Sullivan, R.M. and Lucas, S.G., eds., 2016, Fossil Record 5. New Mexico Museum of Natural History and Science Bulletin 74. 127 THE EARLY CRETACEOUS CLAYTON LAKE DINOSAUR TRACKSITE, NORTHEASTERN NEW MEXICO

SPENCER G. LUCAS and SEBASTIAN G. DALMAN

New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico 87104; -email: [email protected]

Abstract—At Clayton Lake in northeastern New Mexico, an extensive dinosaur tracksite is present across the contact of the upper Albian Mesa Rica and Pajarito formations. Dinosaur footprints and other trace fossils are found at four stratigraphic levels, two in the uppermost Mesa Rica Formation, one on the top bedding surface of the Mesa Rica Formation (the main track level) and one in the lowermost Pajarito Formation. In May 2016, we counted 260 dinosaur footprints in the main track level. The invertebrate ichnoassemblage of the main track level consists of shallow burrows assigned to Taenidium, Arenicolites, Planolites and Thalassinoides. These invertebrate ichnofossils and the vertebrate tracks are a low ichnodiversity assemblage representative of the Scoyenia ichnofacies. The vast majority of the dinosaur tracks at Clayton Lake are of ornithopods, many of which can be assigned to Caririchnium isp. These are the pes tracks of bipeds. Deeply impressed ornithopod tracks at Clayton have long metatarsal impressions and some are associated with tail drags. One quadrupedal ornithischian trackway is present that has characters of Deltapodus and thus may represent a stegosaur trackway. Theropod tracks are of two kinds assigned to Magnoavipes isp. and cf. Irenesauripus isp. What is now needed is a detailed, photogrammetric study of the Clayton Lake tracksite to create an accurate map of track distribution and thereby allow more detailed studies of the information on dinosaur behavior contained at the tracksite.

