Anaglyph Stereo Imaging of Dinosaur Track Morphology and Microtopography
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Palaeontologia Electronica http://palaeo-electronica.org ANAGLYPH STEREO IMAGING OF DINOSAUR TRACK MORPHOLOGY AND MICROTOPOGRAPHY Stephen M. Gatesy, Neil H. Shubin, and Farish A. Jenkins, Jr. ABSTRACT Fossil tracks should be recorded by methods that foster detailed ichnological analysis. Although outline drawings remain the standard currency of footprint illustra- tion, their simplicity entails a tremendous loss of information. By contrast, monocular photographs are highly detailed but often suffer from suboptimal lighting, which can cause misperceptions. Anaglyph stereo imaging offers a compact, scale-independent format for illustrating and presenting the complex three-dimensional (3-D) shape of dinosaur footprints. Using examples from the Upper Triassic Fleming Fjord Formation of East Greenland, we address the benefits of anaglyphs to the exploration and exposi- tion of theropod tracks in both the field and laboratory. We find that the addition of ste- reopsis to other available depth cues (shading, cast shadows) maximizes the information content of a 2-D image while minimizing erroneous or ambiguous percep- tions of shape. Stephen M. Gatesy. Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912, USA [email protected] Neil H. Shubin. Department of Organismal Biology and Anatomy, and Committee on Evolutionary Biology, University of Chicago, Chicago, Illinois 60637, USA [email protected] Farish A. Jenkins, Jr. Department of Organismic and Evolutionary Biology, and Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, USA [email protected] KEY WORDS: anaglyph; track; footprint; dinosaur; Greenland; Triassic, Norian PE Article Number: 8.1.10 Copyright: Society of Vertebrate Paleontology May 2005 Submission: 30 November 2004. Acceptance: 6 March 2005. INTRODUCTION and dragged into a new configuration that variably records aspects of pedal anatomy and locomotor Footprints record the dynamic interaction movement (Padian and Olsen 1984; Thulborn and between an animal’s limb and a malleable sub- Wade 1984, 1989; Gatesy et al. 1999; Gatesy strate (Baird 1957). As the foot penetrates and is 2003). This imperfect mold is further altered, if not extracted, nearby sediment is pushed, sheared, Gatesy, Stephen M., Shubin, Neil H., and Jenkins, Farish A. Jr., 2005. Anaglyph Stereo Imaging of Dinosaur Track Morphology and Microtopography, Palaeontologia Electronica Vol. 8, Issue 1; 10A: 10p, 693KB; http://palaeo-electronica.org/paleo/2005_1/gatesy10/issue1_05.htm GATESY, SHUBIN, & JENKINS: ANAGLYPH IMAGING OF DINOSAUR TRACKS completely destroyed, by pre-burial erosion, graphic techniques for more than 90 years (e.g., diagenesis, post-exposure weathering, collection, Hudson 1913, 1925) and the methodology is well and preparation. A fossil track that survives these described (Gott 1945; Evitt 1949; Feldman 1989; harsh filters can offer unique and valuable evi- Knappertsbusch 2002). Yet, although several ste- dence of behavior in extinct taxa (e.g., Seilacher reo pairs of tetrapod track photographs have been 1967). For ichnological analyses to be well published (Sarjeant and Thulborn 1986; Ishigaki founded, however, footprints must be documented and Fujisaki 1989; McAllister 1989), this approach by methods that minimize loss of information. Inac- has been underutilized relative to the widespread curate representations of track morphology can use of stereophotography to illustrate skeletal fos- distort or obscure potentially important data. sils. There are drawbacks that make traditional ste- Compared to many other fossils, description reo pairs less than ideal for publication and of footprints can be particularly challenging. The presentation (e.g., Evitt 1949). Printed figures are most important distinction is that the internal mor- constrained to relatively small widths (less than ca. phology of bones, eggshells, and soft tissues are 8 cm), which preclude highly detailed images span- preserved when actual biological material serves ning a full page. At the same time, separate left as a template for mineral replacement. By contrast, and right images require at least twice the area of tracks are purely sedimentary structures that only an individual plate. Traditional stereo pairs can only reflect a foot’s external morphology and frequently be projected before an audience using specialized lack discrete borders. Second, images of track sur- polarizing or LCD equipment. faces are informative from a limited range of per- Anaglyph stereo imaging offers an alternative spectives, whereas skeletal elements can often be method of presentation that is scale-independent. figured from many viewpoints. Finally, many foot- An anaglyph is a