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 foot sinks to a considerable depth in the sediment, then the vertical parts, beds in association with rare footprints of larger prosauropods (Jenkins between the bottom of the track and the tracking surface are termed the et al., 1994; Clemmensen et al., 1998; Gatesy et al., 1999; Gatesy, 2001). “track wall” (Brown, 1999). The weight of the track maker’s foot is The underside of the slabs containing some of the studied tracks are transferred radially outward into the sediment around and below the covered with numerous small arthropod trackways (Diplichnites) pre- track maker’s foot (Allen, 1997), causing the formation of “undertracks” served in hyporelief, and examples of the arthropod resting trace (sensu Lockley, 1991) along the subjacent horizons. Undertracks differ Rusophycus, indicating the presence of a fluvial and possibly lacustrine from true tracks in being wider, shallower, and less detailed downward environment before the track-bearing layer was deposited (Bromley and (Milàn and Bromley, 2006). Asgaard, 1979). Theropod dinosaurs were functionally tridactyl and their tracks Early Jurassic, Lavini di Marco, Northern Italy consist mainly of the impressions of digits II, III and IV. Digit I, the hallux, is situated at an elevated position on the metatarsus (Christiansen, Three single theropod tracks, two assigned to the ichnogenus 1997), and impressions of digit I mostly occur in tracks emplaced in deep Kayentapus Welles, 1971 and one to the ichnogenus Grallator Hitchcock, mud (Gatesy et al., 1999). The phalangeal skeleton inside the digits 1858, collected from the Lower Jurassic Monte Zugna Formation (Calcari contains 3, 4 and 5 phalanges respectively for digits II, III and IV. The Grigi Group), Lavini di Marco, northern Italy and currently stored in the digits are covered by fleshy digital pads, which in most cases are situated Museo Tridentino Scienze Naturali (Trento, Italy), were studied. The around the phalangeal joints, and these are recognizable as individual pad Calcari Grigi Group comprises shallow water marine deposits and formed impressions in well preserved tracks. The proximal pad impressions of on an extensive carbonate platform (Trento Platform). The lowermost digit IV in theropod tracks extend further back than those of digits III and part of the Monte Zugna Formation is composed of alternating, subtidal II, giving the tracks a typically asymmetric “heel” area. The claw impres- limestones in meter-thick beds and thin intervals of green clay without sion of digit III is, in many cases, directed inward toward the midline of evidence of tidal influence. Peritidal cycles with structures typical of the trackway (Lockley, 1991; Farlow et al., 2000). This is an important tidal deposition characterize the upper part of the formation; an abun- factor in identifying single footprints as either from a right or left foot; dant and diverse dinosaurian ichnofauna was found in the supratidal these factors can be recognizable in even badly eroded tracks. deposits (Leonardi and Mietto, 2000). The subtidal facies, representing the lower two-thirds of the formation, is mainly made up of homoge- METHODS neous micritic, peloidal-fossiliferous limestones completely reworked Eight of the tracks used for this study were sectioned vertically, by marine bioturbation. Sometimes accumulations of large bivalves (i.e., perpendicular to the length axis of the impression of digit III, using a Gervillia buchii) are present. The supratidal facies are characterized by stationary rock saw with a 3 mm-wide width. The surface of each slice sedimentary structures such as stromatolites, birdseyes, sheet cracks, was subsequently polished to enhance the view of the internal structures mud cracks, teepees, flat pebble breccias, etc. (Masetti, 2000). The algal of the sections. The sections used for the study were subsequently and palynological association of the Monte Zugna Formation suggests a scanned directly on a flatbed scanner at 600 dpi. To protect the glass Hettangian-Early Sinemurian age. The ichnotaxa identified are Grallator, surface of the scanner, a sheet of transparent plastic was placed between Eubrontes and Kayentapus, all pertaining to theropod dinosaurs (Piubelli the rock slice and the glass, and ethanol was used as a contact medium et al., 2005), and Anomoepus, probably made by primitive ornithopods between the rock and the plastic sheet. Digital color and contrast-en- (Avanzini et al., 2001a). There are several tracks that are attributed to hancing was subsequently performed using Corel Photo-paint 11. Other very early sauropods (i.e., ichnogenera Parabrontopodus and Lavinipes) sections were polished and accurate interpretative drawings made. (Avanzini et al., 2004) and some poorly preserved tracks attributed to prosauropods and possibly thyreophoreans (Leonardi and Mietto, 2000; DESCRIPTION OF TRACKS Avanzini et al., 2001b). Triassic Tracks Late Jurassic of Asturias, Northern Spain Track 1 Three single theropod footprints collected from the Upper Juras- sic Lastres Formation, Asturias, northern Spain and currently stored in This theropod track is relatively small, 14.4 cm long and 12.3 cm the Museo del Jurásico de Asturias (Colunga, Spain) are here described. wide (Fig. 1). The track is preserved as a natural cast of a left foot on the The Lastres Formation is Kimmeridgian in age and constitutes the up- underside of a reddish brown mudstone. The cast of the track is well- 354

