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Eutriconodonta is an order of early
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...
s. Eutriconodonts existed in
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,
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, Europe,
North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north'' is ...
and South America during the Jurassic and the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the
paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
Triconodonta. Traditionally seen as the classical
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
small mammalian insectivores, discoveries over the years have shown them to be among the best examples of the diversity of mammals in this time period, including a vast variety of bodyplans, ecological niches and locomotion methods.


Classification

"Triconodonta" had long been used as the name for an order of early
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fu ...
s which were close relatives of the ancestors of all present-day mammals, characterized by molar teeth with three main cusps on a crown that were arranged in a row. The group originally included only the family Triconodontidae and taxa that were later assigned to the separate family
Amphilestidae Amphilestidae is a family of Mesozoic mammals, generally regarded as eutriconodonts. They may form a paraphyletic or polyphyletic assemblage, though they share with gobiconodontids their similar tooth occlusion patterns and may be especially cl ...
, but was later expanded to include other taxa such as ''
Morganucodon ''Morganucodon'' ("Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, ''Morganucodon'' is well represe ...
'' or '' Sinoconodon''. The phylogenetic analyses found that all these taxa did not form a natural group, and that some traditional "triconodonts" were more closely related to
theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...
n mammals than others. Some traditional "triconodonts" do seem to form a natural group (or "clade"), and this was given the name Eutriconodonta, or "true triconodonts). Most analyses use only dental and mandibular characters. Gao ''et al.'' (2010) conducted a second analysis as well, using a modified version of the matrix from the analysis of Luo ''et al.'' (2007); this analysis involved a broader range of Mesozoic mammaliaforms and more characters, including postcranial ones. Both Luo ''et al.'' (2007) and the second analysis of Gao ''et al.'' (2010) recovered a more inclusive monophyletic Eutriconodonta that also contained gobiconodontids and ''Amphilestes''; in the second analysis of Gao ''et al.'' it also contained '' Juchilestes'' (recovered as amphidontid in their first analysis, the only amphidontid included in their second analysis). However, Gao ''et al.'' (2010) stressed that jeholodentids and gobiconodontids are the only eutriconodonts with known postcranial skeletons; according to the authors, it remains uncertain whether the results of their second analysis represent true phylogeny or are merely "a by-product of
long branch attraction In phylogenetics, long branch attraction (LBA) is a form of systematic error whereby distantly related lineages are incorrectly inferred to be closely related. LBA arises when the amount of molecular or morphological change accumulated within a lin ...
of jeholodentids and gobiconodontids". Phylogenetic studies conducted by Zheng ''et al.'' (2013), Zhou ''et al.'' (2013) and Yuan ''et al.'' (2013) recovered monophyletic Eutriconodonta containing triconodontids, gobiconodontids, ''Amphilestes'', ''Jeholodens'' and ''Yanoconodon''. The exact phylogenetic placement of eutriconodonts within
Mammaliaformes Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent com ...
is also uncertain. Zhe-Xi Luo, Zofia Kielan-Jaworowska and Richard Cifelli (2002) conducted an analysis that recovered eutriconodonts within the
crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
of Mammalia, i.e. the least inclusive clade containing
monotreme Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brains ...
s and
theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...
n mammals. The analysis found eutriconodonts to be more closely related to therian mammals than monotremes were, but more distantly than (paraphyletic) amphitheriids, dryolestids, spalacotheriid " symmetrodonts" and multituberculates were. This result was mostly confirmed by Luo ''et al.'' (2007), the second analysis of Gao ''et al.'' (2010), Zheng ''et al.'' (2013), Zhou ''et al.'' (2013) and Yuan ''et al.'' (2013), although in the phylogenies of Luo ''et al.'' (2007) and Yuan ''et al.'' (2013) eutriconodonts were in unresolved
polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...
with multituberculates and trechnotherians. If confirmed this would make eutriconodonts one of the groups that can be classified as mammals by any definition. Several other extinct groups of
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
animals that are traditionally considered to be mammals (such as Morganucodonta and
Docodonta Docodonta is an order of extinct mammaliaforms that lived during the Mesozoic, from the Middle Jurassic to Early Cretaceous. They are distinguished from other early mammaliaforms by their relatively complex molar teeth, from which the order ge ...
) are now placed just outside Mammalia by those who advocate a 'crown-group' definition of the word "mammal". However, Luo, Kielan-Jaworowska and Cifelli (2002) tested alternative possible phylogenies as well, and found that recovering eutriconodonts outside the crown group of Mammalia required only five additional steps compared to the most parsimonious solution. The authors stated that such placement of eutriconodonts is less likely than their placement within the mammalian crown group, but it cannot be rejected on a statistical basis. The most recent cladogram is by Thomas Martin et al. 2015, in their description of '' Spinolestes''. Eutriconodonts are recovered as a largely monophyletic group within
Theriimorpha Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
. A study finds them paraphyletic in regards to crown group
Mammalia Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
.


