| Eutriconodonta | |
|---|---|
 | |
| Examples of several eutriconodonts. Clockwise:Repenomamus,Volaticotherium,Jeholodens andYanoconodon. These occupy vastly different ecological niches: bulky semi-fossorial carnivore, glider, arboreal insectivore and terrestrial carnivore, respectively.[1] | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Clade: | Theriimorpha |
| Order: | †Eutriconodonta Kermacket al., 1973 |
| Subgroups | |
Eutriconodonta is anorder of earlymammals. Eutriconodonts existed inAsia (includingpre-contact India),Africa,Europe,North andSouth America during theJurassic and theCretaceous periods. The order was named by Kermacket al. in 1973[2] as a replacement name for theparaphyleticTriconodonta.[3]
Traditionally seen as the classicalMesozoic 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]
"Triconodonta" had long been used as the name for an order of earlymammals 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 familyTriconodontidae and taxa that were later assigned to the separate familyAmphilestidae,[9] but was later expanded to include other taxa such asMorganucodon orSinoconodon.[4] The phylogenetic analyses found that all these taxa did not form a natural group, and that some traditional "triconodonts" were more closely related totherian 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 onlydental andmandibular characters.[3][10][11] Gaoet al. (2010) conducted a second analysis as well, using a modified version of the matrix from the analysis of Luoet al. (2007);[12] this analysis involved a broader range of Mesozoic mammaliaforms and more characters, including postcranial ones. Both Luoet al. (2007) and the second analysis of Gaoet al. (2010) recovered a more inclusive monophyletic Eutriconodonta that also contained gobiconodontids andAmphilestes;[10][12] in the second analysis of Gaoet al. it also containedJuchilestes (recovered as amphidontid in their first analysis, the only amphidontid included in their second analysis).[10] However, Gaoet 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 oflong branch attraction of jeholodentids and gobiconodontids".[10] Phylogenetic studies conducted by Zhenget al. (2013), Zhouet al. (2013) and Yuanet al. (2013) recovered monophyletic Eutriconodonta containing triconodontids, gobiconodontids,Amphilestes,Jeholodens andYanoconodon.[13][14][15]
The exact phylogenetic placement of eutriconodonts withinMammaliaformes is also uncertain. Zhe-Xi Luo, Zofia Kielan-Jaworowska and Richard Cifelli (2002) conducted an analysis that recovered eutriconodonts within thecrown group of Mammalia, i.e. the least inclusive clade containingmonotremes andtherian 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" andmultituberculates were.[16] This result was mostly confirmed by Luoet al. (2007), the second analysis of Gaoet al. (2010), Zhenget al. (2013), Zhouet al. (2013) and Yuanet al. (2013), although in the phylogenies of Luoet al. (2007) and Yuanet al. (2013) eutriconodonts were in unresolvedpolytomy with multituberculates andtrechnotherians.[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 ofMesozoic animals that are traditionally considered to be mammals (such asMorganucodonta andDocodonta) 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 byThomas Martin et al. 2015, in their description ofSpinolestes. Eutriconodonts are recovered as a largely monophyletic group withinTheriimorpha.[6]
| Eutriconodonta | |
A 2020 study found them paraphyletic in regards to crown groupMammalia.[18]
When eutriconodonts first appeared is unclear. The earliest remains come from the late Early Jurassic (Toarcian), but they already represent a variety of groups: thevolaticotherianArgentoconodon, thealticonodontineVictoriaconodon and thegobiconodontidHuasteconodon, as well as the putative eutriconodont"Dyskritodon" indicus.[19] They achieve their peak diversity across theEarly Cretaceous, before largely disappearing from the fossil record in the earlyLate Cretaceous outside ofNorth America. TheMaastrichtian genusIndotriconodon is the youngest representative of the group, hailing from theintertrappean beds ofIndia;[20] theCampanian/MaastrichtianAustrotriconodon was originally referred to as a late surviving member of the clade, but has since been moved toDryolestoidea.[21]
Most eutriconodont remains occur inlaurasian landmasses. The exceptions areArgentoconodon and slightly youngerCondorodon from theEarly Jurassic ofArgentina, the putativeDyskritodon indicus from theEarly Jurassic ofIndia (Kota Formation), theLate JurassicTendagurodon fromTanzania (Tendaguru Formation), severalEarly Cretaceousnorth African taxa likeIchthyoconodon,Dyskritodon amazighi andGobiconodon palaios, andIndotriconodon magnus fromLate Cretaceous India. Due to the rarity of the Jurassicgondwanan 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 andharamiyidans (generally considered non-mammalian cynodonts) are also present, but nottherians,dryolestoids and other groups considered true mammals.[22]
