| Tarchia | |
|---|---|
 | |
| A cast of specimen PIN 3142/250, the holotype ofT. teresae. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Thyreophora |
| Clade: | †Ankylosauria |
| Family: | †Ankylosauridae |
| Subfamily: | †Ankylosaurinae |
| Genus: | †Tarchia Maryanska, 1977 |
| Type species | |
| †Tarchia kielanae Maryanska, 1977 | |
| Other species | |
| Synonyms | |
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Tarchia (meaning "brainy one") is agenus of herbivorousankylosauriddinosaur from thelate Cretaceous ofMongolia.
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In 1970, a Polish-Mongolian expedition discovered an ankylosaurian skull nearKhulsan. In 1977,Teresa Maryańska named and described thetype speciesTarchia kielanae. The generic name is derived fromMongolian тархи (tarkhi, "brain") andLatin~ia, in reference to a brain size presumed larger than that of the related formSaichania. Thespecific name honours ProfessorZofia Kielan-Jaworowska, the leader of the expedition.
Theholotype,ZPal MgD-I/111, was discovered in the UpperCretaceous (possiblyCampanian-Maastrichtian)Barun Goyot Formation (previously known as the 'Lower Nemegt Beds') of theNemegt Basin of Mongolia. It consists of a skull roof, braincase and rear skull elements.[1] Maryańska referred three additional specimens: ZPAL MgDI/43, a large postcranial skeleton containing three "free" tail vertebrae, twelve tail vertebrae of the "handle" of the tail club and a scute; ZPAL MgDI/49, a right humerus; and PIN 3142/251, a skeleton with skull, that as yet remains undescribed.
Tarchia is the geologically youngest of all knownAsianankylosaurid dinosaurs.In 1977,Tatyana Tumanova named a second species:Tarchia gigantea. This was a renaming ofDyoplosaurus giganteus Maleev 1956, which had been based on specimen PIN 551/29.[2] In 1987, Tumanova concluded that both species were identical. This would makeDyoplosaurus giganteus the senior synonym ofTarchia kielanae.[3] This was generally accepted andTarchia gigantea became the usualspecies name, as acombinatio nova replacingTarchia kielanae. However, recent study byVictoria Megan Arbour indicates thatD. giganteus is indistinguishable from other ankylosaurs from the late Campanian-Maastrichtian of Mongolia, and hence anomen dubium; the study revived the nameTarchia kielanae.[4]
A rump with tail and club, specimen ZPAL MgD I/113, once referred toDyoplosaurus giganteus and subsequently toTarchia gigantea, was by Arbour seen as different from theD. giganteus holotype.[5]The study by Arbour also concluded that specimenPIN 3142/250, in 1977 referred toTarchia by Tumanova, probably belonged toSaichania instead. This would radically change the common image ofTarchia as this exemplar had been by far the best preserved and most illustrations, museum mounts and indeed scientific research had been based on it. Arbour discovered that the holotype ofTarchia shared distinguishing traits with that ofMinotaurasaurus Miles & Miles 2009, concluding that the latter is ajunior synonym ofTarchia.[6]
Subsequently, in 2016, a study conducted by Paul Penkalski and Tatiana Tumanova indicated that PIN 3142/250 is not referable toSaichania due to significant anatomical differences, but instead represents a new species ofTarchia,T. teresae. The study also recognizedMinotaurasaurus as a distinct genus.[7] In 2021, Jin-Young Park and team named a new species ofTarchia,T. tumanovae, known from the holotype MPC-D 100/1353 which consists of a partial skeleton with associated skull. It was found in theNemegt Formation at the Hermiin Tsav locality, making it coeval withT. teresae.[8]
Tarchia was a medium-sized ankylosaur, measuring around 5.5–6 metres (18–20 ft) long and weighing up to 2.5–3 metric tons (2.8–3.3 short tons).[9][10] If ZPAL MgD I/113 indeed belongs to the genus, it would have belonged to an individual measuring 5.8–6.7 metres (19–22 ft) long.[11]
As an ankylosaurid,Tarchia would have had a broad, low-slung body, positioned on strong short legs. The body would have been protected by skin ossifications, namedosteoderms. It probably had a bony tail club, for active defence against predators.
