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Although all major mammal lineages survived the K/Pg(Cretaceous/Paleogene) extinction, they all suffered losses. The severity was different between lineages and some were even set on the path to extinction. Others survived and became the extant lineages we see today. Fossil evidence along with other scientific techniques such as genetic analysis shows exactly how these lineages changed while passing through the K/Pg boundary. For some lineages the evidence gives a clear picture while for others a significant lack of evidence creates uncertainty. The K/Pg boundary was a critical point in mammalian evolution and tracing the evolutionary history of mammalian lineages from this point to the present is a significant area of study.

Description
Multituberculates, also known as multituberculata, are a lineage of mammals similar to rodents, a placental mammal group, in form and with the longest known geological ranges of existence for a mammal group. They survived from the late Jurassic to the early Oligocene which is about a 100 million years. They are now extinct and their extinction is hypothesized to be a result of competition with early rodents. Their name comes from the fact that they had multiple tubercles, rounded protrusions, on their teeth. They are divided into two major suborders called Plagiaulacida and Cimolodonta. The former is paraphyletic and the latter is monophyletic. There also existed a separate family called incertae sedis, a name given to taxonomic groups with unknown broad relationships. The taxonomic classifications of Multituberculates is still uncertain for even some superfamilies in the two main suborders.

They can be characterized by a size ranging from that of the typical mouse to as large as a beaver. Their ecological niches varied including examples such as burrow dwelling and arborealism which means a frequent interaction with trees. Most of the fossil records of Multituberculates were found in what was once Laurasia, the combination of North America and Eurasia minus the Indian subcontinent. The fossils found in the southern continents, once formed Gondwana, are controversial.

Phylogeny
An adaptive radiation began for Multituberculates 20 million years before the extinction of non-avian dinosaurs at the K/Pg boundary and continued on through it. This suggests that the elimination of dinosaur competition and predation pressures did not significantly affect the diversification of Multituberculates although this doesn’t necessarily have to be true for other mammalian orders. Fossil evidence shows disparity of teeth complexity rising, associated with a range of new diets linked to angiosperm ecological diversification. This also coincided with an increase in generic richness (number of different genera) and range of body sizes. The ecological opportunities that arose in the Mesozoic is the likely cause for the diversification of Multituberculates and these opportunities persisted throughout the K/Pg extinction.

Apart from the extant lineages, Multituberculates were the only now extinct orders to recover and maintain their levels after the K/Pg extinction event. The Plagiaulacida lived from the Late Jurassic to the Early Cretaceous and have been distinguished from the other Multituberculates by a greater number of incisors and premolars. The two superfamilies of the Cimolodonta suborder, Ptilodontoidea and Taeniolabidoidea are thought to have derived from the Plagiaulacida. The Ptilodontoidea lived from the Late Cretaceous to the early Oligocene in North America. They are distinguished by longer, procumbent lower incisors and a lower premolar that is enlarged for slicing. The Taeniolabidoidea also arose in the Late Cretaceous but extend shorter to the early Eocene. They are defined by an enlarged pair of lower incisors bearing with a restricted band of enamel and do not have the enlarged premolar.

Description
Marsupials fall under the group Metatheria and the latter group is only slightly more inclusive than the marsupial term as it includes fossil mammals that are more closely related to marsupials than placentals. All extant metatherians are marsupials. Despite being the second most diverse clade of living mammals, 70% of the extant species are found in Australia and the rest in the Americas, with only one being found north of Mexico. They originated in the late Jurassic and have been surviving for 150 million years. The main subgroup Marsupialia is divided into two main superorders, Ameridelphia and Australidelphia, and are named by whether they live or lived on the regions of the Americas or Australia. One order called Microbiotheria is located in South America but is classified under Australidelphia on the basis of it being more similar to Australian marsupials.

Metatherians can be distinguished from other mammals by their teeth which consist of five upper and four lower incisors, a canine, three premolars, and four molars. They differ from Eutheria also in their tooth replacement pattern. Some other defining characteristics are present in the wrist and ankle such as a unique heel which can be easily observed in the feet of Kangaroos. Most notably metatherians give birth to undeveloped young that reside in a pouch on the mother for a certain period of time. Marsupials have diversified into many different ecological niches in both superorders which is exemplified by the presence of arboreal species.

