User:Ankylosaur Enthusiast/sandbox

Ankylosaur Enthusiast/sandbox Temporal range: Late Cretaceous,
Holotype of T. teresae (PIN 3142/250) on display at the Paleontological Institute, Russian Academy of Sciences.
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
†T. teresae Penkalski & Tumanova, 2016 †T. tumanovae Park et al., 2021
Synonyms
Dyoplosaurus giganteus Maleev, 1956 Minotaurasaurus? Miles & Miles, 2009

Discovery and naming

Fossil localities in Mongolia. Tarchia fossils have been collected at Khulsan, Hermiin Tsav II, Hermiin Tsav I and Altan Uul IV (area A)

Description

Cranium

The holotype skull of T. teresae from different angles.

The skull of Tarchia is trapezoidal and, in dorsal view, broader than long. The premaxilla possesses a distinctive rostral notch and a narrow internarial bar. The palatine has a plane that is elevated horizontally. Each pterygoid has an anterior wall that is inclined forward. The occipital condyle is oval in shape, with a joint surface that slightly protrudes posteroventrally. The paroccipital process is perpendicular to the skull roof and the occiput inclines posteriorly. The mandibular condylus of the quadrate is similar in level to the posterior margin of the orbit, with the orbit itself facing anterolaterally. The foramen magnum is taller than wide and the brain cavity has large openings for the cranial nerves. (Tumanova, 2000) The basipterygoid processes are, although distinct, short anteroposteriorly. (Arbour et al., 2014) Projecting well below the ventral border of the paroccipital processes is the occipital condyle. In caudal view, this forms a hemicircle. Situated near the basitubera is the basioccipital foramen, which is also moderately large. The postorbital fossa is poorly defined in rostromedial view. (Penkalski and Tumanova, 2016) The external nares face anteriorly, with the entrance to the airway being large and subcircular.

The holotype skull of T. tumanovae in different views.

Postcranial material of T. tumanovae (WIP heading)

The osseous nasal septum extends dorsally but does not meet the skull roof. The ectopterygoid is small and wedge-like. The pterygoid has a vertical anterior surface with a foramen pierced through the central body. The pterygoid flange projects anterolaterally and contacts the dorsally positioned ectopterygoid. The quadrate ramus contacts the posterolaterally positioned quadrate. The posterolateral edge of the quadrate ramus is damaged on both sides. The posteromedial margin of the main pterygoid body is not fused with the basipterygoid processes of the basisphenoid. The basipterygoid processes are divided from each other. An interpterygoid vacuity is present between the paired pterygoids. In palatal view, the contact between the basisphenoid and the basioccipital forms a rugose transverse ridge. A basioccipital foramen is present on the convex ventral surface of the basioccipital. The basioccipital and the exoccipitals are entirely fused and form a reniform occipital condyle oriented posteroventrally. The ovoid foramen magnum is taller than wide. A small hill-like process lies between the foramen magnum and the nuchal shelf. A horizontal groove is present below the paired exoccipital protuberances. The lateral terminus of the paroccipital process is long but not fused to the quadrate reaching laterally to the squamosal horns. The transversely broad quadrates are inclined anteroventrally toward the distal articular condyles in lateral view. The medial condyle of the quadrate is larger than the lateral condyle.

Only the third and eighth right maxillary teeth and the seventh left maxillary tooth are preserved. Although these teeth are partially embedded within the sockets, up to eleven marginal denticles can be observed. Shallow vertical grooves are present between the denticles, and a shelf-like labial cingulum is also present.

The type specimen of T. tumanovae preserves isolated osteoderms which vary in morphology. One such osteoderm is sharply keeled dorsally, polygonal and has a thin wall. The osteoderm has damaged external surfaces, although the medial surface remains well preserved, and has a pitted surface texture. Also persevered are two osteoderms that are sub-circular, flattened dorsoventrally and have a slightly concave ventral surface. Like the previous osteoderm, the two osteoderms possess a rugose external surface and a thin wall. Forming the tail club knob are two major osteoderms and a single minor osteoderm that envelop the distal end of the tail. The major osteoderm is hemispherical, while the minor osteoderm is rhomboidal in shape.

Classification

Skull of Minotaurasaurus (far right) compared to other ankylosaurids

In the original description, Maryanska (1977) assigned Tarchia as a member of the clade Ankylosauridae, which remains as the current consensus. Maryanska further suggested that North American ankylosaurids may have descended from ankylosaurids similar to Talarurus and Pinacosaurus that migrated from Asia to North America. In 1987, Tat'yana Tumanova synonymised the genus Tarchia with "Dyoplosaurus" giganteus and formed the combination of Tarchia gigantea, while T. kielanae was kept as a valid taxon. Coombs and Maryańska (1990), however, would later synonymise T. kielanae into T. gigantea. A 2014 study on ankylosaurids from the Baruungoyot and Nemegt formations that was performed by Victoria M. Arbour, Phillip J. Currie and Demchig Badamgarav considered T. kielanae to be a valid taxon and the combination T. gigantea to be unnecessary.