FIGURE 2. Partial Late Triassic track of the ichnogenus Grallator (MGUH 27812) preserved as a true track in a reddish brown mudstone. The track is from a left foot and is well-preserved, with recognizable impressions of the individual digital pads and claws. Notice the prominent raised rim of displaced material on the outsides of digit II and III (arrows), resulting from an outward rotation of the digits during the stride. footprint length of 16.0 cm is estimated. A shallow but well-defined undertrack containing all three digits is present on the underside of the slab; it represents a horizon 13 mm below the tracking surface. No claw impressions are present in the undertrack, and what appears to be the middle digital pad of each digit has left the deepest impression in the FIGURE 1. A, Small, well-preserved, Late Triassic theropod track (MGUH undertrack; it is also evident that the middle digital pad in digit III was the 27811) preserved as a natural cast on the underside of reddish brown most deeply impressed in the track. A prominent raised rim of displaced mudstone. The track is from a left foot and belongs to the ichnogenus material is present along the outer sides of the impressions of digits II Grallator. B, When the track is seen in frontal view, the middle part of the cast of digit III shows a prominent outward deformation, indicated by arrow. and III, indicating an outward movement of the foot relative to the walk- ing direction (Fig. 2). preserved, with clear impressions of the individual digital pads and claws (Fig. 1A). When studied in cranial view at a low angle, the cast of the Tracks 3 and 4 impression of digit III is asymmetrical because the bottom of the digit These theropod tracks are preserved in a reddish-brown mudrock impression is offset outward relative to the walking direction (Fig. 1B). and comprise one complete (track 3) and one partial (track 4) track (Fig. 3). Both tracks were selected for vertical sectioning because the tracks Track 2 were preserved with the original sedimentary infilling in place (Fig. 3). This theropod track is a partial imprint, consisting of the distal Furthermore, the infilling of the tracks is of a darker color than the parts of digits II, III and IV preserved on a slab of finely laminated, red sediment of the tracking surface, which makes it ideal for studying struc- and yellow-brown mudstone 130 mm thick in the part containing the tures in cross section. The complete track is a right pes, 17.3 cm long and digit impressions (Fig. 2). Although incomplete, the track can be identi- 8.7 cm wide between the terminations of digits II and IV. The incomplete fied as the imprint of a left pes because the claw of digit III is offset to the track comprises the distal ends of digits II and III of a left pes. The track right of the long axis of the digit. Only the distal digital pad and a small, is identified as a left pes imprint due to the inward orientation of the claw faint claw impression of digit II are present. The impression of digit III imprint of digit III (Lockley, 1991). Like the other track on the slab, this has three well-defined digital pads, the distal one bearing the clear claw one also preserves an infill of darker material. The orientation of the two impression. Only the medial part of the impression of digit II is present. tracks on the slab is toward each other at a high angle, measured along the Parts of neither the metatarsal pad nor the proximal end of the digits are long axis of the impression of digit III. This makes it unlikely that the present in the slab, but by extrapolating the length of the digits, a total tracks represent the right and left foot of the same animal. 355

FIGURE 3. A, Slab of reddish brown, Upper Triassic mudstone containing one complete and one partial theropod track of ichnogenus Grallator (MGUH 27813). The tracks are partially infilled with a darker colored mudstone. B, Interpretative drawing of the slab with the two tracks. The complete track is from a right foot and the incomplete track from a left foot, identified by the inward rotation of the claw of digit III. The darker colored sedimentary infilling of the complete track is encircled by dotted lines. The discontinuous lines A-E represent the studied vertical sections in Figure 4. Four sections were cut through the complete track, all perpen- dicular to the long axis of digit III (Fig. 3B). The first section, which runs through the tip of the claw impression of digit III, reveals that the im- FIGURE 4. Vertical sections through the tracks from Figure 3 (MGUH pression of the claw has cut down through the layers and left a v-shaped 27813). White arrows indicate direction of deformation. A, Section through impression filled with sediment of a darker color (Fig. 4A). In the next the tip of the impressions of digit III. The tip of the claw has left a v-shaped section, cut through the middle of the impression of digit III, the impres- cut down through the layers (black arrow). B, Section through the middle of sion appears as a steep-walled, clearly defined, rectangular depression in digit III. The deep impression of the digit is filled with sediment of a darker the sediment. The digit impression is clearly asymmetrically developed color. Notice the prominent outward orientation of the section through the because the bottom of the impression is offset outward relative to the digit impression. A vertical Skolithos burrow is indicated by an S. C, Sections tracking surface (Fig. 4B). In the third section, cut through the impres- through the impressions of digits II, III, and IV. The digit impressions are sions of digits II, III, and IV, the impressions of digits II and III are deep and filled with sediment of a darker color. The impression of digit IV disturbed and not so well-defined, but the impression of digit IV is shows an initial outward and downward deformation. D, Section through the clearly defined and is, toward the bottom, strongly offset outward, cre- proximal part of the track. Notice the deep downward and outward ating a zone of brecciated subsediment on the outside of the track (Fig. deformation structure created by the impression of digit IV. E, Section 4C). The last section through the complete track, cut through the proxi- through the middle of the impression of digit III from the incomplete track. mal part of the track, shows a weak impression of the proximal end of The sections show the digit impression to be outwardly deformed. digit II. The impression of digit IV is well defined and strongly developed cm below the tracking surface, indicating a pronounced outward move- downward and outward, undercutting the outer side of the track (Fig. ment of the digit during its time in contact with the ground (Fig. 5C). In 4D). The section through the partially preserved track, cut through the a section through digits II, III, and IV, the sediment between