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Tyrannosaurid Paleobiology

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The opening of an exhibit focused on "Jane," a beautifully preserved tyrannosaur collected by the Burpee Museum of Natural History, was the occasion for an international symposium on tyrannosaur paleobiology. This volume, drawn from the symposium, includes studies of the tyrannosaurids Chingkankousaurus fragilis and "Sir William" and the generic status of Nanotyrannus; theropod teeth, pedal proportions, brain size, and craniocervical function; soft tissue reconstruction, including that of "Jane"; paleopathology and tyrannosaurid claws; dating the "Jane" site; and tyrannosaur feeding and hunting strategies. Tyrannosaurid Paleobiology highlights the far ranging and vital state of current tyrannosaurid dinosaur research and discovery.

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1 Phylogenetic Revision of Chingkankousaurus fragilis, a Forgotten Tyrannosauroid from the Late Cretaceous of China

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Stephen L. Brusatte, David W. E. Hone, and Xu Xing

1.1. Photographs and line drawings of the holotype of Chingkankousaurus fragilis Young, 1958 (IVPP V 836, right scapula). A) Photograph in lateral view (dorsal to top). B) Photograph in medial view (dorsal to bottom). C) Line drawing in medial view (dorsal to bottom). D) Cross sections from the three indicated areas (lateral to top). Abbreviations: mr, medial ridge; rug, ruosities on posterior expansion of blade. Top scale bar equals 10 cm; bottom scale bar (for cross sections) equals 2 cm.

Recent discoveries, especially the feathered theropods of the Jehol Biota, have placed China at the forefront of contemporary dinosaur research (e.g., Chen et al. 1998; Xu et al. 2003; Norell and Xu 2005; Xu and Norell 2006). However, vertebrate paleontology has a long history in China, and the country’s rich dinosaur fossil record has been explored for over a century. Much of the pioneering work on China’s dinosaurs was led by C. C. Young (Yang Zhongjian), the “father of Chinese vertebrate paleontology.” For over 40 years, from the early 1930s until his death in 1979, Young spearheaded expeditions across China and discovered many of the country’s most recognizable dinosaurs, such as the colossal sauropod Mamenchisaurus and the prosauropods Lufengosaurus and Yunnanosaurus (Dong 1992).

 

2 The Case for Nanotyrannus

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Peter Larson

2.1. Holotype of Nanotyrannus lancensis, CMNH 7541.

The genus Nanotyrannus was erected in 1988 by Bakker, Williams, and Currie, redescribing a skull (CMNH 7541) from the Maastrichtian (Lancian) Hell Creek Formation of Montana, first described as Gorgosaurus lancensis by Gilmore (1946). In part due to the absence of additional specimens, the validity of Nanotyrannus came under question by various researchers, culminating in 1999 when Carr assigned the specimen to Tyrannosaurus rex. Carr presented a compelling argument that CMNH 7541 was a juvenile and that characters separating Nanotyrannus from Tyrannosaurus were ontogenetic.

In 2001 a second specimen was located that compared very well with the type of Nanotyrannus. This new specimen (BMR P2002.4.1), nicknamed “Jane,” consists of a beautifully preserved partial skull and skeleton. Although some researchers are convinced that BMR P2002.4.1 confirms Carr’s juvenile Tyrannosaurus rex hypothesis, this paper questions that conclusion.

 

3 Preliminary Analysis of a Sub-adult Tyrannosaurid Skeleton from the Judith River Formation of Petroleum County, Montana

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3.1. Site location map for RMDRC 2002.MT-001. The specimen was discovered just north of the Petroleum and Musselshell county line, approximately 15 miles northwest of the town of Melstone, Montana.

In 2002 we discovered an enigmatic theropod skeleton approximately 15 miles northwest of the town of Melstone, Montana, along the Petroleum/Musselshell county line. Based upon the size, robustness, and interpreted stratigraphic position (the lower third of the Hell Creek Formation) of the exposed elements, the skeleton, at the time, was thought to be a sub-adult Tyrannosaurus rex. Recent, detailed geologic mapping in the area, however, places this site within the lower third of the Judith River Formation, and analysis of the recovered skeletal elements leave more questions than answers. Study continues to present.

