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140-527: The Cretaceous–Paleogene extinction event (formerly known as the Cretaceous– Tertiary extinction event) occurred at the end of this age. In this mass extinction , many commonly recognized groups such as non-avian dinosaurs , plesiosaurs and mosasaurs , as well as many other lesser-known groups, died out. The cause of the extinction is most commonly linked to an asteroid about 10 to 15 kilometres (6.2 to 9.3 mi) wide colliding with Earth, ending

280-465: A diverse group of large predatory marine reptiles, also became extinct. Fossil evidence indicates that squamates generally suffered very heavy losses in the K–Pg event, only recovering 10 million years after it. The extinction of Cretaceous lizards and snakes may have led to the evolution of modern groups such as iguanas, monitor lizards, and boas. The diversification of crown group snakes has been linked to

420-465: A fairly composite pterosaur diversity: at least six ("Nyctosaurus" lamegoi , a Mexican humerus, a Jordan humerus and several taxa from Morocco) nyctosaurs date to this period, as do a few pteranodontids , and Navajodactylus , tentatively assigned to Azhdarchidae, lacks any synapomorphies of the group. This seems to underscore a higher diversity of terminal Cretaceous pterosaurs than previously thought. The radiation of angiosperms (flowering plants)

560-413: A few species of ground and water fowl, which radiated into all modern species of birds. Among other groups, teleost fish and perhaps lizards also radiated. The K–Pg extinction event was severe, global, rapid, and selective, eliminating a vast number of species. Based on marine fossils, it is estimated that 75% or more of all species became extinct. The event appears to have affected all continents at

700-411: A flat plane. The most fundamental difference in spiral form is how strongly successive whorls expand and overlap their predecessors. This can be inferred by the size of the umbilicus, the sunken-in inner part of the coil, exposing older and smaller whorls. Evolute shells have very little overlap, a large umbilicus, and many exposed whorls. Involute shells have strong overlap, a small umbilicus, and only

840-526: A general shape to ammonite tentacles. A contemporary study found an ammonite isolated body, offering for the first time a glimpse into these animals' organs. The smallest ammonoid was Maximites from the Upper Carboniferous . Adult specimens reached only 10 mm (0.39 in) in shell diameter. Few of the ammonites occurring in the lower and middle part of the Jurassic period reached

980-524: A group continued through several major extinction events , although often only a few species survived. Each time, however, this handful of species diversified into a multitude of forms. Ammonite fossils became less abundant during the latter part of the Mesozoic , and although they seemingly survived the Cretaceous–Paleogene extinction event , all known Paleocene ammonite lineages are restricted to

1120-534: A period in the earliest part of the Cenozoic of decreased acanthomorph diversity, although acanthomorphs diversified rapidly after the extinction. Teleost fish diversified explosively after the mass extinction, filling the niches left vacant by the extinction. Groups appearing in the Paleocene and Eocene epochs include billfish, tunas, eels, and flatfish. There is limited evidence for extinction of amphibians at

1260-419: A planktonic strategy of reproduction (numerous eggs and planktonic larvae), which would have been devastated by the K–Pg extinction event. Additional research has shown that subsequent to this elimination of ammonoids from the global biota, nautiloids began an evolutionary radiation into shell shapes and complexities theretofore known only from ammonoids. Approximately 35% of echinoderm genera became extinct at

1400-490: A range of different species provide definitive evidence for the persistence of archaic birds to within 300,000 years of the K–Pg boundary. The absence of these birds in the Paleogene is evidence that a mass extinction of archaic birds took place there. The most successful and dominant group of avialans , enantiornithes , were wiped out. Only a small fraction of ground and water-dwelling Cretaceous bird species survived

1540-527: A result of cooling temperatures in the early Paleocene . Approximately 46% of diatom species survived the transition from the Cretaceous to the Upper Paleocene, a significant turnover in species but not a catastrophic extinction. The occurrence of planktonic foraminifera across the K–Pg boundary has been studied since the 1930s. Research spurred by the possibility of an impact event at

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1680-411: A result of limpets attaching themselves to the shells. However, the triangular formation of the holes, their size and shape, and their presence on both sides of the shells, corresponding to the upper and lower jaws, is more likely evidence of the bite of a medium-sized mosasaur preying upon ammonites. Some ammonites appear to have lived in cold seeps and even reproduced there. The chambered part of

1820-626: A section along the Ardour river called Grande Carrière , close to the village of Tercis-les-Bains in southwestern France . The top of the Maastrichtian Stage is defined to be at the iridium anomaly at the Cretaceous–Paleogene boundary (K–Pg boundary), which is also characterised by the extinction of many groups of life. The Maastrichtian is commonly subdivided into two substages (Upper and Lower) and three ammonite biozones . The biozones are (from young to old): The Maastrichtian

1960-558: A single horny plate or a pair of calcitic plates. In the past, these plates were assumed to serve in closing the opening of the shell in much the same way as an operculum , but more recently they are postulated to have been a jaw apparatus. The plates are collectively termed the aptychus or aptychi in the case of a pair of plates, and anaptychus in the case of a single plate. The paired aptychi were symmetric to one another and equal in size and appearance. Anaptychi are relatively rare as fossils. They are found representing ammonites from

2100-650: A size exceeding 23 cm (9.1 in) in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from the Portland Stone of Jurassic of southern England, which is often 53 cm (1.74 ft) in diameter, and Parapuzosia seppenradensis of the Cretaceous period of Germany, which is one of the largest-known ammonites, sometimes reaching 2 m (6.6 ft) in diameter. The largest-documented North American ammonite

2240-454: A small phylum of marine invertebrates, survived the K–Pg extinction event and diversified during the early Paleocene. The numbers bivalve genera exhibited significant diminution after the K–Pg boundary. Entire groups of bivalves, including rudists (reef-building clams) and inoceramids (giant relatives of modern scallops ), became extinct at the K–Pg boundary, with the gradual extinction of most inoceramid bivalves beginning well before

2380-465: Is Baculites , which has a nearly straight shell convergent with the older orthocone nautiloids. Still other species' shells are coiled helically (in two dimensions), similar in appearance to some gastropods (e.g., Turrilites and Bostrychoceras ). Some species' shells are even initially uncoiled, then partially coiled, and finally straight at maturity (as in Australiceras ). Perhaps

