| Gobiconodon | |
|---|---|
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| Gobiconodon ostromi cast of a specimen fromCrow Indian Reservation,Montana. At theAMNH. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | †Eutriconodonta (?) |
| Family: | †Gobiconodontidae |
| Genus: | †Gobiconodon Trofimov, 1978 |
| Species | |
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| Synonyms | |
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Gobiconodon is an extinct genus of carnivorousmammals (or possibly non-mammalianmammaliaforms) belonging to the familyGobiconodontidae. Undisputed records ofGobiconodon are restricted to theEarly 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 theMiddle Jurassic. Species ofGobiconodon varied considerably in size, withG. ostromi, one of the larger species, being around the size of a modernVirginia opossum. Like other gobiconodontids, it possessed several speciations towards carnivory, such as shearingmolariform teeth, largecanine-likeincisors 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 largerRepenomamus there might be some evidence ofscavenging.[1]
In 1978, the speciesGobiconodon borissiaki andGuchinodon hoburensis were described by the Soviet palaeontologist B. A. Trofimov, based on remains from theKhoboor beds (Dzunbain Formation) of the MongolianGobi Desert, which date to theAptian toAlbian ages of theEarly Cretaceous. The specimens were found during a 1969–1973 Soviet-Mongolian palaeontological expedition to the area. TheGobiconodon borissiaki material consisted of theholotype, a partial rightdentary bone, and fragments of eight other dentaries and twomaxillae.Guchinodon hoburensis was represented only by its holotype, a single partial right dentary.[2] Later expeditions to the area have found several more jaw fragments of both species.[3] In 1998,Guchinodon hoburensis was reassigned toGobiconodon byKielan-Jaworowska and Dashzeveg, makingGuchinodon ajunior synonym of that genus.[3]
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Gobiconodon ostromi, from the Aptian–AlbianCloverly Formation of the US state of Montana, was described in 1988 byFarish A. Jenkins and Charles R. Schaff. It is represented by two partial skeletons preserving both cranial and postcranial elements. The holotype, MCZ 19965, consists of both sides of the dentary, as well as several postcranial bones. The referred specimen MCZ 19860 contains both sides of the dentary and parts of the upper jaw, cranium, vertebrae, ribs, shoulders, pelvis and limbs. The specific epithet honours the American palaeontologistJohn Ostrom for his studies on the Cloverly Formation.[2]
Gobiconodon hopsoni, from the Early Cretaceous (Berriasian–Barremian)Öösh Formation of Mongolia, was described in 2001 by Guillermo W. Rougier, Michael J. Novacek,Malcolm McKenna and John R. Wible. The holotype, PSS-MAE 140, consists of a partial right maxilla preserving the fourthmolariform and parts of the second and third molariforms.[3] The referred specimen PSS-MAE 139 consists of a part of the right dentary bone preserving an unerupted molariform andsockets for two other teeth. Two additional specimens from the same locality, both represented by dentary fragments, were at first described merely asGobiconodon sp., but not assigned to any species. The specimens were eventually assigned toG. hopsoni in a 2015 paper by Alexey Lopatin and Alexander Averianov.[3] The species is named after the palaeontologistJames A. Hopson.[4]
Gobiconodon palaios, from the Early Cretaceous (possibly Berriasian) of Morocco, was described in 2003 byDenise Sigogneau-Russell. It is based on isolated upper molariforms and possible premolariforms. The molariforms were assigned to the genusGobiconodon due to their slightly triangulated cusp arrangement. According to Lopatin & Averianov (2015), this feature is not unique to this genus, but is also found in other genera likeRepenomamus and the "amphilestid"Juchilestes. According to these authors,G. palaios may be synonymous withKryptotherium polysphenos, another species found at the same locality, which is known only from lower teeth.[3]
