North America's Largest Dinosaur Trackway Site: Implications for Morrison Formation Paleoecology
North America's largest dinosaur trackway site: Implications for Morrison Formation paleoecology
MARTIN G. LOCKLEY \ KAREN J. HOUCK | Department of Geology, University of Colorado at Denver, Denver, Colorado 80202 NANCY K. PRINCE j
ABSTRACT sity (~5) and predator-prey ratio (1:30) are in eastern Colorado. Although "the Morrison general agreement with the estimates of other Formation has yielded one of the richest dino- Little-known exposures of the Upper Ju- authors based on skeletal remains, and they saur faunas of the world" (Dodson and others, rassic Morrison Formation from the Purga- suggest that the Purgatoire River tracks may 1980), tracksites have hitherto remained un- toire Valley of southeastern Colorado have accurately reflect the composition of the known or essentially undocumented. We there- yielded the world's largest continuously dinosaur fauna. Distinctive groupings of paral- fore document what is the world's largest Late mapped assemblage of dinosaur trackways. lel, non-overlapping trackways suggest gre- Jurassic footprint site, evaluate its paleobiologi- Body fossils include plant, invertebrate, and garious behavior among sauropods and tri- cal significance, and report briefly on five other vertebrate remains indicative of predomi- dactylous forms. Morrison tracksites in Colorado (Fig. 1). For nantly fresh-water conditions. The tracks convenience we have dubbed the large Purga- occur in a lacustrine sequence characterized INTRODUCTION toire River site "Dinosaur Lake." by (1) shallow-water shales; (2) micritic The study also answers the call of Dodson shoreface limestones with ooids, intraclasts, This paper describes the paleoecology and and others (p. 229) for a "fruitful period of re- ripple marks, mud cracks; and (3) minor depositional environment setting of a unique newed multidisciplinary interest in the Morrison quartzose sandstone with salt-crystal casts. Late Jurassic dinosaur trackway site in south- Formation and its biota." Unlike their broad Analysis of these sedimentary facies suggests that in southeastern Colorado, lakes were larger and longer lived than in other Morri- limit of Morrison Formation —>;::' son paleoenvironments. Trackway orientations and footprint-depth contours pinpoint the location of the paleo- shoreline at successive levels in the section. Detailed mapping of 1,300 footprints in bed 2 has revealed more than 100 trackways which • 10 *
testify to the activity of both quadrupedal and u . bipedal dinosaurs. The respective ratio based • 9 on trackway counts is -40/60. The quad- rupedal tracks are attributed to sauropods, and represent the first ever discovered in North America. They also exhibit the first " 3 known manus claw impressions and are in need of formal description. More than 90% of the bipedal tridactyl prints lack claw impres- sions and are tentatively therefore referred to the Ornithopoda, cf. Gypsichnites possibly a Camptosaurus. The small proportion with distinct claw impressions may represent Allo- saurus. Resulting estimates of species diver-
Figure 1. Location of important dinosaur track sites in Colorado and M-11 principal fossil localities after Dodson et al 1980 adjacent states in relation to skeletal-bearing sites 1-11 listed by Dodson and others (1980). A-E, respectively, represent the main Purgatoire * 10 tracks River site 20 km southwest of Higbee, and the Higbee, Fort Collins, é 10C100 tracks - PRINCIPAL FOOTPRINT LOCALITIES State Bridge, and Fruita sites referred to in the text. F-H, respectively, * refer to footprint sites mentioned in the literature by Marsh (1899), 1000 tracks A - H Lockley (1986b), and Hatcher (1903).
Geological Society of America Bulletin, v.97,p. 1163-1176, 10 figs., 3 tables, October 1986.
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study, ours is limited mainly to one geographic PALEONTOLOGY Formation is similar to the lower Morrison, and area and paleoenvironmental setting but focuses the thin discontinuous chert layers sometimes on the entire preserved biota, including plants, Although the dinosaur fauna "is one of the used to define the contact (Oriel and 1 kludge, invertebrates, vertebrates, and trace fossils richest in the world" (Dodson and others, 1956) are of dubious stratigraphic value ( Brady, (Lockley and others, 1984). The study also pro- 1980), the Morrison flora, invertebrate, and 1969). Similarly, the overlying Lytle Formation vides the first detailed account of the lacustrine small vertebrate fauna are generally rather resembles the upper Morrison (Altschuld, 1980) depositional systems in this region. sparse and poorly known. The same applies to and commonly appears conformable (Long, terrestrial vegetation which evidently consisted 1966). Scott (1970), however, suggested an un- GENERAL PALEOGEOGRAPHIC AND mainly of conifers, with additional cycads and conformable contact based on his recognition STRATIGRAPHIC FRAMEWORK ferns known mainly from Utah (Chandler, of upper Albian mollusc zones in the Purga- 1966; Tidwell, 1975) and palynomorphs from toire Formation immediately above the Morri- During the Lale Jurassic, Colorado was part the Brushy Basin Member (Tschudy and others, son. Elsewhere, the lack of useful zone fossils of an extensive (1,000,000 km2) low-lying al- 1980). Although little vegetation has been re- hampers efforts to resolve stratigraphic corre- luvial plain, bordered to the west and southwest ported from the Morrison of Colorado, the Pur- lation and accurately determine the age of the by magmatic arc terrain, which provided the gatoire track site has yielded a number of plant Morrison. main source of sediment (Dodson and others, fossils, including Equisetum sensu lato. Stromat- 1980; Imlay, 1980; Reynolds and Dolly, 1983). olites and charophytes are also known from HISTORY OF THE PURGATOIRE The alluvial basin may have resembled the pres- various lacustrine facies (Peck, 1956; Ott, 1958; RIVER SITE ent Hwang-Ho and Yangtse-Kaiang alluvial Lockley and others, 1984). plains of China (Mook, 1916) and/or the Gran Fresh-water invertebrate remains are known Surprisingly, the spectacular Purgatoire River Chaco Plain of antral South America (Mo- from a number of localities (White, 1886; Stan- site has been known for more than 50 yr but berly, 1960). Local higher relief is indicated by ton, 1915; Yen, 1952) but are not well docu- never studied. It was brought to the attertion of unconformable overlap of Morrison sediments mented from a paleoecological or depositional the scientific community by the Riddennoure onto Precambrian basement rock (Brady, 1969). environment viewpoint. The most common in- family of Higbee, Colorado, in 1935; in 1936, The area was evidently situated in tropical to vertebrates are pulmonate (lung-bearing) and collectors from the Denver Museum of Natural subtropical latitudes, between paleomagnetic lat- prosobranch (gill-bearing) snails, unionid bi- History recovered two isolated tridactyl prints itude 20°-30° (Steiner, 1975). valves, and ostracods, all of which are well rep- (DMNH 1471). In 1937, a section of trackway The Purgatoire River dissects the gently dip- resented in the study area as part of a (DMNH 1498) of a smaller biped (Fig. 8g ping Purgatoire uplift (Heaton, 1939). This up- recognizable paleoecosystem (Lockley and oth- below) was