2520-506: Is Parapuzosia bradyi from the Cretaceous, with specimens measuring 137 cm (4.5 ft) in diameter. Starting from the mid-Devonian, ammonoids were extremely abundant, especially as ammonites during the Mesozoic era. Many genera evolved and ran their course quickly, becoming extinct in a few million years. Due to their rapid evolution and widespread distribution, ammonoids are used by geologists and paleontologists for biostratigraphy . They are excellent index fossils , and it

2660-626: Is a narrow tubular structure that runs along the shell's outer rim, known as the venter, connecting the chambers of the phragmocone to the body or living chamber. This distinguishes them from living nautiloides ( Nautilus and Allonautilus ) and typical Nautilida , in which the siphuncle runs through the center of each chamber. However the very earliest nautiloids from the Late Cambrian and Ordovician typically had ventral siphuncles like ammonites, although often proportionally larger and more internally structured. The word "siphuncle" comes from

2800-476: Is clearly marked at the species level. Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a sharp increase in extinctions than by a decrease in speciation . Major spatial differences existed in calcareous nannoplankton diversity patterns; in the Southern Hemisphere, the extinction was less severe and recovery occurred much faster than in

2940-537: Is estimated that 75% or more of all species on Earth vanished. However, the extinction also provided evolutionary opportunities: in its wake, many groups underwent remarkable adaptive radiation —sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches. Mammals in particular diversified in the Paleogene , evolving new forms such as horses , whales , bats , and primates . The surviving group of dinosaurs were avians,

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3080-411: Is from κέρας ( kéras ) meaning "horn". Ammonites (subclass Ammonoidea) can be distinguished by their septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa join the outer shell wall, and in general by their siphuncles . Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex when seen from

3220-475: Is influenced by a lack of fossil records, rather than extinctions. Ostracods , a class of small crustaceans that were prevalent in the upper Maastrichtian, left fossil deposits in a variety of locations. A review of these fossils shows that ostracod diversity was lower in the Paleocene than any other time in the Cenozoic . Current research cannot ascertain whether the extinctions occurred prior to, or during,

3360-515: Is marked by a thin layer of sediment called the K–Pg boundary, Fatkito boundary or K–T boundary , which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows unusually high levels of the metal iridium , which is more common in asteroids than in the Earth's crust . As originally proposed in 1980 by a team of scientists led by Luis Alvarez and his son Walter , it

3500-494: Is no evidence that late Maastrichtian non-avian dinosaurs could burrow, swim, or dive, which suggests they were unable to shelter themselves from the worst parts of any environmental stress that occurred at the K–Pg boundary. It is possible that small dinosaurs (other than birds) did survive, but they would have been deprived of food, as herbivorous dinosaurs would have found plant material scarce and carnivores would have quickly found prey in short supply. The growing consensus about

3640-515: Is now generally thought that the K–Pg extinction was caused by the impact of a massive asteroid 10 to 15 km (6 to 9 mi) wide, 66 million years ago causing the Chicxulub crater , which devastated the global environment, mainly through a lingering impact winter which halted photosynthesis in plants and plankton . The impact hypothesis, also known as the Alvarez hypothesis ,

3780-422: Is occasionally preserved in fossil specimens. The soft body of the creature occupied the largest segments of the shell at the end of the coil. The smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas. Thus, the smaller sections of the coil would have floated above the larger sections. Many ammonite shells have been found with round holes once interpreted as

3920-467: Is often possible to link the rock layer in which they are found to specific geologic time periods . Due to their free-swimming and/or free-floating habits, ammonites often happened to live directly above seafloor waters so poor in oxygen as to prevent the establishment of animal life on the seafloor. When upon death the ammonites fell to this seafloor and were gradually buried in accumulating sediment, bacterial decomposition of these corpses often tipped

4060-633: Is often preserved. This type of preservation is found in ammonites such as Hoplites from the Cretaceous Gault clay of Folkestone in Kent, England. The Cretaceous Pierre Shale formation of the United States and Canada is well known for the abundant ammonite fauna it yields, including Baculites , Placenticeras , Scaphites , Hoploscaphites and Jeletzkytes , as well as many uncoiled forms. Many of these also have much or all of

4200-451: Is postulated that some early monotremes, marsupials, and placentals were semiaquatic or burrowing, as there are multiple mammalian lineages with such habits today. Any burrowing or semiaquatic mammal would have had additional protection from K–Pg boundary environmental stresses. After the K–Pg extinction, mammals evolved to fill the niches left vacant by the dinosaurs. Some research indicates that mammals did not explosively diversify across

4340-596: Is roughly coeval with the Lancian North American Land Mammal Age . The breakup of Pangaea was nearly complete in the Maastrichtian, with Australia beginning to break away from Antarctica and Madagascar breaking away from India. However, Arabia had not yet rifted away from Africa. North America was separated from Europe by rift basins, but sea floor spreading had not yet commenced between the two continents. The Pacific Plate

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4480-472: Is thought that ammonites were the principal food of mosasaurs , a group of giant marine reptiles that became extinct at the boundary. The K–Pg extinction had a profound effect on the evolution of life on Earth . The elimination of dominant Cretaceous groups allowed other organisms to take their place, causing a remarkable amount of species diversification during the Paleogene Period. After

4620-526: Is thought that body sizes of placental mammalian survivors evolutionarily increased first, allowing them to fill niches after the extinctions, with brain sizes increasing later in the Eocene . Plant fossils illustrate the reduction in plant species across the K–Pg boundary. There is overwhelming evidence of global disruption of plant communities at the K–Pg boundary. Extinctions are seen both in studies of fossil pollen, and fossil leaves. In North America,

4760-538: Is thought to be because the female required a larger body size for egg production. A good example of this sexual variation is found in Bifericeras from the early part of the Jurassic period of Europe . Only recently has sexual variation in the shells of ammonites been recognized. The macroconch and microconch of one species were often previously mistaken for two closely related but different species occurring in

4900-591: The Neo-Latin siphunculus , meaning "little siphon". Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian ( circa 409 million years ago (Mya)) and became extinct shortly after Cretaceous (66 Mya). The classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers. The Ammonoidea can be divided into six orders, listed here starting with

5040-768: The Paleocene epoch (65–61 Ma). Goniatites, which were a dominant component of Early and Middle Permian faunas, became rare in the Late Permian, and no goniatite is thought to have crossed into the Triassic. Ceratitida originated during the Middle Permian, likely from the Daraelitidae , and radiated in the Late Permian. In the aftermath of the Permian–Triassic extinction event , Ceratitids represent

