thirteen back and eight caudal centra, and portions of both hindlimbs). [9] [10] A third species, L. consors, was established by Marsh in 1894 for YPM 1882, which consists of most of one articulated skeleton and part of at least one other individual. [11] The skull was only partially preserved, and the fact that the vertebrae were represented only by centra suggests a partially grown individual. Galton (1983) notes that much of the current mounted skeleton was restored in plaster, or had paint applied. [10]

Galton's taxonomic revisions Edit

These animals attracted little professional attention until the 1970s and 1980s, when Peter Galton reviewed many of the "hypsilophodonts" in a series of papers. In 1973, he and Jim Jensen described a partial skeleton (BYU ESM 163 as of Galton, 2007) missing the head, hands, and tail as Nanosaurus? rex, which had been damaged by other collectors prior to description. [12] By 1977, he had concluded that Nanosaurus agilis was quite different from N. rex and the new skeleton, and coined Othnielia for the latter species. The paper (primarily concerning the transcontinental nature of Dryosaurus) considered Laosaurus consors and L. gracilis synonyms of O. rex without elaboration, and considered L. celer an invalid nomen nudum. [13]

In 1990, Robert Bakker, Peter Galton, James Siegwarth, and James Filla described remains of a dinosaur they named Drinker nisti. The name is somewhat ironic Drinker, named for renowned palaeontologist Edward Drinker Cope whose infamous "bone wars" with rival Othniel Charles Marsh produced many dinosaur fossils which are world-famous today, was described as a probable close relative of Othnielia, named for Marsh. The species name refers to the National Institute of Standards and Technology (NIST). Discovered by Siegwarth and Filla in upper Morrison Formation beds at Como Bluff, Wyoming, it was based on a partial subadult skeleton (listed as CPS 106 originally, then as Tate 4001 by Bakker 1996 [14] ) including partial jaws, vertebrae, and partial limbs. Several other specimens found in the same area were assigned to it, mostly consisting of vertebral and hindlimb remains, and teeth. [15] The holotype specimen's current location is unknown according to Carpenter and Galton (2018), the previous two institutions reported to have had it did not ever curate the specimen, and the collection it was originally said to be in never existed at all. [14]

Several decades later, in his 2007 study of the teeth of Morrison ornithischians, Galton concluded that the holotype femur of Othniela rex is not diagnostic, and reassigned the BYU skeleton to Laosaurus consors, which is based on better material. As the genus Laosaurus is also based on nondiagnostic material, he gave the species L. consors its own genus, Othnielosaurus. As a result, in practical terms, what had been thought of as Othnielia is now known as Othnielosaurus consors. Regarding Nanosaurus agilis, Galton considered it a potentially valid basal ornithopod, and noted similarities to heterodontosaurids in the thigh bone. He tentatively assigned to it some teeth that had been referred to Drinker. [4]

Another decade later, in 2018, Galton, alongside Kenneth Carpenter, described a new ornithischian specimen. They found it very similar to the fragmentary holotype of Nanosaurus, but more clear in its anatomical features. Their new specimen was also found to display extreme similarity with the specimens of Othnielosaurus and Drinker. Due to the new data, they concluded that all three species, alongside Othnielia, represented the same animal, united under the name Nanosaurus agilis. This painted a new picture of a singular, very common small dinosaur known from a large amount of material. [14] This conclusion has been recognized by papers since, some of which incorporating the new, all-encompassing taxon into their phylogenetic analyses. [16] [17] [18]

The cladogram below results from analysis by Herne et al., 2019. [16]


Scientists discover the "Monkeydactyl" &mdash a Jurassic-era flying dinosaur with opposable thumbs

It soared across the skies, had opposable thumbs and lived in China during the Jurassic era: Meet the "Monkeydactyl" &mdash a newly discovered ancient dinosaur.

The bizarre Kunpengopterus antipollicatus, appropriately nicknamed the Monkeydactyl, lived in a forest ecosystem 160 million years ago, an international team of researchers said in a report published in the journal Current Biology on Monday. Antipollicatus means "opposite thumbed" in ancient Greek.

Pterosaurs were the first known vertebrates to fly, researchers said. The arboreal pterosaur species marks the oldest of its kind with true opposable thumbs, a phenomenon never before seen in the species.

