Tarchia

''Tarchia'' (meaning "brainy one") is a genus of herbivorous ankylosaurid dinosaur from the late Cretaceous of Mongolia.

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 тархи (''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 (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 the geologically youngest of all known 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''; the study revived the name ''Tarchia kielanae''.Arbour, Victoria Megan, 2014. ''Systematics, evolution, and biogeography of the ankylosaurid dinosaurs.'' Ph.D thesis, University of Alberta. https://era.library.ualberta.ca/public/.../Arbour_Victoria_Spring2014.pdf

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''.

Subsequently, in 2016, a study conducted by Penkalski & Tumanova indicated that PIN 3142/250 is not referable to ''Saichania'' due to significant anatomical differences, but instead represents a new species of ''Tarchia'', ''T. teresae''. The study also recognized ''Minotaurasaurus'' as a distinct genus. In 2021, Jin-Young Park and team named a new species of ''Tarchia'', ''T. tumanovae'', known from the holotype MPC-D 100/1353 which consists of a partial skeleton with associated skull. It was found in the Nemegt Formation at the Hermiin Tsav locality, making it coeval with ''T. teresae''.

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 .Holtz, Thomas R. Jr. (2011) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,'
Winter 2010 Appendix.
/ref> Confusingly, the skull size often mentioned, with a length of and width of , 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 2016, Gregory S. Paul had estimated a body length of 5.5 meters (18 ft) and a weight of 2.5 tonnes (2.75 short tons) suggesting that Minotaurasaurus was a juvenile of the species.Paul, G.S., 2016, ''The Princeton Field Guide to Dinosaurs 2nd Edition'', Princeton University Press p. 261

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 previously 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. In 2014, Arbour reported two distinguishing traits apart from those known exclusively from the holotype of ''Minotaurasaurus''; 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.

The 2016 redescription of ''Tarchia'' notes that it differs from ''Saichania'' in having a postorbital fossa (which separates the squamosal horn from the supraorbital) and an accessory osteoderm; the occiput being visible in dorsal view; the large, deep braincase; the foramen magnum being higher than it is wide; and the nuchal osteoderms being taller laterally than medially. In addition, it differs from both ''Saichania'' and ''Minotaurasaurus'' in that it lacks postocular caputegulae (or small, polygonal bony plates behind the orbit) and has a proportionally high occiput in caudal view. The study additionally found that PIN 3142/250 (i.e. ''T. teresae'') can be distinguished from ''T. 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.

Much information given about ''Tarchia'' in older work refers to PIN 3142/250 (which was briefly referred to ''Saichania'' until it was named as ''T. teresae'' in 2016). In 2001, it was stated that, in ''Tarchia'', wear facets indicative of tooth-to-tooth occlusion are present; this likely does not refer to the holotype specimen, since in the holotype no teeth are preserved.

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.

The following cladogram is based on a 2015 phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie:

A limited phylogenetic analysis conducted in the 2016 redescription of ''Tarchia'', focusing on the interrelationships between ''Tarchia'', ''Saichania'', and ''Minotaurasaurus'', is reproduced below.

Palaeobiology

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'' would have been preyed upon by '' Tarbosaurus''.

Paleopathology

One skull of ''Tarchia'' shows tooth marks identified as belonging to the tyrannosaurid, ''Tarbosaurus'', indicating the theropod hunted the ankylosaurid.Gallagher W.B., Tumanova T.A., Dodson P., Axel L., 1998, "CT scanning Asian ankylosaurs: paleopathology in a ''Tarchia'' skull", ''Journal of Vertebrate Paleontology'' 18: 44A-45A

See also

* Timeline of ankylosaur research
* 2017 in archosaur paleontology

References

External links

''Tarchia'' in the Dino Directory

{{Taxonbar, from=Q135304

Ankylosaurids
Late Cretaceous dinosaurs of Asia
Fossils of Mongolia
Maastrichtian life
Fossil taxa described in 1977
Taxa named by Teresa Maryańska
Ornithischian genera