Phylogeny
Marsupials are fairly difficult to distinguish from other mammals in the fossil record because their main difference, which is their reproductive structures, don’t get fossilized. Also the distinction between a marsupial and other metatherians is in the subtle differences of the bone and tooth structure. The first metatherian fossil was found in the Early Cretaceous of Asia. Metatherians likely originated in Laurasia and were spread across Asia, Europe and North America by the early Late Cretaceous. During this period in North America, metatherians were dominant and more taxonomically diverse than the Eutherians who would give rise to the now dominant placentals. Fossil evidence showing metatherian diversification patterns indicate the angiosperm ecological diversification during the Cretaceous did not have a significant effect on the lineage and a major extinction was suffered at the K/Pg boundary. In North America there were only 8 species left which is a 66% reduction. During the same period in South America there was a diversification of metatherians.

Despite the number of extant metatherian mammals the fossil evidence of metatherian lineages is quite sparse and the relationships remain unresolved. Part of this is because Antarctica holds the link between South American metatherians and Australian metatherians but as it is covered in ice fossils are limited. After the K/Pg extinction metatherians mostly disappeared from Laurasia, but they remained in the continents that were once Gondwana. The reason for metatherian disappearance in Laurasia has historically been attributed to competition from placentals but has been disregarded as the main factor. All metatherians in Australia are hypothesized to have derived from one species that crossed over from Antarctica shortly after the two land masses separated. This species was likely related to the South American Microbiotheria based on phylogenetic relationships. Australian metatherians thrived because local placental mammals disappeared for an unknown reason in the early Cenozoic. New mitochondrial genome analysis suggests that metatherians colonized Australia twice. The evidence points to a basal split between Didelphimorphia, a marsupial order, and the remaining orders 69 million years ago accounting for the first colonization. The evidence also nests the South American order, Microbiotheria with the Australian orders accounting for the second colonization as the most recent common ancestor of these orders must have crossed over from South America to Australia. The results also show that the timing of the second divergence coincided with the separation of Antarctica and Australia.

Description
Monotremata, commonly referred to as monotremes, is an order of mammals that lays eggs and of which the extant species are only found in Australia and New Guinea. The common ancestor of all mammals was thought to have existed 200-310 million years ago and had milk resource genes called caseins which functioned similarly to the vitellogenin proteins in birds to promote embryonic development. Only the monotremes retained the vitellogenin protein genes that were found in the common ancestor of mammals, birds and reptiles. According to the Theria hypothesis, monotremes form a sister group to Theria which is what the placentals and marsupials derived from. The divergence of the two sister groups occurred 231-217 million years ago and the monotremes have survived since the Late Triassic. This divergence is supported by morphological analysis. The marsupial-placental split was shown to have occurred 193-186 million years ago. There have been studies conducted with nuclear DNA to provide support for the Theria hypothesis and it is now generally accepted. The Marsupionata hypothesis states that monotremes and marsupials are sister groups and more closely related to each other than either is to Eutheria. This hypothesis has significantly less evidence supporting it but one study that was made found significant similarities in mitochondrial protein coding genes between marsupials and monotremes.

Monotremes are most notably characterized by their egg-laying but there are many more features that differentiate them from therians. They have a cloaca which is a single opening for reproductive and waste functions and this is also present in birds. They also do not have teeth when adult and have a spur on their hind limb which the platypus can use to secrete venom. It also doesn’t have nipples and secretes milk through skin. Its limbs are aligned laterally to the body like reptiles and unlike other mammals. Monotremes consist of two extant families, Ornithorhynchidae(platypus) and Tachyglossidae(echidna). The former has one extant species and the latter has 4.

Fossil Record Evidence
Fossil evidence of monotremes is extremely lacking and despite monotremes appearing in the Late Triassic based on genetic studies only a few fossils have been found earlier than the Miocene. This means understanding of Monotremata evolutionary history through the K/Pg extinction is extremely limited. The fossils that have been discovered allow certain hypotheses about Monotremata evolutionary history despite the place of origin being still relatively unknown. Three significant fossils have been found in the Mesozoic and all in Australia. The first one that was found is called Steropodon and has been classified in a family with the third fossil found, Teinolophos, called Steropodontidae. The second fossil found is called Kollikodon and is part of its own family. The classifications of families from the fossils is very tenuous and other classification schemes are possible. The fossils belonging to the different families are significantly different and this suggests that monotremes had already diversified by the early cretaceous. Another important fossil was also found in South America of an ornithorhynchid (similar to platypus) and it was dated to Early Paleocene which suggests that in this period monotremes had a distribution that spanned Gondwana. It is hypothesized that monotremes have an Australian origin but there isn’t any solid evidence besides prevalence of extant and fossil monotreme species in Australia. There isn’t any evidence showing that monotreme presence in South America resulted from an Australian monotreme migration.

