PACE VERSUS TROT: CAN MEDIUM SPEED GAIT BE DETERMINED FROM FOSSIL TRACKWAYS?

Transcription

1 Lucas, Spielmann and Lockley, eds., 2007, Cenozoic Vertebrate Tracks and Traces. New Mexico Museum of Natural History and Science Bulletin 42. PACE VERSUS TROT: CAN MEDIUM SPEED GAIT BE DETERMINED FROM FOSSIL TRACKWAYS? 309 MARY E. THOMPSON 1, RICHARD S. WHITE JR. 1 AND GARY S. MORGAN 2 1 International Wildlife Museum, 4800 West Gates Pass Road, Tucson, AZ 85745; 2 New Mexico Museum of Natural History, 1801 Mountain Road, NW, Albuquerque, NM Abstract Five camelid trackways from Arizona and New Mexico are described. Two trackways from the Bear Springs Badland (early Blancan) of Graham County, Arizona, represent a large camel, possibly Camelops or Megatylopus, and a medium-sized camel, possibly Hemiauchenia or Pleiolama. We also describe three camelid trackways from New Mexico. A medial Miocene (late Barstsovian) trackway from the Benavidez Ranch Local Fauna in Sandoval County represents a primitive, medium-sized camel, probably Protolabis. A late Miocene (early Hemphillian) trackway from the Gabaldon Fauna on the Isleta Pueblo in Valencia County is from a large camel possibly referable to the large lamine Alforjas. A late Pliocene (Blancan) trackway from the Cerros del Rio volcanic field near Santa Fe in Santa Fe County was made by a very large camel such as Gigantocamelus. The morphology for all five of these trackways fits within the definition of the ichnogenus Lamaichnum. We describe the characteristics of the trackways left by pacing and trotting camels and horses, and conclude that they can be differentiated in at least some cases in the fossil record. INTRODUCTION Trackways provide a snapshot of a brief point of time in an animal s life. Interpretation of these snapshots can yield data regarding that life. While several guides to tracks and trackways of extant vertebrates exist (Murie, 1974; Brown and Morgan, 1983; Halfpenny, 1986), few references are available for tracks of extinct mammalian taxa (Sarjeant, 1975). This difficulty is compounded by poor understanding of the morphology and contours of the feet of most extinct vertebrates. Small or juvenile members of one species may leave tracks the size and shape of a smaller, unrelated species. These factors make identification of a track of an extinct animal tenuous at best (LaGarry et al., 1998). Can pace be inferred from tracks alone? Alf (1966) described five Barstovian camelid trackways (Fig. 1) and concluded, based on the stride and the preserved print pattern (hindfoot print occurring ahead of forefoot print for same side), that these animals had an ipsilateral gait. Webb (1972) used this trackway pattern, combined with skeletal morphology changes in extinct camels, as evidence for the evolution of pacing. Dagg (1974) argued that analogous morphological changes are lacking in horses that pace efficiently and disputed that pacing can be inferred from a trackway alone. In an attempt to address this question, we have collected data from five extinct camelid trackways of various geologic ages and compared them to descriptive and observational data of extant camel and horse trackways. METHODS Data were collected and analyzed from five fossil camelid trackways. Measurements of the length and width of individual prints, the overall length of the trackway, the stride and straddle of the trackway, and the relative placement of fore and hind foot were obtained for each trackway. Stride (Fig. 1) is the distance from one footprint to the next print made by the same foot. The measurement is recorded from a specific point on the first print to the same point on the next print. The trailing edge of each print was used in this study. Straddle (Fig. 2) is measured perpendicular to the line of travel at the widest point of a group pattern (all four feet). Measurement of the stride gives a relative estimate of speed. Longer strides indicate faster speed of travel no matter what gait is employed. As speed increases, the stride will increase and the straddle will decrease (Halfpenny, 1986). FIGURE 1. Method of measuring stride, from the trailing edge of a given print to the trailing edge of the next print made by that same foot. Modified from Halfpenny, Footprint morphology was described using the terminology of Sarjeant and Reynolds (1999; Fig. 3). Camelid foot morphology is didactyl (two-toed) and symmetrical. The toes are the third and fourth digits and are supported by fleshy digital cushions. The distal phalanges and the surrounding soft tissue are often referred to as the claw due to the pointed impression that is created in the print. The interclavular gap is the V-shaped space between the claws. The interphalangeal trough is a space that separates the posterior portion of the digital cushions. The interdigital cleft is a ridge between the digital cushions and is visualized only in soft substrate when the phalanges are spread far apart. Locality information, faunal associations, and stratigraphy obtained for each trackway were used to identify potential track makers as well as for geologic age estimations. Fossil track/trackway studies typically represent groupings based solely on the shape of the track because of uncertainties in the identity of the track maker (Vialov, 1972), rather than Linnean groups (i.e. Class, Order, Family, Genus, Species) based on the relatedness of the track makers. Some workers, preferring to retain zoological affinities of trackways, have adopted a combined system in which morphologically-defined ichnogenera and inchnospecies are assigned to Linnean families and orders (Sarjeant, 1990, Sarjeant and Langston, 1994). We tentatively assign all camel footprints from the five Mio-Pliocene trackway sites in Arizona and New Mexico described here to Lamaichnum, following the revised diagnosis and description of this ichnogenus by Sarjeant and Reynolds (1999). Lucas and Hunt (this volume) refer all North American Neogene camelid footprints to Lamaichnum, recognizing several different ichnospecies distinguished primarily on the basis of size. For this study we have inferred the possible identity of track makers based on associated faunal material.

2 310 FIGURE 2. Measurement of straddle. Modified from Halfpenny (1986). tracks of Nannippus, a proboscidean, two carnivores, deer or pronghorn, and a total of four kinds of camel tracks (Thompson et al., 2002), including the two camelid trackways reported in detail in this paper. The tracks are associated with a Pliocene (early Blancan) fauna, the Bear Springs Fauna (Tomida, 1987; Morgan and White, 2005) approximately 3.0 to 3.5 Ma. Currently we have identified four different camelids from this fauna (Morgan and White, 2005): Camelops traviswhitei, Megatylopus gigas, Hemiauchenia sp. and Pleiolama vera. Based on the size of the prints in Trackway 1, the track maker appears to be a medium to giant camel, most likely Camelops traviswhitei or Megatylopus gigas. All prints are didactyl and symmetrical, representing the hoofprints of digits III and IV. Fore prints are larger (15 cm long x 10 cm wide) than hind prints (11 cm long x 6 cm wide). Each print (fore and hind) demonstrates clear digital claws separated by an interclavular gap. The prints are convex rather than flat and indicate the presence of fleshy digital cushions similar to those seen in extant camels. A marginal push up of sediment is apparent in some of the first prints in the trackway. This suggests an interdigital cleft and is an indication of a soft substrate at the time the prints were made. The interphalangeal trough is obvious on all prints. The stride, measured from right fore to right fore (trailing edge), equals 200 cm at the beginning of the trackway and increases to 207 cm in the direction of travel. The straddle, measured from fore foot to fore foot, initially equals 24.5 cm and decreases to 20.5 cm. The hindfoot print occurs in front of the forefoot print throughout the trackway. Trackway 2 This trackway was also discovered in the Bear Springs Badlands of Graham County, near Safford, Arizona. It appears in a block of mudstone that separated from its surface contact approximately 9 m above the arroyo floor. Examination of the remaining intact mudstone ledge revealed no additional tracks. The individual prints are deeper than those in Trackway 1, and some characteristics are not distinct (Fig. 5). Digital claws can be identified and indicate direction of travel. There is a long, narrow ridge of sediment pushed up, suggesting an interdigital cleft. Fore prints measured 14 cm long by 9 cm wide and hind prints measured 10 cm long by 5 cm wide. Placement of hind prints is in front of fore prints. Only one stride length could be obtained at 197 cm. Straddle could not be obtained. We