5180-629: The San Juan River in Colorado, indicate that the animal lived during the Cenozoic, approximately 64.5 Ma (about 1 million years after the K–Pg extinction event). If their existence past the K–Pg boundary can be confirmed, these hadrosaurids would be considered a dead clade walking . The scientific consensus is that these fossils were eroded from their original locations and then re-buried in much later sediments (also known as reworked fossils ). Most paleontologists regard birds as

5320-638: The Solnhofen Limestone , their soft-part record is surprisingly sparse. Beyond a tentative ink sac and possible digestive organs, no soft parts were known until 2021. When neutron imaging was used on a fossil found in 1998, part of the musculature became visible and showed they were able to retract themselves into the shell for protection, and that the retractor muscles and hyponome that work together to enable jet propulsion in nautilus worked independently in ammonites. The reproductive organs show possible traces of spermatophores, which would support

5460-433: The buoyancy of the shell and thereby rise or descend in the water column. A primary difference between ammonites and nautiloids is the siphuncle of ammonites (excepting Clymeniina ) runs along the ventral periphery of the septa and camerae (i.e., the inner surface of the outer axis of the shell), while the siphuncle of nautiloids runs more or less through the center of the septa and camerae. One feature found in shells of

5600-458: The molluscan class Cephalopoda became extinct at the K–Pg boundary. These included the ecologically significant belemnoids , as well as the ammonoids , a group of highly diverse, numerous, and widely distributed shelled cephalopods. The extinction of belemnites enabled surviving cephalopod clades to fill their niches. Ammonite genera became extinct at or near the K–Pg boundary; there was a smaller and slower extinction of ammonite genera prior to

5740-451: The photic zone ) areas of the ocean were less impacted by the K–Pg boundary. Colonial coral species rely upon symbiosis with photosynthetic algae , which collapsed due to the events surrounding the K–Pg boundary, but the use of data from coral fossils to support K–Pg extinction and subsequent Paleocene recovery, must be weighed against the changes that occurred in coral ecosystems through the K–Pg boundary. Most species of brachiopods ,

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5880-653: The Ceratitina from the Triassic; and the Ammonitina, Lytoceratina and Phylloceratina from the Jurassic and Cretaceous. In subsequent taxonomies, these are sometimes regarded as orders within the subclass Ammonoidea. Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are very rarely preserved in any detail. Nonetheless, much has been worked out by examining ammonoid shells and by using models of these shells in water tanks. Many ammonoids probably lived in

6020-459: The Chicxulub peak ring confirmed that the peak ring comprised granite ejected within minutes from deep in the earth, but contained hardly any gypsum , the usual sulfate-containing sea floor rock in the region: the gypsum would have vaporized and dispersed as an aerosol into the atmosphere, causing longer-term effects on the climate and food chain . In October 2019, researchers asserted that

6160-617: The Cretaceous. The Maastrichtian was introduced into scientific literature by Belgian geologist André Hubert Dumont in 1849, after studying rock strata of the Chalk Group close to the Dutch city of Maastricht . These strata are now classified as the Maastricht Formation – both formation and stage derive their names from the city. The Maastricht Formation is known for its fossils from this age, most notably those of

6300-484: The Cretaceous. Along with the aforementioned mosasaurs, plesiosaurs , represented by the families Elasmosauridae and Polycotylidae , became extinct during the event. The ichthyosaurs had disappeared from fossil record tens of millions of years prior to the K-Pg extinction event. Ten families of crocodilians or their close relatives are represented in the Maastrichtian fossil records, of which five died out prior to

6440-540: The Devonian period through those of the Cretaceous period. Calcified aptychi only occur in ammonites from the Mesozoic era. They are almost always found detached from the shell, and are only very rarely preserved in place. Still, sufficient numbers have been found closing the apertures of fossil ammonite shells as to leave no doubt as to their identity as part of the anatomy of an ammonite. Large numbers of detached aptychi occur in certain beds of rock (such as those from

6580-492: The Eocene ants became dominant and diverse, with larger colonies. Butterflies diversified as well, perhaps to take the place of leaf-eating insects wiped out by the extinction. The advanced mound-building termites, Termitidae , also appear to have risen in importance. There are fossil records of jawed fishes across the K–Pg boundary, which provide good evidence of extinction patterns of these classes of marine vertebrates. While

6720-606: The Hell Creek Formation shows a minimum of 75% of turtle species survived. Following the extinction event, turtle diversity exceeded pre-extinction levels in the Danian of North America, although in South America it remained diminished. European turtles likewise recovered rapidly following the mass extinction. The rhynchocephalians which were a globally distributed and diverse group of lepidosaurians during

6860-492: The Jurassic and continued to diversify throughout the Cretaceous. They are currently the most successful and diverse group of living reptiles, with more than 10,000 extant species. The only major group of terrestrial lizards to go extinct at the end of the Cretaceous were the polyglyphanodontians , a diverse group of mainly herbivorous lizards known predominantly from the Northern Hemisphere. The mosasaurs ,

7000-506: The K-Pg boundary known as the Main Fossiliferous Layer (MFL) containing a thanatocoenosis of disarticulated vertebrate fossils, which was likely also caused by a catastrophic flood from the impact. The K–Pg boundary represents one of the most dramatic turnovers in the fossil record for various calcareous nanoplankton that formed the calcium deposits for which the Cretaceous is named. The turnover in this group

7140-538: The K–Pg boundary subsequently becoming extinct in the Miocene . The gharial-like choristodere genus Champsosaurus ' palatal teeth suggest that there were dietary changes among the various species across the K–Pg event. More than 80% of Cretaceous turtle species passed through the K–Pg boundary. All six turtle families in existence at the end of the Cretaceous survived into the Paleogene and are represented by living species. Analysis of turtle survivorship in

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7280-672: The K–Pg boundary resulted in numerous publications detailing planktonic foraminiferal extinction at the boundary; there is ongoing debate between groups which think the evidence indicates substantial extinction of these species at the K–Pg boundary, and those who think the evidence supports a gradual extinction through the boundary. There is strong evidence that local conditions heavily influenced diversity changes in planktonic foraminifera. Low and mid-latitude communities of planktonic foraminifera experienced high extinction rates, while high latitude faunas were relatively unaffected. Numerous species of benthic foraminifera became extinct during

7420-445: The K–Pg boundary, although taxa that thrived in low-latitude, shallow-water environments during the late Cretaceous had the highest extinction rate. Mid-latitude, deep-water echinoderms were much less affected at the K–Pg boundary. The pattern of extinction points to habitat loss, specifically the drowning of carbonate platforms , the shallow-water reefs in existence at that time, by the extinction event. Atelostomatans were affected by