color image formed by superim- prints are studied only in the field, rather than col- posing left and right members of a stereo pair. The lected or cast. Even under controlled laboratory two original images are color-converted so that conditions, a track’s three-dimensional contours each is invisible when viewed through a corre- and textures are notoriously difficult to quantify and spondingly colored gel. Inexpensive and widely illustrate, making these phases of analysis espe- available “3-D glasses” with different lenses (red- cially prone to inaccuracy or bias. blue, red-green, or red-cyan) are worn to provide The human visual system is adept at deter- each eye with its appropriate image. Instructions mining the distance and orientation of surfaces. for creating anaglyphs using imaging software However, just as a camera compresses a 3-D such as Adobe Photoshop are available in Purnell world onto a planar CCD or film, depth information (2003) and on many websites. Anaglyphs can be is lost when the environment is projected onto the printed and projected at any size, making them 2-D receptor array in our retina (e.g., Palmer ideal for journals, websites, museum displays, 1999). Our brain must therefore use a number of poster sessions, small seminars, and large confer- different signals to perceive spatial arrangement ence halls. Sequential anaglyphs can be easily and resolve ambiguity. Some sources of informa- combined to create compelling stereo animations. tion are intrinsically dynamic, such as the differen- Paleontologists have recently taken advantage of tial motion of objects at unequal distances (motion this technique by publishing static and animated parallax). However, most other depth cues are anaglyphs of conodont and invertebrate microfossil static and potentially useful for extracting 3-D infor- material (Knappertsbusch 2002; Purnell 2003). mation from 2-D images on a monitor or in print Herein, we address the utility of anaglyph ste- (Palmer 1999; Ware 2004). These cues include reo imaging to the exploration and exposition of occlusion (near overlaps far), perspective (conver- dinosaur tracks. Our examples are tracks attribut- gence of parallel lines, position relative to the hori- able to small theropods that are preserved in the zon, relative size, atmosphere), focus (depth of Late Triassic Fleming Fjord Formation of Jameson field), shading, cast shadows, and stereopsis. Land, East Greenland (Jenkins et al. 1994; Gatesy Unfortunately, tracks rarely show features with et al. 1999; Gatesy 2001, 2003). We present three enough topography to benefit from occlusion, per- case studies ranging from field to laboratory, from spective, or depth of field. This leaves shading, whole footprints to minute skin impressions, and cast shadows, and stereopsis as signals available across a range of imaging techniques. We include for elucidating and communicating track geometry. specific methods as part of each case study. Our Stereopsis is the extraction of depth informa- goal is to focus on the benefits of anaglyphs for tion from differences between images recorded by footprint studies in general, rather than on specific our two retinas (binocular disparity; e.g., Palmer descriptions or interpretations of these specimens. 1999). Paleontologists have used stereophoto- 2 GATESY, SHUBIN, & JENKINS: ANAGLYPH IMAGING OF DINOSAUR TRACKS Figure 1. Five photographs of the same deep theropod track (MGUH VP 3391) in situ under different field lighting. Arrows depict primary direction of sunlight. Photograph 1.5 was taken under relatively uniform, ambient illumination on an overcast day. Infilling matrix was not yet removed when Figure 1.1 was taken. Scale bars equals 100 mm. MATERIALS this flexibility, many of our track photographs suffer from the commonly encountered flaws of excessive Footprints are now known to be quite common contrast (Figure 1.2-1.4), misleading or concealing in the Ørsted Dal Member of the uppermost Flem- shadows (Figure 1.3-1.4), confusing color artifacts, ing Fjord Formation (Norian-Rhaetian; Clem- or morphological ambiguity due to uniform illumina- mensen 1980; Clemmensen et al. 1998). tion (Figure 1.5). Even when multiple images are Specimens described in this study were photo- captured of the same track under different lighting graphed and collected on the eastern slope of conditions, the topology of the sediment’s surface Wood Bjerg (71° 24.88´N, 22° 33.17´W) and the may not be obvious. Morphological description, western slope of Tait Bjerg (71° 29.08´N, 22° artistic illustration, and scientific interpretation can 39.10´W). All collected material will be housed at be hampered by this variable fidelity, particularly if the Geological Museum at the University of Copen- viewers are unfamiliar with the original material.