Range

When eutriconodonts first appeared is unclear. The earliest remains come from the late Early Jurassic (
Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarc ...
), but they already represent a variety of groups: the
volaticotheria Volaticotherini is a clade of eutriconodont mammals from the Mesozoic. In addition to the type genus ''Volaticotherium'', it includes the genera '' Argentoconodon'', '' Ichthyoconodon'', and potentially '' Triconolestes''. Since most remains ar ...
n ''
Argentoconodon ''Argentoconodon'' (meaning "Argentina cone tooth") is an extinct genus of theriimorph mammal from the Cañadón Asfalto Formation of the Cañadón Asfalto Basin in Patagonia. When originally described, it was known only from a single molarifo ...
'', the alticonodontine '' Victoriaconodon'' and the gobiconodontid '' Huasteconodon'', as well as the putative eutriconodont ''" Dyskritodon" indicus''. They achieve their peak diversity across the
Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous ( chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145  Ma to 100.5 Ma. Geology Pr ...
, before largely disappearing from the fossil record in the early
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', t ...
outside of
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the C ...
. The
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian ...
genus ''
Alticonodon ''Alticonodon'' is a genus of extinct mammal from the Late Cretaceous of North America. It is the geologically youngest known eutriconodont, and is a fairly more specialised animal than earlier representatives of this clade. Description ''Altico ...
'' is the youngest representative of the group; the
Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian ...
/
Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the inter ...
''
Austrotriconodon ''Austrotriconodon'' is a mammal genus from the Campanian and Maastrichtian of South America. It currently contains only the type species, ''A. mckennai''. Originally assumed to be a eutriconodont, more recent studies have recovered it as a merid ...
'' was originally referred to as a late surviving member of the clade, but has since been moved to
Dryolestoidea Dryolestida is an extinct order of mammals, primarily and possibly exclusively known from the Jurassic and Cretaceous. They are considered members of the clade Cladotheria, close to the ancestry of therian mammals. It is also believed that they ...
. Most eutriconodont remains occur in laurasian landmasses. The exceptions are ''
Argentoconodon ''Argentoconodon'' (meaning "Argentina cone tooth") is an extinct genus of theriimorph mammal from the Cañadón Asfalto Formation of the Cañadón Asfalto Basin in Patagonia. When originally described, it was known only from a single molarifo ...
'' and slightly younger '' Condorodon'' from the
Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma&nb ...
of
Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...
, the putative '' Dyskritodon indicus'' from the
Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma&nb ...
of
India India, officially the Republic of India ( Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the ...
( Kota Formation), the
Late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the nam ...
'' Tendagurodon'' from
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands an ...
(
Tendaguru Formation The Tendaguru Formation, or Tendaguru Beds are a highly fossiliferous formation and Lagerstätte located in the Lindi Region of southeastern Tanzania. The formation represents the oldest sedimentary unit of the Mandawa Basin, overlying Neoprote ...
) and several
Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous ( chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145  Ma to 100.5 Ma. Geology Pr ...
north Africa North Africa, or Northern Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, and it is sometimes defined as stretching from the Atlantic shores of Mauritania in t ...
n taxa like ''
Ichthyoconodon ''Ichthyoconodon'' is an extinct genus of eutriconodont mammal from the Lower Cretaceous of Morocco. It is notable for having been found in a unique marine location, and the shape of its teeth suggests an unusual, potentially fish-eating ecologic ...
'', '' Dyskritodon amazighi'' and '' Gobiconodon palaios''. Due to the rarity of the Jurassic
gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period The Jurassic ( ) is a geologic period and st ...
n fossil record the presence of eutriconodonts in southern landmasses may be of interest, due to their comparatively early age. Eutriconodonts are among the few Mesozoic mammals present at Arctic locations; docodonts and
haramiyidan Haramiyida ("thief" from Arabic الحرامية (al ḥarāmiyah), "thief, bandit") is a possibly polyphyletic order of mammaliaform cynodonts or mammals of controversial taxonomic affinites. Their teeth, which are by far the most common remain ...
s (generally considered non-mammalian cynodonts) are also present, but not
theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...
ns,
dryolestoid Dryolestida is an extinct order of mammals, primarily and possibly exclusively known from the Jurassic and Cretaceous. They are considered members of the clade Cladotheria, close to the ancestry of therian mammals. It is also believed that they d ...
s and other groups considered true mammals.


