Journal of the Arkansas Academy of Science Volume 67 Article 21 2013 Morphometric Analysis of Dinosaur Tracks from Southwest Arkansas R. Shell University of Arkansas, Fayetteville Stephen K. Boss University of Arkansas, Fayetteville, [email protected] Follow this and additional works at: http://scholarworks.uark.edu/jaas Part of the Paleontology Commons Recommended Citation Shell, R. and Boss, Stephen K. (2013) "Morphometric Analysis of Dinosaur Tracks from Southwest Arkansas," Journal of the Arkansas Academy of Science: Vol. 67 , Article 21. Available at: http://scholarworks.uark.edu/jaas/vol67/iss1/21 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact [email protected], [email protected]. Journal of the Arkansas Academy of Science, Vol. 67 [2013], Art. 21 Morphometric Analysis of Dinosaur Tracks from Southwest Arkansas R. Shell and S.K. Boss* Department of Geosciences, 113 Ozark Hall, University of Arkansas, Fayetteville, AR 72703 *Correspondence: [email protected] Running Title: Morphometric Analysis of Dinosaur Tracks from Southwest Arkansas Abstract new trackway site exposing innumerable sauropod tracks and trackways similar to those described by Dinosaur trackways were discovered in Cretaceous Pittman and Gillette (1989), but also the first-reported De Queen Limestone strata in Howard County, occurrence in Arkansas of tridactyl tracks and Arkansas, in June 2011. Multiple trackways with trackways of theropod dinosaurs and three poorly variably sized tridactyl tracks were exposed in a preserved tetradactyl tracks of uncertain origin. commercial quarry, suggesting multiple theropod species or adult and juvenile tracks of a single species. Results of morphometric analyses of 32 plaster casts from selected trackways are reported in an effort to identify the specific track-making dinosaurs and differentiate large and small tracks. Track measurements included length and width of each track, the lengths and widths of each digit impression, and the angular spread (divarication) between digit impressions. Twenty-nine plaster casts were of tridactyl theropod tracks whereas three casts were of poorly preserved tracks of a presumed but unknown tetradactyl (and possibly tetrapod) organism. Plaster casts of tridactyl theropod tracks ranged from 0.36 to 0.61 m long and 0.22 to 0.54 m wide. The longest digit impression on each track was the second, or middle, digit (range = 0.15 – 0.35 m long) with total digit divarication ranging from 31 - 57 degrees. The Figure 1. Arkansas Map showing Howard County (shaded). From Arkansas track measurements were compared to tracks http://upload.wikimedia.org/wikipedia/commons/thumb/8/85/Map_ of_Arkansas_highlighting_Howard_County.svg/200px- (Eubrontes glenrosensis Shuler 1935) preserved in the Map_of_Arkansas_highlighting_Howard_County.svg.png) correlative Glen Rose Formation, Texas and attributed to the large Early Cretaceous carnosaur, Acrocanthosaurus atokensis. The E. glenrosensis track The theropod tracks occur in multiple trackways measurements from Texas plotted within the Arkansas up to 40 m long and individual tracks are of several data range, suggesting affinity of the Arkansas tracks distinct size classes suggesting multiple individuals to E. glenrosensis. Relatively poor preservation of responsible for trackways and potentially representing tetradactyl tracks precluded morphometric analysis, but different species or a single species in different stages visual comparison to known Cretaceous crocodilian of development (adults versus juveniles). The largest tracks is suggestive of affinity to such organisms. theropod tracks are morphologically similar to large tridactyl tracks (Eubrontes glenrosensis Shuler 1935; Introduction Lockley, 2000; Lockley et al. 2000; Adams et al. 2010) preserved in the Glen Rose Formation in Texas and Open-pit quarries and mines in Howard County, attributed to the early Cretaceous carnosaur, Arkansas (Fig. 1) have long been known to expose Acrocanthosaurus atokensis (Farlow 2001). Smaller dinosaur tracks and trackways (Pittman and Gillette theropod trackways observed at the Howard County 1989). In June 2011, continued excavation unearthed a site may represent juvenile Eubrontes glenrosensis, or Journal of the Arkansas Academy of Science, Vol. 67, 2013 121 Published by Arkansas Academy of Science, 2013 121 Journal of the Arkansas Academy of Science, Vol. 67 [2013], Art. 21 R. Shell and S.K. Boss a different ichnospecies of Eubrontes, or another in shallow water but being propelled forward