Eutriconodonta

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Eutriconodonta
Temporal range: Early Jurassic - Late Cretaceous, 200–66  Ma
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Examples of several eutriconodonts. Clockwise: Repenomamus , Volaticotherium , Jeholodens and Yanoconodon . These occupy vastly different ecological niches: bulky semi-fossorial carnivore, glider, arboreal insectivore and terrestrial carnivore, respectively. [1]
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Theriimorpha
Order:† Eutriconodonta
Kermack et al., 1973
Subgroups

Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia (including pre-contact India), Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack et al. in 1973 [2] as a replacement name for the paraphyletic Triconodonta. [3]

Contents

Traditionally seen as the classical Mesozoic 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. [4] [5] [6] [1] [7] [8]

Classification

Illustration of the lower jaw of Triconodon mordax, 1861 Triconodon Owen.jpg
Illustration of the lower jaw of Triconodon mordax , 1861

"Triconodonta" had long been used as the name for an order of early mammals 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. [4] The group originally included only the family Triconodontidae and taxa that were later assigned to the separate family Amphilestidae, [9] but was later expanded to include other taxa such as Morganucodon or Sinoconodon . [4] The phylogenetic analyses found that all these taxa did not form a natural group, and that some traditional "triconodonts" were more closely related to therian 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. [3] [10] [11] 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); [12] 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; [10] [12] 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). [10] 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 of jeholodentids and gobiconodontids". [10] 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. [13] [14] [15]

The exact phylogenetic placement of eutriconodonts within Mammaliaformes is also uncertain. Zhe-Xi Luo, Zofia Kielan-Jaworowska and Richard Cifelli (2002) conducted an analysis that recovered eutriconodonts within the crown group of Mammalia, i.e. the least inclusive clade containing monotremes and therian 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. [16] 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 with multituberculates and trechnotherians. [10] [12] [13] [14] [15] 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 animals that are traditionally considered to be mammals (such as Morganucodonta and Docodonta) are now placed just outside Mammalia by those who advocate a 'crown-group' definition of the word "mammal". [17] 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. [16]

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. [6]

 Eutriconodonta 

A 2020 study found them paraphyletic in regards to crown group Mammalia. [18]

Range

When eutriconodonts first appeared is unclear. The earliest remains come from the late Early Jurassic (Toarcian), but they already represent a variety of groups: the volaticotherian Argentoconodon , the alticonodontine Victoriaconodon and the gobiconodontid Huasteconodon , as well as the putative eutriconodont "Dyskritodon" indicus. [19] They achieve their peak diversity across the Early Cretaceous, before largely disappearing from the fossil record in the early Late Cretaceous outside of North America. The Maastrichtian genus Indotriconodon is the youngest representative of the group, hailing from the intertrappean beds of India; [20] the Campanian/Maastrichtian Austrotriconodon was originally referred to as a late surviving member of the clade, but has since been moved to Dryolestoidea. [21]

Most eutriconodont remains occur in laurasian landmasses. The exceptions are Argentoconodon and slightly younger Condorodon from the Early Jurassic of Argentina, the putative Dyskritodon indicus from the Early Jurassic of India (Kota Formation), the Late Jurassic Tendagurodon from Tanzania (Tendaguru Formation), several Early Cretaceous north African taxa like Ichthyoconodon , Dyskritodon amazighi and Gobiconodon palaios, and Indotriconodon magnus from Late Cretaceous India. Due to the rarity of the Jurassic gondwanan fossil record the presence of eutriconodonts in southern landmasses may be of interest, due to their comparatively early age. [19]

Eutriconodonts are among the few Mesozoic mammals present at Arctic locations; docodonts and haramiyidans (generally considered non-mammalian cynodonts) are also present, but not therians, dryolestoids and other groups considered true mammals. [22]

Biology

Anatomy

Like many other non-therian mammals, eutriconodonts retained classical mammalian synapomorphies like epipubic 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. [4] Eutriconodonts had a modern ear anatomy, the main difference from therians being that the ear ossicles were still somewhat connected to the jaw via the Meckel's cartilage. [5] 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. [23]

Soft tissues

Some eutriconodonts like Spinolestes and Volaticotherium were very well preserved, showing evidence of fur, internal organs and, in the latter, of patagia. Spinolestes shows hair similar to that of modern mammals, with compound hair follicles with primary and secondary hair, even preserving traces of a pore infection. It also possesses a clear thoracic diaphragm like modern mammals, as well as spines, dermal scutes and an ossified Meckel's cartilage. Furthermore, Spinolestes may also display signs of dermatophytosis, suggesting that gobiconodontids, like modern mammals, were vulnerable to this type of fungal infection. [6]

Triconodon itself has been the subject to cranial endocast studies, revealing a unique brain anatomy. [4] [24]