To date, approximately 20–25 percent of the skeleton has been recovered, including approximately 20–30 percent of the skull. Major elements of the skeleton consist of the left femur, both ischia, several cervical and dorsal vertebrae, thoracic and cervical ribs, and many gastralia. Important skull elements recovered include both dentaries, the right squamosal, left lachrymal, left postorbital, left ectopterygoid, left pterygoid, left quadratojugal, and a portion of the right jugal. The elements were completely disarticulated, poorly preserved, and encased in sideritic ironstone concretions, and they show strong evidence for both pre-depositional weathering and possible dispersal from scavenging or predation (tooth marks and shed tyrannosaur teeth).

 

4 Internal Structure of Tooth Serrations

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4.1. a) Earliest known mention of a relationship between sharpness of a blade and the transverse curvature of the cutting edge. As seen in cross section, the sharp edge comes to a point, while the dull edge ends in a rounded curve. b) Earliest known mention of the individual cutting action of serrations

a) From Abler (1973:9);

b) From Abler (1973:23).

Serrations on the teeth of vertebrates are functional, but they have only a few characteristic external shapes that are only a partial aid to identification. Internal structure of serrations can be both functional and characteristic. Thus, serrations easily differentiated on the basis of internal structure include those of a phytosaur (with internal peak), Dimetrodon (with rounded interior), Troodon (with radiating interior tubules), and Albertosaurus (with inter-serrational loop). A physical model is described here as an aid to understanding internal structure of serrations that include an inter-serrational loop (ampulla) as protection against breaking under pressure. Serrations of Tyrannosaurus rex will be considered.

 

5 Feet of the Fierce (and Not So Fierce): Pedal Proportions in Large Theropods, Other Non-avian Dinosaurs, and Large Ground Birds

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5.1. Tridactyl footprints attributed to large theropod dinosaurs; all except 5.1C are natural or artificial casts. A) YPM 2098, ichnogenus Eubrontes; digit II impression on left. B) UCM (formerly CU-MWC) 188.25 (ichnogenus Megalosauripus), Summerville Formation, Carrizo Mountains, Arizona; digit II impression on right. C) Right footprint (ichnogenus Buckeburgichnus or Megalosauropus), Enciso Group, Los Cayos, Spain. D) Cast of footprint from the Glen Rose Formation, bed of the Paluxy River, Somervell County, Texas; digit II impression on right. E) TMP 81.34.1, probable tyrannosaurid footprint, Dinosaur Provincial Park, Canada; digit II impression probably on right; McCrea et al. (2005), however, identified this as a large ornithopod print. F) MPC-D 100F/12, probable tyrannosaurid footprint, likely made by Tarbosaurus. One of the digit impressions is missing, making interpretation of the print problematic. Currie et al. (2003) identified it as a left footprint (and so the digit II impression would be on the right), inferring a greater interdigital angle between digits III and IV than between II and III. However, in many large theropod footprints the tip of digit III is directed medially; if this was true here, the print would be a right footprint, and it would be digit II, rather than IV, that is missing. G) CU-MWC 225.1, ichnogenus Tyrannosauripus, likely made by Tyrannosaurus, Raton Formation, New Mexico, digit II on left; also note likely impression of digit I behind digit II. H) Natural casts of possible Tyrannosaurus prints, Laramie Formation, Golden, Colorado. Possible trackway sequence indicated by prints 1 and 2 (direction of motion toward the top of the page), with the identifying numbers adjacent to the digit III impression; if this interpretation is correct, print 1 would likely be a right footprint, with digit II on the right.

 

6 Relative Size of Brain and Cerebrum in Tyrannosaurid Dinosaurs: An Analysis Using Brain-Endocast Quantitative Relationships in Extant Alligators

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Grant R. Hurlburt, Ryan C. Ridgely, and Lawrence M. Witmer

6.1. Left lateral view of endocranial cast of wild Alligator mississippiensis. Skull length 34.3 cm, estimated body length, 2.6 m. Scale bar equals 1 cm.