believe the prints represent one half of the trackway, probably the right side based on the direction of travel. Again these prints are associated with an early Blancan fauna. The prints for Trackway 2 are smaller than those of Trackway 1, and the track maker could be either Hemiauchenia or Pleiolama vera. Both of these are camels are present in the associated fauna. NEW MEXICO FIGURE 3. Descriptive terms for camel footprint morphology (after Sarjeant and Reynolds, 1999). TRACK SITES Arizona Trackway 1 This trackway is located in the Bear Springs Badlands of Graham County, near Safford, Arizona. It consists of five sets (fore and hind) of prints from a medium to large camelid (Fig. 4). The tracks appear in a fine-grained sandstone that is traceable over a wide area, and that contains tracks and trackways at numerous localities. We have recorded Trackway 3 This trackway occurs on a 1.5 m square block on exhibit at the New Mexico Museum of Natural History (site number NMMNH L- 2930; Fig. 6). The block contains not only the trackway of a mediumsized camel but also five smaller individual camelid prints and the trackway of a large cat that crosses the camelid trackway. The trackway consists of five tracks preserved in a well-indurated, reddish-brown finegrained sandstone (catalog number NMMNH 26124) and is associated with the Benavidez Ranch local fauna, middle Miocene (late Barstovian), derived from the Cerro Conejo Member of the Zia Formation (Morgan and Williamson, 2000). Faunal remains of three camels have been described from this fauna, the very small protolabine Michenia and two medium-sized species, the protolabine Protolabis and the more advanced cameline Procamelus. Based on the size and shape of the prints, the suspected track maker is a medium-sized, early camelid, most likely Protolabis. Since only five tracks are preserved, just one stride length could be taken, 101 cm between the anterior tips of the left front foot of the first

3 311 FIGURE 4. Camel Trackway 1, from the Bear Springs Badlands, Safford, Arizona with outline drawing of tracks. Inset, is a set of right fore and hind prints. and fifth tracks. The straddle is 16.5 cm. However, much can be seen regarding the foot morphology. The prints are didactyl and symmetrical with clear ungual claws. The interclavular gap can be seen on all the prints. The prints also suggest the rudimentary development of digital cushions (axial surface is flat to slightly convex) while retaining features that give the appearance of a typical unguligrade stance (pointed toe impressions, minimal interphalageal gap) (Sarjeant and Reynolds, 1999). Trackway 4 This trackway (Fig. 7a) is located in the Gabaldon Badlands on the Isleta Pueblo, Valencia County, south of Albuquerque, New Mexico. It is preserved in a large block of fine-grained sandstone from the Popotosa Formation, and is associated with the late Miocene (early Hemphillan) Gabaldon Fauna (Lozinsky and Tedford, 1991). Three camelids are known from this fauna, the small Michenia, the medium-sized Procamelus, and the large lamine Alforjas. Lozinsky and Tedford (1991, fig. 7) provided a field photograph of this trackway, as well a close-up of a single, wellpreserved track. The trackway is 5.7 m long, semicircular in plane view, dipping in a southerly direction, and includes 14 individual prints with the fore prints ranging in length from 15 cm to 19 cm and in width from 12.7 to 16.5 cm. The hind prints range in length from 12.7 to 16 cm and in width from 10 to 12.7 cm. The stride is cm at the beginning of the trackway, changes to cm in the middle, and is cm at the end. The straddle starts at 30.2 cm, changes to 32.3 cm in the middle and ends at 28.6 cm. The depth and clarity of the prints also change from beginning to end of the trackway with deep unclear prints occurring in the middle of the trackway and becoming shallow and distinct at the end. Digital claws can be visualized on most of the prints, giving a clear indication of direction of travel. Early photographs (e.g. Fig. 7b) show a clear interclavular gap and the sediment ridge formed by the interdigital cleft. Figure 7c (the same print as Fig. 7b) shows the weathering that has occurred over the last 10 years. Variation of print size, change of stride length and straddle, and the semicircular appearance of the track suggest that the track maker entered a soft muddy