The discovery also marks the earliest known record of a true opposed thumb in the history of Earth.

Scientists found the Monkeydactyl fossil in the Tiaojishan Formation of Liaoning, China in September 2019. Using micro-CT scans to enlarge the anatomical features, they found the small fossil preserved both hands, which featured an opposed "pollex," or thumb, on each.

Artist's interpretation of K. antipollicatus. Chuang Zhao

"The fingers of 'Monkeydactyl' are tiny and partly embedded in the slab. Thanks to micro-CT scanning, we could see through the rocks, create digital models and tell how the opposed thumb articulates with the other finger bones," co-author Fion Waisum Ma said in a statement. "This is an interesting discovery. It provides the earliest evidence of a true opposed thumb, and it is from a pterosaur &mdash which wasn't known for having an opposed thumb."

Trending News

An opposed thumb is extremely rare among reptiles &mdash they are commonly found in humans and other mammals.

Researchers say the Monkeydactly, a type of darwinopteran, named for Charles Darwin, used the thumb for climbing and grasping, an adaptation for living in trees. In addition to the thumbs, researchers determined the animal to be very small, with a wingspan of about 33 inches.

It managed to avoid most competition in its complex forest habitat, which featured closely-related species that adapted into different niches.

"Darwinopterans are a group of pterosaurs from the Jurassic of China and Europe, named after Darwin due to their unique transitional anatomy that has revealed how evolution affected the anatomy of pterosaurs throughout time," said co-author Rodrigo V. Pêgas. "On top of that, a particular darwinopteran fossil has been preserved with two associated eggs, revealing clues to pterosaur reproduction. They've always been considered precious fossils for these reasons and it is impressive that new darwinopteran species continue to surprise us!"

First published on April 14, 2021 / 8:18 AM

© 2021 CBS Interactive Inc. All Rights Reserved.

Sophie Lewis is a social media producer and trending writer for CBS News, focusing on space and climate change.


An Opposed-Thumbed “Monkeydactyl” and a Cuddly Porg-Like Pterosaur

A newly discovered species of flying reptile that lived during the Jurassic Period about 160 million years ago has been discovered in China. What’s even more incredible about this new species was that it had an opposed thumb. This is the earliest-known example of an opposed thumb and the first time ever that it’s been discovered on a flying reptile.

The tree-dwelling reptile has been nicknamed “Monkeydactyl” while its proper name is “Kunpengopterus antipollicatus” which means “opposite thumbed” in ancient Greek. It was discovered in the Tiaojishan rock formations in Liaoning, China.

The small pterosaur had a wingspan that measured just 2.8 feet (33.6 inches). Its fingers were the most interesting part of the discovery as they were exceptionally small and “partly embedded in the slab” as explained by Waisum Ma who is a palaeontologist from the University of Birmingham and an author of the study. It was its thumb, though, that was the creature’s most astonishing feature.

While opposed thumbs – also known as a “pollex” – have been found in mammals and some tree frogs, they are very rare in reptiles except for chameleons. The researchers analyzed the fingers by using micro-CT scans that allowed them to look through the rocks and take digital models of the bones. That’s how they were able to confirm that “Darwinopteran” pterosaurs, such as the Monkeydactyl, somehow evolved opposed thumbs.

As for what the Monkeydactyl would have used its opposed thumb for, it could have been used to either hold onto tree branches or food. (Pictures of the remains as well as what the Monkeydactyl would have looked like can be seen here.)

The study was published in the journal Current Biology where it can be read in full.

In other Pterosaur news, a newly discovered species has been compared to the cuddly Porg aliens from Star Wars. This new species, which has been named Sinomacrops bondei, has been classified as an anurognathid. It was unearthed by paleontologists at the Yanliao Biota excavation area in Mutoudeng, located in China’s Hebei province.

It was small in size as it had a wingspan of just 36 inches (3 feet) while it glided through forests. It would have had a short skull, round eyes with large sockets, a short rounded jaw, peg-like teeth, and hair-like filaments on its body. It lived approximately 160 million years ago during the Jurassic Period.

Its remains are currently at China’s Jinzhou Museum of Paleontology. Pictures of the remains and what the Sinomacrops bondei would have looked like can be seen here.


Sophie Lewis – CBS News

It soared across the skies, had opposable thumbs and lived in China during the Jurassic era: Meet the “Monkeydactyl” — a newly discovered ancient dinosaur.

