Theropod Dinosaur Trackways from the Lower Cretaceous of the Chacarilla Formation , Chile

We describe sixteen theropod dinosaur trackways from site III (Lower Cretaceous?) of the Quebrada Chacarilla in the Chacarilla Formation (Jurassic-Lower Cretaceous), northern Chile. The trackbed belongs to a meandering river environment, with recurrent track assemblages. We fi nd that the main direction of the ichnites from site III parallels that of water fl ow. Two theropod footprint morphotypes are recognized: A) digit II and IV occupying about 0.5 times the total footprint length, and B) digit II and IV occupying less than 0.35 times the total footprint length. We estimate similar speed (~7 km/h) for the two parallel theropod trackways of morphotype A, suggesting synchro nized walking. We also describe some of the largest theropod footprints from South America (>60 cm), which are all of morphotype B.

We re-study the different outcrops of this formation, and present here the fi rst detailed information on the vertebrate ichnofauna of the tracksite mentioned by Galli and Dingman (1962) as 'second locality' with dinosaur footprints, named 'site III' in this paper (Fig. 2).The locality shows numerous eastern-western and northern-southern striking outcrops with a pronounced west-dip (44º) and dozens of dinosaur trackways exposed in a thick sequence of sandstones.Their preservation is generally good, depending on the weathering of the different levels.
A detailed track mapping of the Chacarilla Formation is far from complete, thus only preliminary results will be presented here.

General Geologic Context
Mesozoic tectonic evolution in northern Chile was dominated by subduction along the western margin of Gondwana (Coira et al., 1982;Jordan et al., 1983;Mpodozis and Ramos, 1989;Ramos and Kay, 1991;Mpodozis and Allmendinger, 1992).Dur ing the Jurassic and Lower Cretaceous, a magmatic arc was formed along the present-day Cordillera de la Costa.East of the arc, in the Central Depression and Precordillera in Chile, there was a contemporaneous ensialic back-arc basin, the Tarapacá Basin, which comprises thicknesses of thousands of meters of marine and continental sediments (Coira et al., 1982;Groschke et al., 1988;Mpodozis and Ramos, 1989).During Lower and Middle Jurassic, deposition of marine sequences was controlled by post-rift thermal subsidence and global sea-level fl uctuations (Groschke et al., 1988;Ardill et al., 1998), whereas later on, in Middle Jurassic to Cretaceous times, the basin was controlled by tectonic movements related to the break-up of Pangea and fragmentation of Gondwana (Ardill et al., 1998).The latter resulted in a widespread marine regression causing deposition of continental redbeds during Upper Jurassic and Lower Cretaceous in the backarc basin (Rutland, 1971;Chong, 1977;Bogdanic, 1990).Sedimentation stages in Lower Cretaceous times corresponds to the track-bearing deposits of various carnivorous and herbivorous dinosaurs tracks (Blanco et al., 2000;Moreno et al., 2000;Rubilar et al., 2000a and b;Moreno, 2001).
The Chacarilla Formation is mainly a clastic succession with an exposed thickness of 1100 m.The lower part is of marine origin (Oxfordian), whereas the upper part was deposited in a continental setting (Galli and Dingman, 1962).The marine section at the Quebrada Chacarilla crops out in a box-fold anticline centre (Galli and Dingman, 1962), and the continental deposits form an open syncline.The top of the formation is marked by an angular unconformity, overlaid by volcanic and clastic rocks of the Cerro Empexa Formation (Upper Cretaceous-Lower Paleocene, Tomlinson et al., 2001).
A certain discrepancy in age determination has been noted for lithostratigraphically equivalent continental red-bed successions exposed along the Precordillera in Chile's Regions I and II.This variability is due to the scarcity of biostratigraphically signifi cant fossils in the continental facies rocks.As a result, previous workers have reported ages varying from Oxfordian (Galli and Dingman, 1962) or Kimmeridgian (Vergara, 1978) to entirely Lower Cretaceous (Ardill et al., 1998).In addition, 60 km north of the study area, Herbst and Troncoso (1996) placed a part of this formation in the Middle Jurassic, based on a taphofl ora association.However, the discovery of large ornithopod ichnites in the Chacarilla Formation indicates a Cretaceous age at least for the upper part of this formation (Blanco et al., 2000;Rubilar et al., 2000a).These tracks are correlated to the Caririchnium ichnofacies described by Lockley et al. (1994) for the Cretaceous of Laurasia, which are dominated by large ornithopod tracks preserved in silicoclastic rocks from coastal plain environments, in medium to high latitudes (Lockley et al., 1994).The studied strata are located in the uppermost part of the Chacarilla Formation, hence it is likely to be of a Lower Cretaceous age.
The measured section (Fig. 3) contains 140 m of rhythmically alternating shales and red sandstones, and is structured in multiple, superposed fi ning-and thinning-upward sequences of decametric to metric scale.Within this section we observed two facies associations, which represent point bars and fl oodplains of a meandering river environment.
The point bar facies association consists of several superposed fi ning-and thinning-upward cycles, with each cycle being 30-50 cm thick and reaching an overall thickness of 4-10 m.Each cycle is composed of medium-grained quartzose sandstone with trough cross-bedding stratifi cation, which passes gradually into sandstone with parallel plane lamination.The highest part of the cycle shows reddish mudstone with current ripple marks.Sedimentary structures such as trough cross-bedding and parting lineation indicate paleocurrents toward the NW, WNW and W (150 measurements).In sections perpendicular to the paleofl ow, the sandstone shows a sigmoidal geometry that forms a large-scale crossstratifi cation of the epsilon type (Allen, 1963).
The fl oodplain facies association consists of fi nely laminated mudstone (generally shale).Plant remains, carbonaceous levels, and root marks are present, as well as some fossil trunks in life position.Furthermore, arboreal root casts and dinosaur footprints can be found.Intercalated in the mudstones are laterally extensive, but thin (0.1-0.3 m), tabular bodies of fi ne-grained lenticular sandstones, with a planar to slightly concave base, ripple marks, and parallel plane-lamination.These sandstones prob-ably correspond to the distal deposits of crevasse splays covering fl oodplain areas (Miall, 1996).Asymmetric ripples in this association indicate a paleofl ow toward WSW (50 measurements).The orientations of ornithopod and theropod dinosaur trackways are toward WSW and SW, approximately parallel to the water fl ow direction.