digits IV and middle of the impression of digit III, bears evidence of a slight outward III is deformed sideways in an overturned structure (Fig. 5D). A section movement that disturbed the subsurface layers on the outside of the digit through the proximal area of the track shows little more than a slight impression (Fig. 4E). sideways disturbance of the most surficial layers, which has been scraped to the side and crumbled together (Fig. 5E). For further details, see Milàn Tracks 5 and 6 et al. (2004). These two theropod tracks (here termed tracks 5 and 6) were Track 6 is the imprint of a right pes approximately 23 cm long and examined in cross section to reveal the characteristics of undertracks and 14 cm wide preserved on a slab of reddish-brown mudstone and (Fig. 6). other subsurface deformation structures (see also Milàn et al., 2004). Erosion of the surface has erased all finer morphological details of the Track 5 is a left track; it is 21.3 cm long and 14.9 cm wide and is track; only the outline and general shape of the track remain (Fig. 6A). preserved in a greyish mudstone (Fig. 5). The track is relatively shal- Four sections were made through the track. The first section was cut lowly impressed with well-defined impressions of each digit. Claw marks through the tip of the claw impression of digit III where the slender, and impressions of each digital pad are present in the impression of digit sharp ungual has left a clear, vertical cut down through the layers that is III. A prominent rim of displaced material is present along the outer side infilled with a lighter colored matrix. At the tracking surface, the cut mark of digit III (Fig. 5A). Four sections were made through the track. A starts at a bowl-shaped depression. Also at the tracking surface, a shal- section cut through the claw shows a sharp, slightly asymmetrical low, up-bulged area of displaced material is present (Fig. 6B). The sec- downcut through the layers (Fig. 5B). A section through the middle of tion cut through the middle of the impression from digit III displays the digit III shows the development of a prominent, outwardly directed digit impression as a bowl-shaped impression in the tracking surface. deformation structure recognizable down into the undertracks as far as 2 The weight of the digit has caused the formation of small raised rims of 356

FIGURE 6. Poorly preserved, Late Triassic theropod track (Grallator) FIGURE 5. Sections through a Late Triassic theropod track (Grallator) (MGUH 27815) on a slab of reddish brown mudrock. A, The track is somewhat (MGUH 27814), emplaced in a greyish mudstone. A, The track is from a eroded but can be identified as the impression of a right foot. B, Section cut left foot and has well preserved impressions of digital pads and claws. B, through the tip of the impression of the claw of digit III. The deep cut from Section through the tip of the impression of digit III. The impression is the claw is infilled with sediment of a lighter color, indicated by the arrow. preserved as a clear cut through the layers, indicated by the arrow. C, C, Section through the middle of the impression of digit III. Raised ridges of Section through the middle of the impression of digit III, which has created deformed material are present on both sides of the digit impression, but a raised rim of displaced material on the outside of the digit impression. This most prominently toward the outside of the track, indicated by arrows. D, asymmetrical deformation is also prominent in the undertracks along the Section through the impressions of digits II, III, and IV. The divisions of the subjacent horizons. Arrows indicate the sideways-deformed layers. D, Section digits are not visible in the base of the track due to erosion, but the undertracks through the impressions of digits II, III, and IV. Especially the impression of in the subjacent layers show the division of the digits clearly (vertical digit III shows an outward deformation both in the tracking surface and the arrows). The impression of digit IV shows a downward and outward undertracks, indicated by arrows. E, Section through the proximal part of deformation of the subjacent layers, indicated by arrow. E, The outward the track. The foot has created only a slight disturbance in the tracking twist of the proximal end of digit IV has created a prominent outward surface, most prominently developed toward the outside of the track, indicated deformation in the tracking surface as well as a subsediment deformation by the arrow. Figure modified after Milàn et al. (2004). below the tracking surface, indicated by arrows. Figure modified after Milàn displaced material on both sides of the digit. The rim on the outer side of et al. (2004). the digit, however, is more developed than the rim on the inner side of the digit because more pressure was exerted on the outer side of the digit The section through the impressions of digits II, III, and IV shows during its impression in the sediment. The rim is also visible in undertracks the initial division between the digits preserved as small, up-bulged areas formed approximately 1 cm below the tracking surface, again most promi- on the horizon with undertracks approximately 1 cm below the bottom nent on the outer side of the digit impression (Fig. 6C). of the track. A prominent deformation structure projects downward and 357 outward from the impression of digit IV and disturbs and folds the layers as much as 3 cm below the tracking surface (Fig. 6D). The last section, cut through the proximal part of the track, appears as a shallow, flat- bottomed depression in the tracking surface, revealing no anatomical information. However, a prominent, raised, marginal rim of displaced surface material is present at the right (outward) side of the footprint. The lateral disturbance of the layers is evident down to 3 cm below the tracking surface (Fig. 6E). For further details see Milàn et al. (2004). Lower Jurassic Tracks One dinosaur track described in a previous study by Avanzini (1998) was re-examined. Three other, very shallow tracks from the same site were examined in cross-section for the presence of subsurface defor- mation structures.

