Milan Et Al (Theropod Walking)
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Harris et al., eds., 2006, The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History and Science Bulletin 37. 352 THEROPOD FOOT MOVEMENT RECORDED BY LATE TRIASSIC, EARLY JURASSIC AND LATE JURASSIC FOSSIL FOOTPRINTS JESPER MILÀN1, MARCO AVANZINI2, LARS B. CLEMMENSEN3, JOSE CARLOS GARCIÁ-RAMOS4 AND LAURA PIÑUELA4 1Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350, Copenhagen K, Denmark, E-mail: [email protected]; 2Museo Tridentino di Scienze Naturali, Via Calepina 14, I-38100 Trento, Italy, E-mail: [email protected]; 3Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350, Copenhagen K, Denmark, E-mail: [email protected]; 4Museo del Jurásico de Asturias [MUJA], Colunga, Asturias, Spain - Departamento de Geología Universidad de Oviedo, C/Jesús Arias de Velasco, s/n 33005 Oviedo, Asturias, Spain, E-mail: jcgramos.mujagmail.com and [email protected] Abstract—Vertebrate tracks reveal more information than just the shape and anatomy of a track maker’s foot. By studying the deformation of sediment, around and subjacent to tracks, important information about a track maker’s walking kinematics can be obtained. Thirteen theropod tracks spanning the Late Triassic, Early Jurassic and Late Jurassic were examined for deformation induced by foot movements during a stride. The tracks from the Late Triassic and Early Jurassic are preserved as true tracks except one that is preserved as a natural cast. The tracks show two, distinctly different types of deformation: (1) an outward twist of digit III formed during the kick-off, and (2) outward rotation of the proximal parts of the foot, causing the basal part of digit IV to dig down and outward into the sediment. Late Jurassic tracks preserved as deep natural casts show an even, outward deforma- tion of the whole track. All the studied Late Triassic and Early Jurassic tracks and undertracks except one show the same outward deformation, but the tracks from the Late Jurassic are impressed into the sediment in a slightly different way. The base of the track under digits II and IV slopes sharply outward in the Late Jurassic footprints. This seems to demonstrate that different theropods adopted different walking strategies at different times. INTRODUCTION cal pads should contact the substrate with all toes simultaneously, sink vertically, and then extract from the substrate vertically with digit III the A vertebrate track is a complex structure resulting from dynamic last to exit. Such un-deformed, “ideal” footprints have been experimen- contact between a track maker and the sediment upon which it treads. tally obtained by laboratory experiments with model feet, or severed Not only the actual tracking surface (sensu Fornós et al., 2002) is subject animal feet, emplaced in artificial substrates (Allen, 1997; Manning, to deformation, but also the subjacent horizons are deformed as the 2004; Milàn and Bromley, 2006), and they all show a uniform deforma- pressure from the track maker’s foot are transferred radially outward tion in the sediments in and around the tracks. When viewed in vertical into the sediment (Allen, 1997; Gatesy, 2003). In layered sediments, this sections, the undertracks formed along the subjacent horizons in the causes the formation of a stacked succession of undertracks that gradu- artificial substrates are symmetrically developed radiating outward and ally becomes wider, shallower and less detailed downward (Milàn and downward from the tracks (Allen, 1997; Milàn and Bromley, 2006). The Bromley, 2006). herein described footprints all deviate from these “ideal,” undeformed Following a simplified model proposed by Thulborn and Wade tracks in showing various degrees of lateral deformation, caused by foot (1989) and Avanzini (1998), the contact between an animal’s foot and the movements occurring during the time the track maker’s foot is in contact sediment during walk can be divided into three distinct phases. The with the substrate. A similar complex deformation pattern has been re- touch-down phase is when the foot is moved forward and emplaced on ported by Gatesy et al. (1999) and Gatesy (2001, 2003) in deep theropod the sediment surface. This is followed by the weight-bearing phase, footprints from East Greenland. Evidences of foot and toe movement are when the animal’s center of gravity passes over the animal’s foot, which recognizable with high fidelity in well preserved true track surface and, if is consequently pressed into the substrate, forming the track. Last is the the true track is obscured by overlying sediments, sectioning can reveal kick-off phase, when the weight is transferred to the distal parts of the the true contours of the surface (e.g., Loope, 1986). Generally, however, digits