Description of the dinosaur ichnites
Here we describe sixteen trackways (3-6, 8, 10, 12-13, 15-18, 22, 24-26) from Quebrada Chacarilla site III (Fig. 2), which correspond to the theropod morphology.Measurements were taken in situ (Fig. 4, Table 1).Footprint dimensions clearly shows a tendency toward a longer than wide foot print shape (Fig. 5).This characteristic, together with the anatomical features described below support the assignation of these ichnites to the Theropoda Suborder.Most of these footprints exhibit a tridactyl morphology, with impressions of digits II, III and IV.However, trackway 26 also displays the impression of digit I (tetradactyl morphology), which corresponds to a deeper impression of the autopod into the sub strate and subsequent collapse of the sedimentary structure (Fig. 6).Despite of this, two main pedal morphotypes are recognized, A and B, based on digital proportions: In morphotype A, the length of digits II and IV is about half the total footprint length.This morphol ogy is observed in trackways 3, 4 and 18 (plastotype SNGM-311, Servicio Nacional de Geología y Minería).Trackways 3 and 4 show surprising similarities.Footprints have comparable shape and size.Also, these two trackways are parallel (separated by about 80 cm), and have approximately the same length (6 m) and same number of footprints (8 each).Later on, trackways 1, 2 and 5 crossed trackways 3 and 4 (Fig. 7).In morphotype B (trackways 5, 6, 8, 10, 12, 13, 15, 16, 22, 24, 25 and 26), the length of digits II and IV are proportionally smaller than in morphotype A, and are about a third of the total footprint length.Morphotype B displays a wide variety of footprint dimensions, from 28 cm long and 15 cm wide to 65 cm long and 53 cm wide , including some of the largest in South America (see discussion).Also, the tetradactyl footprints (trackway 26) mentioned above are included in this morphotype.These footprints show deep impressions of digits I to IV plus a metatarsal impression.The impression of digit I is directed anteromedially (Fig. 6).The total anteroposterior length (1 m) of this footprint was taken without accounting for the metatarsal impression, which is approximately one third of the total length (Table 1).N: number of footprints; L: footprint length; W: footprint width; II/L: digit II-footprint length ratio; IV/L: digit IV-footprint length ratio; P: pace length; A: pace angle; S: stride length; H: hip height calculated using the correction factor of Thulborn (1984): either 4.5 or 4.9 times the footprint length for small or large theropods respectively.Speed is calculated with the formula developed by Alexander (1976) in MKS units; speed: 2.817 L 1.67 H -1.17 ; †: Theropods with morphology A, in which the length of digits II and IV is half the length of digit III; *: Length of the footprint without considering the metatarsal impression.