The bizarre Kunpengopterus antipollicatus, appropriately nicknamed the Monkeydactyl, lived in a forest ecosystem 160 million years ago, an international team of researchers said in a report published in the journal Current Biology on Monday. Antipollicatus means “opposite thumbed” in ancient Greek.

Pterosaurs were the first known vertebrates to fly, researchers said. The arboreal pterosaur species marks the oldest of its kind with true opposable thumbs, a phenomenon never before seen in the species.

The discovery also marks the earliest known record of a true opposed thumb in the history of Earth.

Scientists found the Monkeydactyl fossil in the Tiaojishan Formation of Liaoning, China in September 2019. Using micro-CT scans to enlarge the anatomical features, they found the small fossil preserved both hands, which featured an opposed “pollex,” or thumb, on each.

“The fingers of ‘Monkeydactyl’ are tiny and partly embedded in the slab. Thanks to micro-CT scanning, we could see through the rocks, create digital models and tell how the opposed thumb articulates with the other finger bones,” co-author Fion Waisum Ma said in a statement. “This is an interesting discovery. It provides the earliest evidence of a true opposed thumb, and it is from a pterosaur — which wasn’t known for having an opposed thumb.”

An opposed thumb is extremely rare among reptiles — they are commonly found in humans and other mammals.

Researchers say the Monkeydactly, a type of darwinopteran, named for Charles Darwin, used the thumb for climbing and grasping, an adaptation for living in trees. In addition to the thumbs, researchers determined the animal to be very small, with a wingspan of about 33 inches.

It managed to avoid most competition in its complex forest habitat, which featured closely-related species that adapted into different niches.

“Darwinopterans are a group of pterosaurs from the Jurassic of China and Europe, named after Darwin due to their unique transitional anatomy that has revealed how evolution affected the anatomy of pterosaurs throughout time,” said co-author Rodrigo V. Pêgas. “On top of that, a particular darwinopteran fossil has been preserved with two associated eggs, revealing clues to pterosaur reproduction. They’ve always been considered precious fossils for these reasons and it is impressive that new darwinopteran species continue to surprise us!”

banner image: artist’s interpretation of K. antipollicatus. Chuang Zhao


Huge Dinosaurs

On land, dinosaurs were making their mark in a big way—literally. The plant-eating sauropod Brachiosaurus stood up to 52 feet (16 meters) tall, stretched some 85 feet (26 meters) long, and weighed more than 80 tons. Diplodocus, another sauropod, was 90 feet (27 meters) long. These dinosaurs' sheer size may have deterred attack from Allosaurus, a bulky, meat-eating dinosaur that walked on two powerful legs. But Allosaurus and other fleet-footed carnivores, such as the coelurosaurs, must have had occasional success. Other prey included the heavily armored stegosaurs.

The earliest known bird, Archaeopteryx, took to the skies in the late Jurassic, most likely evolved from an early coelurosaurian dinosaur. Archaeopteryx had to compete for airspace with pterosaurs, flying reptiles that had been buzzing the skies since the late Triassic. Meanwhile, insects such as leafhoppers and beetles were abundant, and many of Earth's earliest mammals scurried around dinosaur feet—ignorant that their kind would come to dominate Earth once the dinosaurs were wiped out at the end of the Cretaceous.


Sudden Appearance of Jurassic Dinosaurs Demonstrates the Creator’s Work

Only during the last few decades have scientists recognized the many mass extinction and speciation events throughout life’s history. Psalm 104:29–30, however, described these events thousands of years ago:

When you [God] hide your face, they [Earth’s creatures] are terrified. When you take away their breath, they die and return to the dust. When you send your Spirit, they are created, and you renew the face of the earth.