INTRODUCTION and a set of pterosaur manus impressions. However, Bennett (1992) Clayton Lake State Park in northeastern New Mexico is the site re-evaluated the supposed pterosaur tracks reported by Gillette and of an extensive assemblage of dinosaur footprints (Figs. 1-2). Located Thomas (1989). He concluded that they were made by crocodilians. 24 km north of Clayton in Union County, Clayton Lake is a manmade Lockley and Hunt (1995, p. 200) briefly commented on the Clayton reservoir of about 69 hectares (170 acres) extent along Seneca Creek. Lake tracksite, noting that more than 500 tracks have been documented Building of the dam and spillway of the lake uncovered hundreds of at the site. Hunt and Lucas (1998) stated there were more than 400 dinosaur tracks in the spillway (Fig. 2) tracks at Clayton Lake, mostly of ornithopods, and of theropods, large The dinosaur tracks at Clayton Lake were brought to scientific and small. Hunt and Lucas (2004) briefly reported multiple tail drags attention in 1982, and Gillette and Thomas (1983, 1985) and Thomas associated with ornithopod tracks at Clayton Lake. and Gillette (1985) provided the first published analysis. Ironically, no GEOLOGICAL CONTEXT more detailed study of the Clayton Lake tracksite has been published since Gillette and Thomas (1985), though a diverse literature discusses As Lucas et al. (1986) concluded, the Clayton Lake tracksite (Fig. various aspects of the Clayton Lake tracksite (Lucas et al., 1986, 1989; 1) is at the boundary between the Mesa Rica Formation (Sandstone) and Gillette and Thomas, 1989; Bennett, 1992; Lockley and Hunt, 1995; Pajarito Formation (Shale), strata of late Albian age. Although Gillette Hunt and Lucas, 1996, 1998, 2004). Here we provide a reassessment and Thomas (1985) and Lucas et al. (1986) stated that the tracks at of the Clayton Lake tracksite based primarily on fieldwork undertaken Clayton Lake are at three stratigraphic levels, our recent observations in May 2016. Our goals are to document the tetrapod ichnotaxa present indicate there are actually four track-bearing strata (Fig. 1). at the tracksite, as well as to provide a preliminary assessment of the Thus, the track levels are in the upper 2.3 m of the Mesa Rica invertebrate ichnology at the site. Formation and in the lower 0.5 m of the Pajarito Formation (Fig. 1). The lowest track level is about 2.3 m below the top of the Mesa Rica HISTORY Formation. This is the track level with crocodilian tracks documented Gillette and Thomas (1985) noted that the Clayton Lake tracksite by Gillette and Thomas (1989) and Bennett (1992). However, we found was discovered before 1982, but only brought to the scientific attention this lowest track level to have been heavily weathered since their work in that year. They identified nearly 500 dinosaur footprints and assigned was published. Therefore, we are unable to locate and evaluate the tiny them to one ornithopod taxon and three theropod taxa. They stated that theropod track Gillette and Thomas (1989) reported. Similarly, the the tracks occur at three stratigraphic levels in lacustrine or marine crocodilian tracks can no longer be recognized, but a cast of one of sediments of the Aptian-Albian Dakota Formation. Importantly, them is on display at the visitor center of Clayton Lake State Park (Fig. Gillette and Thomas (1985, fig. 3) presented the only published map of 3B). the Clayton Lake tracksite. The next track level is about 0.4 m below the local top of the Mesa Lucas et al. (1986) studied the stratigraphy and age of the Rica Formation (Fig. 1). This track level is little exposed and very Clayton Lake tracksite. They identified the stratigraphic succession poorly preserved. The tracks at this level all appear to be ornithopod at Clayton Lake as (ascending order): Glencairn Shale Member of tracks, amid some remarkably well preserved mudcracks. Purgatoire Formation (including late Albian foraminiferans), Mesa The third track level is the main track-bearing surface (Figs. 1-2). Rica Formation (Sandstone) (three dinosaur footprint levels), Pajarito We now judge this surface to be the top bedding plane of the Mesa Rica Formation (Shale) (upper dinosaur footprint level), Dakota Sandstone Formation; note that Lucas et al. (1986) placed it in the basal Pajarito (later named the Romeroville Formation (Sandstone) by Lucas, 1990), Formation. We counted 260 dinosaur footprints on this surface, and will and Graneros Formation (Shale) (with Cenomanian foraminiferans) discuss them at length below. (Fig. 1). Based on microfossils and regional stratigraphy, they assigned The fourth track level, the highest level, is almost 0.5 m above the Clayton Lake dinosaur tracksite a late Albian age. the base of the Pajarito Formation (Fig. 2). These are a few ornithopod Lucas et al. (1989) reviewed the stratigraphic and biostratigraphic tracks in clayey sandstone at this level. data of Lucas et al. (1986) and put them into a broader, regional The Mesa Rica Formation represents fluvial and deltaic deposition stratigraphic context. Gillette and Thomas (1989) described some along the shoreline of the Western Interior Seaway, and the Pajarito “problematic” traces from the lowest track level at Clayton Lake, Formation mostly represents floodplain deposits that prograded over identifying them as tracks made by a very small dinosaur (cf. that shoreline as it regressed to the east (e.g., Holbrook and Wright Irenichnites gracilis), digit impressions of a small bird or coelurosaur Dunbar, 1992). Thus, the Clayton Lake tracksite formed in a coastal setting in sediments of fluvial origin. 128

FIGURE 2. Photographic overview of the main track surface in the dam spillway at Clayton Lake State Park. Most of the dark depressions in the surface are ornithopod tracks of the main track surface, on the top of the Mesa Rica Formation. Note outcrop of ledgy intercalated sandstone and shale of lowermost Pajarito Formation exposed at end of track surface. The boardwalk allows visitors to Clayton Lake State Park to view the main track surface.

