A SUBADULT MAXILLA OF A TYRANNOSAURIDAE FROM THE TWO MEDICINE FORMATION, MONTANA, UNITED STATES

ABSTRACT

Daspletosaurus is a Campanian genus of Tyrannosauridae from North America. This genus occupied the same geographic area of Albertosaurus, but remains of Albertosaurus are more abundant than Daspletosaurus. Here is described a subadult maxilla (AMNH FARB 5477) of Daspletosaurus sp. from Montana (USA) and possibly from the Two Medicine Formation. The description is based on Carr (1999) that described cranial ontogenetic variations in tyrannosaurids. The maxilla belongs to the ontogenetic Stage 3 sensu Carr (1999), in which the maxilla is thick, the lateral surface of the bone well sculptured, and the maxillary fenestra is subcircular and well separated from the anterior edge of antorbital fossa. Possibly there were more than one species of Daspletosaurus and the locality of the here described subadult specimen suggests that Daspletosaurus species occurred more southern than Albertosaurus.

KEY-WORDS:
Daspletosaurus; Ontogeny; North America; Montana

RESUMO

Daspletosaurus é um gênero de Tyrannosauridae do Campaniano da América do Norte. Esse táxon ocupou a mesma área geográfica que Albertosaurus, no entanto restos de Albertosaurus são mais abundantes que Daspletosaurus. Aqui é descrito um maxilar de um indivíduo sub-adulto (AMNH FARB 5477) do gênero Daspletosaurus sp. de Montana (EUA) e possivelmente da Formação Two Medicine. A descrição é baseada em Carr (1999) que descreveu a variação ontogenética craniana em tiranossaurídeos. O maxilar foi reconhecido como estágio ontogenético 3 de Carr (1999), o qual possui fenestra maxilar sub-circular com grande distância a partir da borda anterior da fossa anterorbital, e maxilar robusto com superfície lateral bem esculpida. Possivelmente existiram mais de uma espécie de Daspletosaurus e a localidade do espécime sub-adulto sugere que essa espécie de Daspletosaurus viveu mais ao sul Albertosaurus.

PALAVRAS-CHAVE:
Daspletosaurus; Ontogeny; North America; Montana

INTRODUCTION

Daspletosaurus is a genus of non-avian theropod erected by Russell (1970) for many materials mainly from the Oldman Formation near Steveville, Alberta. This genus is deeply nested within Tyrannosauridae (Brusatte et al., 2010; Loewen et al., 2013; Lü et al., 2014) and occupied the same geographic area as Albertosaurus, although Daspletosaurus constitutes a much more robust form of Campanian tyrannosaurid than the latter taxon (Russell, 1970; Farlow & Planka, 2002). However, the remains of Albertosaurus are more abundant than Daspletosaurus even among juvenile individuals (Carr, 1999; Farlow & Planka, 2002; Currie, 2003). These two taxa occupied the northern of the continental area of Laramidia during the Late Cretaceous (Loewen et al., 2013), but Albertosaurus was more abundant further north than Daspletosaurus (Farlow & Planka, 2002; Currie, 2003). Farlow & Planka (2002) therefore suggested that Albertosaurus lived and nested within the northern regions of the continent whereas Daspletosaurus was restricted to more southern latitudes.

Carr (1999) described cranial ontogenetic variations in Albertosaurus and taxonomic differences among the tyrannosaurids Albertosaurus, Daspletosaurus and Tyrannosaurus.Carr (1999) identified four ontogenetic stages that can be recognized in the cranial elements. Though remains of juveniles and subadults of Daspletosaurs are scarcer compared to those of Albertosaurus, there are still abundant samplings of the former genus. Here I describe a new subadult maxilla of Daspletosaurus sp. from Montana, possibly from Two Medicine Formation.