Track 7 The first track (Fig. 7) is a medium-sized incomplete theropod track, 19.5 cm long and 19.0 cm wide. The track, a right foot in concave epirelief, is preserved on the surface of a whitish stromatolitic bindstone. The track is incomplete, with only the impressions of the three digits well-preserved. The general features and the relatively high divarications of digits II-IV cautiously suggest attribution to the ichnogenus Kayentapus. The deformation of the stromatolitic laminite indicates a general outward movement of the foot relative to the trackway midline. In cross section C (Fig. 7), the laminae around and subjacent to the impression of digit III are compressed toward the external side of the track, creating an evident ridge. The outward translocation of the digits is most evident in cross sections D and E (Fig. 7).

Track 8 The second track (Fig. 8) is a medium-sized theropod track 22 cm long and 17 cm wide. The track is preserved as concave epirelief of a right foot on the surface of a whitish, stromatolitic bindstone. The track is complete and well-preserved, with recognizable impressions of digital pads and claws (Fig. 8). The track is tridactyl, mesaxonic, and shows a high angle of divarication between digits II and IV, a higher divarication of digits III and IV than digit II and III, a well defined covering the joint between the metatarsals and phalanges of digit IV, impressions of claws on digits II, III, and IV, and a highly projected middle digit like in other grallatorids. These features are typical of the theropod ichnogenus FIGURE 7. Early Jurassic theropod footprint (?Kayentapus isp.) (MTSN Kayentapus. The impression of digit IV shows a slight inward deforma- 5212) from the Lavini di Marco, northern Italy, tracksite sectioned at 1 cm tion in cross sections F, G, and H. In section G, the impression of digit III intervals. The track is an incompletely preserved imprint of a right foot, is apparently directed toward the inside of the print, while in the more comprising only the impressions of the digits. The sections C, D, and E cut distal section L, it seems to cut vertically down in the substrate. through the impressions of digits II, III, and IV all show an outward deformation of the digit impressions, recognizable in the undertracks as well Track 9 as on the tracking surface. Section F, cut through the distal part of digit III, shows an outward deformation of the digit impression. Arrows in sections A- The third footprint (Fig. 9) is a slender-toed theropod footprint F indicate areas of outward deformation. 19 cm long and 14 cm wide. The track is preserved as concave epirelief on the surface of a whitish stromatolitic bindstone. The track is the imprint section I, through the tip of the impression of digit III, shows a pro- of a left foot and is complete and well-preserved, with recognizable nounced inward deformation, contrary to the sections through the rest of digital pads and claws. The impression of digit III extends beyond the the track. axis joining digits II and IV. The divarication between digits II and III is slightly lower than that between digits III and IV. The metatarsopha- Track 10 langeal pad of digit IV is evident. All these characteristics are congruent The fourth footprint (Fig. 10) is a small theropod track 14 cm long with those of one of the morphotypes recently recognized at the Lavini and 10 cm wide. The track is the imprint of a left foot preserved as di Marco site and attributed to the ichnogenus Kayentapus (Piubelli et concave epirelief on the surface of a whitish stromatolitic bindstone. The al., 2005). track is poorly preserved but still attributable to a Grallator-like The cross sections through this track show an overall outward ichnogenus. A small depression, preserved at the base of the impression deformation of the track. Section B shows a slightly outward component of digit II is interpreted as the partial trace of digit I (Fig. 10). This track of movement of digit IV and section D, which shows the section through was previously described by Avanzini (1998) as a right footprint due to the impression of digit II, has preserved a significant outward-directed an incorrect interpretation of a depression (digit I?) at the base of digit II deformation. More evident is the deformation of the stromatolites under as the vestige of the pad covering the metatarsophalangeal joint of digit the impression of digit III. In sections E-H, the outward-directed compo- IV. For this reason, Avanzini’s (1998) interpretation of the movements of nent of external translocation is prominent and well recognizable. Only the track maker based on the deformation of the stromatolite bindstone 358

FIGURE 8. Early Jurassic theropod footprint (Kayentapus isp.) (MTSN 5211) from the Lavini di Marco, northern Italy, tracksite sectioned at 0.5 FIGURE 9. Early Jurassic theropod footprint (Kayentapus isp.) (MTSN cm intervals. The track is a right footprint and is preserved as a true track, 5213) from the Lavini di Marco, northern Italy, tracksite sectioned at 0.5 with well-defined impressions of digital pads and claws. When studied in cm intervals. The track is a left footprint and is preserved as a true track vertical sections, no apparent outward deformation is evident. Only sections still retaining some sedimentary infilling. The sections B to H represent L and G hint at a slight inward deformation of the impression of digit III, sections all through the track; all show prominent outward deformation. indicated by arrows. Section I, cut through the impression of the claw of digit III, exhibits inward deformation of the claw impression. Arrows in sections indicate areas and under the trampled surface was incorrect; we therefore redescribe it in direction of deformation. the present study. The true tracking surface is partly covered in the foot region footprint, and distally the impression of digit III becomes the dominant (post-track surface sensu Gatesy, 2003) by an infill made of dolomitized, agent of deformation in the subjacent layers, deforming the subjacent fine-grained breccia and mud (Avanzini et al., 1997) marked by the gray layers to a greater depth than either digit II or IV in the impact area of the color in Figure 10. The subsurface deformation structures, created by footprint. Deformation of the footprint is especially visible in section H the foot being emplaced on the tracking surface, are recognizable in all where the deflected lamina surrounding the toes III and IV both exhibit a vertical sections cut through the footprint. The proximal sections through pronounced, outward deformation. On the contrary, the section cut the track (A-C) show a deep vertical to sub-vertical penetration of the through the tip of the impression of digit III shows a pronounced inward foot down into the substrate, with a slight outward component in the deformation of the subjacent layers. movement. In the proximal sections of the footprint, the impressions of digits II and IV are the most prominent, indicating that these digits Late Jurassic Tracks carried the bulk of the track maker’s weight, but the pressure seems to have been exerted mostly on digit IV because it has been emplaced diago- Track 11 nally down into the sediment, making the bottom of the footprint appear The first track is a medium-sized theropod track 37 cm long and tilted slightly outward. 