Brain and cerebrum mass are estimated from endocasts of three tyrannosaurid taxa (Tyrannosaurus rex, Gorgosaurus, and Nanotyrannus) using morphological and quantitative brain-endocast relations in a size series of sexually mature alligators (Alligator mississippiensis). The alligator size series (N = 12) ranged from the smallest sexually mature size to the largest size commonly encountered. Alligator brain mass (MBr) increased regularly with increasing body mass, while the ratio of brain mass to endocast volume (MBr:EV) declined regularly from 67 percent to 32 percent. The ratio of cerebrum mass to cerebrocast was 38 percent in the largest alligators and regularly exceeded the MBr:EV ratio by 5.6 percent. For estimates from endocasts of non-avian dinosaurs of unknown sex, a MBr:EV ratio of 37 percent was used, the mean of the ratio of the largest male and female alligators. A corresponding 42 percent ratio was used for the cerebrum-cerebrocast ratio.

 

7 Jane, in the Flesh: The State of Life-Reconstruction in Paleoart

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Tyler Keillor

7.1. Tyler Keillor’s flesh model of Jane.

My goal in creating a flesh reconstruction of an extinct animal is to provide the museum visitor with a sense of what the real live animal was all about. I don’t want to give the exhibit viewer a cliché, a toy, a Hollywood prop, or something that’s been seen in every kid’s dinosaur book. I want the observer to see a restoration that is unique, that shows a creature, frozen in time, that endured various life processes, and that might challenge preconceived notions about the animal and elicit questions or thought. A reconstruction requires not just artistry and imagination but also the input of the latest scientific opinions and comparative observations of extant animals. A life reconstruction is, by nature, highly speculative, and being so is of less value scientifically than artistically (as an exhibit piece for the layperson). Nevertheless, a rigorously executed reconstruction may, through its very creation, yield new insight into paleontological questions and so can be a working model and an aid to scientific understanding. I’ll let the task of bringing the Burpee Museum’s juvenile tyrannosaur “Jane”(BMR P2002.4.1) back to “life” provide a glimpse into the behind-the-scenes aspects of paleoart (the depiction of ancient beasts; see Fig. 7.1). In this reconstruction, in particular, observations of extant reptiles yielded new insights into the external appearance of Jane’s oral margin.

 

8 A Comparative Analysis of Reconstructed Jaw Musculature and Mechanics of Some Large Theropods

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Ralph E. Molnar

8.1. Illustration of the methodology for estimating the lever arm and muscle extension of the jaw muscles of Tyrannosaurus rex, using the M. adductor mandibulae externus superficialis et medialis as an example. A) First the areas of origin (dashed) and insertion of the muscles are determined (or postulated). Then a “center” of each area is estimated. This “center” is chosen subjectively, taking into account the angle of the surface with respect to the plane of the projection, the relative proportion of fibers originating from this area, and so forth. In this example, the “center” (indicated by “x”) is set in the middle of the upper lobe of the infratemporal fenestra. The apparently large area of origin of the squamosal-quadratojugal flange probably contributed relatively few fibers, as the orientation of the muscle was parallel to the surface of the flange. However, the area of origin along the dorsal margin of the infratemporal fenestra is only small in projection since it is situated nearly perpendicular to the plane of projection. Hence, this area presumably contributed more fibers to the muscle than the squamosal-quadratojugal flange, although that area appears to be the greater. B) After the “centers” have been chosen, a line is constructed from the origin “center” to the insertion “center.” The length of this line is taken as a measure of the length of the muscle fibers. The perpendicular distance from this line (OI) to the center of rotation of the quadrate condyles (r) is the lever arm for this muscle at this gape. The measurements were made for angles of 0°–50° of gape, with zero being taken as that gape for which the tips of several of the dentary teeth reach the ventral margin of the maxilla. C) The same for a gape of 10°. D) The same for a gape of 20°. E) Graph of extension vs. gape. The extension plotted here is the extension for zero gape subtracted from the extension for a given gape, so that the extension for zero gape is zero. This graph therefore represents the relation of gape to the length of the muscle extended beyond its (presumed rest) length with the mouth closed. The units of the abscissa are centimeters times 0.2: the reason for this unconventional unit is that the measurements were made on a one-fifth scale projection of the skull. The points on this graph labeled B, C, and D are likewise derived from the extensions illustrated in parts (B), (C), and (D), respectively. F) Graph of the lever arm versus gape. This graph is constructed from the lengths of the lever arms as shown in (B), (C), and (D). The abscissa represents the lever arm, and the ordinate the gape. The units of the axes of this graph are the same as those of the previous graph. The points on the graph labeled B, C, and D are derived from the lever arms illustrated in parts (B), (C), and (D), respectively.