Biology


Anatomy

Like many other non-
theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...
n mammals, eutriconodonts retained classical mammalian synapomorphies like
epipubic Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placental mammals, who lack them). They first oc ...
bones (and likely the associated reproductive constrictions), venomous spurs and sprawling limbs. However, the forelimb and shoulder anatomy of at least some species like '' Jeholodens'' are similar to those of therian mammals, though the hindlimbs remain more conservative. Eutriconodonts had a modern ear anatomy, the main difference from
theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...
ns being that the ear ossicles were still somewhat connected to the jaw via the
Meckel's cartilage In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel's cartilages (right and left) also known as Meckelian cartilages; above this the incus and malleus are developed. Meckel's cartilage arises from the firs ...
. Uniquely among crown-group mammals, gobiconodontids replaced their molariform teeth by successors of similar complexity, while in other mammals less complex replacements are the norm.


Soft tissues

Some eutriconodonts like '' Spinolestes'' and ''
Volaticotherium ''Volaticotherium antiquum'' (meaning "ancient gliding beast") is an extinct, gliding, insectivorous mammal that lived in Asia during the Jurassic period, around 164 mya. It is the only member of the genus ''Volaticotherium''. The discovery o ...
'' were very well preserved, showing evidence of fur, internal organs and, in the latter, of
patagia The patagium (plural: patagia) is a membranous body part that assists an animal in obtaining lift when gliding or flight. The structure is found in extant and extinct groups of flying and gliding animals including bats, birds, some dromaeosa ...
. ''Spinolestes'' shows hair similar to that of modern mammals, with compound
hair follicle The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...
s with primary and secondary hair, even preserving traces of a pore infection. It also possesses a clear
thoracic diaphragm The thoracic diaphragm, or simply the diaphragm ( grc, διάφραγμα, diáphragma, partition), is a sheet of internal skeletal muscle in humans and other mammals that extends across the bottom of the thoracic cavity. The diaphragm is the ...
like modern mammals, as well as spines, dermal
scutes A scute or scutum ( Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior ...
and an ossified
Meckel's cartilage In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel's cartilages (right and left) also known as Meckelian cartilages; above this the incus and malleus are developed. Meckel's cartilage arises from the firs ...
. Furthermore, ''Spinolestes'' may also display signs of
dermatophytosis Dermatophytosis, also known as ringworm, is a fungal infection of the skin. Typically it results in a red, itchy, scaly, circular rash. Hair loss may occur in the area affected. Symptoms begin four to fourteen days after exposure. Multiple ...
, suggesting that gobiconodontids, like modern mammals, were vulnerable to this type of fungal infection. '' Triconodon'' itself has been the subject to cranial
endocast An endocast is the internal cast of a hollow object, often referring to the cranial vault in the study of brain development in humans and other organisms. Endocasts can be artificially made for examining the properties of a hollow, inaccessible sp ...
studies, revealing a unique
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in ...
anatomy.