Description
Placentals vary greatly in size (whales down to shrews), in locomotion (flying, swimming, climbing, burrowing, running, etc.), and diet (meat, leaves, fruit, termites, etc.). They have a high resting temperature (homiothermy) and produce this heat internally (endothermy). They have a chorioallantoic placenta that allows a long gestation for development. Placental mammals are characterized by several unique features:
 * The young are nourished in a placenta prior to birth
 * There are no epipubic bones
 * Different openings are used for urination and defecation
 * There is a malleolus (bony extension) at the bottom of the tibia
 * The back bones of the foot fit into a hole fored by the bottom of the tibia and fibula
 * The back of the first metatarsal bone is further back than the back of the second metatarsal bone
 * A corpus collosum is present in the brain

There are approximately 4800 extinct and extant genera, including 1135 extant genera and over 5000 extant species arrayed in 20 extant orders. Placentals (extant eutharians) are the most taxonomically diverse of the three branches of extant mammals. The earliest known possible fossils of the eutharians come from Asia. They appear in the fossil record by 105 million years ago (Ma). However, extant orders of eutharians (placentals) do not occur in the fossil record until after the dinosaur extinction, the K/Pg extinction event, 65-66 Ma.

Phylogeny
Based on anatomical and developmental studies, the fossil record, and molecular studies, 20 orders of placental are now recognized. Most of the orders appear in the fossil record within the first 15-20 million years of the Cenozoic. The earliest diverging major new taxon is Afrotheria, which groups together size orders that are restricted to Africa and Madagascar, or appear to have originated on this continent. Afrotheria includes paenungulata of hyracoidea, proboscidea (elephants), Sirenia (tropical marine manatees and dugongs), Tubulidentata (aardvark of Africa), Macroscelidea (African elephant shrews), and an new order called Tenrecoidea (tenrecs and golden moles).

A second group is Xenarthra, which includes Cingulata (armadillos) and Pilosa (anteaters and sloths) with a history mostly found in South America. Finally are two groups, Euarchontoglires and Laurasiathera, together known as Boreoeutheria, reflecting a northern or boreal distribution earlier in their evolutionary history. Euarchontoglires includes Glires (Lagomorpha, the rabbits and pikas, as well as Rodentia), which are linked to a modified Archonta, including Scandentia (tree shrews), Dermoptera (the so-called flying lemurs), and Primates but lacking Chiroptera (bats). Laurasiastheria includes Lipotyphla (Erinaceomorpha and Soricomorpha), Chiroptera, Pholidota (pangolins), Carnivora, Perissodactyla (odd-toed ungulates such as horses, rhinos and tapirs), and a new ordinal grouping. This new ordinal grouping includes cetaceans (whales and relatives), and artiodactyls (even-toed ungulates such as deer, antelope, pigs, camels and hippos), and is called Artiodactyla or Cetartiodactyla. Unlike traditional phylogenies that link them as sister taxa, the new molecular studies indicate that the nearest relative of Cetacea is within Artiodactyla, specifically the family Hippopotamidae.

The Explosive Model
This model argues that most, if not all, interordinal origination and diversification, as well as ordinal origination of extant placentals occurred within a very short interval of about 10 million years, mainly following the K/Pg boundary that occurred 65 Ma. This argument is denoted as the “Explosive Model” because of the short interval of 10 million years, given the minimum 100 million year existence of eutharians. In this model, most of the known species of Late Cretaceous eutharian mammals had little to do with the appearance and radiation of placental orders.

The Long Fuse Model
This model basically agrees with the explosive model in placing intraordinal diversifications mostly following the K/Pg boundary. Unlike the explosive model, this model argues that the species or larger clades, which stem to a particular order or are interordinal clades, can be found well back in the Late Cretaceous. This is called the “Long Fuse Model” in reference to this extended interval of evolution for ordinal stem taxa. There is evidence to show an example of the Long fuse model in recognizing splits between extant placental orders extending far back as 115 Ma. This evidence indicates that the oldest definitive records of extant orders of placentals extend backwards only to about the K/Pg boundary.