INVERTEBRATE ICHNOLOGY Other than the mention of the presence of Thalassinoides (Lucas et al., 1986), no description of the invertebrate ichnology of the Clayton Lake tracksite has been attempted. We present here a preliminary analysis of the invertebrate ichnology of the Clayton Lake tracksite, based on the invertebrate ichnoassemblage present on the main track surface (Figs. 4-6). This surface yields a low diversity invertebrate ichnoassemblage that we assign to four well-known ichnogenera. The dominant (most abundant) invertebrate trace fossils are nearly straight, to slightly sinuous horizontal burrows that do not branch though some overcross (Fig. 4). These burrows are unlined, cylindrical in cross section and contain thin, meniscate backfills made of different kinds of material. The external burrow texture is roughened but lacks linear striations. The longest burrow course is at least 58 cm long, and burrow diameters range from 10 to 15 mm. These burrows have all the diagnostic features of Taenidium, to which they are assigned (e.g., D’Alessandro and Bromley, 1987; Keighley and Pickerill, 1994, 1997; Rodríguez-Tovar et al., 2016). Taenidium is interpreted as either a locomotion or feeding structure produced by an arthropod or worm- like organism (Bromley, 1996). Surfaces with Taenidium at Clayton Lake also are also marked by numerous straight, cylindrical burrows that are perpendicular to the bedding plane (Figs. 4-5). Many are paired circular openings on top of the bedding plane. They have a massive fill, and some of these burrows weather as filled cylinders in full relief. Diameters of the cylinders are 4-15 mm, but the lengths of the tubes in the rock could not be measured. Similarly, we could not observe u-shaped cross sections connecting two of these burrows, but the presence of paired openings suggests such a geometry. These burrows lack spreite and have features characteristic of Arenicolites, to which they are assigned (e.g., Fürsich, 1974; Häntzschel, 1975; Mángano et al., 2002). Arenicolites is interpreted as a dwelling trace produced by a worm or worm-like organism (Bromley, 1996). Locally, on the main track surface, cylindrical to subcylindrical, filled burrows are present. These are straight to gently curved, do not branch and are horizontal to slightly oblique to the bedding plane (Fig. 6A). They range from 3 to 16 mm in diameter, but most are 3-5 mm in diameter. These unlined burrows have a fill different from the surrounding matrix and are usually preserved in full relief. Gillette and Thomas (1989) identified these traces as “isolated toe” impression of birds or pterosaurs. However, these traces are burrows readily assigned to Planolites (e.g., Pemberton and Frey, 1982). Planolites is interpreted as a feeding structure of worm-like organisms (Pemberton and Frey, 1982). A fourth kind of burrow exposed parallel to bedding is cylindrical, unlined, has structureless fill and has Y- or T-shaped branching (Fig. 6B). At many of the branch points the burrow is inflated to form FIGURE 1. Index map and stratigraphy of the dinosaur tracksite at expanded nodes. These burrows form horizontal networks and polygons Clayton Lake State Park (modified from Lucas et al., 1986). on bedding planes and are preserved in convex hyporelief. The greatest 129