Institutional Abbreviations

AMNH FARB, American Museum of Natural History, New York; BHI, Black Hills Institute of Geological Research, Hill City, South Dakota; BYU, Brigham Young University Museum of Paleontology, Provo, Utah; CM, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania; FMNH, Field Museum of Natural History, Chicago, Illinois; GIN, Institute of Geology, Ulaanbaatar; MOR, Museum of the Rockies, Bozeman, Montana; NMMNH, New Mexico Museum of Natural History and Science, Albuquerque; TMP, Royal Tyrell Museum of Paleontology, Drumheller, Alberta; UMNH, Utah Museum of Natural History, now the Natural History Museum of Utah, Salt Lake City, Utah; USNM, National Museum of Natural History, Smithsonian Institute, Washington DC.

SYSTEMATIC PALEONTOLOGY

Dinosauria Owen, 1842

Theropoda Marsh, 1881

Tyrannosauroidea Walker, 1964

Tyrannosauridae Osborn, 1906

Daspletosaurus sp. Russell, 1970

Material

AMNH FARB 5477. An incomplete right maxilla, lacking the distal end of the ascending process and some areas of the jugal ramus (= horizontal ramus) (Figure 1).

FIGURE 1:
Right maxilla of Daspletosaurus sp. (AMNH FARB 5477). In, (A) lateral, (B) medial and (C) ventral view. amp, anteromedial process; aof, anteorbital fossa; da, dentary alveoli; idp, interdental plates; ifs, interfenestral strut; mf, maxillary fenestra; mms, medial maxillary shelf.

Locality

The exact locality of AMNH FARB 5477 is missing. However, it is known from AMNH number, that the maxilla came from Montana, USA. Montana has two Campanian formations: the Two Medicine and Judith River formations (Eberth, 1997) (Figure 2). Remains of Daspletosaurus were recovered only from the Two Medicine Formation in USA (Weishampel et al., 2004). Thus, it is more plausible to suppose that AMNH FARB 5477 belongs to this unit. The Two Medicine Formation has yielded multiple Daspletosaurus specimens (Varricchio, 2001; Currie, 2003; Currie et al., 2005).

FIGURE 2:
Map of Upper Cretaceous units in northwestern and north-central Montana showing the Two Medicine and Judith River formations (after Rogers et al., 2010). The exact locality of AMNH FARB 5477 is missing, but possibly it belongs to the Two Medicine Formation (see text).

Description and comparisons

The maxilla (AMNH FARB 5477) is incomplete, lacking the distal extremities of ascending ramus and jugal ramus (= horizontal ramus). The medial wall of the antorbital fossa is also partially preserved and, as a result, the borders of the maxillary foramen are incomplete. It has a preserved length of 38.42 centimeters and is dorsoventrally deep. The tooth row is convex in lateral view and moderately damaged, but all fifteen alveoli are preserved.

The maxilla is transversely thick especially in the anterior portion. The maxillary body is sculptured with large foramina and deep dorsoventral neurovascular sulci as seen in advanced ontogenetic stage of Daspletosaurus (Carr, 1999). The neurovascular sulci are concentrated on the anterior end of the jugal ramus and are more abundant than in Albertosaurus libratus (AMNH 5336; AMNH 5458; AMNH 5432), Bistahieversor (NMMNH 27469), Teratophoneus (BYU 8120/9402; UMNH VP 16690, Loewen et al., 2013: fig. 3A) and Lythronax (Loewen et al., 2013: fig. 2C), but similar to Daspletosaurus (FMNH PR 308; TMP 94.143.1 [Currie, 2003]; MOR 590, TMP 2001.36.1) and Tyrannosaurus (AMNH 2750; USNM 4811033; FMNH 2081; BHI 3033). There are two parallel rows of foramina dorsal to the alveolar margin, as occurs in Albertosaurus, Alioramus, Daspletosaurus and Tyrannosaurus (Currie, 2003). The antorbital fossa is smooth as in other tyrannosaurids, with the lateral interfenestral strut being proportionally wider than that observed in Albertosaurus. This strut is damaged, reaching only the midpoint of the maxillary fenestra. However, its morphology resembles an hourglass shaped as in others tyrannosaurids.