Gobiconodon zofiae was described in 2003 by Li Chuankui, Wang Yuanqing, Hu Yaoming and Meng Jin. The holotype, IVPP V12585, is based on a single skull and lower jaw found near the village of Lujiatun in the province ofLiaoning in Northeast China.[5] The rocks in which the species was found belong to the basal member of theYixian Formation, and have been dated to theBarremian age of the Early Cretaceous.[3] The species is named after the Polish palaeontologistZofia Kielan-Jaworowska.[5] According to Lopatin & Averianov (2015),G. zofiae possesses no features clearly distinguishing it fromG. hopsoni of Mongolia, but they provisionally retained it as a valid taxon until more material of the two species is discovered.[3]
Gobiconodon luoianus was described in 2009 by Yuan Chongxi, Xu Li, Zhang Xingliao, Xi Yunhong, Wu Yanhua and Ji Qiang based on material found near Lujiatun, in the same member of the Yixian Formation that yieldedG. zofiae. It is known from a single specimen, 41H III-0320, consisting of a nearly complete skull with well-preserved upper and lower teeth. The species is named after the Chinese palaeontologistZhe-Xi Luo. Lopatin & Averianov (2015) disputed the validity of this species, suggesting that it represents ajunior synonym ofG. zofiae.[3]
Gobiconodon haizhouensis andGobiconodon tomidai were described in 2015 by Nao Kusuhashi, Wang Yuanqing, Li Chuankui and Jin Xun. Both species are from the Early Cretaceous (Aptian to Albian) of the Liaoning province of China.G. haizhouensis is known from a single specimen (IVPP V14509) from theFuxin Formation, consisting of a nearly complete right dentary. The species is named after Haizhou, an old name for the town ofFuxin where the type specimen was found.[6] Lopatin (2017) commented thatG. haizhouensis has no valid characteristics distinguishing it from the Mongolian speciesG. hoburensis, but did not formally synonymise the two.[7]
G. tomidai is known from one specimen (IVPP V14510) from theShahai Formation, consisting of an incomplete right dentary. The species is named after the Japanese palaeontologist Yukimitsu Tomida.[6]
Gobiconodon bathoniensis was described in 2016 byPercy M. Butler and Denise Sigogneau-Russell based on materials found in theForest Marble Formation of England, which is dated to theBathonian age of theLate Jurassic. The holotype (M46527) consists of an upper right molariform from the Old Cement Works Quarry ofKirtlington, Oxfordshire. Other specimens include two upper left molariforms from Kirtlington and a possible upper right molariform fromSwyre, Dorset. As withG. palaios, the attribution ofG. bathoniensis to the genusGobiconodon is based mainly on the triangulated cusps of the teeth.[8] A 2020 paper by Kusuhashiet al. therefore recommended that its attribution toGobiconodon should be reexamined.[9]
In 2005, two new species ofGobiconodon were described based on material from the Bol'shoi Kemchug 3 locality in theKrasnoyarsk Krai of Russia, which belongs to the Early CretaceousIlek Formation. Both species are based on isolated teeth and/or jaw fragments, and were described in open nomenclature asGobiconodon sp. A and B.[10] Two indeterminate species of the genus of distinct size have also been reported from the Early CretaceousBatylykh Formation in Yakutia, Russia, which represents the northernmost record of the genus.[11]
In 2006, an isolated premolariform tooth from Barremian-aged strata of theWessex Formation of England was described as possibly belonging toGobiconodon. The tooth has a mostly symmetrical crown with three cusps, and a single, strongly curved root.[12] Lopatin & Averianov (2015) disputed the assignment of this tooth toGobiconodon, as it differs markedly from typicalGobiconodon premolariforms.[3] Teeth from the Early CretaceousAngeac-Charente bonebed in France have also been referred to the genus, though like the British specimens, their placement in the genus has been questioned because they lack tricuspid premolars.[11]
Different species ofGobiconodon ranged from small to medium-sized.[4]G. ostromi, one of the largest and most well-preserved species ofGobiconodon, had a skull length of around 10 centimetres (3.9 in) and an estimated presacral body length of around 35 centimetres (14 in), making it comparable to a modernVirginia opossum.[2][13] Size may not be a reliable way of distinguishing between species ofGobiconodon, as the animal is thought to have had a long, protracted growth period compared to extant mammals.[3]