FIGURE 3. A, example of dispersed foliage impressions on the main track layer at Clayton Lake State Park. B, cast of a crocodilian track from lowermost track layer (see Bennett, 1992). Cast is on display at the Park visitor center. length of a burrow course is 40 cm, and burrow diameters are 4-13 mm. Based on the latest ichnotaxonomic revision of large ornithopod These burrows thus show features characteristic of Thalassinoides, tracks by Díaz-Martínez et al. (2015), the better preserved Clayton to which they are assigned (e.g., Howard and Frey, 1984; Yamin and ornithopod tracks can be assigned to Caririchnium because the pes Baraboshkin, 2013). Thalassinoides is interpreted as a crustacean tracks have a large, centered, wide and rounded heel impression burrow (Howard and Frey, 1984). and relatively short and wide digit impressions. Importantly, poor The invertebrate ichnoassemblage at Clayton Lake (main preservation precludes discernment of any digital pad impressions, track surface) consists of (in order of abundance, highest to lowest) and some tracks are so poorly preserved that any ichnotaxonomic Taenidium, Arenicolites, Planolites and Thalassinoides. Together identification is impossible. Díaz-Martínez et al. (2015) recognized with the vertebrate tracks, the ichnoassemblage has many aspects of three ichnospecies of Caririchnium, two with pointed digits (C. the Scoyenia ichnofacies (e.g., MacEachern et al., 2007; Buatois and kortmeyeri and C. billsarjeanti), but the digital pad features used to Mángano, 2011). Of the ichnoassemblage present at Clayton Lake, only distinguish those ichnospecies cannot be discerned on the Clayton Thalassinoides suggests some marine influence, but is well known to be ornithopod tracks. Therefore, identification of the Clayton ornithopod a facies-crossing ichnotaxon. Otherwise, all the traces are characteristic tracks as Caririchnium isp. is all that can be verified. Lockley and of a freshwater setting and well represent a low ichnodiversity Hunt (1995, fig. 5.25) illustrated a two-step quadrupedal ornithopod assemblage of the Scoyenia ichnofacies. trackway at Clayton Lake, but we did not recognize that trackway in May 2016. DINOSAUR TRACKS A common extramorphological variant of ornithopod pes track at The main track surface preserves 260 tracks, almost all in concave Clayton Lake has a long metatarsal impression (Figs. 8, 12C-D). These epirelief (Figs. 7-13). This number, however, is an underestimate, as are very deeply impressed undertracks that are about the same size as when counted (in May 2016), parts of the track surface were covered the other ornithopod tracks (length = 35-38 cm, width = 20-22 cm). by washed-in sediment. Furthermore, the boardwalk built around the They are diamond-shaped pes impressions that have short, pointed digit tracksite (Fig. 2) is concealing some tracks (C. Jordan, oral commun. impressions. Importantly, if the metatarsal impression is not considered, 2016). Of the ~ 260 tracks, almost all are of ornithopods (242). One the remainder of the track is very much the same morphology as some quadrupedal trackway is ornithischian, possibly stegosaur. The other 14 of the better preserved ornithopod tracks. They fall into two size tracks are theropod tracks, of two distinct morphologies. groups, smaller (27 tracks) and larger (52 tracks). Each footprint has an associated elongate metatarsal impression. Similar elongate metatarsal Ornithopod Tracks impressions have been recognized on other ornithopod tracks from the Most of the Clayton Lake tracks are of ornithopods (Figs. 7-8); we Cretaceous of Colorado (Lockley et al., 2014c), at Cerro de Cristo Rey counted 242 of the 260 tracks on the main track level as ornithopod. They in southern New Mexico (Kappus et al., 2011) and at the Paluxy Rver have previously been identified as Caririchnium isp. (e. g., Lockley in Texas (Kuban, 1989). and Hunt, 1995; Hunt and Lucas, 1998). The pes tracks assigned to A trackway of these undertracks consisting of at least seven Caririchnium at Clayton Lake are almost all slightly longer than wide footprints is approximately 3.8 m long (Fig. 12C). The tracks within (about 5-10% longer), although a few tracks are slightly wider (5-10%) this trackway are approximately 30 cm long. The estimated average than long. Pes lengths range from 39 to 67 cm, and widths range from pace length R (right) to L (left) = 58. 3 cm, whereas the estimated 36 to 62 cm, though most of the pes tracks are 45-50 cm long and 36- average stride R to R = 112.6 cm and L to L = 114 cm. The estimated 44 cm wide. All of the well-preserved footprints have pointed digit tips average pace angle of the track maker R-L-R = 149° and L-R-L = 135°. and large heels that are rounded posteriorly, though a few heels are The hip height of the trackmaker is estimated at 1.2 m based on the squared off or broadly triangular in shape (Fig. 7). Almost all of the formula of Thulborn (1990). Alexander’s (1976) formula for the speed Clayton ornithopod tracks are pes tracks without any associated manus estimate (stride/hip-height) = 0.9 m/s, which suggests slow walking. impressions. However, there is one trackway in which there is a clear Another trackway of long-metatarsal undertracks consisting of association of manus and pes tracks (Lockley and Hunt, 1995). All of four footprints is preserved (Fig. 12D). The tracks within this particular the ornithopod tracks at Clayton Lake clearly are undertracks, some trackway are large and each has an elongated metatarsal impression. made in a very soupy substrate. Thus, some tracks have extremely short The length of the trackway is 4.3 m. In some of the tracks the digits digit impressions that project forward from a large and overall rounded cannot be clearly defined, because of the poor preservation. The pace footprint impression (Fig. 7). length R to L = 128 cm, L to R = 110, and R to L = 118, whereas the 130