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 triconodontids (as well as the unrelated morganucodontan mammals), in which lower cusp "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. [19] A study on Priacodon however suggests that only the latter arrangement was present. [25]

However, it's clear that most if not all eutriconodonts were primarily carnivorous, given the presence of long, sharp canines, [note 1] 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. [4] Eutriconodont teeth are known to have had a shearing function, [4] [19] allowing the animal to tear through flesh much like carnassial teeth of therian mammals. [4] In a study about Mesozoic mammalian diets the taxa Repenomamus , Gobiconodon , Argentoconodon , Phascolotherium , Triconodon and Liaoconodon rank among carnivorous mammal species, while Volaticotherium , Liaotherium , Amphilestes and Jeholodens ranked among insectivorous mammals, while Yanoconodon , Priacodon and Trioracodon ranked somewhere in between. [26] A study on Priacodon suggests that the jaw roll was more passive for eutriconodonts than modern therian carnivores. [25]

Eutriconodonts are often among the largest mammals in Mesozoic faunal assemblages, displaying a broad size range from small shrew-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 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. [4] Evidence of predation on significantly larger dinosaurs is also known. [27]

At least in carnivorous niches, eutriconodonts were probably replaced by deltatheroidean metatherians, which are the dominant carnivorous mammals in Late Cretaceous faunal assemblages. [28] 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. [29] 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 in the northern continents. [26]

Volaticotherins, such as Volaticotherium are particularly notable for their specializations towards gliding. Volaticotherium Life Reconstruction.png
Volaticotherins, such as Volaticotherium are particularly notable for their specializations towards gliding.

Some eutriconodonts were instead among the most specialised of Mesozoic mammals. Several taxa like Astroconodon , Dyskritodon and Ichthyoconodon may show adaptations for piscivory and occur in aquatic settings with their molars being compared to those of seals and cetaceans. Caution has been advised in these comparisons, however; as many researchers like Zofia Kielan-Jaworowska have noted, eutriconodont molars are more functionally similar to those of terrestrial carnivorans than pinnipeds and cetaceans, occluding in a shearing motion instead of not-occluding and providing a grasping function. [4] However, Dyskritodon and Ichthyoconodon's teeth shows no erosion associated with aquatic transportation, meaning that the animals died in situ or close. [30] Studies on Liaoconodon show that it has adaptations for an aquatic lifestyle, possessing a barrel-like body and paddle-like limbs, [1] and analysis of the postcrania of Yanoconodon shows adaptations towards multiple forms of locomotion, with traits in common with fossorial, arboreal, and semiaquatic mammals. [7]

Additionally, Volaticotherium and Argentoconodon show adaptations for aerial locomotion. Both genera are closely related, implying a long lived lineage of gliding mammals. [31]

At least Spinolestes had xenarthrous vertebrae and osseous scutes, convergent to those of modern xenarthrans and to a lesser extent the hero shrew. This genus may have displayed an ecological role similar to that of modern anteaters, pangolins, echidnas, aardvark, aardwolf and numbat, being the second known Mesozoic mammal after Fruitafossor to have done so. [6]

Reproductive biology

Triconodon shows dental replacement patterns consistent with milk-drinking mammals. [24]

Notes

  1. ↑ In gobinocontids, though, the incisors are also long and fang-like; in Gobiconodon , the lower incisors outrightly replace the canines, which are vestigial. [4]

Related Research Articles

<i>Repenomamus</i> Extinct genus of mammals

Repenomamus is a genus of opossum- to badger-sized gobiconodontid mammal containing two species, Repenomamus robustus and Repenomamus giganticus. Both species are known from fossils found in China that date to the early Cretaceous period, about 125-123.2 million years ago. R. robustus is one of several Mesozoic mammals for which there is good evidence that it fed on vertebrates, including dinosaurs. Though it is not entirely clear whether or not these animals primarily hunted live dinosaurs or scavenged dead ones, evidence for the former is present in fossilized remains showcasing the results of what was most likely a predation attempt by R. robustus directed at a specimen of the dinosaur Psittacosaurus lujiatunensis. R. giganticus is among the largest mammals known from the Mesozoic era, only surpassed by Patagomaia.

<span class="mw-page-title-main">Allotheria</span> Extinct subclass of mammals

Allotheria is an extinct clade of mammals known from the Mesozoic and early Cenozoic. Shared characteristics of the group are the presence of lower molariform teeth equipped with longitudinal rows of cusps and enlarged incisors. Typically, the canine teeth are also lost. Allotheria includes Multituberculata, Gondwanatheria, and probably Haramiyida, although some studies have recovered haramiyidans to be basal mammaliaforms unrelated to multituberculates. Allotherians are often placed as crown group mammals, more closely related to living marsupials and placentals (Theria) than to monotremes or eutriconodonts, though some studies place the entirety of Allotheria outside of crown Mammalia.

