Tyrannosaurus rex Osborn 1905

This article develops a functional model for the arctometatarsus that incorporates inferences of soft tissues. Our format follows the logical progression involved in developing and testing a complex hypothesis of locomotion: 1) We first introduce aspects of bone and connective organ function in large terrestrial vertebrates that suggest hypotheses of arctometatarsus morphology and action. 2) Our investigations and results test these proposals and lead inductively to a model of overall arctometatarsus function. 3) Because the model is emergent from the results, we present it in a separate section. 4) The subsequent discussion tests the model against modern analogs and addresses implications of the proposed kinematics for tyrannosaurids. The model therefore acts as a prism that focuses multiple lines of evidence into a coherent functional picture and a fulcrum over which morphological detail facilitates discussion of behavior. Our construction and application of the model exemplify the integrated approach necessary for reconstructing locomotion of extinct vertebrates. Arctometatarsus as an ElasticallyDamped System As with foot structures in modern animals, the arctometatarsus’ role in tyrannosaurid locomotion hinged on the specific morphology of its bones and connective tissues. In vertebrates, tendons that join bone to muscle, and ligaments that connect bone to bone, transmit and dissipate locomotor forces acting on the skeleton (Hildebrand, 1988). Tendons and ligaments are variably elastic, depending on their relative proportions of collagen and elastin proteins (Alexander, 1988) and the magnitude and period of imposed loadings (Pollock, 1991). The bones of the arctometatarsus and its unmineralized tissues would have been elastic to some degree, but understanding its precise function depends, in part, on consideration of locomotor elasticity in modern vertebrates. A number of studies have explicated the locomotor role of elastic ligaments and tendons and their interactions with associated bones. Elastic fore- and hindfoot connective elements store, return, and distribute footfall energies and forces. Ligaments of the feet of humans (Kerr et al., 1987; Alexander, 1988; Deane and Davies, 1995) and the wrists of horses (Rubeli, 1925) are noteworthy examples. Ligaments absorb shock under specific physical conditions. Paradoxically, they display greater strength and resiliency when subject to high magnitude, sudden loadings, such as those incurred during rapid locomotion (Frank and Shrive, 1994). In animals of large body size, the extensibility of connective elements increases because their cross sectional area is lower relative to mass than ligaments and tendons of smaller animals (Pollock, 1991). The ligaments of large vertebrates store and return relatively more elastic strain energy, which increases locomotor efficiency and decreases strain energy transmitted to bones (Pollock, 1991). These conditions probably existed in adult tyrannosaurids, which ranged from two (Paul, 1988) to an upper estimate of eight tons (cross scaling of measurements from fragmentary metatarsals: University of California Museum of Paleontology, locality number V91181). These considerations suggest that elastic mechanisms may have operated efficiently within the tyrannosaurid foot. Two contingencies are critical for assessing possible elastic functions: the type of connective tissues associated with the arctometatarsus and the freedom of movement between metatarsals. These factors lead to hypotheses of anatomy and movement necessary for erecting a comprehensive model of tyrannosaurid arctometatarsus function. Prerequisites for Modeling Arctometatarsus Function: Reconstructing Soft Tissues and Intermetatarsal Displacement Soft tissue reconstruction of extinct vertebrates entails comparison with extant relatives, ideally by bracketing the extinct taxon between modern, derived representatives of its clade and a more archaic living sister group (Bryant and Russell, 1992; Witmer, 1995). Although there are no extant arctometatarsalian theropods, unpreserved tissues are still inferable through broader phylogenetic comparison, mechanical considerations, and universal correlates of soft parts that are present on mineralized structures (Bryant and Seymour, 1990; Bryant and Russell, 1992). Soft tissues leave consistent marks on bone, termed osteological correlates (Witmer, 1995), that are evident in both extant and fossil specimens. Ligament or tendon attachments display two primary correlates: rugosity, and rough or smooth faceted areas. Rugosity marks the location of Sharpey’s fibers, mineralized collagen fibers within the bone that are continuous with fibers of the attaching connective element (Woo et al., 1987). Ligaments and tendons also attach through a gradient of fibrocartilage, mineralized fibrocartilage, and bone. These so-called direct insertions occur on bone surfaces that are