7560-421: The K–Pg boundary, despite the ecological niches made available by the extinction of dinosaurs. Several mammalian orders have been interpreted as diversifying immediately after the K–Pg boundary, including Chiroptera ( bats ) and Cetartiodactyla (a diverse group that today includes whales and dolphins and even-toed ungulates ), although recent research concludes that only marsupial orders diversified soon after

7700-506: The K–Pg boundary. Deposit feeders were the most common bivalves in the catastrophe's aftermath. Abundance was not a factor that affected whether a bivalve taxon went extinct, according to evidence from North America. Veneroid bivalves developed deeper burrowing habitats as the recovery from the crisis ensued. Except for nautiloids (represented by the modern order Nautilida ) and coleoids (which had already diverged into modern octopodes , squids , and cuttlefish ) all other species of

7840-521: The K–Pg boundary. Five families have both Maastrichtian and Paleocene fossil representatives. All of the surviving families of crocodyliforms inhabited freshwater and terrestrial environments—except for the Dyrosauridae , which lived in freshwater and marine locations. Approximately 50% of crocodyliform representatives survived across the K–Pg boundary, the only apparent trend being that no large crocodiles survived. Crocodyliform survivability across

7980-412: The K–Pg boundary. However, morphological diversification rates among eutherians after the extinction event were thrice those of before it. Also significant, within the mammalian genera, new species were approximately 9.1% larger after the K–Pg boundary. After about 700,000 years, some mammals had reached 50 kilos (110 pounds), a 100-fold increase over the weight of those which survived the extinction. It

8120-493: The K–Pg boundary. A study of fossil vertebrates across the K–Pg boundary in Montana concluded that no species of amphibian became extinct. Yet there are several species of Maastrichtian amphibian, not included as part of this study, which are unknown from the Paleocene. These include the frog Theatonius lancensis and the albanerpetontid Albanerpeton galaktion ; therefore, some amphibians do seem to have become extinct at

8260-555: The K–Pg boundary. Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of non-avian dinosaurs. Based on molecular sequencing and fossil dating, many species of birds (the Neoaves group in particular) appeared to radiate after the K–Pg boundary. The open niche space and relative scarcity of predators following the K-Pg extinction allowed for adaptive radiation of various avian groups. Ratites , for example, rapidly diversified in

8400-418: The K–Pg event. Scientists agree that all non-avian dinosaurs became extinct at the K–Pg boundary. The dinosaur fossil record has been interpreted to show both a decline in diversity and no decline in diversity during the last few million years of the Cretaceous, and it may be that the quality of the dinosaur fossil record is simply not good enough to permit researchers to distinguish between the options. There

8540-532: The K–Pg extinction event as marine environments were. Among the terrestrial clade Notosuchia , only the family Sebecidae survived; the exact reasons for this pattern are not known. Sebecids were large terrestrial predators, are known from the Eocene of Europe, and would survive in South America into the Miocene. Tethysuchians radiated explosively after the extinction event. Two families of pterosaurs, Azhdarchidae and Nyctosauridae , were definitely present in

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8680-663: The K–Pg extinction event, although they suffered losses. In particular, metatherians largely disappeared from North America, and the Asian deltatheroidans became extinct (aside from the lineage leading to Gurbanodelta ). In the Hell Creek beds of North America, at least half of the ten known multituberculate species and all eleven metatherians species are not found above the boundary. Multituberculates in Europe and North America survived relatively unscathed and quickly bounced back in

8820-496: The K–Pg extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches . Evidence from the Salamanca Formation suggests that biotic recovery was more rapid in the Southern Hemisphere than in the Northern Hemisphere. Despite the massive loss of life inferred to have occurred during the extinction, and a number of geologic formations worldwide that span

8960-601: The Lilliput effect. Insect damage to the fossilized leaves of flowering plants from fourteen sites in North America was used as a proxy for insect diversity across the K–Pg boundary and analyzed to determine the rate of extinction. Researchers found that Cretaceous sites, prior to the extinction event, had rich plant and insect-feeding diversity. During the early Paleocene, flora were relatively diverse with little predation from insects, even 1.7 million years after

9100-541: The Maastrichtian age, 28  shark families and 13 batoid families thrived, of which 25 and 9, respectively, survived the K–T boundary event. Forty-seven of all neoselachian genera cross the K–T boundary, with 85% being sharks. Batoids display with 15%, a comparably low survival rate. Among elasmobranchs, those species that inhabited higher latitudes and lived pelagic lifestyles were more likely to survive, whereas epibenthic lifestyles and durophagy were strongly associated with

9240-417: The Maastrichtian, and they likely became extinct at the K–Pg boundary. Several other pterosaur lineages may have been present during the Maastrichtian, such as the ornithocheirids , pteranodontids , a possible tapejarid , a possible thalassodromid and a basal toothed taxon of uncertain affinities, though they are represented by fragmentary remains that are difficult to assign to any given group. While this

9380-500: The Maastrichtian. Several archaic clades of birds, such as Enantiornithes , Ichthyornithes , and Hesperornithes , persisted to the latest Maastrichtian but became extinct during the Cretaceous-Paleogene extinction event. Traditionally, pterosaur faunas of the Maastrichtian were assumed to be dominated by azhdarchids , with other pterosaur groups having become extinct earlier on. However, more recent findings suggest

9520-519: The Mesozoic in the Alps ). These rocks are usually accumulated at great depths. The modern Nautilus lacks any calcitic plate for closing its shell, and only one extinct nautiloid genus is known to have borne anything similar. Nautilus does, however, have a leathery head shield (the hood) which it uses to cover the opening when it retreats inside. There are many forms of aptychus, varying in shape and

9660-604: The Northern Hemisphere. Following the extinction, survivor communities dominated for several hundred thousand years. The North Pacific acted as a diversity hotspot from which later nannoplankton communities radiated as they replaced survivor faunas across the globe. The K–Pg boundary record of dinoflagellates is not so well understood, mainly because only microbial cysts provide a fossil record, and not all dinoflagellate species have cyst-forming stages, which likely causes diversity to be underestimated. Recent studies indicate that there were no major shifts in dinoflagellates through

9800-447: The Paleocene, but Asian forms were devastated, never again to represent a significant component of mammalian fauna. A recent study indicates that metatherians suffered the heaviest losses at the K–Pg event, followed by multituberculates, while eutherians recovered the quickest. K–Pg boundary mammalian species were generally small, comparable in size to rats ; this small size would have helped them find shelter in protected environments. It

