Tarchia

*Tarchia gigantea*

***Tarchia*** (meaning "brainy one") is an extinct genus of herbivorous ankylosaurid dinosaur from the late Cretaceous of Mongolia.

A new species to this genus; Tarchia tumanovae has been officially added at 2021.

## Discovery and naming

In 1970, a Polish-Mongolian expedition discovered an ankylosaurian skull near Khulsan.

In 1977, Teresa Maryańska named and described the type species *Tarchia kielanae*. The generic name is derived from Mongolian *tarkhi*, "brain" and Latin *~ia*, in reference to a brain size presumed larger than that of the related form *Saichania*. The specific name honours Professor Zofia Kielan-Jaworowska, the leader of the expedition. The holotype, **ZPal MgD-I/111**, was discovered in the Upper Cretaceous Barun Goyot Formation (previously known as the 'Lower Nemegt Beds') of the Nemegt Basin of Mongolia.

The holotype, ZPal MgD-I/111, was discovered in the Upper Cretaceous (possibly Campanian-Maastrichtian) Barun Goyot Formation (previously known as the 'Lower Nemegt Beds') of the Nemegt Basin of Mongolia. It consists of a skull roof, braincase and rear skull elements. 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 currently, with *Saichania*, among the geologically youngest known of all the Asian ankylosaurid dinosaurs.

In 1977, Tatyana Tumanova named a second species: *Tarchia gigantea*. This was a renaming of *Dyoplosaurus giganteus* Maleev 1956, which had been based on specimen PIN 551/29. In 1987, Tumanova concluded that both species were identical. This would make Dyoplosaurus giganteus the senior synonym of *Tarchia kielanae*. This was generally accepted and *Tarchia gigantea* became the usual species name, as a *combinatio nova* replacing *Tarchia kielanae*.

However, recent study by Victoria Megan Arbour indicates that *D. giganteus* is indistinguishable from other ankylosaurs from the late Campanian-Maastrichtian of Mongolia, and hence a *nomen dubium*, reviving *Tarchia kielanae*.

A rump with tail and club, specimen ZPAL MgD I/113, once referred to *Dyoplosaurus giganteus* and subsequently to *Tarchia gigantea*, was by Arbour seen as different from the *D. giganteus* holotype.

The study by Arbour also concluded that specimen PIN 3142/250, in 1977 referred to *Tarchia* by Tumanova, probably belonged to *Saichania* instead. This would radically change the common image of *Tarchia* 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 of *Tarchia* shared distinguishing traits with that of Minotaurasaurus Miles & Miles 2009, concluding that the latter is a junior synonym of *Tarchia*.

## Description

Size estimates of *Tarchia* have been largely based on *Dyoplosaurus giganteus*, the holotype of which is one of the largest ankylosaurian individuals known. This would make *Tarchia* the longest known Asian ankylosaur, with an estimated body length of 8 metres (26 ft). Confusingly, the skull size often mentioned, with a length of 40 centimetres (16 in) and width of 45 centimetres (18 in), was again based on specimen PIN 3142/250, a much smaller individual. The holotypes of *Tarchia kielanae* and *Minotaurasaurus* also indicate a medium size. In 2010, Gregory S. Paul had estimated a body length of 4.5 metres and a weight of 1.5 tonnes.

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, named osteoderms. It probably had a bony tail club, for active defence against predators. *Tarchia* had been distinguished from *Saichania* on the basis of its relatively larger basicranium, an unfused paroccipital process-quadrate contact and, based on PIN 3142/250, the fact that the premaxillary rostrum is wider than the maximum distance between the tooth rows in the maxillaries. Apart from the unique traits known exclusively from the holotype of Minotaurasaurus, Arbour in 2014 established two distinguishing traits.

The back of the head is visible in top view. A deep groove runs along the front and outer side of the squamosal horn. At the front this groove runs around an accessory osteoderm placed on the rear supraorbital, forming a deep furrow. Much information given about *Tarchia* in older work actually refers to PIN 3142/250. In 2001, it was stated that in *Tarchia*, wear facets indicative of tooth-to-tooth occlusion are present, but in the holotype no teeth are preserved. The morphology of cranial sculpturing seen in *Tarchia* was indicated as an assortment of bulbous polygons, reminiscent of that of Saichania chulsanensis, the other ankylosaurid from the Barun Goyot Formation, but this can be explained by PIN 3142/250 belonging to that taxon.

## Phylogeny

Vickaryous *et alii* in 2004 stated that *Tarchia* was basal to two distinct clades of Late Cretaceous ankylosaurids: one comprising North American taxa (Ankylosaurus, Euoplocephalus) and the other comprising Asian taxa (Pinacosaurus spp., Saichania, Tianzhenosaurus, Talarurus). However, this was again based on PIN 3142/250, the characters of which usually defined the operational taxonomic unit named *Tarchia* in the various cladistic analyses. Remarkably, *Tarchia* and *Saichania* nevertheless in these analyses often occupied very different positions.

## Paleobiology

The rocks in which *Tarchia* fossils were found likely represent eolian dunes and interdune environments, with small intermittent lakes and seasonal streams. Hence, we know that *Tarchia* was a desert animal. On the other hand, well-watered forest would have been present. Tarchia, theoretically, might have preyed upon by Tarbosaurus; albeit unlikely.

## JPInstitute.com Description

Tarchia was a large, armored dinosaur that lived in Asia. This plant-eater seems to have been a common dinosaur and quite was similar to Ankylosaurus. It was another member of the family of dinosaurs that were built low to the ground with thick, bony scutes covering much of their back and neck. It also had a fearsome weapon to protect itself, a heavy, bony club on the end of its tail that when swung could have broken the bones of any animal that tried to attack it.

Tarchia seemed to have a larger brain than other members of its family do, and its skull seems to support the theory that ankylosaur heads became more massive as they evolved.

## Links

http://web.archive.org/web/20030625223108/http://www.jpinstitute.com/dinopedia/dinocards/dc\_tarch.html