collected from a locality 14 mi lift covers much the same area as the late ers, 1984; Lockley, 1986a, 1986b, and Fig. 6 downstream (Markman, 1936, 1937). All are Paleozoic Apishapa uplift, which was eroded below). currently on display. The only publications to down before Mesozoic time (Maher, 1945) and appear at this time were the extremely short probably provided little relief during the Jurassic HISTORY OF RESEARCH IN accounts by MacClary (1936, 1938, 1939) and (Tweto, 1980). Like the associated Muddy SOUTHEASTERN COLORADO Bird (1939a, 1944, p. 67), illustrating trackways Creek Monocline, it is a young Laramide/post- from the main Purgatoire site. Archival library Laramide feature (Taylor, 1974). Reconnaissance of the geology of southeast- records from MacClary's home town of Pueblo, Cross (1894) first applied the name "Morri- ern Colorado indicated that "Juratrias" and Cre- Colorado, indicate that Roland T. Bird visited son Formation" to the drab fresh-water marls, taceous strata were exposed along the Purgatoire the site in the fall of 1938 and that Barnum with minor sandstone and limestone, between River (Gilbert, 1896). Lee (1901, 1902) subse- Brown proposed to do so, also. In November, the brown and pink Triassic sandstone and the quently concluded that the "Juratrias" shales of however, all attention was diverted to Texas fol- Dakota in Colorado. In 1896, Emmons and this region were of Morrison age. Stanton lowing the discovery of Early Cretaceous sau- others described the type section near Morrison, (1905) further bracketed the Morrison by col- ropod tracks in the Glen Rose (Bird, 1939b, Colorado. The type locality was redefined in lecting marine Comanchean fauna from 200 ft 1985). Because of uncertainty about the I rue af- 1944 by Waldschmidt and Leroy, who subdi- above the dinosaur-bearing beds and Triassic finity of the Purgatoire sauropod tracks (Bird, vided the stratotype into six informal lithologic bone from 150 ft below; Heaton (1939) corre- 1939a), a thorough account was never pub- units. On the Colorado plateau, however, the lated the "red beds" of southeastern Colorado lished. Until recently, the site had not been men- Morrison Formation was formally divided into with the lower Morrison. Except for brief refer- tioned again except by Pearl (1969) and the Salt Wash, Ri;capture, Westwater Canyon, ences to the dinosaur tracksite (MacClary, 1936, Langston (1974, p. 97), who noted that, with the and Brushy Basin Members by Craig and others 1938, 1939; Bird, 1939a, 1944), most studies exception of the Glen Rose tracks in Texas, (1955), who noted that much of the "undifferen- have focused on Cretaceous formations and the "those from the Purgatory River in Colorado are tiated Morrison" east of the Rocky Mountains is ambiguous Jurassic/Cretaceous boundary. Tay- the only sauropod ichnites presently exiant in "similar to the Brushy Basin Member" (that is, lor (1974) measured several Morrison sections North America." When reinvestigated in 1982, 2 variegated claystone with conglomeratic sand- and recognized facies which he interpreted as it became evident that, since 1938, ~ 10,000 m stone and limestone lenses containing dinosaur small fluvial channel or levee, overbank, and of the footprint-bearing layer had been de- 1 bone, wood, fresh-water gastropods, and algae). shallow fresh-water (ephemeral) lake deposits. stroyed by flooding (Fig. 2, inset), and that a controlled irrigation regime now extends the Recent studies (Jackson, 1979; Dodson and The reported thickness of the Morrison ex- spring runoff period from April to October, others, 1980) have avoided stratigraphic no- posed in the canyons of southeastern Colorado menclature and used informal lithofacies desig- ranges from 85 to 137 ft, with the variations due nations to characterize various lacustrine, in part to the difficulty in picking the upper and 'Figure 2 is a folded insert that accompanies this flood-plain, and fluvial deposits. lower contacts. The gypsiferous Ralston Creek issue.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/10/1163/3434479/i0016-7606-97-10-1163.pdf by guest on 30 September 2021 Figure 2. Map of Purgatoire River track site with inset (top right) showing location of main track-bearing strata (bed no. 2) in relation to river and overburden. Inset also shows contours for depth of sauropod tracks. Map also shows location of trackways in underlying and overlying beds no. 1 and no. 3, respectively. Rose diagram (left) shows orientation of wave ripple crests in bed no. 2. The LOCKLEY AND OTHERS, FIGURE 2 paleochannel north of the 230 m mark along the baseline, total trackway orientations (lower left), and schematic water table/track depth relationship (below site map) are also shown. Geological Society of America Bulletin, v. 97, no. 10
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thereby submerging much of the site for this pe- are indicative of a lacustrine ecosystem (see Loveland, Colorado; he defined five deposi- riod. Because the runoff destroys and obscures Lockley 1986a and 1986b for brief synopses). tional facies: the open lacustrine fades, the the footprint layer by undercutting it and expos- Dinosaur bone is also known from the region marginal-lacustrine, the interdeltaic facies, the ing it to abrasive bed-load sediment (Fig. 3), we (Lee, 1901; Taylor, 1974), and an articulated interdistributary facies with associated distribu- had to work in winter to sandbag portions of the sauropod limb was recovered from section B tary channels, and the flood-plain facies, in river and remove alluvial deposits. Only at low (Fig. 4). which soil development features are common. water could we construct a meter grid, make our Dodson and others (1980) delineated similar map (Fig. 2), and examine 6 m of section SEDIMENTARY FACIES depositional facies for the Morrison Formation (Fig. 4). in a broad regional study (compare with Fig. 1), A few preliminary reports (Frazier and oth- Previous Work but unlike Jackson, interpreted the sand bodies ers, 1983; Lockley, 1984; Prince, 1983) and as fluvial rather than deltaic. Preliminary studies magazine articles (Blonston, 1983; Stein, 1983) Little information is available on the deposi- (N. K. Prince, unpub. data) in the Purgatoire focused attention on the extensiveness of parallel tional environments of the Morrison Formation uplift (Black Hills monocline) area generally sauropod trackways and the implications for so- east of the Front Range. Jackson (1979) en- support the lithofacies divisions of Jackson cial behavior. Lockley and others (1984) also visioned a lacustrine and deltaic environment for (1979) and Dodson and others (1980). The noted that associated floral and faunal remains Morrison Formation sediments in the vicinity of lower Morrison Formation in this area consists of two distinguishable facies: drab calcareous shales and claystones overlain by interbedded gray micritic limestones, mudstones, and clay- stones with algal stromatolites, ostracods, and charophytes. Wave ripples and mudcracks, par- ticularly in the latter facies, suggest that shallow- water conditions predominated throughout the lacustrine phase of Morrison deposition, not just at the tracksite. The upper portion of the Morri- son Formation is composed of fluvial sandstone and variegated flood-plain sediments of the type described by Dodson and others (1980) and Jackson (1979).
