Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack et al. in 1973 as a replacement name for the paraphyletic Triconodonta.
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.
|Examples of several eutriconodonts. Clockwise: Repenomamus, Volaticotherium, Jeholodens and Yanoconodon. These occupy vastly different ecological niches: bulky semi-fossorial carnivore, glider, arboreal insectivore and adaptive terrestrial carnivore, respectively.|
Kermack et al., 1973
"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. The group originally included only the family Triconodontidae and taxa that were later assigned to the separate family Amphilestidae, but was later expanded to include other taxa such as Morganucodon 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 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. 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 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 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. 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. 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". 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.
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. 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 Campanian genus Alticonodon is the youngest representative of the group; the Campanian/Maastrichtian Austrotriconodon was originally referred to as a late surviving member of the clade, but has since been moved to Dryolestoidea.
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) and several Early Cretaceous north African taxa like Ichthyoconodon, Dyskritodon amazighi and Gobiconodon palaios. 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.
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.
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. 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.
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.
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.
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. Triconodont teeth are known to have had a shearing function, allowing the animal to tear through flesh much like carnassial teeth of therian mammals. 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.
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.
At least in carnivorous niches, eutriconodonts were probably replaced by deltatheroidean metatherians, which are the dominant carnivorous mammals in Late Cretaceous 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, 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. However, Dyskritodon and Ichthyoconodon's teeth shows no erosion associated with aquatic transportation, meaning that the animals died in situ or close. Studies on Liaoconodon 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 shows adaptations towards multiple forms of locomotion, with traits in common with fossorial, arboreal, and semiaquatic mammals.
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.
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.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 eutriconodont found.Condorodon
Condorodon is a genus of extinct mammals from the Middle Jurassic Cañadón Asfalto Formation of the Cañadón Asfalto Basin in Patagonia, Argentina, South America. The type species is C. spanios, described by Gaetano and Rougier in 2012.Dryolestes
Dryolestes is an extinct genus of Late Jurassic mammal from the Morrison Formation and the Alcobaça Formation of Portugal.
Present in stratigraphic zones 2, 5, and 6.Dyskritodon
Dyskritodon ("tooth of unknown origin", from Greek δυσκρίτος, "dyskritos") is a genus of extinct mammal from the Lower Cretaceous of Morocco, and possibly the Early Jurassic of India. Of uncertain affinities, it is tentatively described as a eutriconodont.Gobiconodonta
Gobiconodonta is an order of extinct mammals known from the Early Jurassic (such as Huasteconodon) to early Late Cretaceous. They are generally held to be part of Eutriconodonta.Henkelotherium
Henkelotherium is an extinct genus of mammal from the Late Jurassic (Kimmeridgian) Camadas de Guimarota, in Portugal. It differs from most other paurodontids in having more postcanine teeth.Holotheria
Holotheria are a diverse group of mammals that are descendants of the last common ancestor of Kuehneotherium and Theria (the group that includes marsupials and placental mammals).Jeholodens
Jeholodens was a primitive mammal belonging to the order Eutriconodonta, and which lived in present-day China during the Middle Cretaceous about 125 million years ago.Only one specimen has been formally described. This specimen (the holotype) consists of a virtually complete articulated skull and skeleton, it shared its corporal characteristics with most other Mesozoic mammals; it was a long-tailed, nocturnal tetrapod (with prehensile fingers and toes) which hunted insects, its food, during the night.
It is suspected to be a nocturnal creature because it had very large eyes which were roughly 5 cm across. This would have allowed it to have better night vision for catching insects. It is notable for its relatively derived forelimb morphology, having shoulder blades and other pectoral girdle elements comparable to those of modern therians like opossums. It also had grasping hands. By contrast, however, the hindlimbs retained primitive characters, suggesting a sprawling stance.Recent studies show that it was specialised to an arboreal lifestyle, possessing prehensile hands.Jugulator (mammal)
Jugulator is a genus of extinct mammal from the Cretaceous of North America. 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.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 holotype IVPP V 16051, which consists of nearly complete skeleton and skull. It was found in the Jiufotang Formation (Jehol Biota) near Xiaotaizi, Lamadong. It was first named by Jin Meng, Yuanqing Wang and Chuankui Li in 2011 and the type species is Liaoconodon hui.Studies on its anatomy show that it was a semi-aquatic mammal, having a long body and paddle-like limbs.Meiconodon
Meiconodon is an extinct genus of alticonodontine triconodontid which existed in China during the early Cretaceous period (Aptian/Albian age). It was described by Nao Kusuhashi, Yaoming Hu, Yuanqing Wang, Satoshi Hirasawa and Hiroshige Matsuoka in 2009 and the type species is Meiconodon lii.Priacodon
Priacodon is an extinct genus of Late Jurassic mammal from Portugal and the Morrison Formation.
Present in stratigraphic zones 4–6.
Known species: Priacodon ferox, Priacodon fruitaensis, Priacodon lulli, Priacodon robustus.Spinolestes
Spinolestes is an extinct mammal genus from the Early Cretaceous of Spain. A gobiconodontid eutriconodont, it is notable for the remarkable degree of preservation, offering profound insights to the biology of non-therian mammals.Trechnotheria
Trechnotheria is a group of mammals that includes the therians and some fossil mammals from the Mesozoic Era. In the Jurassic through Cretaceous periods, the group was endemic to what would be Asia and Africa.Trechnotheria has been assigned various ranks, but was originally called a "superlegion" by the original author.
One reference has defined the Trechnotheria as the clade comprising the last common ancestor of Zhangheotherium and living therian mammals, and all its descendants.Triconodon
Triconodon ("three coned tooth") is a genus of extinct mammal from the Early Cretaceous of Europe. 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.
Triconodon, therefore, is significant in the understanding of the evolution of mammals by originating the understanding of the "triconodont" grade and eutriconodont clade. Further discoveries on its skeletal anatomy also offer further insights on the palaeobiology of Mesozoic mammals.Triconolestes
Triconolestes is an extinct genus of Late Jurassic mammal from the Morrison Formation.
Present in stratigraphic zones 4. It is possibly related to Volaticotherium, Argentoconodon, Ichthyoconodon and Jugulator, meaning it could possibly have been capable of gliding.Trioracodon
Trioracodon is an extinct genus of Late Jurassic mammal from the Morrison Formation.
Present in stratigraphic zone 5.Volaticotherini
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 are primarily teeth, they are foremostly diagnosticated by their highly distinctive molars. However, the remains of one species, Volaticotherium antiquum, show that at least some members of this clade were capable of gliding. and Argentoconodon shares similar post-cranial features that also indicate aerial locomotion. As such, this clade contains some of the oldest known aerial mammals, alongside the various gliding haramiyidans.