A 1998 study by paleontologist James Kirkland found Tarchia to be sister taxon to Saichania. Various studies like Carpenter (2001), Thompson and colleagues (2012), and Hill and colleagues (2013) would recover Tarchia in a similar position. Vickaryous and colleagues (2004), however, found it to be outside a large clade containing more derived taxa like Ankylosaurus and Pinacosaurus. Tarchia would later be used in a phylogenetic analysis by Victoria Arbour and Phillip J. Currie in 2015, which recovered it outside the newly coined clade Ankylosaurini and as sister taxon to Zaraapelta within Ankylosaurinae. In this analysis, INBR 21004, a specimen previously assigned to Minotaurasaurus, was used to represent Tarchia. A study that was published in 2016 by Paul Penkalski and Tat'yana Tumanova once more recovered Tarchia as sister taxon to Saichania. In addition, INBR 21004 was found to not represent Tarchia. Park and colleagues (2021) recovered the genus within a similar position to Penkalski and Tumanova (2016), while Frauenfelder and colleagues recovered it in a similar position to Arbour and Currie (2015).

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

Ankylosaurinae	Crichtonpelta Tsagantegia Zhejiangosaurus Pinacosaurus Saichania Tarchia Zaraapelta Ankylosaurini Dyoplosaurus Talarurus Nodocephalosaurus Ankylosaurus Anodontosaurus Euoplocephalus Scolosaurus Ziapelta

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

	Minotaurasaurus Zaraapelta Saichania T. kielanae T. teresae Pinacosaurus

Palaeobiology

Feeding

Snout morphology and diet of Mongolian ankylosaurids; T. teresae and T. tumanovae in right

Based on its anteriorly protruded shovel-shaped muzzle, Tarchia was a selective-feeder. Park et al. (2021) noted a dietary shift in Mongolian ankylosaurines from bulk feeding to selective feeding that occurred sometime during the middle Campanian to lower Maastrichtian stages. The authors suggested that this may have either been caused by the change from semi-arid and arid to humid climates or interspecific competition with saurolophine hadrosaurids that immigrated from North America to Central Asia during the Campanian stage.

Ősi and colleagues (2016) performed a study on the feeding adaptations in ankylosaurs and noted that ankylosaurs living in dry environments, such as Tarchia, may have either relied more on hindgut fermentation than oral processing for the digestion of plant matter or, alternatively, consumed succulent plants that did not require complex chewing. They also noted that they may have been restricted to simple orthal pulping, with the main component of jaw action being the movement of the jaws in a vertical plane. As indicated by the higher proportions of microwear pits, ankylosaurs living in dry climates may have had to deal with more grit in their environment during feeding than ankylosaurs that lived in tropical to subtropical climates.

Ontogeny

Arbour et al. (2014) suggested that during ontogeny, the external layer of the squamosal horn and the underlying squamosal horn propers would fuse together into a single, large horn with a keel and smooth texture. However, a 2016 study by Penkalski and Tumanova instead suggested that the smooth texture signifies the beginning of resorption and that the external layer of the squamosal horn disappears entirely in skeletally mature individuals, implying that ankylosaurines underwent extreme remodelling of the squamosal horns during growth. The squamosal horns of the holotype skull of T. tumanovae are divided into the external layer and the proper, with the base of the external dermal layer having an irregular ventral margin which may represent resorption.

In 2019, a Canadian Society of Vertebrate Palaeontology abstract book briefly mentioned the discovery of hatchling material from the Hermiin Tsav locality of the Nemegt Formation that likely represents either Tarchia or Saichania, although the authors noted that taxonomic identification of juvenile postcranial material is difficult. At least two individuals are preserved and were found in close association with each other, which may either represent multiple eggs hatching within the same nest at the same time, or eggs that hatched at almost the same time but from multiple nests.

Paleopathology

Tarchia pathology

The holotype skull of T. teresae catalogued as PIN 3142/250 exhibits an open hole on the dorsal surface of the right orbit. It has been suggested to either be caused by predation from tyrannosaurs such as Tarbosaurus, as the anterior-most premaxillary teeth are of the right shape and size, or intraspecific combat. CT scans have revealed mineralised concretions within the airway and sinuses of the skull that are as a result of a traumatically introduced infection.