9940-548: The adaptations of many dinosaurs to cold environments. Whether the extinction occurred gradually or suddenly has been debated, as both views have support from the fossil record. A highly informative sequence of dinosaur-bearing rocks from the K–Pg boundary is found in western North America, particularly the late Maastrichtian-age Hell Creek Formation of Montana . Comparison with the older Judith River Formation (Montana) and Dinosaur Park Formation ( Alberta ), which both date from approximately 75 Ma, provides information on

10080-440: The ammonite shell is called a phragmocone . It contains a series of progressively larger chambers, called camerae (sing. camera) that are divided by thin walls called septa (sing. septum). Only the last and largest chamber, the body chamber , was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the coil. Where the outer whorl of an ammonite shell largely covers

10220-456: The ammonoid suture line (the intersection of the septum with the outer shell) is variably folded, forming saddles ("peaks" that point towards the aperture) and lobes ("valleys" which point away from the aperture). The suture line has four main regions. The external or ventral region refers to sutures along the lower (outer) edge of the shell, where the left and right suture lines meet. The external (or ventral) saddle, when present, lies directly on

10360-474: The animal's life; additional shell layers covered it. The majority of ammonoid specimens, especially those of the Paleozoic era, are preserved only as internal molds; the outer shell (composed of aragonite ) has been lost during the fossilization process. Only in these internal-mould specimens can the suture lines be observed; in life, the sutures would have been hidden by the outer shell. The ammonoids as

10500-461: The asteroid impact and not volcanism . A wide range of terrestrial species perished in the K–Pg extinction, the best-known being the non-avian dinosaurs, along with many mammals, birds, lizards, insects , plants, and all the pterosaurs . In the oceans, the K–Pg extinction killed off plesiosaurs and mosasaurs and devastated teleost fish, sharks , mollusks (especially ammonites , which became extinct), and many species of plankton. It

10640-402: The biotic recovery in the aftermath of the K-Pg extinction event. Pan-Gekkotans weathered the extinction event well, with multiple lineages likely surviving. ∆ Ca values indicate that prior to the mass extinction, marine reptiles at the top of food webs were feeding on only one source of calcium, suggesting their populations exhibited heightened vulnerability to extinctions at the terminus of

10780-402: The boundary associated with a late Cretaceous marine regression, and a small, gradual reduction in ammonite diversity occurred throughout the very late Cretaceous. Researchers have pointed out that the reproductive strategy of the surviving nautiloids, which rely upon few and larger eggs, played a role in outsurviving their ammonoid counterparts through the extinction event. The ammonoids utilized

10920-495: The boundary interval. Ostracods that were heavily sexually selected were more vulnerable to extinction, and ostracod sexual dimorphism was significantly rarer following the mass extinction. Among decapods , extinction patterns were highly heterogeneous and cannot be neatly attributed to any particular factor. Decapods that inhabited the Western Interior Seaway were especially hard-hit, while other regions of

11060-498: The boundary layer. There were blooms of the taxa Thoracosphaera operculata and Braarudosphaera bigelowii at the boundary. Radiolaria have left a geological record since at least the Ordovician times, and their mineral fossil skeletons can be tracked across the K–Pg boundary. There is no evidence of mass extinction of these organisms, and there is support for high productivity of these species in southern high latitudes as

11200-450: The boundary may have resulted from their aquatic niche and ability to burrow, which reduced susceptibility to negative environmental effects at the boundary. Jouve and colleagues suggested in 2008 that juvenile marine crocodyliforms lived in freshwater environments as do modern marine crocodile juveniles, which would have helped them survive where other marine reptiles became extinct; freshwater environments were not so strongly affected by

11340-780: The boundary, only a few fossil sites contain direct evidence of the mass mortality that occurred exactly at the K-Pg boundary. These include the Tanis site of the Hell Creek Formation in North Dakota , USA, which contains a high number of well-preserved fossils that appear to have buried in a catastrophic flood event that was likely caused by the impact. Another important site is the Hornerstown Formation in New Jersey , USA, which has prominent layer at

11480-418: The boundary. The relatively low levels of extinction seen among amphibians probably reflect the low extinction rates seen in freshwater animals. Following the mass extinction, frogs radiated substantially, with 88% of modern anuran diversity being traced back to three lineages of frogs that evolved after the cataclysm. The choristoderes (a group of semi-aquatic diapsids of uncertain position) survived across

11620-487: The changes in dinosaur populations over the last 10 million years of the Cretaceous. These fossil beds are geographically limited, covering only part of one continent. The middle–late Campanian formations show a greater diversity of dinosaurs than any other single group of rocks. The late Maastrichtian rocks contain the largest members of several major clades: Tyrannosaurus , Ankylosaurus , Pachycephalosaurus , Triceratops , and Torosaurus , which suggests food

11760-438: The circumstances of food chain disruption previously mentioned, non-avian dinosaurs died out, while some crocodiles survived. In this context, the survival of other endothermic animals, such as some birds and mammals, could be due, among other reasons, to their smaller needs for food, related to their small size at the extinction epoch. Prolonged cold is unlikely to have been a reason for the extinction of non-avian dinosaurs given

11900-423: The data suggests massive devastation and mass extinction of plants at the K–Pg boundary sections, although there were substantial megafloral changes before the boundary. In North America, approximately 57% of plant species became extinct. In high southern hemisphere latitudes, such as New Zealand and Antarctica, the mass die-off of flora caused no significant turnover in species, but dramatic and short-term changes in

12040-498: The deep-sea realm was able to remain seemingly unaffected, there was an equal loss between the open marine apex predators and the durophagous demersal feeders on the continental shelf. Within cartilaginous fish , approximately 7 out of the 41 families of neoselachians (modern sharks , skates, and rays) disappeared after this event and batoids (skates and rays) lost nearly all the identifiable species, while more than 90% of teleost fish (bony fish) families survived. In

12180-419: The delicate balance of local redox conditions sufficiently to lower the local solubility of minerals dissolved in the seawater, notably phosphates and carbonates . The resulting spontaneous concentric precipitation of minerals around a fossil, a concretion , is responsible for the outstanding preservation of many ammonite fossils. When ammonites are found in clays , their original mother-of-pearl coating

