Daspletosaurus

Daspletosaurus wilsoni. Credit: Rudolf Hima.

*Daspletosaurus* is an extinct genus of tyrannosaurid that lived in North America during the Middle to Late Cretaceous.

## History

*D. wilsoni* was named from the Judith River Formation and after John Wilson. discoverer of the specimen at the "Jack's B2" site in 2017. The holotype, BDM 107 is a partial disarticulated skull and postcranium, with 2 premaxillae, a right maxilla, jugal, lacrimal, quadrate, quadratojugal, dentary, splenial, left postorbital, squamosal, partial cervical-sacral-caudal series, a rib, chevron and first metatarsal. The fossil bears great intricacies. As of it's naming in 2022, the sacrals and caudals were encased in a concretion that was left unprepared. The description paper relies heavily on character states and morphological combinations to avoid misattribution of ontogenetic or synapomorphous data.

## Description

*Daspletosaurus* was a medium-sized tyrannosaurid, being slightly taller than an average human. *Daspletosaurus* had a large skull, and had sturdier bones than other large theropods. Large fenestrae in the skull reduced overall body weight. A mature *Daspletosaurus* had was armed with about six dozen blade-like teeth, and had a long oval shaped cross section rather than resembling a blade. Even the nasal openings on the front tip of the skull were reinforced. Unlike all other teeth in the jaws of *Daspletosaurus*, the teeth lining the premaxilla had a D-shaped cross section. The irregularity in the teeth is a type of heterodonty common in tyrannosaurs. *Daspletosaurus* is unique as in the following features: a rough premaxilla exterior, pronounced crests around the eyes, especially the jugal, lacrimal and postorbital bones. The eye orbit takes the shape of an oval, between the orbit shapes of *Gorgosaurus* and the 'keyhole' shape of *Tyrannosaurus*. Split carinae edges have been found in the teeth of *Daspletosaurus*. Studies on the skull of *Daspletosaurus* counteract many depictions of tyrannosaurs; *Daspletosaurus* did not bear lips. Research claims (in comparison with modern crocodiles) that the rough texture on the snout bore large flat scales, rather than fleshy lips. The rugose texture leads almost all the way down to the teeth, and it is theorized that there would have been no room for the animal to possess lips. Because *D.* "horneri" lacks a ZooBank, it is a nomen nudum. *D. wilsoni* is differentiated from other upper and lower taxa through the subsutaneous surface of the maxilla being very coarse, no elevated ridges over the fossae, postorbital cornual processes reaching near to the laterotemporal fenestra, the dorsal postorbital process on the squamosal terminates caudally to the laterotemporal rostral margin and the symphyseal dentary surface is also very coarse. It also presents one autapomorphous character, that is the elongate rostrocaudally/dorsoventrally narrow mylohyoid foramen on the splenial, being much deeper than other members of the species. Together, this makes a unique mosaic of derived and ancestral traits as well as numerous shared traits of *D. torosus* and *D.* "horneri".

## Paleobiology

### Social Behaviour

Evidence suggests that *Daspletosaurus* lived in social groups, which may have led to interspecies conflict. Teeth marks in the skull of a young *Daspletosaurus* show that the wounds were inflicted by other tyrannosaurs. Some argue that the marks could be from another genus of tyrannosaur, it is also likely that the marks were created by other *Daspletosaurus*; via facial biting. The wounds were healed over, meaning that the animal outlived the inflicted wounds and survived. Another, mature, specimen had the same marks, and the face biting was not limited to only younger group members. Face biting is common in predators, and other tyrannosaurids like *Gorgosaurus* and *Tyrannosaurus*, and other theropods like *Sinraptor* and *Saurornitholestes*. Darren Tanke argues that the conflict revolves around competition for resources like food or water, or even territory and dominance displays.

Fossils of *Daspletosaurus* family groups have been found, namely a specimen consisting of juvenile, an adult of indeterminate size and a large adult. About five hadrosaurs were fossilized around the same location the *Daspletosaurus* family was found. The hadrosaur remains are scattered, and bear evidence of tyrannosaur teeth serrations. The cause of death is indeterminate, but it is known that the family died while eating. Some claim that the animals formed a pack, and others are skeptical of the evidence. *Daspletosaurus*' social behaviour is similar to komodo dragon social interactions, according to Brian Roach and Danial Brinkman.