The maxillary fenestra of AMNH FARB 5477 is incomplete and seemingly was subcircular, contrasting with more elongated maxillary fenestra of Daspletosaurus (MOR 590, CM 8506, TMP 2001.36.1), but similar to TMP 94.1431, AMNH 5432, BHI 3033, UMNH VP 16690, NMMNH 27469 and Appalachiosaurus (Carr et al., 2005: fig. 6A). It does not contact the anterior and posterior border of this fossa as seen in Tyrannosaurus and Tarbosaurus (Larson, 2013). As in Daspletosaurus (MOR 590, TMP 2001.36.1, CM 8506: Carr, 1999: fig. 2G), juvenile Tarbosaurus (GIN 100/177) and Shanshanosaurus huoyanshanensis (Carr & Williamsom, 2004) the maxillary fenestra of AMNH FARB 5477 is separated from the anterior edge by a narrow apron of bone. In Appalachiosaurs, Albertosaurus (Carr et al., 2005), juveniles Tyrannosaurus and Daspletosaurus (Carr & Williamsom, 2004), Bistahieversor, Alioramus altai (Brusatte et al., 2012), Lythronax and Teraphoneus (Loewen et al., 2013) all display a maxillary fenestra that is separated from the border by a proportionally wider apron of bone.

The promaxillary fenestra is covert in lateral view by the lateral lamina of the ascending ramus, as in Tyrannosaurus, Tarbosaurus, Daspletosaurus and Eotyrannus (Brusatte et al., 2010). Although this fenestra is filled with matrix and the anterior rim of the antorbital fossa is damaged, AMNH FARB 5477 shows a foramen-like promaxillary fenestra, as occurs in adults of Daspletosaurus, Tyrannosaurus and Tarbosaurus, whereas this fenestra is more developed in Albertosaurus and Bistahieversor.

The vestibular bulla, between the subnarial foramen ventrally and the contact surface for the premaxilla dorsally (broken in AMNH FARB 5477), is well developed and convex in transverse section as in Daspletosaurus and Tyrannosaurus whereas in juveniles and subadults of Albertosaurus is laterally flat or transversely convex (Carr, 1999). The subnarial foramen is a notch anterior to the row of foramina and dorsal to the margin of the premaxillary buttress as seen in others tyrannosaurids.

The medial surface of the bone is damage, but preserves the anteromedial process, medial maxillary shelf and the interdental plates. The medial bounding wall of the maxillary antrum was lost after the dead of the animal as it is in Alioramus altai, Daspletosaurus (TMP 97.12.223) and Albertosaurus (ROM 1247), however, this wall is present in diverse tyrannosauroids (Brusatte et al., 2012).

The interdental plates are large and unfused as in Daspletosaurus as showed by Carr (1999), Alioramus altai showed by Brusatte et al. (2012), Tarbosaurus showed by Hurum & Sabath (2003), Tyrannosaurus, Raptorex and Albertosaurus. The plates possess striations on its medial surface and become reduced triangles posteriorly and ventral to articular facet for palatine as seen in derivates tyrannosauroids (Brusatte et al., 2012).

The anteromedial process (= palatal process) is damaged and the grooves on its surface are poorly preserved. It starts ventrally to the third alveoli and a more ventral inclination is more accentuated in AMNH FARB 5477, Daspletosaurus, Albertosaurus, Tarbosaurus, Tyrannosaurus and Appalachiosaurus (Carr et al., 2005: fig. 6) than in A. altai and Raptorex. The grooves progressively pinch out posteriorly merging to the medial maxillary shelf.

The medial shelf is robust, flat and well developed reaching, its greatest dorsoventral height below the maxillary fenestra as in others tyrannosauroids (e.g., Tarbosaurus, Daspletosaurus and Tyrannosaurus). The medial shelf is preserved just in the midpoint of the interfenestral strut and reveals the posterior anteromaxillary fenestra ventral to the posterior edge of the anterorbital fenestra. This fenestra is rounded and its diameter is nearly half of the anteroposteriorly length of the interfenestral strut.

The medial shelf becomes posteriorly the articular facet for the palatine (Brusatte et al., 2012); however, in AMNH FARB 5477 this structure is not preserved, neither the jugal articulation of maxilla.