<i>Jeholodens</i> Extinct family of mammals

Jeholodens is an extinct genus of primitive mammal belonging to the order Eutriconodonta, and which lived in present-day China during the Middle Cretaceous about 125 million years ago.

<span class="mw-page-title-main">Docodonta</span> Extinct order of mammaliaforms

Docodonta is an order of extinct Mesozoic mammaliaforms. They were among the most common mammaliaforms of their time, persisting from the Middle Jurassic to the Early Cretaceous across the continent of Laurasia. They are distinguished from other early mammaliaforms by their relatively complex molar teeth. Docodont teeth have been described as "pseudotribosphenic": a cusp on the inner half of the upper molar grinds into a basin on the front half of the lower molar, like a mortar-and-pestle. This is a case of convergent evolution with the tribosphenic teeth of therian mammals. There is much uncertainty for how docodont teeth developed from their simpler ancestors. Their closest relatives may have been certain Triassic "symmetrodonts", namely Woutersia, Delsatia, and Tikitherium.

<span class="mw-page-title-main">Mammaliaformes</span> Clade of mammals and extinct relatives

Mammaliaformes 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 common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia. Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium as well as the Triassic Tikitherium, the earliest known member of the group.

<span class="mw-page-title-main">Australosphenida</span> Subclass of mammals

The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. Although they have often been suggested to have acquired tribosphenic molars independently from those of Tribosphenida, this has been disputed. Fossils of australosphenidans have been found from the Jurassic of Madagascar and Argentina, and Cretaceous of Australia and Argentina. Monotremes have also been considered a part of this group in many studies, but this is also disputed.

<i>Volaticotherium</i> Extinct family of mammals

Volaticotherium antiquum 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.

<span class="mw-page-title-main">Volaticotherini</span> Extinct clade of mammals

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.

<i>Gobiconodon</i> Extinct genus of mammals

Gobiconodon is an extinct genus of carnivorous mammals belonging to the family Gobiconodontidae. Undisputed records of Gobiconodon are restricted to the Early Cretaceous of Asia and North America, but isolated teeth attributed to the genus have also been described from formations in England and Morocco dating as far back as the Middle Jurassic. Species of Gobiconodon varied considerably in size, with G. ostromi, one of the larger species, being around the size of a modern Virginia opossum. Like other gobiconodontids, it possessed several speciations towards carnivory, such as shearing molariform teeth, large canine-like incisors and powerful jaw and forelimb musculature, indicating that it probably fed on vertebrate prey. Unusually among predatory mammals and other eutriconodonts, the lower canines were vestigial, with the first lower incisor pair having become massive and canine-like. Like the larger Repenomamus there might be some evidence of scavenging.

<span class="mw-page-title-main">Deltatheroida</span> Extinct order of mammals

Deltatheroida is an extinct group of basal metatherians that were distantly related to modern marsupials. The majority of known members of the group lived in the Cretaceous; one species, Gurbanodelta kara, is known from the late Paleocene (Gashatan) of China. Their fossils are restricted to Central Asia and North America. This order can be defined as all metatherians closer to Deltatheridium than to Marsupialia.

Shuotherium is a fossil mammaliaform known from Middle-Late Jurassic of the Forest Marble Formation of England, and the Shaximiao Formation of Sichuan, China.

<span class="mw-page-title-main">Triconodontidae</span> Extinct family of mammals

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 periods at least from 190–66 mya.

<span class="mw-page-title-main">Gobiconodontidae</span> Extinct family of mammals

Gobiconodontidae is a family of extinct mammals that ranged from the mid-Jurassic to the early Late Cretaceous, though most common during the Early Cretaceous. The Gobiconodontids form a diverse lineage of carnivorous non-therian mammals, and include some of the best preserved Mesozoic mammal specimens.

<span class="mw-page-title-main">Yinotheria</span> Subclass of mammals

Yinotheria is a proposed basal subclass clade of crown mammals uniting the Shuotheriidae, an extinct group of mammals from the Jurassic of Eurasia, with Australosphenida, a group of mammals known from the Jurassic to Cretaceous of Gondwana, which possibly include living monotremes. Today, there are only five surviving species of monotremes which live in Australia and New Guinea, consisting of the platypus and four species of echidna. Fossils of yinotheres have been found in Britain, China, Russia, Madagascar and Argentina. Contrary to other known crown mammals, they retained postdentary bones as shown by the presence of a postdentary trough. The extant members (monotremes) developed the mammalian middle ear independently.

<i>Ichthyoconodon</i> Extinct family of mammals

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 ecological niche. Analysis suggests it is part of a group of gliding mammals that includes Volaticotherium.