 

9 Tyrannosaurid Craniocervical Mobility: A Preliminary Qualitative Assessment

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Tanya Samman

9.1. Cervical vertebra of an indeterminate tyrannosaur (TMP 2002.12.02), estimated to be C3 or C4. A) Anterior view. B) Left ventrolateral view. Abbreviations: ctm, centrum; diap, diapophysis; ep, epipophysis; ft, foramen transversarium; nc, neural canal; ns; neural spine; parp, parapophysis; poz, postzygapophysis; prz, prezygapophysis. The diapophysis articulates with the tuberculum of the cervical rib (not figured), and the parapophysis with the capitulum. Scale in cm.

Tyrannosaurs were dynamic predators, and the analysis of craniocervical mobility has implications for the biomechanics of their foraging and feeding. The cervical vertebrae of tyrannosaurids are anteroposteriorly shorter than those of many other coelurosaurs, as well as some extant birds, and the neck is correspondingly less flexible. Variation of vertebral shape along the vertebral column results in differences of mobility, with the posterior portion of the neck being much less flexible than the anterior. The software package DinoMorph™ was used, in collaboration with Dr. Kent Stevens from the Department of Computer and Information Science, University of Oregon, to digitally model tyrannosaur vertebrae as complex three-dimensional surfaces. The digital model was manipulated into various flexion poses. The “neutral pose” of theropods requires further study, and the Tyrannosaurus rex model needs refinement before any quantitative interpretations can be made. Soft-tissue data obtained from the study of birds helps to constrain the range-of-motion limits. Evidence from “death-pose” specimens helps to establish limits of dorsiflexion and suggests intergeneric mobility differences. Assessing the neutral pose, maximum dorsiflexion, ventriflexion, and lateral flexion gives insight into the biomechanical controls that influence the behavior and ecology of these animals.

 

10 Clawing Their Way to the Top: Tyrannosaurid Pathology and Lifestyle

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Bruce M. Rothschild

10.1. Lateral view of maxillary grooves (A) in “Scotty” (RSM P2523.8). Tooth in position (B) shows apparent manner of attack.

Facial scars in tyrannosaurids have been attributed to intraspecific biting behavior. Remodeled bone surrounding the lesions document survival of these attacks/interactions. While that is a reasonable hypothesis, examination of recently discovered specimens suggests an alternative explanation. The “Jane” and “Peck” Tyrannosaurus rex specimens have substantial evidence of trauma by sharp objects. However, the width and breadth of noted lesions match neither Tyrannosaurus tooth size nor those of other tyrannosaurids. The right side of the surangular of the Peck T. rex provides insight. It has a penetrating hole that aligns with a smaller one in the subjacent clinal. The holes are much too large for a tyrannosaurid tooth but did accommodate a T. rex toe claw. The potential of claws to produce bone damage has not been previously considered because of the hardness differential. Given the lack of correlation of some bone damage with tooth parameters and demonstration that claws could penetrate bone, claw damage remains the residual hypothesis.

