Tyrannosaurus rex, Osborn, 1905

Michael D. Henderson & William H. Harrison, 2008, Taphonomy and environment of deposition of a juvenile tyrannosaurid skeleton from the Hell Creek Formation (latest Maastridhtian) of southeastern Montana, Tyrannosaurus rex, the tyrant king, lndiana University Press, pp. 82-90 : 5-11

Extensive outcrops of the Hell Creek Formation (uppermost Maastrichtian) occur in eastern Montana. The formation is widely regarded as having been deposited in a lowland fluviolacustrine svstem; these sections consist of a stacked series of fining-upward sedimentary sequences formed by channels meandering across a lowland floodplain (Kirk Johnson, personal communication). The soft texture of the Hell Creek strata, combined with the sporadic rainfall in the northern Great Plains, has resulted in the development of extensive badlands throughout its outcrop area. These badlands contain an abundant and diverse fossil flora and fauna, including dinosaurs.

In 2001, an expedition from the Burpee Museum of Natural History, with permission from the Bureau of Land Management (BLM) to survey for vertebrate fossils, discovered several foot and lower limb elements of a theropod dinosaur weathering from an exposure of the Hell Creek Formation in Carter County, MT. After initial evaluation, the site was winterized and an application made to the BLM to open a quarry the following year. In the summer of 2002, a field crew returned to excavate the specimen. During the course of the excavation, a sizable quarry was created, which yielded major portions of the skeleton of a tyrannosaur (BMR P2002.4.1), nicknamed Jane, approximately 7 m in length (Fig. 6.1). In addition to Jane, many associated plant, invertebrate, and vertebrate fossils were recovered from the quarry.

Lack of fusion of the neural arches to their respective centra in the vertebral column indicates that Jane is a juvenile animal. Examination of histological sections from a rib, fibula, and metatarsal indicates that at death, Jane was 11 years old and still in a phase of rapid growth (G. Erickson, personal communication 2003). Recovered skull bones and teeth of Jane ( BMR P2002.4.1 ) bear a close resemblance to those of CMNH 7541 , a controversial tyrannosaurid skull that has been interpreted as belonging to either a juvenile Tyrannosaurus rex ( Carr 1999; Carr and Williamson 2004) or a separate taxon, Nanotyrannus lancensis ( Bakker et al. 1988; Currie 2003a, 20003b; Currie et al. 2003). Research is currently ongoing into the systematic position of both specimens.

In spite of many taxa of dinosaurs known from the Hell Creek Formation, most finds consist of isolated bones ( Pearson et al. 2002). The discovery of a substantial portion of a juvenile tvrannosaurid skeleton in the formation is so unusual that the circumstances of its burial and preservation merit careful analysis so that like environments can be explored.

Stratigraphy

Extensive exposures of the Hell Creek Formation occur in southeastern Montana. Widely regarded as a prograding, clastic wedge associated with the retreat of the Western Interior Sea, the Hell Creek primarily consists of poorly cemented channel and crevasse splav sandstones, overbank mudstones and siltstones, paleosols, carbonaceous claystones, and thin and sparse lignite beds deposited during the last years of the Cretaceous Period ( Murphy et al. 2002; Johnson this volume).

Near its type section in Garfield County, north-central Montana, the Hell Creek Formation is 170 m thick ( Johnson et al. 2002). Its thickness in southeastern Montana is estimated to be 150 m; however, no complete sections of the formation are exposed. The nearest complete sections are in southwestern North Dakota (about 100 km from the quarry), where the formation is approximately 100 m thick ( Murphy et al. 2002). Unfortunately, no identifiable marker beds occur in the Hell Creek, with the exception of the top and bottom contacts. Within the formation, there is little lateral continuity of beds and bentonitic surface weathering obscures bedding ( Johnson et al. 2002). Consequently, the exact stratigraphic placement of Jane within the formation is complicated. Pollen and plant megafossils from the Jane Quarry correlate with a stratigraphic level in southwestern North Dakota that is 28 to 35 m below the top of the formation.

The Jane Quarry is located on the northern side of an elongate eastwest-trending ridge in northwestern Carter County, MT (Fig. 6.2); the exact locality is available from us. The tyrannosaur was discovered weathering out near the base of the butte. The Jane Quarry section exposes a fining-upward sequence of clastic sediments approximately 8 m thick (Fig. 6.3).

At the base of the section is massive, poorly cemented, dirty, tanbrown, crossbedded sandstone. Its total thickness is unknown because it is incompletely exposed. Abundant wood and coniferous and deciduous leaf impressions are present on or near the upper surface of the sandstone. Deciduous leaves recovered from this unit are identified as belonging to Dryophyllum subfalcatum and “ Vitis ” stantoni. The only vertebrate fossil encountered in the unit was a single midseries cervical vertebrae of a large azhdarchid pterosaur (Henderson and Peterson in press).