Discussion
There are two dinosaur groups with tridactyl footprints: theropods and ornithopods.Diagnostic features which permit the discrimination between both groups are the marked differences of footprint length/width ratio, the lower interdigital angles and lower trackway pace angulations, and the presence of an indentation along the medial edge of digit two just distal to the 'heel' apex seen in theropods as opposite to ornithopods (Fig. 4).However, these characters do not allow the identifi cation of a spe cifi c trackmaker, due to the weak correlation between footprint morphology and skeletal features (Farlow and Chapman, 1997).
The ichnofauna identifi ed in site III of Quebrada Chacarilla is composed of about 80% theropods and 20% ornithopod footprints.Therefore, it is distinct from other Late Jurassic-Early Cretaceous tracksites, such as San Salvador and Baños del Flaco, which present a mostly small theropod and a small ornithopod-small theropod-narrow gauge sauropod ichnocoenosis respectively (Moreno and Pino, 2002;Moreno et al., 2004;Moreno and Benton, 2005).This variation is diffi cult to interpret since many factors are involved in the preservation of the various tracksites, being dinosaur behaviour, distribution, palaeogeography and chronological differences among the most important.In this respect, much more work is needed in order to identify possible ichnofacies and chronological accuracy.
The general orientation of the trackways in site III is similar to the direction of water fl ow (NE-SW).
Based on the proportions of the digital impressions, two morphotypes have been recognized: Morphotype A, with a prominent digit III (lateral digits II and IV about a half of digit III length), which is comparable to tracks attributed to 'coelurosaurs' in South America (Leonardi, 1989;Calvo, 1991;Leonardi and Spezzamonte, 1994).Within this morphotype we observed two trackways (3 and 4) that are walking parallely at a similar speed (~7 km/h), and have a regular separation between both trackways, suggesting that both individuals were walking in a synchronized manner (Fig. 7).
Morphotype B, with lateral digits (II and IV) about a third of digit III, is similar to the ichnogenus Abelichnus (Calvo, 1991(Calvo, , 1999;;see Figs. 10-11), where claw impressions are very prominent and the digit impressions occupy most of the length of the tracks, and may be tentatively referred to that ichnotaxon.However, the pace angulation recorded is 180° in contrast to ~150° for Abelichnus.
Morphotype B includes one of the largest theropod ichnites known in South America.These ichnites are comparable in size (mean hip height ~3 m) with carcharodontosaurids such as Giganotosaurus, as was previously referred (Calvo and Mazzetta, 2004) from the early Late Cretaceous of Argentina (Coria and Salgado, 1995;Calvo and Coria, 1998;Novas et al., 2005).However, theropod remains of this size have not yet been found on the western portion of Gondwana.Consequently, the Chacarilla footprints represent the fi rst evidence of their existence in that area.In addition, a trackway of seven tetradactyl footprints (trackway 26) with metatarsal impression is included in this morphotype.The particular morphology of these footprints suggests a quasi-plantigrade locomotion (Kuban, 1989) and presents independent evidence that these theropods powered the early stance phase by femoral retraction, rather than by knee fl exion as in living birds (Gatesy et al., 1999).Subtracting the posterior footprint elongation from the total footprint length (1 m), we obtain similar dimensions (60 cm long, 53 cm wide) as those as in other trackways of morphotype B (Fig. 6).
The average speed estimated for all the trackways is 5.8 km/h, which is consistent with a walking gait.The only trackway that has a trotting gait is 17, with an estimated speed of 11.2 km/h.

Conclusions
The ichnofossils from the Chacarilla Formation as a whole, but particularly in site III, are a unique record of dinosaurian fauna which inhabited a fl ood plain environment of a meandering river in the north of Chile during the Early Cretaceous (as indicated by the presence of large ornithopod footprints characteristical of this period).Here, described some of ichnites from site III, which show a main direction NE-SW, and therefore are parallel to the direction of the water fl ow.Two theropod morphotypes were recognized, including some of the largest of South America, which indicate that large theropods lived on the western portion of Gondwana in earlier times than the suggested by the oldest carcharodontosaurid record (early Late Cretaceous).In addition, among these large theropods traces we found two parallel trackways with similar speed and footprint morphology, which are evidence of synchronized walking, a behaviour previously not described for these big animals.
The quality, abundance and time span of the foot print record of the Chacarilla Formation makes it potentially useful for the study of paleogeographic, behavioural and evolutionary patterns during the Late Jurassic and Early Cretaceous.This should be addressed in future research.

FIG. 4 .
FIG. 4. Diagram showing the footprint and trackway measurements that were taken.I: length of digit I; IV: length of digit IV; L: footprint length; W: footprint width; P: pace length; S: stride length; A: pace angle.

FIG. 11 .
FIG. 11.Photo of the fi fth footprint of trackway 22. Scale is 50 cm long.
The present study only describes theropod trackways of an outcrop in Quebrada Chacarilla.The cast of a footprint from this site is housed in the Paleontological Collection of the Servicio Nacional de Geología y Minería, Chile (SNGM-311).