FIGURE 4. Taenidium on the main track surface at Clayton Lake State Park. A, overview of long burrow courses and overcrossing; also note Arenicolites (Ar) on the surface. B, eroded burrows showing meniscate fill. 131

FIGURE 5. Arenicolites on the main track surface at Clayton Lake State Park. Note paired nature of most of the burrows. 132

FIGURE 6. A, Planolites on the main track surface at Clayton Lake State Park. B, Thalassinoides on the main track surface at Clayton Lake State Park. 133

FIGURE 7. Selected ornithopod pes tracks on the main track surface at Clayton Lake State Park. stride R to R = 235 and L to L = 219 cm. The pace angle R-L-R = 165° relatively large, broader than long and semicircular (e.g., Gierlinski and L-R-L = 145°. The hip height of the trackmaker is estimated at and Sabath, 2008; Milàn and Chiappe, 2009; Pascual et al., 2012). If 2.5 m based on the formula of Thulborn (1990). Alexander’s (1976) this trackway is Deltapodus, then it indicates a late Albian stegosaur, a formula for the speed estimate (stride/hip-height) = 1.8 m/s, which considerable extension of the stratigraphic record of the group in North suggests slow walking. America. Stegosaur Trackway? Theropod Tracks At Clayton Lake, a four-step, quadrupedal trackway is clearly There are two kinds of theropod tracks at the Clayton Lake ornithischian but not obviously ornithopod (Fig. 9). The pes tracks tracksite. of this trackway are very long, rectangular to broadly triangular Magnoavipes impressions with short, pointed digits. Pes lengths are 28-37 cm, and widths are 24-26 cm, which is relatively small compared to the other There are six tracks assignable to Magnoavipes exposed at the ornithopod pes tracks at Clayton Lake. Clayton Lake tracksite (Figs. 10, 12A). These are relatively small The manus impressions are much smaller than the pes impressions. (length 17-19 cm, width 20-21.5 cm) tridactyl tracks, with thin, distally They are semicircular to slightly crescentric and located antero-lateral pointed digit impressions. The middle digit III is much longer than the to the pes tracks (Fig. 9). Manus lengths are 6-13 cm, and widths are lateral digits II and IV. The metatarsophalangeal impression is small; 18-22 cm. The trackway has a stride of 100 cm, pace of 48 cm, pes pace in some tracks it is faintly impressed, and in others it has a nearly width of 36 cm and a pace angulation of 120o. triangular shape. It comprises the pads of all three digits and pads of Although it is possible that this is the poorly preserved quadrupedal the distal ends of the metatarsals. The relatively small surface area of trackway of an ornithopod, Hendrik Klein (written commun., 2016) the metatarsophalangeal impression suggests that all three functional has suggested to us that it may be assignable to Deltapodus and thus metatarsals were tightly appressed to each other. The posterior ends the trackway of a stegosaur. Indeed, this trackway has the following of digits II and IV are at the same level, whereas that of digit III is features characteristic of Deltapodus: quadrupedal; pes prints outwardly anterior to them. However, that of digit IV lies slightly on the long axis directed with short digits and very large, triangular heel; and manus of digit III, whereas that of digit II does not. The estimated average foot 134