Sedimentary Facies at the Purgatoire River Site The Purgatoire River site falls near the top of the lower lacustrine sequence in the Morrison stratigraphic succession. Aside from the exten- sive dinosaur trackways, the site is unique in many other ways. A well-developed shoreline sequence is present, consisting of thick beds of graded intraclastic oosparite and oomicrite, in many instances with large-amplitude wave rip- ples. To our knowledge, such a shoreline facies has not been described from the Morrison For- mation, although oolitic limestone deposits are known from Tertiary lacustrine sequences (Bradley, 1926), from modern Salt Lake, Utah (Sandberg, 1975), and Pyramid Lake, Nevada (Popp and Wilkinson, 1983). The succession of limestone-shale couplets, representing shoreline and shallow-lake facies, respectively, provide ev- idence for cyclic episodes of restriction and re- plenishment at Dinosaur Lake. Multiple foot- print horizons demonstrate that the lakeshores were frequented by dinosaurs over an extended period of time (Figs. 4 and 5). On the basis of lithology, bedding characteristics, lateral extent, sedimentary structures, flora, and fauna in the four measured sections, we have described the following three lithofacies: (1) shallow lacustrine Figure 3. Aerial view of most of the Purgatoire River tracksite (see Fig. 2 for map), showing shale facies, (2) shoreline carbonate facies, and submerged and covered portions of Bed 2. People provide scale. (3) shoreline clastic sandstone facies.
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LOCALITY MAP
Figure 4. Stratigraphie sections in the immediate vicinity of the Purgatoire tracksite. Note that four successive footprint-bearing beds are numbered 1-4.
Shallow Lacustrine Shale Facies A shallow lake is also suggested by the low significant bias, mixing, or damage, we have also faunal diversity indicative of only one ecological assumed that we are dealing with the remains of The most common lithologies are gray, fissile community. Modern tropical lakes lacking pro- a single autochthonous or in situ community. claystones and mudstones (not variegated flood- nounced thermoclines probably provide the best Our evidence suggests the following. plain shales), containing typical fresh-water fos- analogues. For example, in Lake Chad, the shal- 1. The Lacustrine Shale Was Deposited in sils, including Chara (calcareous green algae egg low, uniform nature of the lake bottom limits Shallow Water. Modern green algae, such as cases), the pulmo iiate gastropods Lymnaea and the number of benthic habitats and hence the Chara, live totally submerged in an aquaic en- Gyraulus, the prosobranch gastropod Amploval- number of communities (Beadle, 1981). This vironment, yet cannot live below the photic vata, and abundant ostracods of several species lack of variety stands in contrast to ecological zone (Ott, 1958). Modern Lymnaea and Gyrau- (Fig. 5). As man)' of the genera found in these models of Reid (1976), which predict the occur- lus are pulmonates which prefer shallow water sediments are also typical of modern lakes, it is rence of several communities, controlled by and rooted, emergent vegetation on which to possible to infer the probable environmental depth and substrate, across the diameter of tem- climb to the lake surface. Consequently they are preferences of the fossil species. Taken as a perate lakes. Because we have observed similar rarely found in water greater than 3 m in depth group, they indicate shallow, open waters with lacustrine lithofacies extending for 40 km or (Pennak, 1978). good circulation, adequate oxygen, and an alka- more, we have assumed that lake-bottom condi- 2. Stable, Open Waters with Good Circula- line, basic water chemistry. tions varied very little. As there is no evidence of tion Prevailed. The pulmonate gastropod lung
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allows for a much wider ecological tolerance a> cs than that of the gill-bearing prosobranch gas- co CO ® fish remains also sug- O o 3 Q gest a well-established lake. 3. The Water Chemistry Was Alkaline Sa- • o line in the Sense That There Is Evidence for High Solute Concentrations. As prosobranch • gastropods have lower tolerances to water chem- • istry fluctuations than do pulmonate gastropods, • • O • they provide the most definitive ranges of chem- ical conditions. Valvata, the best modern anal- • ogy for Jurassic Amplovalvata, requires a slightly basic pH range of 7.3-8.4, and alkaline waters with carbonate concentrations of 80-250 ppm (Harman, 1974). Chara also requires alka- line waters for growth and reproduction, be- • • O • cause they build their egg cases out of calcite. Evidence for saline conditions is also found in the other facies discussed below. 4. A Well-Oxygenated Water Column in Di- nosaur Lake Is Suggested The lack of lamina- tion and organic-rich layers commonly found in lakes with anoxic bottoms (Grande, 1980; Olsen • O • and others, 1978) suggest that there was a good supply of oxygen. An oxygenated bottom is also necessary to sustain active benthos and rooted vegetation such as Chara
Shoreline Carbonate Facies • 1m m Thickly bedded limestones, including sparsely fossiliferous micrite, oomicrite (Fig. 6), intra- SHALLOW WATER SHORELINE micrudite, oosparite, intrasparrudite, and algal biolithite (Folk, 1974) are almost as abundant as Figure 5. Faunal remains and traces observed in and adjacent to section C, Purgatoire River the lacustrine shales. Regardless of allochem site. Presence-absence data based on a combination of field observations, thin sections, and content, the limestones as a group show shallow- sample analyses. Typical shallow-water and shoreline forms are separated, and a schematic water to emergent features. Where underlain by curve to indicate depositional cyclicity is presented. Data represent only a preliminary shales, basal loading and salt crystal casts are presence-absence survey for section C and do not encompass known lateral variation. also common. Fossils include unionid bivalves, ostracods, conchostracans, fragmented fish re- mains, dinosaur bone, blue-green algae, charo- presence of conchostracans also indicates very waters from rivers or springs with carbonate- phytes, terrestrial plant impressions, and hori- shallow water, as modern species typically live rich lake waters. Micrite is the usual product of zontal root casts (compare with Cohen, 1982). in shallow ephemeral pools, and their eggs must this reaction, but ooid coatings may also occur if These limestones formed at the lake margin, go through a period of dessication before hatch- sufficient wave energy is present (Popp and in shallow-water shoals and on mud flats, and ing (Beadle, 1981; Pennak, 1978). Wilkinson, 1983). Algal stromatolites also con- we infer that wave activity and influx of solute- Carbonate precipitation often occurs in the tribute to carbonate precipitation near lacustrine rich waters were both important processes in central, deep portion of a lake, as has been de- shorelines (Smoot, 1978). As micrite, ooids, and their genesis. We suggest the following scribed for modern (Kelts and Hsu, 1978) and algal stromatolites are all present, we envision a conclusions: Tertiary lakes (Grande, 1980). Extensive car- depositional situation similar to that described 1. The Limestones Formed at the Lake Mar- bonate precipitation, however, can also take by Popp and Wilkinson (1983) for modern Pyr- gin. Conditions of shallow water to emergence place at lake margins (Eugster, 1980; Wilkinson amid Lake, Nevada, where ooid formation oc- are indicated by roots, footprints, mud cracks, and others, 1980; Popp and Wilkinson, 1983) as curs as calcium-rich spring waters mix with intraclasts, ooids (Fig. 6), and ripple marks. The a result of the mixing of inflowing calcium-rich carbonate-rich lake waters in a nearshore, wave-