28 cm wide (Fig. 11). The track is preserved as a natural cast of a left foot From around the middle of the footprint, the deformation caused and so only the penetration modality of the foot in the substrate can be by the impression of digit III becomes visible in the layers below the recognized as no subsurface structures are preserved. The track is well 359

FIGURE 10. Early Jurassic theropod footprint (Grallator isp.) (MTSN 5210) from the Lavini di Marco, northern Italy, tracksite sectioned at 0.5 cm intervals. The track is a left footprint and contains the weak impression of the posteromedially directed digit I. The true track is covered by an infill FIGURE 11. Late Jurassic theropod footprint (MUJA 1080) from Asturias, made of dolomitized, fine-grained breccia and mud marked by the gray color. Sections C-H through the track show a prominent outward and Spain, preserved as a deep natural cast. A, In frontal view, the foot has been downward deformation caused by digit IV. The impressions of digits II and pressed outward and down through the sediment. The cast of the impression of digit III is the deepest, followed digits II and IV. B, When viewed from III seem not to possess any outward deformation. The tip of the impression of digit III, sections I and L, show the claw impression to be directed inward. below, the cast of the track has the typical inward twist of the of the claw The arrows in all sections indicate areas and directions of deformation. impression of digit III. Arrows indicate direction of deformation. Figure modified from Avanzini (1998). incomplete digit III seems relatively greater than 60% of the footprint length, and the proximal edge of the metatarsophalangeal pad on digit III preserved and deeply impressed to a depth of 13 cm. The track lacks is not anterior to the posterior part of the second phalangeal pad on digit discrete digital pad impressions, but this appears to be a function of IV as it is in the ichnogenus Megalosauripus (Lockley et al., 2000). The preservation and not a primary morphological feature. The impression footprint is relatively broad and wide with an elongate digit III, and of digit III spans about 60% of the total footprint length. The general seems very close to a “Eubrontes-like” morphotype (Lockley, 2000). morphology suggests a cautious attribution to the ichnogenus The cast of the track shows a pronounced outward deformation, espe- Megalosauripus (Lockley et al., 2000). The track is most deeply im- cially in the naturally-occurring cross section through the tip of digit III, pressed on the inside of the track, corresponding to digit II, and the which displays the cast of the digit protruding down from the tracking whole cast of the track is deformed slightly toward the outside of the surface at a 65° angle. The track was emplaced on a semi-lithified, 1-2 cm track. The cast of the impression of digit II shows that the digit pen- thick sandstone layer that has been dragged around the digits during their etrated the ground in a sinuous-shaped way, while digit III was initially impression in the substrate. This layer is preserved draping the inside brought down vertically, and then, at about 4 cm depth, curved outward and bottom of the cast of digit III, while the layer has been cut by a through the sediment at an angle of 80°. reverse fault on the outside of the digit (Fig. 12B). Track 12 Track 13 The second track is a relatively large theropod track 40 cm long The third track is a wide theropod track 30 cm long and 23 cm and 33 cm wide. The track is preserved as the natural cast of a left foot. wide. The track is preserved as a natural cast of a left foot. The track is The track is incomplete in that the tips of the casts of digits III and IV are complete and is identified as an undertrack because the impressions of missing, but it is otherwise well-preserved, with recognizable casts of the individual digits are undefined and broad (see Milàn and Bromley, digital pads and claws (Fig. 12). The phalangeal pad formula is 2, 3, and 2006). The footprint bears an elongate “heel” associated with a small, 4 corresponding to digits II, III and IV respectively. The length of the 360

FIGURE 12. Late Jurassic theropod track (MUJA 1261) from Asturias, Spain. A, View from the front. B, Interpretative drawing of view from front. C, View from below. D, Interpretative drawing of view from below. The track is preserved as a natural cast and is complete except for the distal part of digit III. The cast of the footprint exhibits outward deformation created by the foot being forced down and outward through the sediment. The base of the cast of the track is draped by a 1 cm thick layer of sandstone that was impressed into the sediment when the foot was emplaced. The sandstone layer was semi-consolidated when the foot was impressed into the sediment, as evidenced by the layer following the contour of the digit impression. The impact of the foot has caused the development of a fault in the sand layer on the outside of digit III. The frontal view (A, B) shows how the sand layer, indicated by shading, has been shaped and faulted by the impression of the digit. The view from below (C, D) shows the parts of the footprint that show outward deformation, indicated by arrows. When the track was emplaced, digit III was the most deeply impressed, followed by digits II and IV. Arrows indicate directions of deformation. posteromedially-directed protrusion that is identified as the cast of digit I (Fig. 13). The track lacks discrete