area (possibly a shallow shoreline), slowed as it entered deeper softer mud, and then quickened its step and changed direction. Hind print placement is always in front of the fore print. Based on faunal remains from the Gabaldon Fauna, the probable track maker is a very large camelid, either Alforjas, which occurs in this fauna, or Megatylopus, which is known from other Hemphillian faunas in New Mexico. Trackway 5 This trackway (Fig. 8) occurs on the eastern edge of the Cerros del Rio volcanic field near the Santa Fe Airport, Santa Fe County, New Mexico, with a estimated age of late Pliocene (late Blancan) based on radioisotopic dates of Ma on correlative volcanic strata (Koning et al., 2001). There are no faunal remains associated with this trackway. The track layer is in a buff-colored, fine-grained volcaniclastic sandstone bordered above and below by a black basaltic ash. The trackway is 23.9 m long and has a total of 52 individual tracks and appears to continue into the bank. It is on BLM property and was opened to the public until theft of individual tracks occurred in the late 1970s. Fore print lengths vary from 18.5 to 20 cm and 15 to 17 cm in width. Hind prints range in length from 15 to 17 cm and in width from 12 to 15 cm. The depth of prints also varies suggesting a soft substrate when the prints were made. The stride

4 312 FIGURE 5. Camelid Trackway 2, from Bear Springs Badlands, Safford, Arizona with outline tracing of tracks. Insets show details of individual tracks. FIGURE 6. Camelid Trackway 3 on exhibit at the NMMNH. The fifth print in this trackway, a partial left front track is not visible in the photo. Notice the cat prints, with the hind print overlapping the fore print. length is fairly consistent throughout the trackway, with the average being cm. Straddle is also consistent, with an average of 30.5 cm. Hind print placement is in front of the fore print. While most of the prints have weathered into nondescript rounded blobs, a few of the prints do provide clear morphology. Interdigital clefts and claws can be discerned on 20 of the prints showing the direction of travel was south towards the bank visible at the top of the photograph (Fig. 8). These prints also show an interclavular gap and a proximal gap. These prints were made by a very large individual. Since there are no faunal remains nearby it is difficult to determine a possible track maker. Based on the age of the trackway (Pliocene) and the size of the prints the most likely candidate would be Gigantocamelus, which has been described from similarly-aged sediments in Arizona and New Mexico (Morgan and Lucas, 2003; Thompson and White, 2003; White and Morgan, 2005). FIGURE 7. Isleta Pueblo camelid Trackway 4. Note the semicircular pattern and depth of prints. Also note the weathering of prints, B, pre-1991 from Lozinsky and Tedford 1991, C, FIGURE 8. A, Santa Fe camelid trackway, with outline tracing of a portion. B, shows a closeup of prints. Note the large area at top of inset where tracks were removed by theft. DISCUSSION Trackways may be the key to determining whether an animal is pacing or trotting. Figure 9 shows a computer generated trackway of a horse pacing and trotting. However, the computer generated pattern is at odds with a number of direct observations. Both the pace and trot show placement of the ipsilateral hind print in front of the fore print. In a trot, the hind foot can be placed on top (capping) (Fig. 10) or in front (overstride) of the fore print, depending on the speed (slow or fast trot) (Zeigler, 2005). In a pace, the hind foot is always placed in front of the fore print, with the distance of overstride increasing with the speed of the gait (Fig. 9). All of the camelid trackways examined demonstrate the footfall pat-