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agitated environment. We suspect that some Colorado, at least for the lower part of the present in the form of abundant salt-casts found solute-rich water entered the lake as surface Morrison Formation, it is necessary to revise the on the basal surfaces of sandstone and allochem- runoff rather than as spring emanations, because opinion "that sub-aerial environments were rich limestone beds that overlie impermeable we have evidence of a small paleodrainage en- common if not predominant." shales. The identity of the mineral that f ormed tering the lake (Fig. 2). Sauropod footprint-depth contours and the casts is as yet unknown. It is rhombohedral Carbonate may also have precipitated on trackway orientations for bed 2 suggest a gener- in cross section (Fig. 6), a form that could indi- water-soaked mud flats, as it does at Deep ally east-west-trending shoreline with an em- cate calcite, dolomite, glauberite, or even de- Springs, California (Jones, 1965), thus produc- bayment extending toward the south (Fig. 2). formed halite. Their occurrence at a permea- ing the abundant micrite found in the shoreline Trackway configurations in other footprint- bility barrier suggests that the casts prabably carbonate facies a: Dinosaur Lake. Periodic dry- bearing beds suggest that the shorelines probably formed by evaporitic concentration following ing and subsequent cracking of the mud flats occupied similar positions during each episode capillary action (Eugster, 1980; Eugster and may have produced the abundant intraclasts of track-making. The orientations of wave ripple Hardie, 1975; Courel and Demathieu, 1984), found in the limestones. crests for the footprint beds 3 and 4 are predom- not as crystals precipitated at the sedimeni -water 2. Wave Activity Was an Important Process inantly shore-parallel, (east-west), as expected if interface (Beadle, 1981). in the Formation and Deposition of the Lime- the prevailing wind direction was from the stones. The presence of abundant ooids and north-northeast (Tanner, 1974). Ripple orienta- Shoreline Clastic Sandstone Facies intraclasts require:» wave activity, as does forma- tions for bed 2, however, show predominantly tion of the rather large wave ripples on bedding shore-perpendicular (north-south) orientations, Thin clastic sandstones at Dinosaur Luke are planes and within units. Periods of very high with only a subordinate shore-parallel (east- minor lithologies in all four sections (Fig. 4). wave activity, such as storms, are indicated by west) component. Because bed 2 is also graded, The sandstones are medium to fine grained, well normally graded teds 8-10 cm in thickness. The we infer that there was a change in prevailing sorted, and mainly quartzose, commonly with bases of the beds contain abundant rip-up clasts, wind direction during a storm. We particularly well-developed wave ripples on the upper sur- algal biscuits, shell debris, and unionid bivalves favor a storm genesis interpretation for footprint faces, and salt crystal casts on the lower surfaces. in the hydrodynarnically stable, convex-up posi- beds 2 and 3. The waning storm clay drape They are usually bounded by shales and occur at tion. Toward the i:op, these units become coarse would provide the ideal separating medium to a position in the sequence where a shoreline oolitic limestones that are cleanly washed and preserve the crisp footprint impressions which limestone might be expected (for example, the sparry, further suggesting reworking by waves. were subsequently made. The abundance offish upper sandstone in section D grades laterally The admixture of abraded and fragmented fos- debris (especially in bed 3) attests to widespread into sandy shoreline limestone in sections B and sils from differer.t habitats (for example, bi- disturbance in the lake. C). The lower sandstone bed in section D can be valves, fish, dinosaurs, and plants) also suggests In contrast to the disarticulated bivalves in the traced westward for at least 1.5 km into sections transportation and mixing. higher energy facies, footprint bed 1 yielded an C and A. The sandstones are well sorted, wave Ripple-mark measurements have provided assemblage of articulated, spar-filled Unio shells rippled, and in stratigraphic proximity to stro- some insight into t he size and shape of Dinosaur partially buried in low-energy micrite in and matolites and salt casts. This suggests that they Lake. Following the method of Tanner (1971, around a dinosaur trackway. The fate of these formed as reworked sheets along the shoreline 1974), amplitude!>, wavelengths, and grain sizes bivalves was evidently similar to that of modern during or following periods of unusual clastic of 41 fields of ripples were measured at various bivalves trampled by cows at river crossings influx, such as rainstorms and seasonal flooding. horizons, allowing; for consistent wave-fetch es- (Baker, 1901). This unique example of "dino- Because sandstone beds constitute only a timates greater than 100 km. Because of the turbation" (sensu Dodson and others, 1980) has small portion of the exposed sequence, w e infer small data base, we interpret the results cau- provided an unusual taphonomic example of that terrigenous sand influx was an unusual oc- tiously. Tanner had a larger data base with vertebrates contributing to the autochthonous currence. The abundant carbonate sediments which to work (200 ripple fields), and his sands preservation of an invertebrate assemblage suggest that the lake basin was sediment starved were clean and well sorted, whereas ours were (Lockley, 1986a), and we infer that this part of during portions of its history. If the shallow- somewhat muddy, making an estimate of aver- bed 1 represents a preferred shoreline habitat of water shales represent lake highstand and rela- age grain size difficult. Evidence of storm-graded lacustrine unionids. tively neutral water chemistry, and the shoreline beds suggest that some of the fetch distances 3. Inflow of Solute-Rich (Saline) Waters En- limestones represent lake lowstand and nore al- derived from Tanner's equations are overesti- hanced Chemical Precipitation along the Shore- kaline saline water chemistry, the terrigenous mates, rather than being indicative of average line of Dinosaur Lake. The requisite ions for clastics probably entered the lake as silt and clay windiness. Nonetheless, we envision a fairly carbonate precipitation were evidently present during periods of heavy rainfall. During periods large lake, because Tanner (1971) observed that only at the lake margin and were attributable to of low-water stand clastic supply was cut off, an average wave-type ripple mark spacing of the influx of calcium-rich ground water or sur- and carbonate precipitation prevailed except 4-5 cm separates, with fairly good reliability, face water along the lake shore. According to when terrigenous sand was periodically re- large lakes and seas from ponds, small lakes, and Eugster (1980) and Jones (1965), composition worked along the shoreline. Some of these sand other restricted bodies of water. Nearly all of the of water flowing into a lake is strongly con- grains served as nuclei for ooid growth when Purgatoire site ripple mark spacings are greater trolled by the kind of bedrock it travels over, or carbonate precipitation was reactivated (Fig. 6). than 5 cm, and about half are greater than 10 through. As no nearby limestones or igneous cm. The Purgatoire data contrast with the ob- intrusions are known to exist in the vicinity of DINOSAUR TRACKWAY ANALYSIS servation of Dodson and others (1980, p. 211) Dinosaur Lake, we speculate that a ready supply that "there is little published evidence for of calcium may have come from the underlying In order to provide thorough documentation large scale (50 km) lakes ... in areas where gypsiferous Ralston Creek Formation. of the Purgatoire site, we mapped the entire area vertebrate fossils occur." In southeastern Further evidence for solute-enriched water is (Fig. 2), initially at 1:100 scale. Using appro-
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Figure 6. Salt casts (left) on underside of bed 3 (section D of Fig. 4). Photomicrographs (center and right) of ooid-rich limestone of bed 2 (both x!40). Note quartz-grain nucleus (top center) and extensive sparry matrix.