Park et al. (2022) reported that a specimen of T. tumanovae has evidence of fractured healing on both sides of the first dorsosacral ribs in the anterolateral part of the pelvic region. They noted that injuries localized to the pelvic region in ankylosaurines were likely the result of intraspecific combat. In addition, a poorly healed ossified tendon of the tail club handle is present, which may have also been caused by injury. The authors also reported grooves that are present along the lateral sides of each major knob osteoderm and, in addition, the tail club knob is asymmetric as the left major knob osteoderm is shorter in mediolateral width than the right major knob osteoderm. This may relate to side preferences in tail use and further suggests that ankylosaurines used their tail clubs for agonistic behaviour.

Neuroanatomy

A 2017 study by palaeontologist Ariana Paulina-Carabajal and colleagues analysed the neuroanatomy of T. tumanovae and noted the presence of the cerebellum’s flocculus on the back wall of the vestibular eminence. The presence of a flocculus might relate to capabilities for gaze stabilization, while the presence of a longer anterior semicircular canal may correlate with the ability to hear a greater range of sound frequencies, most notably in the lower range. Schade et al. (2022) further suggested that the combination of a longer anterior semicircular canal and the presence of a floccular recess might be associated with more active kind of protective behaviours, such as digging and the ability to target their tail clubs.

Paleoenvironment

Baruungoyot Formation

Khermen Tsav locality

The type species, T. kielanae, is known from the Khulsan and Hermiin Tsav II localities at the Baruungoyot Formation. The formation overlies the Djadokhta Formation and consists of a series of red beds, reddish-brown sandy claystones, light-brown siltstones and light-brown to yellowish-orange fine-grained sandstones. The upper Baruungoyot Formation and lower Nemegt Formation overlapped in time, and may have been a singular ecosystem. The sediments of the formation were deposited in various conditions, with the lower part consisting of alternating dune deposits and lakes that existed in interdune areas, while the upper part consisted of sediments that were deposited over a takyr-like area that was flooded at irregular intervals. Overall, the formation had an arid to semi-arid climate with significant rainfall. The formation dates to the middle to late Campanian stage of the Late Cretaceous period, ca. ~72-71 Ma. Arbour and colleagues (2014) noted that there was a great diversity of ankylosaurids in the Baruungoyot Formation and suggested that this may have either been due to the lack of direct competition for food resources with large herbivores, or through sexual selection. Although, the authors also noted that their relative niches were unclear and there is little indications of sexual dimorphism.

The Baruungoyot Formation has yielded specimens of the alvarezsaurids Ceratonykus, Khulsanurus, Ondogurvel and Parvicursor; the ceratopsians Bagaceratops and Breviceratops; the dromaeosaurids Kuru, Shri and an indeterminate velociraptorine; the halszkaraptorines Hulsanpes and Natovenator the pachycephalosaur Tylocephale; the oviraptorosaurs Conchoraptor, Heyuannia and Nemegtomaia; the ankylosaurids Saichania, Zaraapelta and an indeterminate ankylosaurid; and the sauropod Quaesitosaurus.

Nemegt Formation

Nemegt and Barun Goyot formations intersection

Specimens attributable to T. teresae and T. tumanovae have been recovered from the Hermiin Tsav I and Altan Uul IV localities at the Nemegt Formation. The formation consists of a variety of sediments such as such as reddish-brown to grey-green mudstones, grey-to-brown sandstones, light grey to-tan coloured siltstones and granular beds. Stream and river channels, mudflats, and shallow lakes were present in the formation as indicated by the rock facies, although droughts periodically occurred as indicated by the presence of caliche deposits. Woodlands were also present in the Nemegt Formation and were represented by large, enclosed, Araucaria forests, as evidenced by the numerous petrified wood found in the outcrops of the formation. The formation had mean annual temperatures of 7.6 and 8.7 °C, and was influenced by monsoons with cold, dry winters and hot summers. It has been said that the environment was similar to the Okavango Delta of present-day Botswana. Due to the discontinuity of exposures, absence of microfossil biostratigraphy, and absence of datable volcanic rock facies, the precise age of the Nemegt Formation cannot be attained through radiometric dating. Although, it has been suggested to be of late Campanian to early Maastrichtian age based on the fauna present in the fossil record.

The Nemegt Formation has yielded specimens of the ankylosaurid Saichania; the alvarezsaurids Mononykus and Nemegtonykus; the dromaeosaurid Adasaurus; the hadrosaurids Saurolophus angustirostris and Barsboldia; the ornithomimosaurs Anserimimus, Deinocheirus and Gallimimus; the oviraptorosaurians Avimimus, Conchoraptor, Elmisaurus, Gobiraptor, Nemegtomaia, Oksoko and Rinchenia; the pachycephalosaurs Homalocephale and Prenocephale; the sauropods Nemegtosaurus and Opisthocoelicaudia; the therizinosaur Therizinosaurus; the troodontids Borgovia, Tochisaurus and Zanabazar; the tyrannosaurids Alioramus and Tarbosaurus; and an indeterminate azhdarchid.

See also

• Timeline of ankylosaur research
• 2017 in archosaur paleontology