digital pad impressions, but its general morphology suggests a tentative referral to the ichnogenus Megalosauripus. Even though the print is preserved as an undertrack, and thus lacks many anatomical details, a pronounced outward deforma- tion is evident in the casts of digits II, III, and IV. The depths of the casts of the digits vary substantially: the cast of digit III is deepest, followed by the cast of digit II: the cast of digit IV is the shallowest. The cast of digit II penetrates the tracking surface sub-vertically and is directed slightly outward. The angle of impression, measured from the cast of FIGURE 13. Late Jurassic theropod track (MUJA 1199) from Asturias, digit III, is a constant 75°, and the cast of digit IV, though impressed in a Spain with a cast of digit I preserved. The track is preserved as a natural cast sinuous-shaped way, is on average directed outward. and represents an undertrack because of its wide and undefined appearance. DISCUSSION A, View from front. B, View from below. The track shows an outward deformation of the whole track, most prominent in the casts of digits III All five Late Triassic tracks, including undertracks described herein, and IV. The cast of digit III protrudes the deepest from the tracking surface, show the same pronounced outward deformation of both the tracks and followed by digits II and IV. Arrows indicate direction of deformation. the surrounding sediment. The tracks were randomly collected from different bedding planes and are of different dimensions, which indicate all isolated tracks whose differences in size, shape and time also exclude that they originated from animals of different sizes and times. This the possibility that these tracks were made by the same species of track excludes the possibility that the tracks pertain to the same animal (al- maker. though the tracks could be from different-sized individuals of the same Undoubtedly, the different tracks described were created by dif- species) or that the outward deformation could be a result of either an ferent theropod taxa. The Late Triassic tracks from Greenland are be- abnormal gait of an individual track maker or a special gait adopted to suit lieved to be have been made by small- to medium-sized ceratosaurian local sedimentary conditions. The Early and Late Jurassic tracks are also theropods, but the only body fossil material found at the locality is a 361 partial, small, indeterminate theropod skeleton (Jenkins et al., 1994). The Early Jurassic and Late Jurassic tracks from Italy and northern Spain could be from either ceratosaurs or more advanced tetanurans or both. One large, incomplete theropod skeleton is known from the Early Jurassic of Italy (Nicosia et al., 2005). The skeleton belongs to one of the oldest-known, large-bodied tetanurans and is still undergoing study; it has been informally named “Saltriosaurus” (nomen nudum) (Dal Sasso, 2001). From the Late Jurassic of Spain, several theropod remains are known (Martínez et al., 2000, 2001; Garcia Ramos et al., 2002; Canudo and Ruiz-Omeñaca, 2004). Most of the finds are incomplete and not diagnostic, which makes it difficult to identify all the taxa that may be represented. The identified taxa include the Ceratosauria and Tetanurae. At first glance, there seem to be little variation in the observed sideways deformation of the tracks and undertracks, but closer inspec- tion reveals slight differences in the mode and geometry of the track and surrounding sediment deformation from the different time periods. The Late Triassic tracks from Greenland (Figs. 1-6) all show a pronounced outward deformation relative to the inferred midline of the trackway. FIGURE 14. Schematic representations of the three different types of The parts of the foot responsible for this deformation differ, however, deformation encountered in the 13 investigated tracks. Types 1 and 2 are between the studied specimens. In track 1, the main deformation is found in the Late Triassic and Early Jurassic tracks; type 3 is from the Late caused by the distal two-thirds of digit III, which also is impressed to a Jurassic tracks. Type 1 deformation comprises a pronounced outward twist greater depth than digits II and IV (Fig. 1). In track 2, most of the of digit III, and less deformation caused by digit IV. Type 2 displays outward deformation is caused by the middle and distal parts of digit III, and less deformation of digit III and a pronounced outward and downward deformation by digit II. As in the previous track, digit III is impressed to a greater caused by the proximal parts of digit IV. Type 3 shows an even, outward depth than digits II and IV (Fig. 2). The sections through these tracks deformation along the whole length of the foot. In all investigated tracks, show in more detail the different forms of deformation that occurred. the claw of digit III showed a more or less inward orientation. Track 8 falls outside of these three types in showing a slight inward deformation, and The sections through track 3 show a prominent outward deformation of track 4 was only represented by a section through the middle of digit III, and digit III, which is also the most deeply impressed digit. In this case, digit could thus be either type 1 or 2. The thickest arrows indicate the areas of IV, especially its proximal part, also caused a pronounced, outward de- strongest deformation. formation of the sediment. The whole foot was dislocated sideways during the stride (Fig. 4), contrary to the two first tracks in which the theropod tracks (Lockley, 2000). most significant deformation was caused by digit III only. Track 4 is only The nature of the outward deformation of the Late Triassic and represented by a section through digit III, but this section also shows a Early Jurassic tracks falls into two distinct groups (Fig. 14). In the group pronounced outward deformation. As the track is incomplete, no further containing tracks 1, 2, 4, 5, 7 and 9, the predominant outward deforma- deformation data from the other digits are obtainable. The two tracks tion has been caused by digit III. The other type of deformation, wit- previously described by Milàn et al. (2004) show two different types of nessed in tracks 4, 6, and 10, shows an outward deformation of the whole sideways deformation. Track 5 shows the main deformation to be caused length of digit III and a stronger, more prominent, outward and down- by digit III, with only a little deformation