priate symbols, we distinguished between sau- not been previously described from the Jurassic Identification of the tracks as sauropod is ropod and tridactyl trackway segments (Figs. 7 of North America, although they were photo- based on the distinctive claw impressions asso- and 8) and in each case provided a numerical graphed (Bird, 1939a). Some display the first ciated with some of the better-preserved pes and designation before collecting relevant size fre- recorded manus claw impressions (Fig. 7k). Such manus impressions (see Fig. 7). There are two quency, orientation, depth and other data (Table evidence shows that these sauropods did not distinct sauropod ichnites. One is a larger, 1). The net result (Fig. 2) indicates more than 40 walk on their metacarpals, as Beaumont and broad-footed form (Fig. 7b) which often shows distinctive trackway segments in each category, Demathieu (1980) suggested for one of the recognizable claw impressions; the second is a which, together with various isolated tracks, Texas trackmakers. smaller, narrow-footed form (Figs. 7a and 9c) in total more than 100 distinct trackways, exclu- Bird was puzzled by the Purgatoire tracks, which claw impressions are less obvious. The sive of repeat segments. In addition, we have which appeared to indicate a "two-footed dino- distinction between these two morphotypes is mapped trackway segments preserved in under- saur," and asked if they might not represent a highlighted by the fact that they tend to be dif- lying and overlying beds, recorded sauropod "swimming sauropod." Our studies solved this ferentiated by group, and may represent differ- track depth and ripple-mark data for bed 2 puzzle by showing that one can trace deep, ap- ent species or at least distinct age groups of a (Fig. 2), and obtained plaster and rubberized parently two-footed trackways into shallow, species exhibiting significant allometric broaden- casts of selected tracks. well-defined quadrupedal trackways. The differ- ing of the foot with growth. We stress the importance of a footprint map ent configurations simply show that quadrupeds None of the sauropod trackways have asso- in any tracksite study (Bird 1944, 1985; Thul- can partially overlap or totally overprint their ciated tail-drag impressions. In the past, recon- born and Wade, 1984) which seeks to preserve manus tracks with the pes (primary overlap structions have frequently shown sauropods evidence which may subsequently be destroyed sensu Peabody, 1959) as their speed varies. The dragging their tails. Even Bird (1944), who by erosion. After this, one of the first steps in the range of sauropod track depths (2-25 cm) cor- found no evidence of tail-drag impressions at the synthesis of trackway data is footprint identifica- responds closely with the 1- to 23-cm range re- famous Paluxy sites, attempted to explain their tion. At the Purgatoire site, there are quadrape- corded by Laporte and Behrensmeyer (1980) for absence by suggesting that the sauropods were dal (sauropod) and bipedal tridactyl trackways, large mammal tracks in Quaternary sediments. walking in shallow water with their tails floating both of which show variations attributable to Such track depth differentials also provide sensi- behind them. Evidence from the Purgatoire site substrate consistency as well as differences tive paleowater-table indicators (Fig. 2). Even shows that this interpretation is also incorrect. which reflect taxonomic and size differences the trackway density can be indicative of pa- Even when trackways are traced from areas of among the trackmakers (Figs. 7, 8, and 9). leoenvironmental conditions, possibly, in this softer substrate, which may have been in a shal- case, falling closer to the heavily trampled shore- low subaqueous setting when the tracks were Tracks of Quadrupedal Dinosaurs line of a fresh lake than the less frequented mud- made, to firmer, presumably subaerial, settings flats associated with slightly alkaline and with mudcracks, no signs of tail-drag impres- All quadrupedal trackways appear to be at- ephemeral lakes (Laporte and Behrensmeyer, sions are noted. Because all trackways, including tributable to sauropods, whose footprints have 1980). small bipedal forms, also indicate normal walk-
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ing progression, the hypothesis of progression through anything aut the shallowest water seems unlikely. There is, however, limited potential for preservation of tracks made underwater (see Peabody in McKee, 1947). The narrow configuration of sauropod track- ways, where both pes and manus impressions impinge on, or are very close to the mid-line, saggital plane (Fig. 7), support the revised inter- pretations of brontosaur ecology suggested by Bakker (1971), who favors an erect terrestrial adaptation over the outdated notion of sprawl- ing, aquatic sauropods (see Coombs, 1975, for a review). Sauropod tracks are extremely rare in North America. They are known from the Glen Rose of Texas (Bird, 1939b-1985), the De Queen of Arkansas (Pittman, 1984), and the Morrison Formation discussed herein. The only other documented occurrences are in North Africa (Ginsburg and others, 1966; Dutuit and Ouaz- zou, 1980; Jenny and others, 1981; Jenny and Jossen, 1982), Germany (Kaever and Lappar- ent, 1974; Hendricks, 1980), Portugal (Antunes, 1976), and Bolivia (Leonardi, 1984). Unfortu- nately, many of the most distinctive sauropod tracks have no formal ichnological names, whereas several indistinct, poorly preserved tracks have form;il designations. Therefore, a strong case exists l:or publishing formal descrip- tions of the well-preserved Morrison and Glen Rose tracks. We cannot determine which sauropods made the tracks at the Purgatoire site; however, "Camarasaurus and Diplodocus were gregar- ious, with juveniles and subadults of the former particularly common" (Dodson and others, 1980, p. 208). Footprint and glenoacetabular data suggest a large proportion of subadults (Table 1). Ginsbuig and others (1966) also cited these two genera as