 

11 Brodie Abscess Involving a Tyrannosaur Phalanx: Imaging and Implications

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Christopher P. Vittore, MD, and Michael D. Henderson

11.1. Left pedal digit II phalanx I. Side (left) and dorsal (right) views show an eccentric osseous protuberance (arrows).

Osteomyelitis is rarely found in dinosaur fossils. When it is identified, the bone lesion shows characteristic, chronic changes of disorganized osseous overgrowth resulting in distortion of the bone. We present a focal bone lesion compatible with osteomyelitis involving a tyrannosaur phalanx. Computed tomographic (CT) imaging disclosed typical findings of a type of subacute osteomyelitis known as a Brodie abscess. This has not been reported in a dinosaur previously. CT scanning of recovered bone fossils is advised for further assessment, particularly if there is any visible surface anomaly. Osteomyelitis, Brodie abscess, and the potential impact on an animal with this lesion are discussed.

During the summer of 2002, field crews from the Burpee Museum of Natural History collected the partially articulated skeleton of a juvenile tyrannosaurid, nicknamed “Jane” (BMR P2002.4.1), approximately 60 m above the base of the Hell Creek Formation (latest Maastrichtian) of Carter County, Montana. The collection site exposes a fining-upward sequence of clastic sediments that record an active channel and the subsequent formation of an oxbow lake. One hundred forty-five skeletal elements representing about 52 percent of the skeleton (by bone count) of an approximately 7 m long juvenile tyrannosaurid were recovered near the base of this sequence. Recovered skeletal elements show excellent preservation. The dinosaur lay on its right side on top of a point of bar sand. Sixteen proximal caudal vertebrae arced over the back, while the neck was pulled back with the skull positioned over the hips. This posture, the common avian/dinosaur “death pose,” is most likely a result of perimortem muscle contractions (Faux and Padian 2007). The skull, pectoral girdle, ribs, presacral vertebrae and distal caudal vertebrae were disarticulated. However, these elements were generally found near their life positions. Although the cause of death is unknown, it was apparently not a result of predation or agonistic interaction given the completeness of the remains and their relatively undisturbed nature. Burial took place after decomposition was advanced, and the animal largely skeletonized, as evidenced by the extensive disarticulation. Sediments surrounding the skeleton (a clay ball conglomerate) are consistent with burial during a flood event, which probably occurred several weeks to months after the death of the animal.

 

12 Using Pollen, Leaves, and Paleomagnetism to Date a Juvenile Tyrannosaurid in Upper Cretaceous Rock

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William F. Harrison, †Douglas J. Nichols, Michael D. Henderson, and Reed P. Scherer

12.1. Restored skeleton of juvenile tyrannosaurid Jane in Burpee Museum of Natural History, Rockford, Illinois.

The juvenile tryrannosaurid from the Hell Creek Formation (Upper Cretaceous: Maastrichtian) in southeastern Montana, informally named “Jane” (BMR P2002.4.1), is determined to be from a zone in the formation that dates to about 66 Ma. The stratigraphic position of the Jane site is established on the basis of palynology and paleobotany by comparison with correlative sections in southwestern North Dakota and is supported by paleomagnetic data. The palynological and paleobotanical data tightly constrain the age and stratigraphic position of this unique fossil.

In June 2001, an expedition from the Burpee Museum of Natural History in Rockford, Illinois, discovered the skeleton of a juvenile tryrannosaurid (BMR P2002.4.1; see Fig. 12.1), approximately 7 m in length, in the Hell Creek Formation (Upper Cretaceous: Maastrichtian) in northwestern Carter County, southeastern Montana (45°46′N, 104°56′W; see Fig. 12.2). The specimen, nicknamed “Jane,” was initially identified as either a young Tyrannosaurus rex (Carr 2005; Henderson 2005; Parrish et al. 2005) or a Nanotyrannus lancensis (Larson 2005), which was known from a single skull found earlier in the same Montana county.