using ichnospecies altogether. Morphological differences only their front feet (Gillette and Lockley 1991, Bird among theropod tracks observed at this site are 1944, Lee and Huh 2002, Henderson 2004, Lee and described in detail below. Brief mention is also made Lee 2006; contra Lockley and Rice 1990, Hwang et al., regarding morphology of the three enigmatic 2008). Carnosaur tracks coincident with sauropod tetradactyl tracks, though ichnospecies identification tracks provide evidence of predator-prey relationships and attribution to a particular trackmaker was difficult that body fossils do not (Gillette and Lockley 1991). due to poor preservation of these tracks. Farlow (1981) and Alexander (1991) also outlined the The purpose of this investigation was to assess uses of dinosaur tracks for determining the mechanics how the observed theropod track morphologies related of walking dinosaurs, their speeds, and their weights. to one another, and how they related to other Albian- aged theropod tracks from Texas and Australia. It was Geologic Setting not known if the Arkansas tracks represented a single unknown ichnospecies, a single known ichnospecies, Dinosaur tracks and trackways in Arkansas are several unknown ichnospecies, or some combination known exclusively from the De Queen Limestone thereof. Morphometric data detailed below were (Pittman 1984). The formation as a whole is exposed in obtained from 32 plaster casts from the Arkansas southwest Arkansas (Sevier, Howard, and Pike trackway site and compared to similar tracks observed Counties) (Pittman 1984, Pittman and Gillette 1989) in correlative strata of Texas (Farlow et al. 2012, and generally consists of interbedded claystone, Farlow 1982) and a known track site of similar age gypsum, and limestone (Fig. 2). It is correlative to the from Australia (Romilio and Salisbury 2011). An Glen Rose Formation (Trinity Group; Forgotson, 1957) attempt to identify the tetradactyl tracks was also of Texas. made, but this attempt was based strictly on visual comparison to known Cretaceous tetradactyl tracks, owing to the small number and poor preservation of these tracks. Previous Work Shuler made the first published report of dinosaur tracks from the Gulf Coast region in 1917 (Shuler 1917). This report described sites along the Paluxy River in Somervell County, Texas. Reports of other sites in the region began in 1922 and have continued to this day (Pittman 1989). Much of the early work on dinosaur footprints in Texas comes from Roland T. Bird, who described, mapped, and even excavated sauropod and carnosaur tracks from the Paluxy River (Bird 1939 and 1941). Sauropod tracks and trackways in Arkansas were thoroughly described by Pittman and Gillette (1989) and compared to similar tracks and trackways exposed in the correlative Glen Rose formation of Texas (Forgotson, 1957; Langston 1974). Dinosaur tracks and trackways provide crucial evidence of biomechanics (movement and locomotive behavior) of dinosaurs (Farlow et al. 2012). For example, Gulf Coast sauropod tracks proved that the leg structure of this group of very large dinosaurs was Figure 2. The wall of the Howard County, Arkansas open-pit of sufficient design and strength to permit walking on quarry showing gypsiferous claystone and limestone of the Early Cretaceous De Queen Limestone correlative to the Glen Rose land, though it is also evident from some trackways the Formation, Texas (Trinity Group; Forgotson, 1957). Dinosaur animals were wading in shallow water, and even track-bearing limestone at top of section (arrows). Scale bar is controversially suggest they were partially swimming approximately 5 meters. Journal of the Arkansas Academy of Science, Vol. 67, 2013 122 http://scholarworks.uark.edu/jaas/vol67/iss1/21 122 Journal of the Arkansas Academy of Science, Vol. 67 [2013], Art. 21 Morphometric Analysis of Dinosaur Tracks from Southwest Arkansas The unit is assigned to the Albian stage (Early Methods Cretaceous) based on presence of Douveilliceras ammonites and the foramiferan, Orbitolina texana. During summer 2011, 32 dinosaur tracks (29 (Pittman 1984, Loucks and Longman 1982). theropod and 3 tetradactyl tracks) were cast using Sedimentary structures observed at the site and by plaster, burlap, and wire mesh. The casts were made by Pittman (1989) indicated that the tracks were made greasing dinosaur track impressions in the trackway close to the shoreline of the Cretaceous Gulf Coastal limestone and carefully coating the track interior with Plain.