possible trackmakers at a Figure 7. Sauropod tracks and trackways. A and B. Segments of trackways 31 and 34, site in North Africa, and Antunes (1976) men- respectively, from the Purgatoire River site, with dashed outline around tracks which have tioned Camarasaurus in the context of Early been cast. C. Sauropod trackway from the Morrison Formation near State Bridge, Colorado; Cretaceous tracks from Portugal. Apatosaurus stippled area (right) represents edge of outcrop of track-bearing layer. DMNH archive photo- was another relatively ubiquitous genus but may graphs reveal that a portion of the trackway, enclosed by dotted line, has been lost to ero- have been "more solitary in its habits" (Dodson sion. D. Indistinct ?sauropod trackway from the Morrison Formation near Fort Collins, and others, 1980, p. 208). Colorado. E. Breviparopus taghbaloutensis from the Late Jurassic/Early Cretaceous of Mo- A single trackway of a ?sauropod from near rocco (Dutuit and Ouazzou, 1980). F and G. Unnamed sauropod tracks from the l^ower Fort Collins in Larimer County (Fig. 7d) occurs Cretaceous Glen Rose of Texas. F. Modified after Langston's 1974 and 1979 reproductions of in topset sandstones of a small prograding delta Bird's work. G. Large and small sauropod tracks from the Davenport Ranch site (sec: Bird, capped by algal slxomatolites. The coincidence 1944). H. Unnamed sauropod tracks from the Lower Cretaceous De Queen Formation of of the trackway with the strike of the underlying Arkansas after Pittman (1984). I. Unnamed sauropod track from the Late Jurassic/Upper foresets indicates a shore-parallel progression Cretaceous of Niger after Ginsburg and others (1966). J. Undiagnostic circular depression toward the southwest. Another site, at State assigned to Elephantopoides barkhausensis, a ?sauropod track from the Upper Jurassic of Bridge in Eagle County, also indicates the activ- Germany after Kaever and Lapparent (1974); also, this same figure represents the outline of ity of sauropods and large tridactyl bipeds in a Neosauropus ¡agosteños, a poorly preserved ?sauropod track from the Lower Cretaceaus of lake shore setting. Tracks occur as impressions Portugal after Antunes (1976). K. Detail of sauropod manus track from trackway no. 33, in at least six successive sandstone beds in the showing digit impressions. All tracks at the same scale except K, and all orientation!! with lower part of the Morrison Formation of this anterior toward top of page.
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Tracks of Bipedal Dinosaurs
Tridactyl trackways at the Purgatoire site outnumber the sauropod trackways by a ratio of é about 6 to 4. As shown in Table 1, the tracks range in size (width) from 9.5 to 47.3 cm. Size frequency data (Fig. 10) suggest that both the very small and very large tracks fall well outside the main cluster, possibly indicating that they H were made by different species. Only a small proportion of tridactyl footprints show unequiv- ocal claw impressions indicative of theropod af- finities. We therefore provisionally assign the other tridactyl forms to the Ornithopoda, even though it is known that many tracksites exhibit a preponderance of carnivore tracks (Leonardi, 1984). The largest and most distinctive tracks resem-
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TABLE 1. DINOSAUR FOOTPRINT DATA, PURGATOIRE RIVER SITE, COLORADO 1975). These conclusions emphasize the impor- tance of the Purgatoire site (bed 2) as a represen- Trackway Identificaiion Foot size Average h Average X° no. Width Length stride speed v tative ichnological sample (ichnocoenosis) of the (w) (1) (X) (m/s) Morrison dinosaur community resulting from (cm) (m) the activity of animals in a localized area during 1 Omithopod 24.3 28.5 2.25 1.14 2.60 254 a time period of short duration (Mac Clary, 2 Ornithopod 29.7 44.3 2.90 1.77 2.37 260 3 Ornithopcd 28.4 42.6 2.20 1.70 1.57 260 1938). 4 Ornithopcd 27.0 37.0 2.30 1.48 1.99 115 5 Ornithopcd 26.4 42.4 2.32 1.70 1.71 270 6 Ornithopcd 30.0 40.0 2.50 1.60 2.06 315 Synthesis of Trackway Data 7 Ornithopcd 27.5 33.5 2.45 1.34 2.48 313 S Ornithopcd 27.7 37.3 2.40 1.49 2.11 292 9 Ornithopcd 33.7 42.0 2.58 1.68 2.08 264 10 Ornithopcd 32.3 36.3 2.50 1.45 2.34 294 Synthesis of the abundant footprint dita re- 12 Ornithopcd 22.0 28.0 2.06 1.12 2.29 290 quires a clear distinction between individual 13 Ornithopcd 32.7 43.7 2.68 1.75 2.11 346 14b Ornithopcd 31.0 40.0 2.68 1.60 2.34 280 trackways and repetitious trackway segments 16 Ornithopcd 33.0 37.0 2.24 1.48 1.90 60 17 Ornithopcd 29.0 36.0 2.00 1.44 1.63 110 which may represent the activity of the same 18 Ornithopcd 34.0 40.0 2.48 1.60 2.06 105 individual at two locations within the study 19 Ornithopcd 32.0 39.2 2.43 1.57 2.04 43 20 Ornithopcd 29.0 36.3 2.54 1.45 2.40 220 area. To do this effectively, both the size of 22 Large omithopod 45.7 44.7 3.27 1.79 2.87 234 23 Ornithopcd 30.0 38.0 2.42 1.52 2.09 300 tracks and the orientation of trackway segments 26 Small omithopod 9.5 13.5 1.29 0.54 2.46 253 must be taken into consideration. (Compare 27 Ornithopcd 32.5 37.5 2.82 1.50 2.75 240 29 Ornithopcd 30.0 37.0 2.48 1.48 2.25 280 Table 1 and Fig. 2 to identify segments that have 31 Large omithopod 47.3 50.8 3.50 2.03 2.77 145 32 Ornithopcd 28.0 36.0 2.48 1.44 2.33 175 been excluded from the synthesis.) 33 Ornithopcd 32.0 37.0 2.84 1.48 2.83 255 33a Small omithopod 10.0 14.0 1.40 0.56 2.70 280 Speed Estimates. Following Alexander 34 Carnosaur 28.0 35.0 2.20 1.40 1.97 82 (1976), we infer that all of the tridactyl biped 35 Ornithopcd 35.0 40.0 2.80 1.60 2.52 165 36 Omithopod 38.0 42.0 2.64 1.68 2.16 225 trackways indicate normal walking progression 37 Ornithopcd 35.0 42.0 2.50 1.68 1.97 300 38 Ornithopcd 35.0 2.80 280 at speeds between 1.5 and 3.0 m/s. When at- 39 Ornithopcd 25.0 30.0 2.40 1.20 2.73 315 tempting to estimate sauropod speeds, it became 40 Omithopod 35.0 40.0 2.72 1.60 2.40 340 41 Carnosaur 31.0 2.20 55 evident that the elongate pes tracks resulted in 42 Carnosaur 36.0 2.72 225 overestimates of hip height (h) which failed to