Earth’s organisms have experienced several catastrophic upheavals. The Triassic-Jurassic mass extinction event (TJEE) is the second greatest mass extinction event on record. It eliminated more than 95 percent of terrestrial megaflora species, 1 plus all the animal species dependent upon these plants. For continental organisms it was at least as devastating as the Permian-Triassic event, 2 which wiped out over 70 percent of land species. 3

The TJEE is the only mass extinction event in deep time for which an accurate absolute date exists because the strata containing the fossils and footprints of creatures living just before and just after the TJEE shows both the cyclic variations in Earth’s orbit and several reversals in Earth’s magnetic field. Thus, geophysicists have established that the TJEE occurred 201.564±0.015 million years ago. 4

Despite the destruction caused by the TJEE, the mass speciation that followed was surprisingly quick and robust. Within 10,000 years or less large theropod dinosaurs appeared, 5 and in less than 100,000 years dinosaur species diversity attained a stable maximum. 6 Especially astounding is not just the body size and complexity of the new species but the fact that they appeared amid hostile environmental conditions. The TJEE coincided with the first and greatest of the four volcanic eruptions that formed the Central Atlantic Magmatic Province. 7 These eruptions poured out 2–3 million cubic kilometers of lava—enough to pave Earth’s entire surface 4–6 meters (13–26 feet) deep. The first eruption had hardly subsided when the first large theropod dinosaurs appeared. And the dinosaur species diversity reached a stable maximum while climate changes were still extreme. 8

Mass Speciation Reflects the Creator and Challenges Evolution

The rapidity and diversity with which large dinosaurs appeared fits what one would expect from the actions of a Creator intent on taking advantage of shallow seas, lakes, and swamps resulting from the breaking up of the Pangaea supercontinent. It is consistent, too, with the Creator’s desire to endow Earth with all the necessary resources humans would need to fulfill His purposes.

However, the short time between the TJEE and the Jurassic speciation is far too brief to allow for a naturalistic explanation. Long-term evolution experiments performed in real time show that even under extreme laboratory pressure the most evolutionally flexible species experience nothing more than microevolution. 9 Evolutionists responded with the suggestion that a few small-bodied reptiles survived the TJEE and, due to the hostile conditions and lack of competing species, rapidly evolved into a huge population of diverse, complex, large-bodied dinosaurs. 10 Conservation biology research refutes this hypothesis.

Rapid and extreme climate change, which characterized the TJEE, stymies evolutionary development. An experiment performed on tiny crustaceans with a reputation for evolvability demonstrated that their rate of evolution (assisted by guided selective breeding) was unable to keep pace with even a modest change in average temperature. 11

Numerous studies also confirm that when an animal species suffers a population collapse and faces environmental stress, it rapidly goes extinct without human intervention. 12 Furthermore, the extinction risk and the speed with which extinction occurs rises dramatically with adult body mass. 13

God’s Purposes for Extinction and Speciation

Why did God wipe out and replace life so frequently? For one thing, these mass creation events perfectly compensated for the Sun’s increasing luminosity by producing new life-forms that could draw more greenhouse gases out of the atmosphere. Another reason is keeping Earth packed with the greatest possible biomass, biodiversity, and biocomplexity. This ensures that humans have all the biodeposits they need to launch and sustain global high-technology civilization—which allows Christ’s followers to fulfill the Great Commission in the shortest time possible.


Ancient giant rhino was one of the largest mammals ever to walk Earth

The remains of a 26.5-million-year-old giant, hornless rhino — one of the largest mammals ever to walk Earth — have been discovered in northwestern China, a new study finds.

The newly identified species, Paraceratherium linxiaense — named after its discovery spot in the Linxia Basin in Gansu province — towered over other animals during its lifetime. The 26-foot-long (8 meters) beast had a shoulder height of 16.4 feet (5 m), and it weighed as much as 24 tons (21.7 metric tons), the same as four African elephants, the researchers said.

The new species is larger than other giant rhinos in the extinct genus Paraceratherium, said study lead researcher Deng Tao, director and professor at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences in Beijing. A new family tree analysis of Paraceratherium species, including P. linxiaense, reveals how these ancient beasts evolved as they migrated across Central and South Asia at a time when the Tibetan Plateau was lower than it is today, Tao told Live Science in an email.

Researchers have known about the fossil trove at Linxia Basin, located at the northeastern border of the Tibetan Plateau, since the 1950s, when farmers there began discovering "dragon bones," Tao said. Digs in the 1980s revealed rare, but fragmentary giant rhino fossils. That changed in 2015, with the discovery of a complete skull and jaw of one giant rhino individual, and three vertebrae from another individual, both dating to the late Oligocene epoch (33.9 million to 23 million years ago).