MostGobiconodon species are known from highly incomplete skull material, usually only preserving parts of the jaws. The speciesG. zofiae and its possible synonymG. luoianus are known from more complete skulls.[5][14] Material from those species shows thatGobiconodon had a somewhat narrowskull roof with a flat upper surface. Thesuture between thefrontal andparietal bones was V-shaped. Thesagittal crest was short and low. At the rear end of the skull, thelambdoidal crest was turned forwards, rendering theocciput visible from above.[14] Unusually, part of the lower border of theeye socket appears to have been formed by the maxilla, as there was seemingly no connection between thelacrimal andjugal bones in this region.[3] Thezygomatic arches are badly preserved in most specimens, but are complete in the holotype ofG. luoianus. In this specimen, the zygomatic arch was mostly straight, and had a protuberance near the front end.[14] On thepalate, there was a set of large depressions located next to the upper molariforms, and a smaller one next to the final premolariform.[5] These depressions would have housed the teeth of the lower jaw when the mouth was closed.[3] On thebraincase, the anterior lamina of thepetrosal bone bore a largeforamen (hole), which may have served as a common exit for themaxillary (V2) andmandibular (V3) branches of thetrigeminal nerve.[5]
The lower jaw was formed mostly by the dentary bone. Themandibular symphysis (the joint between the two halves of the dentary) was unfused.[14] Thecoronoid process was tall and backwards-pointing. Themasseteric fossa was large and rather deep. Jenkins & Schaff (1988) identified a facet for thecoronoid bone inG. ostromi,[2] but this seems to be absent in some of the other species.[5][6] Unlike in more basalcynodonts, themiddle ear bones were not attached directly to the dentary, and apostdentary trough for the attachment of these bones was therefore absent. Unlike in moderntherians andmonotremes, there was however still an indirect connection between the middle ear and the jaw, formed by anossified (bony)Meckel's cartilage. Meckel's cartilage was housed in aMeckelian groove which ran along the inner surface of the dentary.[5] Like othereutriconodonts,Gobiconodon lacked anangular process at the rear end of the dentary.[6]
Gobiconodon and other gobiconodontids can be distinguished from their relatives by their specialised anterior (front) dentition. Theincisors,canines and anteriorpremolariforms were all rather simple, conical teeth; accordingly, it is debated how many teeth of each type were present. For consistency, the interpretation by Lopatin & Averianov (2015) is followed here; these authors interpretedGobiconodon as having 3 incisors, 1 canine and 2 premolariforms in both the upper and lower jaws.[3]
As mostGobiconodon specimens consist of dentary fragments, the lower dentition is better known than the upper. The incisors and canine were all single-rooted. The first lower incisor (i1) was large and strongly procumbent (forwards-pointing). The second incisor (i2) was also procumbent, but was smaller than the first one. The third incisor (i3) and canine (c) were smaller and less procumbent than the first two incisors. The first premolariform (p1) was rather similar to the last incisor and canine, being dominated by a large main cusp. In some specimens, there was a second relatively large cusp located distally to (behind) the main cusp, but in other specimens this cusp was much smaller. The premolariforms with prominent distal cusps may representdeciduous teeth and be labelled as dp1.[3] The first premolariform was single-rooted in most species, but double-rooted inG. tomidai.[6][7] The second premolariform was more complex than p1, bearing three well-developed cusps. This tooth was possibly not replaced inGobiconodon, and in some specimens the tooth is missing and the socket is filled with bone. As a result, the second premolariform may be considered a deciduous tooth and labelled as dp2. The second premolariform was double-rooted in some species, likeG. hoburensis, but single-rooted in others likeG. borissiaki.[3]