caused by digits II and IV (Fig. ward deformation along the outside of digit IV that, in some instances, 5). Track 6 shows a similar deformation to that of track 4 in that digit III has caused the outer edge of the digit to dig into the substrate (Figs. 4, 6 caused some sideways deformation, but the basal parts of digit IV also and 10). created a prominent outward deformation (Fig. 6). These two types of deformation result from different phases of Apparently, the observed lateral deformation of the Triassic tracks the walk. The outward twist of digit III occurs during the kick-off phase falls into two different groups: one in which the deformation is caused by (sensu Thulborn and Wade, 1989) during which the animal’s center of an outward movement of digit III during the kick-off, and one where the gravity is positioned anterior to the foot (Roberts and Scales, 2002) and whole foot is rotated slightly outward and the median and basal parts of the weight of the animal is transmitted to the distal part of the longer, the foot are displaced outward relative to the walking direction (Fig. 14). middle digit, which becomes more deeply imprinted into the substrate The tracks from the Lower Jurassic show a less clear pattern of than the outer digits. The sideways deformation of the proximal part of deformation. The incomplete track 7 exhibits primarily outward defor- digit IV occurs during the stance phase, when the animal’s center of mation caused by digits III and IV (Fig. 7), but because the track com- gravity is moved anterior to the point of force while progressing toward prises only the digit impressions, no deformation from the proximal part the kick-off phase. In this case, this must have caused an outward rota- of the foot is observable. Track 8 shows a slight inward deformation in tion of the proximal part of the foot. Within these two groups of defor- the impression of the proximal parts of digit III, contrary to what is mation, tracks 5, 8, 9, and 10 show an inward deformation of the trace observed in the other tracks that are all deformed outward and have a from the distal phalanx of digit III. This deformation pattern is inter- more normal, inward rotation at the tip of the same digit (see other preted as having formed when the distal part of the foot (in this case digit tracks). Track 9 shows the dominant outward deformation caused by III) twisted outward during the kick-off (implying rotation), causing the digit III, and less by digits II and IV. Like in track 8, the tip of digit III is digit bearing the claw to experience a slight twist inward due to drag from deformed inward, as if the tip with the claw had been twisted inward the claw in the sediment. The deformation of the Late Jurassic tracks while the middle of the digit is turned outward during the kick-off. In differs from the two types exhibited by the Late Triassic and Early track 10, the deformation is much like that of tracks 4 and 6 in that the Jurassic tracks in that their outward deformation is even and concen- outward deformation is most prominent in digit III, but also the whole trated along the whole length of digits III and IV and subordinately by length of digit IV displays a prominent outward twist. The tip of digit III digit II (Fig. 14). There is thus a clear tendency among the studied tracks shows an inward deformation in its distal part (Fig. 10). The Late Juras- from Late Triassic, Early Jurassic and Late Jurassic theropods to have an sic tracks (11-13) all show a similar, even, outward deformation, most outward component in their stride; only track 8 show no apparent out- prominent in digits III and IV (Figs. 11-13). These tracks further show ward deformation, but is instead slightly deformed inward in the middle the inward rotation of the tip of digit III, which is a common feature of part of digit III. This might merely be the result of a local, unusual foot 362

FIGURE 15. Schematics showing the different penetrations of the left foot into the sediment between the Late Triassic, Early Jurassic and Late Jurassic tracks (ideal section of the central part of the track perpendicular to the long axis of digit III). A, The base level, here defined as the line between the bottom of the impressions of digits II and IV, below the Late Triassic and Early Jurassic tracks are horizontal to sub-horizontal. In tracks with deformation type 1, the bases are horizontal; in the tracks with deformation type 2, the impression of digit IV is the deepest, making the base level slope slightly outward relative to the midline of the trackway. Base line is indicated by broken line below the foot. B, The base below the Late Jurassic tracks is sloping strongly inward relative to the midline of the trackway. This is due to digit II being significantly more deeply impressed into the sediment than digit IV. Base line is indicated with broken line below the foot. Even though the depths of the different parts of the feet are impressed into the sediment and the base lines show different angles, all the tracks (except track 8) are deformed outward relative to the midline of the trackway. movement related to differences in gait or posture during walk. A possi- etries and mechanics of the limbs between Late Triassic and Late Jurassic bility that should be taken into account is that the differences in deforma- theropods. Holistic studies of evolution of theropod tracks by Lockley tion pattern observed in the Late Jurassic tracks could be due to the foot (in press) show a trend in which the digital pads on digit II become having been impressed to a greater depth in the substrate. Future studies compressed and fuse in one large fleshy pad among the larger theropods. including more tracks from more localities, time periods, and sedimen- In non-avian theropod dinosaurs, the metatarsophalangeal joints of dig- tary conditions will shed light on this issue. its II and III must have been held off the ground, while digit IV was Theropod dinosaurs shared a common locomotory design (Farlow impressed over its entire length. This results in a typical asymmetry of et al., 2000). Despite an enormous range in body size, theropods all had the proximal portion of the footprint (Lockley, in press). Bird footprints very similar hindlimb proportions. Even so, there are distinctive varia- are much more symmetrical than those of non-avian dinosaurs. The tions on the overall theropod morphological theme along the various “heel” is