When the researchers saw the fossils, the bones' completeness and "huge size … [were] a great surprise for us," Tao said. An anatomical analysis, in addition to the fact that the fossils were larger than those from other known species in the Paraceratherium genus, revealed that they belonged to a previously unknown Paraceratherium species.

The skull and jaw bones showed that P. linxiaense had a giant, 3.7-foot-long (1.1 m) head a long neck two tusk-like incisors that pointed downward and a deep nasal notch, indicating that the animal had a trunk like that of a tapir. The giant rhino likely wrapped its trunk around branches so it could easily strip off leaves with its front teeth, Tao said.

P. linxiaense stood on four long legs that were good for running, and its head could reach heights of 23 feet (7 m) "to browse leaves of treetops," Tao said.


The Five Mass Extinctions

The fossil record of the mass extinctions was the basis for defining periods of geological history, so they typically occur at the transition point between geological periods. The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species.

Figure (PageIndex<1>): Five mass extinctions: The transitions between the five main mass extinctions can be seen in the rock strata. The table shows the time that elapsed between each period.

The Ordovician-Silurian extinction event is the first-recorded mass extinction and the second largest. During this period, about 85 percent of marine species (few species lived outside the oceans) became extinct. The main hypothesis for its cause was a period of glaciation followed by warming. These two extinction events, cooling and warming, were separated by about 1 million years the climate changes affected temperatures and sea levels. Some researchers have suggested that a gamma-ray burst caused by a nearby supernova is a possible cause of the Ordovician-Silurian extinction. The gamma-ray burst would have stripped away the earth&rsquos ozone layer, causing intense ultraviolet radiation from the sun. It may account for climate changes observed at the time.

The late Devonian extinction may have occurred over a relatively long period of time. Its causes are poorly-understood and it appears to have have affected only marine species.

The end-Permian extinction was the largest in the history of life. Estimates predict that 96 percent of all marine species and 70 percent of all terrestrial species were lost.The causes for this mass extinction are not clear, but the leading suspect is extended and widespread volcanic activity that led to a runaway global-warming event. The oceans became largely anoxic, suffocating marine life. Terrestrial tetrapod diversity took 30 million years to recover after the end-Permian extinction. The Permian extinction dramatically altered earth&rsquos biodiversity composition and the course of evolution.

The causes of the Triassic&ndashJurassic extinction event are not clear. Hypotheses of climate change, asteroid impact, and volcanic eruptions have been argued. The extinction event occurred just before the breakup of the supercontinent Pangaea although, recent scholarship suggests that the extinctions may have occurred more gradually throughout the Triassic.

The causes of the end-Cretaceous extinction event are the ones that are best understood. It was during this extinction event, about 65 million years ago, that the dinosaurs, the dominant vertebrate group for millions of years, disappeared from the planet (with the exception of a theropod clade that gave rise to birds). Indeed, every land animal that weighed more then 25 kg became extinct. The cause of this extinction is now understood to be the result of a cataclysmic impact of a large meteorite or asteroid off the coast of what is now the Yucatán Peninsula. This hypothesis, proposed first in 1980, was a radical explanation based on a sharp spike in the levels of iridium (which rains down from space in meteors at a fairly constant rate, but is otherwise absent on earth&rsquos surface) at the rock stratum that marks the boundary between the Cretaceous and Paleogene periods. The Cretaceous-Paleogene (K-Pg) boundary marked the disappearance of the dinosaurs in fossils, as well as many other taxa. The researchers who discovered the iridium spike interpreted it as a rapid influx of iridium from space to the atmosphere (in the form of a large asteroid), rather than a slowing in the deposition of sediments during that period. It was a radical explanation, but the report of an appropriately aged and sized impact crater in 1991 made the hypothesis more credible. Now, an abundance of geological evidence supports the hypothesis. Recovery times for biodiversity after the end-Cretaceous extinction were shorter, in geological time, than for the end-Permian extinction: on the order of 10 million years.

Figure (PageIndex<1>): K-Pg mass extinction: In 1980, Luis and Walter Alvarez, Frank Asaro, and Helen Michels discovered, across the world, a spike in the concentration of iridium within the sedimentary layer at the K&ndashPg boundary. These researchers hypothesized that this iridium spike was caused by an asteroid impact that resulted in the K&ndashPg mass extinction. In the photo, the iridium layer is the light band.