Paleobiology

The eutriconodont triconodont dentition has no analogue among living mammals, so comparisons are difficult. There are two main types of occlusion patterns: one present in
triconodontid Triconodontidae is an extinct family of small, carnivorous mammals belonging to the order Eutriconodonta, endemic to what would become Asia, Europe, North America and probably also Africa and South America during the Jurassic through Cretaceous ...
s (as well as the unrelated morganucodontan mammals), in which lower
cusp A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth. Cusp or CUSP may also refer to: Mathematics * Cusp (singularity), a singular point of a curve * Cusp catastrophe, a branch of bifurc ...
"a" occludes anterior to upper cusp "A", between "A" and "B", and one present in amphilestids and gobiconodontids, in which the molars basically alternate, with the lower cusp "a" occluding further forward, near the junction between two upper molars. A study on '' Priacodon'' however suggests that only the latter arrangement was present. However, it's clear that most if not all eutriconodonts were primarily carnivorous, given the presence of long, sharp canines, premolars with trenchant main cusps that were well suited to grasp and pierce prey, strong development of the madibular abductor musculature, bone crushing ability in at least some species and several other features. Triconodont teeth are known to have had a shearing function, allowing the animal to tear through flesh much like
carnassial Carnassials are paired upper and lower teeth modified in such a way as to allow enlarged and often self-sharpening edges to pass by each other in a shearing manner. This adaptation is found in carnivorans, where the carnassials are the modified ...
teeth of
theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. ...
n mammals. In a study about Mesozoic mammalian diets the taxa '' Repenomamus'', '' Gobiconodon'', ''
Argentoconodon ''Argentoconodon'' (meaning "Argentina cone tooth") is an extinct genus of theriimorph mammal from the Cañadón Asfalto Formation of the Cañadón Asfalto Basin in Patagonia. When originally described, it was known only from a single molarifo ...
'', ''
Phascolotherium ''Phascolotherium'' is a genus of extinct eutriconodont mammal from the Middle Jurassic of the United Kingdom. Found in the Stonesfield Slate, it was one of the first Mesozoic mammals ever found and described, although like the other mammal jaws ...
'', '' Triconodon'' and ''
Liaoconodon ''Liaoconodon'' is an extinct genus of early mammal from the early Cretaceous (early Aptian stage, approximately 120 Ma). It is a eutriconodont which lived in what is now the Jianchang of Liaoning Province, eastern China. It is known from the h ...
'' rank among carnivorous mammal species, while ''
Volaticotherium ''Volaticotherium antiquum'' (meaning "ancient gliding beast") is an extinct, gliding, insectivorous mammal that lived in Asia during the Jurassic period, around 164 mya. It is the only member of the genus ''Volaticotherium''. The discovery o ...
'', '' Liaotherium'', ''
Amphilestes ''Amphilestes'' is a genus of extinct eutriconodont mammal from the Middle Jurassic of the United Kingdom. It was one of the first Mesozoic mammals discovered and described. Discovery The first specimen of ''Amphilestes'' was discovered along w ...
'' and '' Jeholodens'' ranked among insectivorous mammals, while ''
Yanoconodon ''Yanoconodon'' is a monotypic genus of extinct early mammal whose representative species ''Yanoconodon allini'' lived during the Mesozoic in what is now China. The holotype fossil of ''Yanoconodon'' was excavated in the Yan Mountains about 300 k ...
'', '' Priacodon'' and '' Trioracodon'' ranked somewhere in between. A study on '' Priacodon'' suggests that the jaw roll was more passive for eutriconodonts than modern therian carnivores. Eutriconodonts are often among the largest mammals in Mesozoic faunal assemblages, displaying a broad size range from small
shrew Shrews (family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to differen ...
-like insectivores to large forms like '' Repenomamus'', '' Gobiconodon'', '' Triconodon'' and '' Jugulator''. They were among the first mammals to be specialised for vertebrate prey, and likely occupied the highest
trophic Trophic, from Ancient Greek τροφικός (''trophikos'') "pertaining to food or nourishment", may refer to: * Trophic cascade * Trophic coherence * Trophic egg * Trophic function * Trophic hormone * Trophic level index * Trophic level * ...
levels among mammals in their faunal communities. Several forms like ''Gobiconodon'' and ''Repenomamus'' show evidence of scavenging, being among the few Mesozoic mammals to have significantly exploited that. At least in carnivorous niches, eutriconodonts were probably replaced by deltatheroidean
metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well ...
ns, which are the dominant carnivorous mammals in
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', t ...