smooth and slightly concave (Doglo-Saburoff, 1929). Sutured or closely conforming bones invariably indicate ligament attachments, and more widely spaced adjacent facets may indicate tendon attachments (Gray and Goss, 1959). Using these criteria we identified osteological correlates along intermetatarsal articular surfaces of large theropods. Ligaments would indicate passive elasticity and tendons would signify controlled function. Our study tested the following hypotheses: H1: Connective tissue scars on adjacent tyrannosaurid metatarsals indicate tendons. H2: Scar configuration was identical in tyrannosaurids and in the nonarctometatarsalian theropod, Allosaurus fragilis. Freedom of movement between tyrannosaurid or Allosaurus fragilis metatarsals dictated the function of their associated ligaments or muscles. Therefore, this study sought to test hypotheses of possible intermetatarsal displacement. Preliminary observations of tyrannosaurid metatarsals, and the work of Wilson and Currie (1985) and Holtz (1995), suggested this hypothesized pattern of movement: H3: In the tyrannosaurid metatarsus, the distal end of MT III was free to move anteriorly to a small degree about a pivot point at the proximal end. The proximal articulation between metatarsals, and their associated soft tissues, would constrain this potential displacement of MT III. Detailed consideration of both osteological and soft tissue anatomy was therefore necessary to test the hypotheses and come to a more complete understanding of tyrannosauruid arctometatarsus function. Institutional abbreviations. MOR: Museum of the Rockies, Bozeman, Montana. IVPP: Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China. NMC: National Museum of Canada, Ottawa. PJC: Philip J. Currie, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada. TMP: Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta. UCMP: University of California Museum of Paleontology, Berkeley, California. ROM: Royal Ontario Museum, Toronto, Ontario. MATERIALS AND METHODS For comparative purposes, specimens of large and small arctometatarsalian forms were examined at UCMP, MOR, and TMP. Metatarsal specimens of Allosaurus fragilis (MOR 693), and others at MOR and TMP, provided control representatives of the primitive condition for theropods. The specimens (Table 1) were sufficiently complete and well preserved for evaluation of osteological correlate position and/or resolution of possible intermetatarsal movement. Assessment of metatarsus dynamics in tyrannosaurids entailed three related lines of inquiry: 1) We identified and measured osteological correlates of soft tissues in tyrannosaurids and Allosaurus fragilis. The proximity and complexity of joint surfaces tested whether the correlates indicated tendons or ligaments. 2) To ascertain the probable range of motion between elements in physical specimens, we manipulated casts of Tyrannosaurus rex metatarsals. 3) To evaluate intermetatarsal freedom of movement in other tyrannosaurids, we examined com- puted tomographic (CT) images of Albertosaurus sarcophagus and Gorgosaurus libratus metatarsals. Assessment of Osteological Correlates We ascertained the distribution and extent of osteological correlates on theropod metatarsals by identifying probable scars and measuring their areas. Likely correlates of soft tissues were identified on specimens in a satisfactory state of preservation, using the criteria of rugosity and delineated faceting outlined above. Surfaces had to be continuous with cortical bone that had not been taphonomically eroded, to avoid the potential of infilled spongy bone being mistaken for rugosity. Problematic degeneration was not present on the metatarsals chosen for area measurement. We measured the area of the correlates only when their boundaries could be repeatably determined, but because soft tissues are not preserved, our techniques are subject to some inaccuracy. Our results, therefore, reflect the position and relative area of correlates more than their absolute extent. Surface areas of osteological correlates were measured on specimens of Albertosaurus sarcophagus (MOR 657), Allosaurus fragilis (MOR 693), Daspletosaurus torosus (MOR 590), and Tyrannosaurus rex (MOR 555). The bones were wrapped in plastic cling film and attachment surface areas traced with a water-based marking pen. This technique facilitates area measurement of complexly contoured surfaces (Snively, 2000). The cling wrap was removed from the bone, pulled gently taut, and smoothed with a ruler. The markings were then retraced onto white paper and digitized. From these scans we determined the areas of the representations in cm 2 using NIH Object-Image for Macintosh. The average of apparent attachment areas on adjacent bones was used to approximate the cross sectional area of intervening soft tissues. A disarticulated MT IV was not present in MOR 555. To estimate the a

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