### Life History

    Gregory Erikson et. al. have been studying the life history of tyrannosaurids based on bone histology determining age in life. He plotted the age and weights of tyrannosaurids on a graph to determine the growth rate of different tyrannosaur genera. The graph tells scientists that juvenile tyrannosaurs underwent massive growth spurts, for about four years, in the middle of their lives. After the individuals hit sexual maturity, they then tremendously slowed their growth. *Daspletosaurus* showed a faster growth rate due to having a higher mature weight. *Daspletosaurus*' maximum growth rate is about 180 kilograms (400 pounds) per year. This estimate is based on the 1800 kilograms (2 tons) rough mass estimate in mature individuals. Other estimates have been given, but still do not change the overall curve. By tabulating the results, Erikson et. al. were able to determine a rough conclusion of life history in a population of *Albertosaurus*. They concluded: juveniles are rare in the fossil record, but rapidly growing subadults and adults were far more common. The little number of juvenile could be possible because of a low mortality rate in juvenile animals, due to low predation. Not enough material of *Daspletosaurus* have been found, but a similar hypothesis can be applied. Traces of a Trichomonas gallinae-like infection have been found in the jaws of *Daspletosaurus*.

Warshaw and Fowler (2022) examined the holotype of *D. wilsoni* to determine ontogenetic maturity before publishing to ensure their defining traits could not be confused as developmental. They found that the specimen is likely of advanced growth due to it being one of the largest specimens attributed to the genus and the characters present are typically found solely in mature tyrannosaurines. Although Carr (2020) suggested that size could not be a wholly reliable marker of *Tyrannosaurus*, they suggest that his study did not include correlation between an individual's age and mature traits, which they suggest together indicates advanced growth. They also find that the traits found (eg. a deeply scalloped maxilla-nasal suture, rostrally positioned maxillary fenestra within the antorbital fenestra, an inflated cornual process of the lacrimal that is dorsal to the ventral ramus and a greatly prominant cornual process of the postorbital) suggest that the specimen is an adult (as defined by Car (2020)) or a Stage 4 (Carr (1999)), but can still be confirmed by histological work or other comparison not within the original study's scope.

### Senses

Daspletosaurus possessed integumentary sensory organs. Evidence points towards the following senses:

* Strong touch sense
* The ability to precisely move the jaws
* Temperature sensing
* The detection of prey

Studies on the scales show they protected the weak elements of the face during prey capture and inter-species conflicts.

## Classification

Warshaw and Fowler (2022) suggest that an anagenetic lineage between species is seen, following the order of *D. torosus*, to *D. wilsoni* and ending in *D.* "horneri", which they postulate is possible due to *D. wilsoni* existing between the two former species. They also find great overlap with *D. wilsoni* and the Dinosaur Park taxon, which may or may not be evidence of taxonomic synonymy or a genuine sister relationship. The former's existence confirms Carr et al. (2017)'s predictions of anagenesis in *Daspletosaurus*. For a general phylogeny:

## Notable Specimens

* RMDRC 06-005 "Pete III": A specimen found around 2005 from the Judith River. Pete III, a *D.* sp., took about 13 years to fully excavate. A toe fragment was first found in Central Montana. A mount was created and housed at the Cincinnati Museum Center, where the specimen was excavated, prepared, scanned, molded, casted and assembled. The mount also incorporates real bone, which was held together with subtle steel rods and super-glue. It is 11 meters long.
* FMNH PR308 "Gorgeous George": A mostly-complete specimen once thought to be of *Gorgosaurus*. Because of this, most reconstructed skulls are inaccurate since they are modeled after *Gorgosaurus*; some sources claim it is an *Albertosaurus*. However, it was later assigned to *Daspletosaurus torosus*. Some think it is a new taxon.
* "Denver's Tyranno/tyrannosaur": A *Daspletosaurus* skeleton found curled in a 9000-pound block that was airlifted from Montana by a Chinook helicopter. It was excavated by Denver and Liz Fowler for 4 years. This specimen likely exploded during decomposition due to gastric gases, which would have scattered the remains, which were encased by sediment that washed over it. it then slowly eroded from its block, with the feet first spotted by the Fowlers, with the tail vertebrae uncovered next. From there, they predict a skull, ribcage, the rest of the legs and the neck are also present. Excavation began in 2018, dug for 2 summers, and returned in 2019 to finish. Many trace fossils such as burrow traces are found on the underside of this specimen, not pushed up against the bone.
* Aguja Formation: Remains of a *D.* sp. were described from the Aguja Formation by Langston, Standhardt and Stevens (1989). These encompass the upper and lower sectors. Unnamed tyrannosaurid teeth are also known from this formation as well as indeterminate remains described by Lehman and Wick (2013), but it is unknown if they correlate. The latter is of a tyrannosaur 5-6 meters long and 700 kilograms heavy.