Tarchia had previously been distinguished fromSaichania on the basis of its relatively largerbasicranium, an unfused paroccipital process-quadrate contact and, based on PIN 3142/250, the fact that thepremaxillary rostrum is wider than the maximum distance between the tooth rows in the maxillaries. In 2014, Arbour reported two distinguishing traits apart from those known exclusively from the holotype ofMinotaurasaurus; the back of the head is visible in top view; and a deep groove runs along the front and outer side of the squamosal horn, and at the front it surrounds around an accessory osteoderm placed on the rear supraorbital, forming a deep furrow.[4]
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The 2016 redescription ofTarchia notes that it differs fromSaichania in having a postorbital fossa (which separates the squamosal horn from thesupraorbital) and an accessory osteoderm; theocciput being visible in dorsal view; the large, deep braincase; theforamen magnum being higher than it is wide; and the nuchal osteoderms being taller laterally than medially. In addition, it differs from bothSaichania andMinotaurasaurus in that it lacks postocular caputegulae (or small, polygonal bony plates behind the orbit) and has a proportionally high occiput in caudal view.[7] The study additionally found that PIN 3142/250 (i.e.T. teresae) can be distinguished fromT. kielanae in that the accessory osteoderm is not fused to the roof of the skull, the quadrate and paroccipital process are not fused, the back of the skull roof is strongly sculptured, and the openings for the fourth to twelfth cranial nerves is bifurcated.[7]
Much information given aboutTarchia in older work refers to PIN 3142/250 (which was briefly referred toSaichania until it was named asT. teresae in 2016). In 2001, it was stated that, inTarchia, wearfacets indicative of tooth-to-toothocclusion are present;[12] this likely does not refer to the holotype specimen, since in the holotype no teeth are preserved.
Vickaryouset al. in 2004 stated thatTarchia wasbasal to two distinctclades of Late Cretaceous ankylosaurids: one comprisingNorth Americantaxa (Ankylosaurus,Euoplocephalus) and the other comprising Asian taxa (Pinacosaurus spp.,Saichania,Tianzhenosaurus,Talarurus).[13] However, this was again based on PIN 3142/250, the characters of which usually defined theoperational taxonomic unit namedTarchia in the variouscladistic analyses. Remarkably,Tarchia andSaichania nevertheless in these analyses often occupied very different positions.
The followingcladogram is based on a 2015phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie:[14]
A limited phylogenetic analysis conducted in the 2016 redescription ofTarchia, focusing on the interrelationships betweenTarchia,Saichania, andMinotaurasaurus, is reproduced below.[7]
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The rocks in whichTarchia fossils were found likely representeolian dunes and interdune environments, with small intermittent lakes and seasonal streams.[8]
Tarchia was, like other Mongolian ankylosaurines,herbivorous and a low-level bulk feeder based on its sub-rectangular broad muzzle.[8] Instead of oral processing, ankylosaurids living in dry environments such asTarchia may have relied more onhindgut fermentation for digestion or, alternatively, consumed succulent plants that did not require complex chewing. These ankylosaurids may have also been restricted to simple orthal pulping and might have had to deal with more grit during feeding compared to ankylosaurs that lived in tropical to subtropical climates, as indicated by themicrowear pits.[15] Parket al. (2021) suggested that there was a shift from bulk feeding to selective feeding in Mongolian ankylosaurines during the Campanian and Maastrichtian stages which may have either been caused by the change in habitat, as the climate changed from semi-arid and arid to humid, or interspecific competition withsaurolophinehadrosaurids that immigrated fromNorth America toCentral Asia during the Campanian stage.[8]
One skull ofTarchia shows tooth marks identified as belonging to thetyrannosaurid,Tarbosaurus, indicating the theropod hunted the ankylosaurid.[16][17]