FIGURE 8. Selected ornithopod pes tracks with long metatarsal impressions on the main track surface at Clayton Lake State Park. Note that these are very deeply impressed undertracks. length/foot width ratio (FL/FW) is 0.92. The digits are fully impressed, right and left footprints are nearly in a single line, whereas the last left but no hallux impression can be observed. The estimated average digit footprint in the trackway strongly deviates to the left of the midline of divarication is 100° (range = 60-130o). Elongate and triangular-shaped the trackway, suggesting a slight turn by the trackmaker. The pace and digital claw impressions are evident on all digits. stride are not uniform. The pace lengths are: R to L = 90 cm, L to R = These tracks display all of the diagnostic features of Magnoavipes, 98 cm, R to L = 118 cm, whereas the stride lengths are: R to R = 182 generally considered to be the track of an ornithomimosaur (e.g., cm, L to L = 190 cm. The pace angle R-L-R = 160° and L-R-L = 125°. Lockley et al., 2001), to which they are assigned. Thus, they are the According to the formula of Thulborn (1990), the average hip-height tridactyl tracks of a biped with very narrow toes, a footprint length/ ratio of the trackmaker is 75 cm, and Alexander’s (1976) formula for width = 0.8-1.0, digit divarication is wide (65-105o), and the tracks are the speed estimate (stride/hip-height) = 2.48 m/s, a walking pace. slightly asymmetrical. Lockley et al. (2001, fig. 1) also referred a track from Clayton Lake to Magnoavipes, as did Hunt and Lucas (1998). Cf. Irenesauripus The Magnoavipes footprints include a trackway that is There are eight of these tracks (Figs. 11, 12B). These are relatively approximately 3 m long (Fig. 12A). The trackway consist of two right large (length = 38-60 cm, width = 33-38 cm) tridactyl tracks with and two left footprint impressions. At the beginning of the trackway the pointed digit tips and relatively short and thick digit impressions. The 135 metatarsophalangeal imprint is large and triangular to subtriangular posteriorly. Some of the largest tracks, with a length of 60 cm and width of 33 cm, have posteriorly oriented metatarsal impressions. However, these tracks do not preserve detailed morphology, whereas the smaller tracks with lengths between 38 cm and 44.2 cm are much better preserved and include digital pad impressions. The digit impressions in these tracks are robust. The angle of digit divarication is 35° to 85°. Some of the tracks preserve digital claw impressions, which are short and triangular-shaped. Theropod tracks like these have been described from the Cretaceous Dakota Group in Colorado and the Cedar Mountain Formation in Utah (Lockley et al., 2014a, b, 2015) and were either left unassigned or assigned to Irenesauripus (cf. Sternberg, 1932). These Clayton theropod tracks have relatively robust digits and moderate divarication angles, features characteristic of Irenesauripus. However, because so few tracks of these theropods are available at Clayton, and preservation is very variable, we only assign them to cf. Irenesauripus isp. Acrocanthosaurus or another, similar-sized theropod is the likely trackmaker (Lockley et al., 2014a, b) At Clayton Lake, a trackway consisting of three footprints of this morphology is preserved (Fig. 12B). The footprints within the trackway form a single line. The pace R to L = 88 cm and L to R = 98 cm, whereas the stride R to R = 182 cm. The estimated pace angle R-L-R = 166°. According to the formula of Thulborn (1990) the average hip-height ratio of the trackmaker is 148 cm, and Alexander’s (1976) formula for the speed estimate (stride/hip-height) = 1.2 m/s of the trackmaker, presumably a walking pace. Dinosaur Tail Drags Both Gillette and Thomas (1985) and Hunt and Lucas (2004) reported sinuous dinosaur tail drags associated with some ornithopod tracks at Clayton Lake. An example is one tail drag associated with ornithopod tracks that is ~ 210 cm long and is a continuous, sinuous concave impression between the right and left pes impressions (Fig. 13). The dinosaur tail traces at Clayton Lake are tail drag impressions sensu Kim and Lockley (2013) or protracted tail traces sensu Platt and Hasiotis (2008). They are essentially continuous, so the PIM (percent interruption metric of Platt and Hasiotis, 2008) is very low. The tail drags at Clayton Lake are all in the same area of the tracksite and headed northerly. They are associated with very deeply impressed ornithopod tracks with long metatarsal impressions. It seems likely that substrate conditions (soupy) in this area explain the presence of these tail drags. However, a more in depth analysis is needed to fully evaluate this hypothesis. DISCUSSION The Clayton Lake tracksite is characteristic of what Lockley has long called the “Dinosaur Freeway”—numerous dinosaur tracksites in the upper Albian-lower Cenomanian strata of the Dakota Group extending from the Colorado Front Range near Denver through southeastern Colorado, northeastern New Mexico and western Oklahoma (e.g., Lockley and Hunt, 1995). Thus, Clayton Lake is an ornithopod-dominated tracksite with lesser numbers of theropod tracks assignable to two ichnotaxa. Crocodile tracks also are present but rare at Clayton Lake. Turtle, pterosaur, ankylosaur and bird tracks are present at some of the Dinosaur Freeway tracksites (e. g., Lockley et al., 2010, table 6), but are not known at Clayton Lake. The present evaluation of the Clayton Lake tracksite, based primarily on field observations made in May 2016, indicates that there have been some substantial changes since Gillette and Thomas (1985) first described the tracksite. Thus, there are far fewer tracks present than originally reported. This is the result of about 30 years of weathering and erosion, which has obliterated or covered many tracks, while uncovering others. Indeed, an examination of the map of the Clayton Lake tracksite published by Gillette and Thomas (1985) indicates that many of the tracks visible in 1985 are either gone or covered in 2016. The Clayton Lake tracksite is scientifically significant for preserving an extensive record of late Albian ornithopod and theropod footprints and associated invertebrate traces. It also preserves relatively rare examples of dinosaur tail traces. Like other Dinosaur Freeway tracksites, it lacks sauropod tracks, so it is significant in documenting the extirpation of sauropods in North America by Cenomanian time, starting the “sauropod hiatus,” which lasted until the Maastrichtian, when sauropods immigrated back to North America, coming either FIGURE 9. Quadrupedal ornithischian trackway on the main track from South America or Asia (Lucas and Hunt, 1989). surface at Clayton Lake State Park, possibly Deltapodus. The Clayton Lake tracksite is also one of the dinosaur tracksites 136