5 FIGURE 9. Computer generated trackway for a horse pacing (top) and trotting (bottom). From Alexander (2003). FIGURE 10. A, Overstride and B, capping. Modified from Ziegler (2005). tern of the hind foot placement in front of the fore print. Stride and straddle can be used to estimate relative speed. As speed increases, stride will increase and straddle will decrease. Stride and straddle obtained from three of the trackways demonstrate a fairly fast medium speed gait being used by the track maker. Because a fast trot will have overstriding of the hindfoot beyond the fore print like a fast pace, it is impossible to differentiate whether the track maker is pacing or trotting. Webb (1974) used the published description of a trackway described by Raymond Alf and his students from the Webb School in California to provide evidence combined with skeletal morphological changes for the evolution of pacing in the family Camelidae. Alf stated that the study of the stride indicates the animal then as now had an ipsilateral gait as it moved across the ancient Barstovian mudflat. This 313 statement was based on the placement of the ipsilateral hindfoot in front of the fore print in the trackway examined. Alf also measured step length, not stride length (measurement from right front print to the left front print). We believe this trackway represents the footfall pattern of a medium speed gait based on comparison to numerous personal observations of trackways made by modern horses and compared to the fossil trackways examined in this study. Placement of prints on the drawing suggests a long stride and narrow straddle, which would indicate the track maker was moving at a relatively fast gait. Since there is no evidence of capping, and compared to the computer generated pattern, it is difficult to discern whether the animal was moving at a fast trot or fast pace. SUMMARY AND CONCLUSIONS Trackways of extinct animals can provide a reliable method to determine the locomotor pattern of the individual. However, caution needs to be exercised when using trackways to differentiate between two gaits within a category such as a pace and a trot. The researcher can be reasonably sure that if capping (placement of hindfoot on top of fore print) is observed that it represents a trot and not a pace. However, if the stride length and straddle indicates a faster medium speed gait it is difficult to ascertain whether the track maker is pacing or fast trotting. Further work with extant species, examining the correlation of distance of overstride occurring in trotting and pacing trackways, and other parameters such as straddle, may provide insight regarding this problem. ACKNOWLEDGMENTS We thank the U. S. Bureau of Land Management (BLM) who allowed us access to the both the Bear Springs and Santa Fe camel trackway sites, located on lands under their jurisdiction. Larry Thrasher of the BLM office in Safford, Arizona was instrumental in our study of the Bear Springs camel trackways. The BLM office in Albuquerque, New Mexico provided personnel and equipment to help excavate and study the Santa Fe camel trackway, and in particular, we thank Mike O Neill, Patricia Hester, Danny Wiese, and Toby Lucero. The Pueblo of Isleta gave us permission to study the camel trackway on their land in the Gabaldon badlands. We especially thank John Sorrell, a geologist for Isleta Pueblo, who made arrangements for us to visit and study the Gabaldon trackway. Benjamin Benavidez gave us permission to study the Benavidez Ranch camel trackway located on his private ranch. REFERENCES Alexander, R.M., 2003, How animals move: Focus Multimedia Ltd. Alf, R. 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6 314 invertebrate traces from the Chadronian (Late Eocene) of Trans-Pecos Texas: Texas Memorial Museum Bulletin, v. 36 p Sarjeant, W.A.S., and Reynolds, R.E., 1999, Camel and horse footprints from the Miocene of California: San Bernadino Couty Museum Quarterly v. 46(2) p Tedford, R. H., 1970, Principles and practices of mammalian geochronology in North America: Proceedings of the North American Paleontological Convention, Pt F, p Thompson, M.E., Meldrum, D.J., White, R.S., and Thrasher, L., 2002, Camel tracks and trackways from Late Pliocene Deposits, Graham County, Arizona (abs.): Journal of Vertebrate Paleontology, v. 22 (3, supplement), p. 114A. Thompson, M. E., and White, R.S., 2003, Late Cenozoic camelids (Mammalia: Artiodactyla) from Graham County, southeastern Arizona (abs.): Journal of Vertebrate Paleontology, v. 23(3, supplement), p. 103A. Tomida, Y., 1987, Small Mammal Fossils and Correlation of Continental Deposits, Safford and Duncan Basins, Arizona, USA: Tokyo, National Science Museum, 141 p. Vialov, O. S., 1972, The classification of the fossil traces of life: Proceedings of the 24 th International Geologic Conference, Section 7, p Webb, S. D., 1972, Locomotor evolution in camels: Forma et Functio, v. 5, p Ziegler, L., 2005, Easy-Gaited Horses: North Adams, Storey Publishing, 247 pp.