3 Sauropod 71.0 290 agree with the glenoacetabular (shoulder :o hip) 4 Sauropod 87.0 290 size estimates obtained from the trackway data. 5 Sauropod 53.0 75.0 2.41 2.12 2.82 290 6 Sauropod 47.0 67.0 1.88 290 We therefore used estimates of h derived from 7 Sauropod 57.0 75.0 2.28 275 8 Sauropod 62.0 83.0 2.48 195 foot width (not length) to obtain reliable size and 11 Sauropod 47.0 63.0 1.88 290 speed estimates. The results indicate sauropod 12 Sauropod 54.0 71.0 2.16 284 13 Sauropod 48.0 75.0 1.92 170 speeds in the range of 1.7 to 3.1 m/s. 14 Sauropod 49.0 60.0 1.96 155 IS Sauropod 51.0 66.0 2.04 155 Orientation of Trackways. We identified at 20 Sauropod 46.0 61.0 1.84 160 21 Sauropod 45.0 60.0 1.80 162 least 25-30 distinctive sauropod trackways 22 Sauropod 49.0 62.0 1.96 335 among the 40 trackway segments shown on the 23 Sauropod (37.0) (48.0) 1.48 342 25 Sauropod 49.0 61.0 1.70 1.96 1.69 05 map. Some of these true trackways fall into ob- 29 Sauropod 34.0 56.0 1.65 1.36 2.52 255 30 Sauropod 40.0 60.0 2.10 1.60 3.12 255 33 Sauropod 38.0 56.0 1.85 1.52 2.68 255 34 Sauropod 38.0 60.0 1.50 1.52 1.89 255 35 Sauropod 32.0 52.0 (2.20) 1.28 (4.38) 255 32a Sauropod 45.0 70.0 2.25 1.80 3.05 255 32 Sauropod 61.0 81.0 2.44 355 31 Sauropod 56.0 78.0 2.24 288 36 Sauropod 300 Figure 9. Representative Morrison Forma- Note: hip height (h) and spesl (u) are estimated using the method of Alexander (1976) where u = 0.25g°-5-x' 67-h"''17. X° represents direction of progression in tion dinosaur tracks and trackways. A Den- degrees. Tracks and trackway segments numbered I, 2. 9.37.38.39 and 40 may represent discrete sauropod trackways in addition to those listed above. The remainder ( 10, ver Museum archival photograph of ¡¡auro- 16-19. 24, 26. 27. 28 and 321 ) are probably repeat segments. For sauropods, h is calculated as 4 w, not 41 (see text for details). pod trackway from State Bridge, Colorado (see Fig. 7C for line drawing). B. Denver Museum archival photograph of trackway 1937. This small tridactyl trackway (Fig. 8g) is The trackway evidence suggests that at least specimen DMNH 1498, from Higbee (s Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/10/1163/3434479/i0016-7606-97-10-1163.pdf by guest on 30 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/10/1163/3434479/i0016-7606-97-10-1163.pdf by guest on 30 September 2021 1174 LOCKLEY AND OTHERS TABLE 2. C ROWINGS OF SUBPARALLEL resent individual trackways, which fall into the SAUROPOD TRACKWAYS The best evidence for gregarious behavior groupings shown in Table 3. The trackways in comes from analysis of intertrackway spacing Group Tnckways Orientation range these groups are all tentatively identified as those (1.2-1.4 m) in group I. With the exception of (degrees) of ornithopods which may have been engaged in no. 32a, all match closely for size and can be gregarious activity. The similarity in over-all traced through equivalent trackway segments 1 3,4, 6, 7 275-290 II 29,30,33, 34, 35, 32» 255-270 orientation trend between tridactyl and sau- 16-19 for a distance of more than 200 ra with- ropod trackways, however, probably indicates out deviating by more than a few degrees or that the physical characteristics of the paleoen- crossing over. The consistent changes ia foot- vironment (for example, shoreline configura- print depth along each trackway as substrate TABLE 3. GROUPINGS OF SUBPARALLEL TRIDACTYL TRACKWAYS tion) exerted some measure of influence on the conditions changed also support the conclusion observed trends (Lockley, 1986a). that the animals passed as a group within the Group Trackway numbers Orientation range Interpretation of Orientational Data. The confines of the "relatively narrow band af lake (degrees) orientation data for sauropod trackways (Fig. 2 margin sediment" favorable to track preserva- A 1.2,3,5,8 254-292 and Table 1) suggest that, like the Cretaceous tion (Laporte and Behrensmeyer, 1980). B 9, 10, 11, 12, 14a 264-300 species of Bird (1944), the Morrison sauropods C 16, 19,21 32-35 The local close convergence (less than 1 m) of D 20,22,27 220-240 were gregarious. We need to consider whether a trackways in group I suggests that the ani- E 24,31,32 145-175 preferred orientation trend implies herd behav- mals were not "spread out on a wide front" as ior, the presence of physical barriers which inferred by Currie (1983, p. 69) for hadrosaurs. vious groupings listed in Table 2. Many of the obliged animals to move in a certain direction Instead, the Purgatoire sauropods were probably remaining trackways are associated in parallel (Ostrom, 1972), or a combination of both fac- walking in some type of staggered or spearhead pair or other distinctive groupings, for example, tors. At the Purgatoire site, the relationship be- formation (Lockley, in press) in the 23 Daven- 11 and 12, 13-li, 20-23, and 38 and 39 and tween preferred orientation and shoreline trend port Ranch trackways in Texas, where there was may, like the group I and II data, suggest gregar- might appear to support the physically con- considerable following in line. The Purgatoire ious behavior. trolled pathway explanation and undermine the sauropod trackways indicate a smaller, less- As shown on the map, 43 tridactyl trackway herd hypothesis; however, "the same constraints congested herd. Groups I and II, respectively, segments and various isolated tracks have been which control the progression of an individual represent larger and smaller individuals, all trav- identified. As there are no obvious repetitions, also are likely to affect the path chosen by a eling westward, possibly as a single herd of at the numbered trackway segments probably rep- herd" (Lockley, 1986a, p. 41). least 11 individuals, or as two separate groups. If Figure 10. A and B orientation of sauropod and tridac- tyl trackways, respectively, with a distinction, for sauro- N = 30 N = 64 pods, between known trackways (black) and segments (white); see text for details. In B, white distinguishes; car- nosaur tracks with distinct claw impressions