as the body moves forward and the foot subsequently is lifted and in the fossil record, theropod dinosaur footprints, being shallow impres- swung toward a new touch-down phase. As evidenced by Gatesy et al. sions of the plantar surface, provide little evidence of the details of limb (1999) and Gatesy (2001, 2003) however, these phases are more com- excursion. For this reason the analysis of the subsurface deformation plex and represent a continuum of interactions between foot and sub- seem to be useful in identifying dinosaur foot movements in shallow and/ strate. In fact, a ground-based reaction force (GRF) perspective (Roberts or eroded footprints. and Scales, 2002) makes it clear that there are no “distinct” phases of the Avanzini (1998) examined dinosaur foot motion by sectioning a stance phase, but a gradual shift from a low magnitude force pointing shallowly impressed Lower Jurassic theropod track vertically, and de- backward and up (decelerating the animal) to a peak magnitude force as it scribed an apparent inward twist of digit II and the proximal part of the becomes more vertical, to a forward and upward force reaccelerating the foot, relative to the midline of the trackway, made during the weight- body. The foot bears weight throughout the contact phase, not just bearing phase of the stride. A similar study of Late Triassic theropod during the middle portion. Similarly, in a dynamically stable walker, the footprints and undertracks in vertical sections revealed an outward de- body can pass lateral or medially to the point of ground contact as it formation of all the studied specimens (Milàn et al., 2004). advances throughout the stance phase. As a footprint is the result of the The aim of this study is to compare and expand on the studies of dynamic contact between the track maker and the substrate, any simul- Avanzini (1998) and Milàn et al. (2004) and describe the lateral deforma- taneous movement of the foot will be captured in the sediment and tion of footprints resulting from walking kinematics as observed in arti- subsequently be recognizable as a zone of disturbance within or around ficial (1-10) and natural (11-13) sectioned theropod footprints from the the track (Brown, 1999). Late Triassic of Jameson Land, East Greenland, Early Jurassic of Lavini To create a perfect, undisturbed footprint, a foot with symmetri- di Marco, northern Italy, and Late Jurassic of Asturias, Spain. Institu- 353 tional abbreviations: MGUH, Geological Museum, University of permost part of the Asturian Jurassic sedimentary succession. It repre- Copenhagen; MTSN, Museo Tridentino Science Naturali; MUJA, Museo sents deltaic sediments that accumulated along the coast of an inland sea del Jurásico de Asturias. separated from the open ocean by a tectonic threshold that served as protection against storms at (García-Ramos and Gutiérrez Claverol, 1995; MATERIAL García-Ramos et al., 2004). The dinosaur footprints are mainly found in Late Triassic Tracks, Jameson Land, East Greenland sand bar deposits; these deposits formed during crevasse splays along distributary channels in the fluvially-dominated deltaic complex. The Five single footprints, all assigned to ichnogenus Grallator Lastres Formation has also yielded a high number of reptile footprints Hitchcock, 1858, collected from the Upper Triassic Fleming Fjord For- including those of pterosaurs, crocodylians, turtles, and lizards (Piñuela, mation, Jameson Land, East Greenland, were studied. The specimens are 2000; Lires, 2000; Garcia Ramos et al., 2002; Avanzini et al., 2005). stored at the Geological Museum, University of Copenhagen. The Up- per Triassic Fleming Fjord Formation consists of a well-exposed 200- TERMINOLOGY 300 meter thick succession of lake deposits. The track-bearing upper The dinosaur that makes an actual footprint is termed the “track part of the formation, the Carlsberg Fjord beds of the Ørsted Dal Mem- maker” and a sediment surface on which a track is emplaced is termed the ber is 80-115 m thick and is composed of structureless red mudstones “tracking surface,” sensu Fornós et al. (2002). The track emplaced on the rhythmically broken by thin, greyish siltstones (Clemmensen et al., 1998). actual tracking surface is termed the “true track,” or just “track,” and The thin siltstone beds represent episodes where the mudflats were several consecutive tracks from the same track maker constitute a “track- flooded by lake water of a depth sufficient to allow formation of small way” (Leonardi, 1987; Lockley, 1991). The sediment infilling the track wave ripples. The flooding was followed by periods of subaerial expo- forms a “natural cast” of the track (Lockley, 1991). If the track maker’s sure and desiccation. The theropod tracks are all found in these siltstone