As seen above AMNH FARB 5477 has 15 tooth positions. This number varies among derived tyrannosauroids. There are 11-12 alveoli in Tyrannosaurus, 12-13 in Tarbosaurus, 13-17 in Daspletosaurus, 13-15 in Albertosaurus (see Currie, 2003), 15 in Appalachiosaurus (Carr et al., 2005), 12 in Teratophoneus (Carr et al., 2011), 17 in Alioramus altai (see Brusatte et al., 2012), and11 in Bistahieversor and Lythronax (see Loewen et al., 2013). There are preserved two replacement teeth on the third and the fifth alveoli. They are flatted labiolingually as seen in others tyrannosaurids. It is not possible to see others replacement teeth, however, it is possible that they are covered by matrix.

DISCUSSION

AMNH FARB 5477 has more characters shared with Daspletosaurus than others tyrannosauroids. The combination of abundance of anterior neurovascular sulci, number of maxillary teeth, promaxillary foramen-like covert, narrow separation between maxillary fenestra and anterior edge of antorbital fossa, wide interfenestral strut, and, finally, vestibular bulla well developed and convex in transverse section argues for referral of AMNH FARB 5477 to Daspletosaurus sp.

The estimated total length of AMNH FARB 5477 (~ 48 cm) suggests that it was an immature individual possibly in the ontogenetic Stage 3 of Carr (1999) with subcircular maxillary fenestra as in TMP 94.143.1 instead of elongated as in mature individuals (CM 8506, TMP 2001.36.1 and MOR 590), and greater distance from the anterior edge of antorbital fossa. However, AMNH FARB 5477 shows characters of mature individuals, such as maxilla thickened and lateral surface well sculptured (Carr, 1999).

Daspletosaurus and Albertosaurus libratus coexistent and occupied the northern of Laramidia whereas the others Campanian tyrannosaurs (Bistahieversor, Lithronax and Teratophoneus) occupied the southern region (Loewen et al., 2013). AMNH FARB 5477 was recovered from Montana, but it was not possible to recognize the geological formation that this material belongs (Two Medicine Formation or Judith River Formations). Nevertheless, Daspletosaurus remains were recovered only in Two Medicine Formation so far (Weishampel et al., 2004).

Possibly there were more than one species of Daspletosaurus (Farlow & Planka, 2002; Currie, 2003; Holtz, 2004) and seemingly the species from Oldman Formation, Dinosaur Park Formation (both from Alberta, Canada) and Two Medicine Formation (Montana, USA) were three different species (Currie, 2003). If there were at least three different species of Daspletosaurus and the fact that AMNH FARB 5477 is a subadult individual, shows that the Daspletosaurus species may have distributed more to the south than the others genus, whereas Albertosaurus lived and nested north, as suggested by Farlow & Planka (2002).

CONCLUSIONS

The specimen AMNH FARB 5477 is here best considered as Daspletosaurus sp. The size, subcircular maxillary fenestra with greater distance from the anterior edge of antorbital fossa, and maxilla thickened and lateral surface well sculptured suggest that AMNH FARB 5477 was a subadult individual in ontogenetic Stage 3 of Carr (1999). The locality of the subadult specimen also suggests that this Daspletosaurus species nested southernmost than Albertosaurus.

ACKNOWLEDGEMENTS

I would like to thank very much Mark Norell and Carl Mehling (Americam Museum of Natural History), Bill Simpson (Field Museum of Natural History), Scott Willians (Buerpee Museum of Natural History) and David Bohaska (National Museum of Natural History) for allowing the access to Paleontological Collection. I thanks to my advisor Hussam Zaher (Museu de Zoologia da Universidade de São Paulo, São Paulo) for providing important comments and guidance in my research and I thanks to Wellton Araújo Pinto (Museu de Zoologia da Universidade de São Paulo, São Paulo) for reviewing the English language. This study was funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo (Processo: 2012/09370-2).

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• 1
  Editor Responsável: Marcelo Duarte

Publication Dates

• Publication in this collection
  2017

History

• Received
  28 Nov 2016
• Accepted
  16 Mar 2017