formed by a thick metatarsal pad located beneath the distal end groups. Theropod hips and hindlimbs show marked morphological of the trochlea of digit III on the tarsometatarsus and the metatarsopha- changes that are consistent with functional changes during their evolu- langeal joints of digits II and IV are elevated off the ground (Lucas and tion. Holtz (1995) demonstrated that arctometatarsalians markedly in- Stettenheim, 1972). Padian and Olsen (1989) observed that the relatively creased the length of the distal hindlimb elements relative to the femur longer metatarsi of birds have to be more vertically oriented than in length compared with other theropods. In addition, these derived typical non-avian theropods to keep the animal’s center of mass posi- theropods also developed a tightly interlocked proximal metatarsus that tioned above the feet. Another complicating factor is the size and orien- more effectively transmitted locomotory forces from the foot to the tation of the femur (Padian and Olsen, 1989; Gatesy, 1991) which is lower leg than did the ancestral theropod metatarsus. Trackway data generally elongate and slender in non-avian theropods and short and suggest conservatism in some aspects of locomotory activity during stout in birds. Non-avian theropod tracks tend to have low interdigital theropod evolution (Farlow et al., 2000). The trackway data provide no angles, while bird footprints often have higher values. evidence for changes in stride lengths (at least compared with footprint Sideways deformation is not observed in studies of tracks and length) during walking or running in comparisons of Early, as opposed to trackways from extant cursorial theropods, like the rhea, Rhea americana later, Mesozoic theropods. Conservatism also seems to characterize the (Padian and Olsen 1989). Similar studies of tracks and trackways from relationship between the total leg length and the total length of digit III (a an emu, Dromaius novaehollandae, also revealed no such sideways de- very rough proxy for footprint length). So the apparent similarity of the formation, but it was observed that the emu carries significantly less walking kinematics in the footprints studied here is, as a result, not weight on digit II, which in many cases becomes less impressed into the surprising. However, while the general tendency for the tracks to show substrate (Milàn, 2006). The ostrich, Struthio camelus, has lost digit II an outward component in the foot movement seems consistent in all the and has a functionally didactyl foot consisting of digits III and IV (Davies, studied tracks (except track 8), the pattern in which the foot penetrated 2002); studies of its tracks and trackways also showed no apparent down into the substrate shows some difference from the Late Triassic outward deformation (Farlow, 1989). and Early Jurassic tracks to the Late Jurassic tracks. In the Late Triassic The subsurface deformation in theropod tracks described here and Early Jurassic tracks, the base of the footprint is of almost equal demonstrates that there was apparently a change in the walking kinemat- depth below the imprints of digits II and IV, while digit II is relatively ics from Late Triassic and Early Jurassic to Late Jurassic theropods. The more deeply impressed into the substrate than digit IV in the Late Juras- former show either an outward component in the distal or proximal part sic tracks, causing the base of the track to slope sharply toward the of the foot during the stride or an outward rotation of the basal part of the inside of the tracks (Fig. 15). This suggests an analogous shifting of the foot; the latter were created by an even, outward component of foot weight during walk, but a different geometry of penetration of the distal movement during the stride. Contrary to the outward deformations ob- part of the foot into the ground, with implications for different geom- served in the Mesozoic theropod tracks, extant cursorial birds appar- 363 ently do not incorporate an outward component in their stride. IV, which becomes deeply impressed into the substrate. The tracks from the Late Jurassic of Spain show an even, unidirectional, outward defor- CONCLUSION mation of the whole track created by the foot being pushed outward and Careful study of the deformation structures occurring in and around down into the sediment during the stride. The bases of the Late Triassic dinosaur tracks provides valuable information about the walking kine- and Early Jurassic tracks below the impressions of digits II and IV differ matics and gait of the track maker. The method of studying tracks in from the Late Jurassic tracks. In the former, the base is almost horizon- vertical section has proven successful in obtaining additional details about tal, with digit IV being only slightly more deeply impressed than digit II. the walking kinematics that rarely could be obtained from studying the In the Late Jurassic tracks, the base is sloping inward as digit II is true track at the surface, especially if these have been exposed to erosion significantly more deeply impressed than digit IV. Most of the studied or are poorly preserved. tracks show an inward rotation of the distal end of digit III created by the Taken all together or singularly, true tracks, undertracks, natural claw being rotated inward while the foot is turned outward during the casts, and subsediment deformation structures observed in vertical sec- stride. tions, provides unique information about the walking kinematics of track ACKNOWLEDGMENTS makers, and are thus important factors to incorporate in the study of fossil footprints. The research of J.M. is sponsored by a Ph.D. grant from the The studied tracks from the Late Triassic of Greenland and the Faculty of Natural Science, University of Copenhagen. Niels Bonde Early Jurassic of Italy both show an outward deformation resulting from kindly placed Late Triassic footprints at our disposition and allowed us an outward rotation of the foot during the stride. This deformation falls to cut sections through them. Jerry Harris, Martin G. Lockley, and an into two distinct types. One type is formed by digit III being twisted anonymous reviewer are thanked for their critical and constructive re- outward during the kick-off and the second type is formed by an out- views. ward translocation of the proximal parts of the foot and especially digit

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