12320-643: The early Mesozoic , had begun to decline by the mid-Cretaceous, although they remained successful in the Late Cretaceous of southern South America . They are represented today by a single species, the tuatara ( Sphenodon punctatus ) found in New Zealand . Outside of New Zealand, one rhynchocephalian is known to have crossed the K-Pg boundary, Kawasphenodon peligrensis , known from the earliest Paleocene (Danian) of Patagonia. The order Squamata comprising lizards and snakes first diversified during

12460-422: The early Paleocene provided the food source to support large benthic foraminiferal assemblages, which are mainly detritus-feeding. Ultimate recovery of the benthic populations occurred over several stages lasting several hundred thousand years into the early Paleocene. There is significant variation in the fossil record as to the extinction rate of marine invertebrates across the K–Pg boundary. The apparent rate

12600-630: The early Paleogene and are believed to have convergently developed flightlessness at least three to six times, often fulfilling the niche space for large herbivores once occupied by non-avian dinosaurs. Mammalian species began diversifying approximately 30 million years prior to the K–Pg boundary. Diversification of mammals stalled across the boundary. All major Late Cretaceous mammalian lineages, including monotremes (egg-laying mammals), multituberculates , metatherians (which includes modern marsupials), eutherians (which includes modern placentals), meridiolestidans , and gondwanatheres survived

12740-642: The end of the Cretaceous and underwent sudden extinction after the Cretaceous–Paleogene extinction event. Alternatively, interpretation based on the fossil-bearing rocks along the Red Deer River in Alberta, Canada, supports the gradual extinction of non-avian dinosaurs; during the last 10 million years of the Cretaceous layers there, the number of dinosaur species seems to have decreased from about 45 to approximately 12. Other scientists have made

12880-405: The endothermy of dinosaurs (see dinosaur physiology ) helps to understand their full extinction in contrast with their close relatives, the crocodilians. Ectothermic ("cold-blooded") crocodiles have very limited needs for food (they can survive several months without eating), while endothermic ("warm-blooded") animals of similar size need much more food to sustain their faster metabolism. Thus, under

13020-518: The event rapidly acidified the oceans and produced long-lasting effects on the climate, detailing the mechanisms of the mass extinction. Other causal or contributing factors to the extinction may have been the Deccan Traps and other volcanic eruptions, climate change , and sea level change. However, in January 2020, scientists reported that climate-modeling of the extinction event favored

13160-422: The event's severity, there was significant variability in the rate of extinction between and within different clades . Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked sunlight and reduced the solar energy reaching the ground. This plant extinction caused a major reshuffling of the dominant plant groups. Omnivores , insectivores , and carrion -eaters survived

13300-409: The event, presumably because they depend on organic debris for nutrients, while biomass in the ocean is thought to have decreased. As the marine microbiota recovered, it is thought that increased speciation of benthic foraminifera resulted from the increase in food sources. In some areas, such as Texas, benthic foraminifera show no sign of any major extinction event, however. Phytoplankton recovery in

13440-449: The extinction event is best represented by the marked discrepancy between the rich and relatively abundant late-Maastrichtian pollen record and the post-boundary fern spike. Polyploidy appears to have enhanced the ability of flowering plants to survive the extinction, probably because the additional copies of the genome such plants possessed allowed them to more readily adapt to the rapidly changing environmental conditions that followed

13580-586: The extinction event, perhaps because of the increased availability of their food sources. Neither strictly herbivorous nor strictly carnivorous mammals seem to have survived. Rather, the surviving mammals and birds fed on insects , worms , and snails , which in turn fed on detritus (dead plant and animal matter). In stream communities and lake ecosystems , few animal groups became extinct, including large forms like crocodyliforms and champsosaurs , because such communities rely less directly on food from living plants, and more on detritus washed in from

13720-471: The extinction event. Studies of the size of the ichnotaxon Naktodemasis bowni , produced by either cicada nymphs or beetle larvae, over the course of the K-Pg transition show that the Lilliput effect occurred in terrestrial invertebrates thanks to the extinction event. The extinction event produced major changes in Paleogene insect communities. Many groups of ants were present in the Cretaceous, but in

13860-406: The extinction of all non-avian dinosaurs . Most other tetrapods weighing more than 25 kg (55 lb) also became extinct, with the exception of some ectothermic species such as sea turtles and crocodilians . It marked the end of the Cretaceous period, and with it the Mesozoic era, while heralding the beginning of the current era, the Cenozoic . In the geologic record , the K–Pg event

14000-466: The first heteromorph ammonoid fossils belongs to the genus Rhabdoceras. The three other heteromorphic genera were Hannaoceras, Cochloceras and Choristoceras. All of them went extinct at the end of Triassic. In the Jurassic an uncoiled shell was found in the Spiroceratoidea, but by the end of Cretaceous the only heteromorph ammonites remaining belonged to the suborder Ancyloceratina. One example

14140-420: The first saddle and lobe pair past the external region as the suture line extends up the side of the shell. The lateral saddle and lobe are usually larger than the ventral saddle and lobe. Additional lobes developing towards the inner edge of a whorl are labelled umbilical lobes, which increase in number through ammonoid evolution as well as an individual ammonoid's development. In many cases the distinction between

14280-433: The front, distinguishing them from nautiloid septa, which are typically simple concave, dish-shaped structures. The topology of the septa, especially around the rim, results in the various suture patterns found. The septal curvature in nautiloids and ammonoids also differ in that the septa curves towards the opening in nautiloids, and away from the opening in ammоnoids. While nearly all nautiloids show gently curving sutures,

14420-417: The giant sea reptile Mosasaurus , which in turn derives its name from the nearby river Maas ( mosa being Latin for the river Maas). The base of the Maastrichtian Stage is at the first appearance of ammonite species Pachydiscus neubergicus . At the original type locality near Maastricht, the stratigraphic record was later found to be incomplete. A reference profile for the base was then appointed in

14560-503: The hypothesis that the microconchs were males. They likely bore a radula and beak, a marginal siphuncle and ten arms. They operated by direct development with sexual reproduction, were carnivorous, and had a crop for food storage. They are unlikely to have dwelt in fresh or brackish water. Many ammonites were likely filter feeders , so adaptations associated with this lifestyle like sieves probably occurred. A 2021 study found ammonite specimens with preserved hook-like suckers, providing

14700-467: The impact, giving rise to today's birds. The only bird group known for certain to have survived the K–Pg boundary is the Aves. Avians may have been able to survive the extinction as a result of their abilities to dive, swim, or seek shelter in water and marshlands. Many species of avians can build burrows, or nest in tree holes, or termite nests, all of which provided shelter from the environmental effects at