FIGURE 10. Selected theropod tracks on the main track surface at Clayton Lake State Park referred to Magnoavipes isp. 137

FIGURE 11. Selected theropod tracks on the main track surface at Clayton Lake State Park referred to cf. Irenesauripus. 138

FIGURE 12. Traced drawings of selected trackways on the main track surface at Clayton Lake State Park. A, Magnoavipes isp. B, cf. Irenesauripus isp. C-D, Ornithopod tracks with elongate metatarsal impressions. 139

FIGURE 13. Dinosaur tail drags on the main track surface at Clayton Lake State Park associated with ornithopod tracks. most accessible to the public in the United States. As such, it is Díaz-Martinez, I., Pereda-Superbiola, X., Pérez-Lorente, F. and Canudo, J. a remarkable resource for public education. What is now needed I., 2015, Ichnotaxonomic review of large ornithopod dinosaur tracks: is a detailed, photogrammetric study of the tracksite to create an Temporal and geographic implications: PLOS ONE, DOI:10.1371/ accurate map of track distribution. This will establish a baseline for journal.pone.0115477. understanding further “evolution” of the tracksite due to erosion and Fürsich, F., 1974, On Diplocraterion Torell 1870 and the significance of weathering. It will also allow more detailed studies of the information morphological features in vertical, spreiten-bearing, u-shaped trace on dinosaur behavior contained at the Clayton Lake tracksite. fossils: Journal of Paleontology, v. 48, p. 952-962. Gierlinski, G.D.. and Sabath, K., 2008, Stegosaurian footprints from the ACKNOWLEDGMENTS Morrison Formation of Utah and their implications for interpreting other We are grateful to Charles Jordan for making our research at the ornithischian tracks: Oryctos, v. 8, p. 29-46. Clayton lake tracksite possible, and for assisting us in many other ways. Gillette, D.D. and Thomas, D.A., 1983, Dinosaur footprints in the Dakota Adrian Hunt, Eric Kappus, Hendrik Klein and Robert Sullivan provided Sandstone (Cretaceous) of northeastern New Mexico: Abstracts on the helpful reviews of the manuscript. Annual Symposium on Southwestern Geology and Paleontology, Museum of Northern Arizona, Flagstaff, p. 7A. 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