Like many other non-therian mammals, eutriconodonts retained classical mammalian synapomorphies likeepipubic bones (and likely the associated reproductive constrictions),venomous spurs and sprawling limbs. However, the forelimb and shoulder anatomy of at least some species likeJeholodens are similar to those of therian mammals, though the hindlimbs remain more conservative.[4] Eutriconodonts hada modern ear anatomy, the main difference fromtherians being that the ear ossicles were still somewhat connected to the jaw via theMeckel'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]
Some eutriconodonts likeSpinolestes andVolaticotherium were very well preserved, showing evidence of fur, internal organs and, in the latter, ofpatagia.Spinolestes shows hair similar to that of modern mammals, with compoundhair follicles with primary and secondary hair, even preserving traces of a pore infection. It also possesses a clearthoracic diaphragm like modern mammals, as well asspines, dermalscutes and an ossifiedMeckel's cartilage. Furthermore,Spinolestes may also display signs ofdermatophytosis, suggesting that gobiconodontids, like modern mammals, were vulnerable to this type of fungal infection.[6]
Triconodon itself has been the subject to cranialendocast studies, revealing a uniquebrain anatomy.[4][24]
The eutriconodont triconodont dentition has no analogue among living mammals, so comparisons are difficult. There are two main types ofocclusion patterns: one present intriconodontids (as well as the unrelatedmorganucodontan mammals), in which lowercusp "a" occludes anterior to upper cusp "A", between "A" and "B", and one present inamphilestids andgobiconodontids, in which the molars basically alternate, with the lower cusp "a" occluding further forward, near the junction between two upper molars.[19] A study onPriacodon 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 likecarnassial teeth oftherian mammals.[4] In a study about Mesozoic mammalian diets the taxaRepenomamus,Gobiconodon,Argentoconodon,Phascolotherium,Triconodon andLiaoconodon rank among carnivorous mammal species, whileVolaticotherium,Liaotherium,Amphilestes andJeholodens ranked among insectivorous mammals, whileYanoconodon,Priacodon andTrioracodon ranked somewhere in between.[26] A study onPriacodon suggests that the jaw roll was more passive for eutriconodonts than modern therian carnivores.[25]
Eutriconodonts displayed a broad size range from smallshrew-like insectivores with body masses of as little as 2 grams (0.071 oz), comparable to the smallest known modern mammals, to large forms likeRepenomamus, which is estimated to have had a body mass of 12–14 kilograms (26–31 lb), comparable to a badger.[27] They were among the first mammals to be specialised for vertebrate prey, and likely occupied the highesttrophic levels among mammals in their faunal communities. Several forms likeGobiconodon andRepenomamus 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.[28]
At least in carnivorous niches, eutriconodonts were probably replaced bydeltatheroideanmetatherians, which are the dominant carnivorous mammals inLate Cretaceous faunal assemblages.[29] Competition between both groups is unattested, but in Asia the Early Cretaceous gobiconodontid diversity is replaced entirely by a deltatheroidean one, while in North AmericaNanocuris appears after the absence ofGobiconodon and other larger eutriconodonts.[30] 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]
Some eutriconodonts were instead among the most specialised of Mesozoic mammals. Several taxa likeAstroconodon,Dyskritodon andIchthyoconodon may show adaptations for piscivory and occur in aquatic settings with their molars being compared to those ofseals andcetaceans. Caution has been advised in these comparisons, however; as many researchers likeZofia Kielan-Jaworowska have noted, eutriconodont molars are more functionally similar to those of terrestrialcarnivorans thanpinnipeds andcetaceans, occluding in a shearing motion instead of not-occluding and providing a grasping function.[4] However,Dyskritodon andIchthyoconodon's teeth shows no erosion associated with aquatic transportation, meaning that the animals diedin situ or close.[31] Studies onLiaoconodon show that it has adaptations for an aquatic lifestyle, possessing a barrel-like body and paddle-like limbs,[1] and analysis of the postcrania ofYanoconodon shows adaptations towards multiple forms of locomotion, with traits in common with fossorial, arboreal, and semiaquatic mammals.[7]
Additionally,Volaticotherium andArgentoconodon show adaptations for aerial locomotion. Both genera are closely related, implying a long lived lineage of gliding mammals.[32]
At leastSpinolestes had xenarthrous vertebrae and osseous scutes, convergent to those of modernxenarthrans and to a lesser extent thehero shrew. This genus may have displayed an ecological role similar to that of modernanteaters,pangolins,echidnas,aardvark,aardwolf andnumbat, being the second known Mesozoic mammal afterFruitafossor to have done so.[6]
Triconodon shows dental replacement patterns consistent withmilk-drinking mammals.[24]