Jugulator is an extinct genus of mammals from the Cretaceous of North America. It contains one species, Jugulator amplissimus. A eutriconodont, it is known from the Cedar Mountain Formation, and is both a large sized and possibly ecologically specialised taxon, showcasing the diversity of mammals in the Mesozoic.

<i>Dyskritodon</i> Extinct family of mammals

Dyskritodon is a genus of extinct mammal from the Early Cretaceous of Morocco, and possibly the Early Jurassic of India. Of uncertain affinities, it is tentatively described as a eutriconodont.

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.

<i>Triconodon</i> Extinct family of mammals

Triconodon is a genus of extinct mammal from the Early Cretaceous of England and France with two known species: T. mordax and T. averianovi. First described in 1859 by Richard Owen, it is the type genus for the order Triconodonta, a group of mammals characterised by their three-cusped (triconodont) molar teeth. Since then, this "simplistic" type of dentition has been understood to be either ancestral for mammals or else to have evolved multiple times, rendering "triconodonts" a paraphyletic or polyphyletic assemblage respectively, but several lineages of "triconodont" mammals do form a natural, monophyletic group, known as Eutriconodonta, of which Triconodon is indeed part of.

Alticonodon is a genus of extinct mammal from the Late Cretaceous of North America. It is one of the geologically youngest known eutriconodonts, and is a fairly more specialised animal than earlier representatives of this clade.

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  22. ↑ Averianov, Alexander; Martin, Thomas; Lopatin, Alexey; Skutschas, Pavel; Schellhorn, Rico; Kolosov, Petr; Vitenko, Dmitry (25 July 2018). "A high-latitude fauna of mid-Mesozoic mammals from Yakutia, Russia". PLOS ONE. 13 (7): e0199983. Bibcode:2018PLoSO..1399983A. doi: 10.1371/journal.pone.0199983 . PMC   6059412 . PMID   30044817.
  23. ↑ Jäger, Kai R. K.; Cifelli, Richard L.; Martin, Thomas (2 August 2020). "Tooth eruption in the Early Cretaceous British mammal Triconodon and description of a new species". Papers in Palaeontology. 7 (2): 1065–1080. doi: 10.1002/spp2.1329 .
  24. 1 2 Simpson, George Gaylord (1928). A Catalogue of the Mesozoic Mammalia in the Geological Department of the British Museum. OCLC   1041779295.[ page needed ]
  25. 1 2 Jäger, Kai R. K.; Cifelli, Richard L.; Martin, Thomas (24 December 2020). "Molar occlusion and jaw roll in early crown mammals". Scientific Reports. 10 (1): 22378. Bibcode:2020NatSR..1022378J. doi: 10.1038/s41598-020-79159-4 . PMC   7759581 . PMID   33361774.
  26. 1 2 Grossnickle, David M.; Polly, P. David (22 November 2013). "Mammal disparity decreases during the Cretaceous angiosperm radiation". Proceedings of the Royal Society B: Biological Sciences. 280 (1771): 20132110. doi:10.1098/rspb.2013.2110. PMC   3790494 . PMID   24089340.
  27. ↑ Han, Gang; Mallon, Jordan C.; Lussier, Aaron J.; Wu, Xiao-Chun; Mitchell, Robert; Li, Ling-Ji (2023). "An extraordinary fossil captures the struggle for existence during the Mesozoic". Scientific Reports. 13 (1): 11221. doi:10.1038/s41598-023-37545-8. PMC   10354204 . PMID   37464026.
  28. ↑ Kielan-Jaworowska, Zofia; Cifelli, Richard L.; Luo, Zhe-Xi (2004). "Metatherians". Mammals from the Age of Dinosaurs. Columbia University Press. pp. 425–462. doi:10.7312/kiel11918. ISBN   978-0-231-11918-4. JSTOR   10.7312/kiel11918.16.
  29. ↑ Rougier, Guillermo W.; Davis, Brian M.; Novacek, Michael J. (January 2015). "A deltatheroidan mammal from the Upper Cretaceous Baynshiree Formation, eastern Mongolia". Cretaceous Research. 52: 167–177. Bibcode:2015CrRes..52..167R. doi:10.1016/j.cretres.2014.09.009.
  30. ↑ Sigogneau-Russell, Denise (1995). "Two possibly aquatic triconodont mammals from the Early Cretaceous of Morocco". Acta Palaeontologica Polonica. 40 (2): 149–162. CiteSeerX   10.1.1.818.1039 .
  31. ↑ Gaetano, Leandro C.; Rougier, Guillermo W. (July 2011). "New materials of Argentoconodon fariasorum (Mammaliaformes, Triconodontidae) from the Jurassic of Argentina and its bearing on triconodont phylogeny". Journal of Vertebrate Paleontology. 31 (4): 829–843. Bibcode:2011JVPal..31..829G. doi:10.1080/02724634.2011.589877. hdl: 11336/68497 . S2CID   85069761.
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