## Paleoecology

*Daspletosaurus* lived in North America, between the Middle and Late Cretaceous. *Daspletosaurus* lived in a vast floodplain along the western shore of the interior seaway that lied in the middle of North America during the Middle/Late Cretaceous. The region sometimes flooded, and filled the area in new sediment. When water was abundant, the area could sustain large amounts of flora and fauna. Other times, however, droughts happened, increasing animal and plant mortality rates. These droughts spanned across both the Two Medicine and Judith River Formations, including the *Daspletosaurus* bonebeds. Similar events sometimes happen in today's East Africa. Volcanic eruptions frequently blanketed the area in ash, killing of many organisms. *Daspletosaurus* lived with a vast amount of animals, but most interesting of all, is the coexistence with another apex predator; *Gorgosaurus*. Some authors claim that the co-operation between species are based on geographical separation, while others claim that *Gorgosaurus* fed on hadrosaurs while *Daspletosaurus* preyed on ceratopsians (due to their bulkier build, being able to take on more armored dinosaurs). The true method that both *Gorgosaurus* and *Daspletosaurus* coexisted is still unknown. Indeterminate tyrannosaur material from the Aguja Formation may represent *D. torosus*.

## References

1. ↑ 1.0 1.1 1.2 1.3 1.4 https://peerj.com/articles/14461/
2. ↑ http://www.palarch.nl/2020/09/yun-chan-gyu-2020-a-subadult-frontal-of-daspletosaurus-torosus-theropoda-tyrannosauridae-from-the-late-cretaceous-of-alberta-canada-with-implications-for-tyrannosaurid-ontogeny-and-taxonomy/
3. ↑ Paul, G. S., 1988, Predatory Dinosaurs of the World, a complete Illustrated guide: New York Academy of sciences book
4. ↑ https://en.wikipedia.org/wiki/Daspletosaurus
5. ↑ https://rmdrc.blogspot.com/2016/05/pete-iii-finished-for-now.html
6. ↑ https://twitter.com/TrieboldPaleo/status/1430900874205822985
7. ↑ http://rmdrc.blogspot.com/2018/12/pete-iii-final-update-in-its-forever.html?m=1
8. ↑ https://rmdrc.blogspot.com/2016/04/daspletosaurus-assembly-building.html
9. ↑ https://t.co/ieDkf8ZTjR?amp=1
10. ↑ https://www.youtube.com/watch?v=HdOU6XMcH14&list=WL&index=4&t=377s&ab\_channel=CityofDickinson-CityHall
11. ↑ https://theindependent.com/news/national/dino-soaring-nearly-complete-tyrannosaur-flies-out-of-valley-county/article\_cc0c282b-95ac-5010-b60e-d154a48b3120.html
12. ↑ https://twitter.com/df9465/status/1451258080587485185
13. ↑ https://web.archive.org/web/20130929074932/http://archosaur.us/theropoddatabase/Tyrannosauroidea.html
14. ↑ https://www.cambridge.org/core/journals/earth-and-environmental-science-transactions-of-royal-society-of-edinburgh/article/abs/tyrannosauroid-dinosaurs-from-the-aguja-formation-upper-cretaceous-of-big-bend-national-park-texas/0503E2FBA29E7E2C147A4F1C7B67AEE9
15. ↑ http://dontmesswithdinosaurs.com/?p=2320