 

13 The Biomechanics of a Plausible Hunting Strategy for Tyrannosaurus rex

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David A. Krauss and †John M. Robinson

13.1. Proposed model of a Tyrannosaurus–Triceratops collision. The Triceratops is represented as a rectangular mass, while the Tyrannosaurus applies rotational momentum in order to pivot it about the rotational axis established with the feet on the opposite side and tip it over. In this diagram, FT is the force applied by the Tyrannosaurus to point A (or point A1 as an alternate impact point with a larger Tyrannosaurus and/or a smaller Triceratops), and FG is the force of gravity; B and C represent two points at which the rear feet touch the ground, establishing a reference plane for the calculations described in the text.

We present here a biomechanical analysis of a hunting strategy that Tyrannosaurus rex could have employed effectively. The modern analogy for this hunting strategy is “cow tipping,” in which reckless people ambush and tip cows over. Although this analogy seems odd, it is apt. Anatomical analysis of Triceratops indicates that, like a cow, if it were knocked over on its side it would have experienced difficulty in getting up. It seems likely that tyrannosaurs could have exploited this weakness in a hunting strategy.

 

14 A Closer Look at the Hypothesis of Scavenging versus Predation by Tyrannosaurus rex

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Kenneth Carpenter

14.1. Skeleton of Tyrannosaurus rex, represented by “Stan.”

Photograph courtesy of the Black Hills Institute of Geological Research.

Controversy surrounds the feeding behavior of the large theropod Tyrannosaurus: Was it an obligate scavenger, a predator, or an opportunist that scavenged as well as hunted? Evidence for an obligate scavenging lifestyle is examined: Enlarged olfactory lobes, allegedly for carrion detection, are shown to also occur in extant non-scavenging predators and in other dinosaurs. Eyesight may have been poor in low light but otherwise acute. Hindlimb segment lengths are significantly greater than potential prey, thus Tyrannosaurus could outrun prey. Despite the proportionally short arms, various stress fractures of the furcula show that the forelimbs generated great stresses, as would be expected with holding struggling prey. Finally, evidence of failed attacks on prey demonstrates conclusively that Tyrannosaurus was a predator.

 

15 New Evidence for Predation by a Large Tyrannosaurid

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Nate L. Murphy, Kenneth Carpenter, and David Trexler

15.1. Partial pelvic and caudal regions of Brachylophosaurus cf. canadensis, JRF 1002, showing evidence of a failed attack by a predator, possibly Daspletosaurus. Arrow denotes the neural spine that was bitten off.

A partial skeleton of a hadrosaur believed to be Brachylophosaurus canadensis shows evidence of a failed attack by a large theropod, possibly Daspletosaurus sp. The injury consists of damage to the neural spines of the last sacral vertebra and first two caudal vertebrae. Remodeled bone, even at the site of the traumatic amputation of the caudal neural spine, demonstrates that the individual survived the attack. In addition, the attack came from the rear, suggesting that the Brachylophosaurus was fleeing the attacker. This specimen adds to the growing body of knowledge that tyrannosaurids were capable of active predation.

The relative importance of predation and scavenging by tyrannosaurid theropods has been controversial. Horner and Lessem (1993) have argued that Tyrannosaurus was an obligate scavenger because of the large body size, short arms, large eyes, and large olfactory lobes. However, Carpenter (2000) described a failed attack on an adult hadrosaur, Edmontosaurus. Carpenter argued from morphological features that the bite was most parsimoniously ascribed to Tyrannosaurus. The features of Tyrannosaurus that Horner and Lessem (1993) cited as evidence for scavenging apply equally to other tyrannosaurids as well. By inference then, the other tyrannosaurids (e.g., Gorgosaurus, Daspletosaurus and Albertosaurus) were scavengers in the absence of evidence to the contrary. That contrary evidence is now slowly emerging. Wegweiser et al. (2004) report evidence of an attack on a lambeosaurine hadrosaur by an unidentified tyrannosaurid. That specimen consists of a partially healed rib bearing the impression of a tooth. And now, newly discovered articulated caudal vertebrae of the hadrosaurid Brachylophosaurus cf. canadensis provides evidence that another tyrannosaurid, probably Daspletosaurus, was an active predator as well.

 

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