Unlike the anterior dentition, the molariforms ofGobiconodon were relatively unspecialised. There were 5 molariforms in both the upper and lower jaws. The lower molariforms had a typical "triconodont" shape quite similar to that of the "amphilestids", with three large main cusps arranged in a straight line.[2] The middle cusp (cusp a) was the largest, while the front cusp (cusp b) and rear cusp (cusp c) were smaller. Cusps b and c were similar in size, but one of the cusps was often slightly bigger than the other. The size ratio between the main cusps varied depending on species and tooth generation. The lower molariforms bore a cingulum on the lingual (inner) side, which could either be continuous or be divided into two parts in the middle. On the cingulum, there was a small cusp d behind cusp c. In front of cusp b there were two cingular cusps e and f, which often were reduced to small ridges, though cusp e could be more prominent in some of the teeth. The molariforms had an interlocking mechanism in which cusp d of one tooth fit into a groove formed by cusps e and f of the tooth behind it. The upper molariforms were similar to the lower ones in overall shape, having a central cusp A flanked by smaller cusps B and C. Unlike the lower teeth, the upper molariforms had a continuous cingulum fully encircling the tooth. The cusps of the first two upper molariforms (M1–M2) were lineally arranged like in the lower teeth, but M3–M5 differed in that the three main cusps were slightly triangulated, with the central cusp A being located more lingually than the side cusps B and C.[3] Theocclusion between the upper and lower molariforms is thought to have been similar to that of other eutriconodonts,[15] with the central cusp (A/a) fitting between two of the opposing molariforms.[3]
Before their formal description, the remains of the type speciesGobiconodon borissiaki were interpreted as belonging to the familyTriconodontidae, but they were assigned toAmphilestidae in the describing paper.[2] In 1984, the subfamily Gobiconodontinae was erected within Amphilestidae, in which the generaGobiconodon,Guchinodon andKlamelia were included.[16] In 1988, Gobiconodontinae was raised to family rank asGobiconodontidae.[2] In later decades, multiple new genera have been assigned to Gobiconodontidae, includingFuxinoconodon,Hangjinia,Meemannodon andRepenomamus from China,[3][9] andSpinolestes from Spain.[17]Huasteconodon, from theEarly Jurassic of Mexico, was also assigned to Gobiconodontidae when first described, but its inclusion within the family has been disputed.[3]
Along with other gobiconodontids,Gobiconodon has often been placed within Eutriconodonta, a group characterised by their "triconodont" molariforms with cusps placed in a straight line. Phylogenetic analyses have often found eutriconodonts to be closer totherians (the group containingmarsupials andplacentals) than tomonotremes,[17] but they are sometimes alternatively placed outside the mammaliancrown group (the clade formed by monotremes and therians), within the larger cladeMammaliaformes.[18] Some analyses have found Eutriconodonta to be amonophyletic group containing all descendants of their common ancestor, while others recover it as aparaphyletic grade ancestral toTrechnotheria and other more derived groups.[3][18]
Thepostcanines (teeth behind the canine) ofGobiconodon can be divided into two types depending on their shape. The ones in front were relatively simple, and are classified aspremolariforms. The ones further back were more complex, and are known asmolariforms. The terms premolariform and molariform should not be confused with the termspremolar andmolar, which are based on tooth replacement rather than shape. Premolars are generally replaced once, while molar teeth by definition are not replaced at all. InGobiconodon, however, most or all of the molariforms were replaced, technically making them premolars rather than molars by that definition.[19] Molariform replacement was first observed inG. ostromi, which preserves molariforms of one generation in the process of replacing molariforms of the previous generation, and has been determined in otherGobiconodon species through indirect evidence such as tooth wear patterns.[3] In 2022, direct evidence of molariform replacement was also described in a specimen ofG. borissiaki.[20] Some species ofGobiconodon have been inferred to have replaced some of their molariforms twice, withG. borissiaki replacing the first two andG. ostromi replacing the first three. Replacement of a tooth position more than one time is known aspolyphyodonty and is rare in extant mammals. The extensive replacement of molariforms may be related to the animal's relatively large size, with larger species likeG. ostromi replacing their teeth more times than smaller species likeG. hoburensis.[3] Molariform replacement has also been observed in the other gobiconodontidsRepenomamus andSpinolestes, indicating that it may be a general feature of the group.[17]
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