faunal assemblages. Competition between both groups is unattested, but in Asia the Early Cretaceous gobiconodontid diversity is replaced entirely by a deltatheroidean one, while in North America '' Nanocuris'' appears after the absence of '' Gobiconodon'' and other larger eutriconodonts. Given that all insectivorous and carnivorous mammals groups suffered heavy losses during the mid-Cretaceous, it seems likely these metatherians simply occupied niches left after the extinction of eutriconodonts. Some eutriconodonts were instead among the most specialised of Mesozoic mammals. Several taxa like ''
Astroconodon ''Astroconodon'' is an extinct genus of mammal from the Cretaceous of North America. Part of Eutriconodonta, it was a small sized predator, either a terrestrial insectivore and carnivore, or a semi-aquatic piscivore. It is the first Cretaceous ...
'', '' Dyskritodon'' and ''
Ichthyoconodon ''Ichthyoconodon'' is an extinct genus of eutriconodont mammal from the Lower Cretaceous of Morocco. It is notable for having been found in a unique marine location, and the shape of its teeth suggests an unusual, potentially fish-eating ecologic ...
'' may show adaptations for piscivory and occur in aquatic settings with their molars being compared to those of seals and
cetaceans Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...
. Caution has been advised in these comparisons, however; as many researchers like
Zofia Kielan-Jaworowska Zofia Kielan-Jaworowska (25 April 1925 – 13 March 2015) was a Polish paleobiologist. In the mid-1960s, she led a series of Polish-Mongolian paleontological expeditions to the Gobi Desert. She was the first woman to serve on the executive commit ...
have noted, eutriconodont molars are more functionally similar to those of terrestrial carnivorans than
pinnipeds Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae (whose only living member is the w ...
and
cetaceans Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...
, occluding in a shearing motion instead of not-occluding and providing a grasping function. However, '' Dyskritodon'' and ''Ichthyoconodons teeth shows no erosion associated with aquatic transportation, meaning that the animals died ''in situ'' or close. Studies on ''
Liaoconodon ''Liaoconodon'' is an extinct genus of early mammal from the early Cretaceous (early Aptian stage, approximately 120 Ma). It is a eutriconodont which lived in what is now the Jianchang of Liaoning Province, eastern China. It is known from the h ...
'' show that it has adaptations for an aquatic lifestyle, possessing a barrel-like body and paddle-like limbs, and analysis of the postcrania of ''
Yanoconodon ''Yanoconodon'' is a monotypic genus of extinct early mammal whose representative species ''Yanoconodon allini'' lived during the Mesozoic in what is now China. The holotype fossil of ''Yanoconodon'' was excavated in the Yan Mountains about 300 k ...
'' shows adaptations towards multiple forms of locomotion, with traits in common with fossorial, arboreal, and semiaquatic mammals. Additionally, ''
Volaticotherium ''Volaticotherium antiquum'' (meaning "ancient gliding beast") is an extinct, gliding, insectivorous mammal that lived in Asia during the Jurassic period, around 164 mya. It is the only member of the genus ''Volaticotherium''. The discovery o ...
'' and ''
Argentoconodon ''Argentoconodon'' (meaning "Argentina cone tooth") is an extinct genus of theriimorph mammal from the Cañadón Asfalto Formation of the Cañadón Asfalto Basin in Patagonia. When originally described, it was known only from a single molarifo ...
'' show adaptations for aerial locomotion. Both genera are closely related, implying a long lived lineage of gliding mammals. At least ''Spinolestes'' had xenarthrous vertebrae and osseous scutes, convergent to those of modern xenarthrans and to a lesser extent the
hero shrew The hero shrew (''Scutisorex somereni''), also known as the armored shrew, is a large shrew native to the Congo Basin of Africa. Its features are typical of a white-toothed shrew − short legs, slender snout, dense fur − except for a highly ...
. This genus may have displayed an ecological role similar to that of modern anteaters, pangolins, echidnas,
aardvark The aardvark ( ; ''Orycteropus afer'') is a medium-sized, burrowing, nocturnal mammal native to Africa. It is the only living species of the order Tubulidentata, although other prehistoric species and genera of Tubulidentata are known. Unlike ...
,
aardwolf The aardwolf (''Proteles cristata'') is an insectivorous species of hyena, native to East and Southern Africa. Its name means "earth-wolf" in Afrikaans and Dutch. It is also called maanhaar-jackal (Afrikaans for " mane-jackal"), termite-eating ...
and
numbat The numbat (''Myrmecobius fasciatus''), also known as the noombat or walpurti, is an insectivorous marsupial. It is diurnal and its diet consists almost exclusively of termites. The species was once widespread across southern Australia, but i ...
, being the second known Mesozoic mammal after '' Fruitafossor'' to have done so.


Reproductive biology

'' Triconodon'' shows dental replacement patterns consistent with
milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digest solid food. Immune factors and immune-modul ...
-drinking mammals.


Notes


References

{{Taxonbar, from=Q135419 Mammal orders Maastrichtian extinctions Bathonian first appearances Fossil taxa described in 1973 Taxa named by Kenneth A. Kermack Taxa named by Frances Mussett