The Short Fuse Model
This model suggests that the extension of intraordinal clades into the Late Cretaceous, based on molecular results that are based on intraordinal comparisons. It concludes rate-adjusted intraordianal divergence times of from 81.7-107.2 MA within Lipotyphla, 70.7 Ma within Chiroptera, 85.9 to 108.8 Ma within Rodentia, 69.6 within Primates and 85 within Xenarthra. Some scientists suggest that the origin of some extant placental orders, or at least interordinal diversification within Placentalia, occurred near or before the earliest known fossils of eutharians at about 105 ma. The name of this model emerged because of the argument it asserts, which is that some placental ordinal crown groups originated and began to diversify well back into the Cretaceous, before or shortly after the appearance of eutharians in the fossil record.

Current Model
The current view is that extant orders of placental mammals appeared and began an intraordinal diversification at 65 Ma, following the extinction of non-avian dinosaurs. The general belief is that the interordinal radiation of extant placentals may have begun earlier within the Late Creataceous. The consensus is that either the Explosive Model or Long Fuse Model might explain the pattern of diversification seen in the fossil record.

Recently, the fossil support for the Long Fuse Model has been challenged. Based on geochronologic ranged and cladistics patterns, it is argued that there is no evidence of an extant clades of placentals in the Cretaceous, with one possible exception of the zhelestids. This suggests that the Long Fuse Model may be correct for some clades (ungulares and their relatives), but beyond this, the fossil record cannot distinguish between the explosive and Long Fuse Models. Thus, the fossil record cannot be used to support or argue against molecular studies that extend superordinal clades of placentals back into the Cretaceous.

Description
Dryolestoideans are an extinct Mesozoic mammal group that is distantly related to modern marsupials and placentals. On the basis of dental and postcranial features, they are more closely related to extant marsupials and placentals than the more ancestral symmetrodonts. It has been suggested that this group contained the ancestors of modern therian mammals.

Phylogeny
The Dryolestoidea are an important group of petribosphenic stem therians that is nested among cladotherians between symmetrodonts and Prototribosphenida. Their fossil record extends from the Middle Jurassic to the Late Cretaceous with the highest diversity in the Late Jurassic. Dryolestoids have been reported from Europe, North America, and recently from Asia. In South America, dryolestoids are known since the Late Cretaceous and survived into the early Paleocene. Dryolestoidea are characterized by the unequal height of the lingual and buccal alveolar border in upper and lower jaws. This feature is also expressed in the strongly curved crown-root transition of the molars, which is explicitly bent cervically on the buccal side in lower molars and on the lingual side in the upper molars.

Fossil Record Evidence
The oldest undisputed fossils of Dryolestidae were found in the Guimarota coal mine near Leiria, Portugal, the largest known deposit of Jurassic mammals. The tooth pattern of these fossils (8–9 mesiodistally compressed molars with small, cusp-like talonids) distinguish them from tribosphenic mammals, and they form a sister group of the latter and Peramus together with Amphitherium, Paurodontidae, and Henkelotheriidae. The youngest fossil in the northern hemisphere is Crusafontia cuencana from early Cretaceous of Uña and Galve, Spain, though a fragmentary lower molar from the late Cretaceous Mesaverde Formation in Wyoming has been attributed to Dryolestidae. In South America, dryolestids survived until the late Cretaceous when they underwent a remarkable radiation.

Description
Gondwantheria are a group of Late Cretaceous and early Paleocene mammals from Gonwana landmasses. The two South American gonwanatherian genera, characterized by hypsodont molars and very thick enamel, were initially assigned to Edentata. However, after further investigation it was concluded that enamel microstructure and hypsodonty had been acquired by Gondwanatheria independently from that of therian mammals. Relationships of Gondwantheria to other mammals are unresolved, although it is evident that this group is monophyletic with several highly distinctive dental autapomorphies. They are poorly known, small mammals with a rodentlike dentary. They share with multituberculates propalinal jaw movement with backward power stroke and normal prismatic enamel. Gonwantherians declined after the K/Pg extinction event and eventually went extinct.

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