14840-461: The impact. Beyond extinction impacts, the event also caused more general changes of flora such as giving rise to neotropical rainforest biomes like the Amazonia , replacing species composition and structure of local forests during ~6 million years of recovery to former levels of plant diversity . Ammonites Ammonoids are extinct spiral shelled cephalopods comprising

14980-414: The land, protecting them from extinction. Modern crocodilians can live as scavengers and survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous. Similar, but more complex patterns have been found in the oceans. Extinction

15120-493: The landscape for centuries after the event. In the sediments below the K–Pg boundary the dominant plant remains are angiosperm pollen grains, but the boundary layer contains little pollen and is dominated by fern spores. More usual pollen levels gradually resume above the boundary layer. This is reminiscent of areas blighted by modern volcanic eruptions, where the recovery is led by ferns, which are later replaced by larger angiosperm plants. In North American terrestrial sequences,

15260-511: The largest and most recent whorls are exposed. Shell structure can be broken down further by the width of the shell, with implications for hydrodynamic efficiency. Major shell forms include: Ammonites vary greatly in the ornamentation (surface relief) of their shells. Some may be smooth and relatively featureless, except for growth lines, resembling that of the modern Nautilus . In others, various patterns of spiral ridges, ribs, nodes, or spines are presented. This type of complex ornamentation of

15400-510: The lateral and umbilical regions are unclear; new umbilical features can develop from subdivisions of other umbilical features, or from subdivisions of lateral features. Lobes and saddles which are so far towards the center of the whorl that they are covered up by succeeding whorls are labelled internal (or dorsal) lobes and saddles. Three major types of suture patterns are found in the Ammonoidea: The siphuncle in most ammonoids

15540-412: The likelihood of perishing during the extinction event. There is evidence of a mass extinction of bony fishes at a fossil site immediately above the K–Pg boundary layer on Seymour Island near Antarctica , apparently precipitated by the K–Pg extinction event; the marine and freshwater environments of fishes mitigated the environmental effects of the extinction event. The result was Patterson's Gap,

15680-434: The lower midline of the shell. As a result, it is often called the median saddle. On suture diagrams the median saddle is supplied with an arrow which points towards the aperture. The median saddle is edged by fairly small external (or ventral) lobes. The earliest ammonoids lacked a median saddle and instead had a single midline ventral lobe, which in later forms is split into two or more components. The lateral region involves

15820-432: The modern Nautilus is the variation in the shape and size of the shell according to the sex of the animal, the shell of the male being slightly smaller and wider than that of the female. This sexual dimorphism is thought to be an explanation for the variation in size of certain ammonite shells of the same species, the larger shell (the macroconch ) being female, and the smaller shell (the microconch ) being male. This

15960-485: The most extreme and bizarre-looking example of a heteromorph is Nipponites , which appears to be a tangle of irregular whorls lacking any obvious symmetric coiling. Upon closer inspection, though, the shell proves to be a three-dimensional network of connected "U" shapes. Nipponites occurs in rocks of the upper part of the Cretaceous in Japan and the United States. Some ammonites have been found in association with

16100-575: The most primitive and going to the more derived: In some classifications, these are left as suborders, included in only three orders: Goniatitida , Ceratitida and Ammonitida . The Treatise on Invertebrate Paleontology (Part L, 1957) divides the Ammonoidea, regarded simply as an order, into eight suborders, the Anarcestina, Clymeniina, Goniatitina and Prolecanitina from the Paleozoic;

16240-622: The nascent North Atlantic to the south. This initiated thermohaline circulation similar to that of the modern oceans. At the same time, the Laramide orogeny drained the Western Interior Seaway of North America, further contributing to global cooling. Nonetheless, the latest Maastrichtian featured a sharp, pronounced warming, which was caused by the activity of the Deccan Traps. Northern Alaska's mean annual temperature

16380-444: The number of flowering plants. However, phylogenetic evidence shows no mass angiosperm extinction. Due to the wholesale destruction of plants at the K–Pg boundary, there was a proliferation of saprotrophic organisms, such as fungi , that do not require photosynthesis and use nutrients from decaying vegetation. The dominance of fungal species lasted only a few years while the atmosphere cleared and plenty of organic matter to feed on

16520-426: The only surviving dinosaurs (see Origin of birds ). It is thought that all non-avian theropods became extinct, including then-flourishing groups such as enantiornithines and hesperornithiforms . Several analyses of bird fossils show divergence of species prior to the K–Pg boundary, and that duck, chicken, and ratite bird relatives coexisted with non-avian dinosaurs. Large collections of bird fossils representing

16660-781: The open water of ancient seas, rather than at the sea bottom, because their fossils are often found in rocks laid down under conditions where no bottom-dwelling life is found. In general, they appear to have inhabited the upper 250 meters of the water column. Many of them (such as Oxynoticeras ) are thought to have been good swimmers, with flattened, discus-shaped, streamlined shells, although some ammonoids were less effective swimmers and were likely to have been slow-swimming bottom-dwellers. Synchrotron analysis of an aptychophoran ammonite revealed remains of isopod and mollusc larvae in its buccal cavity, indicating at least this kind of ammonite fed on plankton . They may have avoided predation by squirting ink , much like modern cephalopods; ink

16800-478: The order Ammonitida , the only remaining group of ammonoids from the Jurassic up until their extinction. Ammonites are excellent index fossils , and linking the rock layer in which a particular species or genus is found to specific geologic time periods is often possible. Their fossil shells usually take the form of planispirals , although some helically spiraled and nonspiraled forms (known as heteromorphs ) have been found. The name "ammonite", from which

16940-478: The original shell, as well as the complete body chamber, still intact. Many Pierre Shale ammonites, and indeed many ammonites throughout earth history, are found inside concretions . Other fossils, such as many found in Madagascar and Alberta , Canada display iridescence . These iridescent ammonites are often of gem quality ( ammolite ) when polished. In no case would this iridescence have been visible during

17080-461: The preceding whorls, the specimen is said to be involute (e.g., Anahoplites ). Where it does not cover those preceding, the specimen is said to be evolute (e.g., Dactylioceras ). A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers. Through a hyperosmotic active transport process, the ammonite emptied water out of these shell chambers. This enabled it to control