A clay-ball conglomerate composed of poorly sorted sand, silt, and rounded greenish-colored clay clasts overlies the sandstone (Fig. 6.4). This unit is lenticular, showing rapid lateral variation in thickness (12-40 cm). The tyrannosaur skeleton was discovered in the lower part of this conglomerate, at its contact with the underlying sandstone. Diagenetically produced siderite nodules occur within the conglomerate and partly encased several bones of BMR P2002.4.1 . Plant fossils recovered from this unit include wood and bark impressions, conifer cones and needles, and numerous small, round to oval seeds, preserved as internal casts. In addition, an abundant, diverse, and well-preserved palynoflora occurs in siderite nodules and clay balls within the conglomerate. To date, 51 genera of pollen, spores, and cysts have been recovered. These indicate the presence of a diverse flora of flowering plants, conifers, ferns, cycads, and palms. Pollen of Gunnera, a herbaceous plant, is especially common. Recovered pollen and spores are typical of the Aquilapollenites palynofloral province, which is found in Upper Cretaceous rocks from western North America westward into northeastern China. Invertebrates are represented by poorly preserved internal casts of unionid bivalves (2 species) and high-spired gastropods (1 species). Remains of vertebrates (dinosaurs, lizards, freshwater fish, crocodilians, champsosaurs, turtles) are common and represented by disarticulated skeletal elements randomly distributed within the unit. Preservation of these bones and teeth range from pristine to significantly worn. A 2- to 4-cm-thick layer of siderite caps the conglomerate.

Above the siderite cap is a siltstone. The basal 30 to 35 cm of the siltstone is finely laminated and contains extremely abundant fossils of aquatic monocots, principally a kind of water lettuce, Pistia corrugata , many preserved as whole plants (Fig. 6.5). A second 2- to 4-cm stratum of siderite caps the Pistiabearing layers. Higher in the unit, sporadic sandstone lenses (up to 1.5 m thick) occur. Crossbedding within the sand units indicates water flow from south to north. The top 0.5 m of the quarry is sandy siltstone, which is not laminated, and contains vertically oriented root casts.

Environment of Deposition

The Jane Quarry section appears to be a typical floodplain sequence. We interpret the massive, crossbedded sandstone on which the tvrannosaur lay as a point bar sand. The clay-ball conglomerate that contained the juvenile tyrannosaur records a mudflow from a flood event, or a bank collapse. Above Jane, the laminated siltstone containing reeds and Pistia indicate a stream avulsion and the subsequent development of an oxbow lake on the site, while sandstone lenses higher in the section are thought to have been produced by underwater dunes migrating through the lake during times of high water when the abandoned channel was temporarily reconnected to the river system. Vertically oriented root casts present at the top of the quarry indicate that the lake eventually silted up and vegetation was established.

Fossils of plants and animals associated with Jane ( BMR P2002.4.1 ) correspond closely with those collected in association with a Tyrannosaurus rex (Peek’s Rex) from the upper Hell Creek Formation of McCone County, MT (Derstler and Myers this volume), and another T. rex (known as Scottv) from the contemporaneous Frenchman Formation of southwestern Saskatchewan, Canada (Tokarvk and Bryant 2004). The environment ofdeposition ofall these specimens indicates burial took place on a warm, wet, lowland floodplain.

Taphonomy

All tyrannosaur skeletal elements recovered from the Jane Quarry are consistent with derivation from a single individual. The 145 bones, representing approximately 52% of the skeleton, were collected from a 4 m2 area (Fig. 6.6). The tyrannosaur lay on its right side when buried. Portions of the right foot remained in articulation. Skull bones were disarticulated but concentrated in a limited area over the hips. A segment of 16 proximal caudal vertebrae, with their associated hemal arches, was found arcing over the back. This distribution of bones indicates that after death, shrinkage of muscles and ligaments along the vertebral column contorted the carcass into the classic dinosaur-avian death pose. Loose teeth from the right dentary were found north of it in a pattern consistent with movement by water or mud from south to north. Many of Jane’s ribs and presacral vertebrae were scattered or missing. This could be the result of movement by water or mud, bloating then bursting of the carcass, or scavenging of the carcass. The right humerus was found about a meter from the main bone concentration, upstream from inferred paleocurrent direction and in direct contact with a shed tyrannosaur tooth. The context suggests scavenging, but the completeness of the skeleton and concentration of bones indicate scavenging was not extensive. No tooth marks were observed on preserved bones.

The bone preservation of Jane is excellent, with no signs of postmortem weathering. This indicates that the skeleton was not exposed on the point bar for an extended period of time. Rapid burial of Jane’s skeleton by a mudflow appears to be the key event responsible for its completeness. The finely laminated siltstones of the oxbow lake deposited on top of the remains provided further protection from disturbance.

Summary

The sequence of events leading to burial may be summarized as follows:

1.

A juvenile tyrannosaur died. At the time of death, or shortlv thereafter, its body came to rest on the point bar of a channel on a forested, lowland floodplain.

2.

Shrinking muscles and ligaments contorted the tvrannosaur's corpse into the classic dinosaur-avian death pose concurrent with, or soon after, minor scavenging of the corpse occurred.

3 As decay proceeded, disarticulation of the skeleton reached an advanced stage. Ligaments remaining along the hips, base of the tail, and feet kept these elements in place.

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4. Burial was accomplished by a viscous mudflow composed of poorly sorted sand, silt, and clay balls. Entrained in these sediments were pieces of wood, seeds, leaves, and a variety of vertebrate bones and teeth. Possibly, burial was related to a cutbank failure triggered by a flood event. Some skeletal elements were moved by the mudflow.

5. After deposition, diagenetic siderite nodules formed around some of the dinosaur’s bones and in the conglomerate.

6. The meander channel was abandoned and became a deep oxbow lake. Under quiet water conditions, aquatic plants flourished and laminated silts were deposited.