17220-489: The relative abundance of plant groups. European flora was also less affected, most likely due to its distance from the site of the Chicxulub impact. In northern Alaska and the Anadyr-Koryak region of Russia, the flora was minimally impacted. Another line of evidence of a major floral extinction is that the divergence rate of subviral pathogens of angiosperms sharply decreased, which indicates an enormous reduction in

17360-494: The same assessment following their research. Several researchers support the existence of Paleocene non-avian dinosaurs . Evidence of this existence is based on the discovery of dinosaur remains in the Hell Creek Formation up to 1.3 m (4.3 ft) above and 40,000 years later than the K–Pg boundary. Pollen samples recovered near a fossilized hadrosaur femur recovered in the Ojo Alamo Sandstone at

17500-494: The same rocks. However, because the dimorphic sizes are so consistently found together, they are more likely an example of sexual dimorphism within the same species. Whorl width in the body chamber of many groups of ammonites, as expressed by the width:diameter ratio, is another sign of dimorphism. This character has been used to separate "male" (Largiventer conch "L") from "female" (Leviventer conch "l"). The majority of ammonite species feature planispiral shells, tightly coiled in

17640-531: The same time. Non-avian dinosaurs , for example, are known from the Maastrichtian of North America, Europe , Asia, Africa , South America, and Antarctica , but are unknown from the Cenozoic anywhere in the world. Similarly, fossil pollen shows devastation of the plant communities in areas as far apart as New Mexico , Alaska , China , and New Zealand . Nevertheless, high latitudes appear to have been less strongly affected than low latitudes. Despite

17780-519: The scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams ' horns. Pliny the Elder ( d. 79 AD near Pompeii) called fossils of these animals ammonis cornua (" horns of Ammon ") because the Egyptian god Ammon ( Amun ) was typically depicted wearing rams' horns. Often, the name of an ammonite genus ends in - ceras , which

17920-479: The sculpture of the inner and outer surfaces, but because they are so rarely found in position within the shell of the ammonite it is often unclear to which species of ammonite one kind of aptychus belongs. A number of aptychi have been given their own genus and even species names independent of their unknown owners' genus and species, pending future discovery of verified occurrences within ammonite shells. Although ammonites do occur in exceptional lagerstatten such as

18060-415: The shell is especially evident in the later ammonites of the Cretaceous. Ammonoids with a shell shape diverging from the typical planispiral form are known as heteromorphs , instead forming a conch with detached whorls (open coiling) or non-planispiral coiling. These types of shells evolved four times in ammonoids, with the first forms appearing already in the Devonian period. In late Norian age in Triassic

18200-432: The subclass Ammonoidea . They are more closely related to living coleoids (i.e., octopuses , squid and cuttlefish ) than they are to shelled nautiloids (such as the living Nautilus ). The earliest ammonoids appeared during the Devonian , with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event . They are often called ammonites , which is most frequently used for members of

18340-507: The warm and humid climate of the Mesozoic to the colder and more arid climate of the Cenozoic. Variation of climate with latitude also became greater. This was likely caused by a major reorganization of oceanic circulation that took place at the boundary between the early and late Maastrichtian. This reorganization was triggered by the breach of tectonic barriers in the South Atlantic, permitting deep ocean water to begin circulating from

18480-597: The world's oceans were refugia that increased chances of survival into the Palaeocene. Among retroplumid crabs, the genus Costacopluma was a notable survivor. Approximately 60% of late-Cretaceous scleractinian coral genera failed to cross the K–Pg boundary into the Paleocene. Further analysis of the coral extinctions shows that approximately 98% of colonial species, ones that inhabit warm, shallow tropical waters, became extinct. The solitary corals, which generally do not form reefs and inhabit colder and deeper (below

18620-423: Was 6.3 °C. South-central Alaska had a mean annual temperature of 7.42 ± 1.2 °C, a warm monthly mean temperature of 17.08 ± 1.6 °C, and a cold monthly mean temperature of − 2.31 ± 1.9 °C. Dinosaurs remained the dominant large terrestrial animals throughout the Maastrichtian, though mammals with internal organs similar to modern mammals were also present. Both ammonites and pterosaurs were in serious decline during

18760-457: Was bolstered by the discovery of the 180 km (112 mi) Chicxulub crater in the Gulf of Mexico 's Yucatán Peninsula in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact . The fact that the extinctions occurred simultaneously provides strong evidence that they were caused by the asteroid. A 2016 drilling project into

18900-527: Was more severe among animals living in the water column than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton , while animals on the ocean floor always or sometimes feed on detritus. Coccolithophorids and mollusks (including ammonites , rudists , freshwater snails , and mussels ), and those organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it

19040-480: Was occurring, modern birds were undergoing diversification; traditionally it was thought that they replaced archaic birds and pterosaur groups, possibly due to direct competition, or they simply filled empty niches, but there is no correlation between pterosaur and avian diversities that are conclusive to a competition hypothesis, and small pterosaurs were present in the Late Cretaceous. At least some niches previously held by birds were reclaimed by pterosaurs prior to

19180-614: Was plentiful immediately prior to the extinction. A study of 29 fossil sites in Catalan Pyrenees of Europe in 2010 supports the view that dinosaurs there had great diversity until the asteroid impact, with more than 100 living species. More recent research indicates that this figure is obscured by taphonomic biases and the sparsity of the continental fossil record. The results of this study, which were based on estimated real global biodiversity, showed that between 628 and 1,078 non-avian dinosaur species were alive at

19320-428: Was present. Once the atmosphere cleared photosynthetic organisms returned – initially ferns and other ground-level plants. In some regions, the Paleocene recovery of plants began with recolonizations by fern species, represented as a fern spike in the geologic record; this same pattern of fern recolonization was observed after the 1980 Mount St. Helens eruption . Just two species of fern appear to have dominated

19460-550: Was rapidly growing in size as the surrounding oceanic plates were consumed by subduction , and the Pacific-Izanagi Ridge was rapidly approaching Asia. Eruption of the Deccan Traps large igneous province began during the Maastrichtian, at around 67 million years ago. This is thought to be a consequence of India drifting over the Réunion hotspot . During the Maastrichtian, the global climate began to shift from

19600-533: Was well under way in the Maastrichtian. From 50% to 80% of all genera of land plants were angiosperms, though gymnosperms and ferns still covered larger areas of the land surface. Cretaceous%E2%80%93Paleogene extinction event The Cretaceous–Paleogene ( K–Pg ) extinction event , also known as the K–T extinction , was the mass extinction of three-quarters of the plant and animal species on Earth approximately 66 million years ago. The event caused

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