from other tridactyles. C and D, respectively, represent size frequency plots of foot width for the same data; in C, white portions F lAii, , represent trackway segments with reliable foot-width data; 30 so 70 track width 20 40 eo in D, white segments represent carnosaur tracks. E. Plot of foot length against stride for tridactyl trackways. •, and respectively, represent carnosaur tracks from the Purgatoire River site, Higbee (DMNH 1498), and Fruita (M.W.C. 8S.26.1). F. Pie diagram showing relative abun- dance of sauropod, clawed tridactyl, and other tridactyl trackways. -1 1 1 1 1 1 r- 100 stride Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/10/1163/3434479/i0016-7606-97-10-1163.pdf by guest on 30 September 2021 NORTH AMERICA'S LARGEST TRACKWAY SITE 1175 evolution: San Francisco, Freeman and Company, p. 117-133. we assume a single herd, the larger individuals son Formation and its tracks are necessary to Beadle, L. C., 1981, The inland waters of tropical Africa (2nd edition): New (nos. 3-7) appear to have been nearest to the refine and test the conclusions presented above. York, Van Nostrand, 485 p. Beaumont, G. de, and Demathieu, G., 1980, Remarques sur les extremites lake, whereas the smaller individuals walked on Given the potential of the Colorado tracksites antérieurs des sauropodes (reptiles saurischiens): Comptes Rendus des Seances, SPHN Geneve, NS. v. 15, p. 191-198. the shoreward side with at least one larger indi- discussed herein, Morrison tracks elsewhere in Bird, R. T., 1939a, Untitled letter: Natural History, v. 43, p. 245. vidual (no. 32a) on the southern flank. This in- Colorado (Hatcher, 1903), South Dakota 1939b, Thunder in his footsteps: Natural History, v. 43, p. 254-261. 1944, Did Brontosaurus ever walk on land?: Natural History, v. 53, terpretation offers greater support for Bakker's (Marsh, 1899), Oklahoma (Lockley, 1986b; p. 61-67. Schoff, 1943, PI. 11; Langston, 1974, p. 97), and 1985, Bones for Barnum Brown: Fort Worth, Texas, Texas Christian 1968 hypothesis of protective adults than does University Press, 225 p. the Texas trackway data. Utah (W. D. Tidwell, 1985, personal commun.) Blonston, G„ 1983, Sociable dinosaurs: Science 83, v. 4, p. 84. Bradley, W. H., 1926, Shore phases of the Green River Formation in northern Population Dynamics. Western and Moss should also prove worthy of investigation. Sweetwater County, Wyoming: U.S. Geological Survey Professional Paper 140-D, p. 121-131. (1983) have recently suggested that it is possible Tracks have been a neglected topic, but are an Brady, L. L., 1969, Stratigraphy and petrology of the Morrison Formation of to use elephant footprints to estimate population integral part of any multidisciplinary paleoeco- the Canon City, Colorado, area: Journal of Sedimentary Petrology, v. 39, p. 632-648. age structure. Their study raises the obvious logic and paleoenvironmental study (Lockley, Chandler, M.E.J., 1966, Fruiting organs from the Morrison Formation of Utah: British Museum of Natural History Bulletin (Geology), v. 12(4), question, can the same be done for dinosaurs? 1986a). p. 137-171. The answer would seem to be yes, if one can (a) Cohen, A. S., 1982, Paleoenvironments of root casts from the Koobi Fora Formation, Kenya: Journal of Sedimentary Petrology, v. 52, find footprints which represent an actual popula- ACKNOWLEDGMENTS p. 401-414. Coombs, W. P., Jr., 1975, Sauropod habits and habitats: Palaeogeography, tion or herd and (b) make reasonable estimates Palaeoclimatology, and Palaeoecology, v. 17, p. 1-33. of growth rate to estimate age classes. Footprints Courel, L., and Demathieu, G. R., 1984, Les inversions de releif dans les traces Among the numerous people who have as- fossils, leur signification: 109e Congres national des Sociétés Savantes have the obvious potential for time-specific age sisted with this project, we wish to thank Debbie Dijon, sciences fasc 1, p. 373-383. Craig, L. C., Holmes, C. N„ Cadigan, R. A., Freeman, V. L., Müller, T. E„ and structure analysis as do mass bone accumula- Adelsperger, Kenneth Carpenter, Kelly Conrad, Weir, G. W., 1955, Stratigraphy of the Morrison and related forma- tions, Colorado Plateau region, a preliminary report: U.S. Geological tions (Currie and Dodson, 1984). Although Mark Jones, Beverley Harrison, Gary Miller, Survey Bulletin 1009E, p. 129-168. such an analysis is beyond the scope of this Andy Rindsberg, and Walter Vest for their con- Cross, W., 1894, Description of the Pikes Peak Sheet. U.S. Geological Survey Geological Atlas, folio 7. paper, the Purgatoire sauropod data (groups I siderable help during field work. Robert Bakker, Currie, P. J., 1983, Hadrosaur trackways from the Lower Cretaceous of Can- and II) and Bird's Davenport Ranch herd evi- ada: Acta Palaeontologica Polonica, v. 28, p. 63-73. William Bilodeau, George Callison, Dan Chure, Currie, P. J., and Dodson, P., 1984, Mass death of a herd of ceratopsian dently record populations that include a variety Andy Cohen, Emmet Evanoff, James Farlow, dinosaurs, in Reif, W. E., and Westphal, F., eds.. Symposium on Meso- zoic Terrestrial Ecosystems, 3rd, p. 61-66. of individuals in subadult- to adult-size classes. Rick Forester, David and Lynette Gillette, Dodson, P., Behrensmeyer, A. K , Bakker, R. T., and Mcintosh, J. S . 1980, The lack of very small individuals could indicate Taphonomy and paleoecology of the dinosaur beds of the Jurassic James Madsen, Mike Parrish, Andy Taylor, Morrison Formation: Paleobiology, v. 6, p. 208-232. type I survivorship curves (Krebs, 1978) indica- Richard Thulborn, and Robert Schaeffer assisted Dutuit, J. M., and Oua2zou, A., 1980, Découverte d'une piste de Dinosaure Sauropode sur le site d'empreintes de Demrat (Haut Atlas Marocain), in tive of rapid early growth rates. through correspondence, discussion, and identi- Ecosystems continentaux du Mesosoique: Memoire Société Géologique de France, n. ser., v. 59, p. 95-102. fication of material. Emmons, S. F., Cross, W., and Eldridge, G. H., 1896, Geology of the Denver basin in Colorado: U.S. Geological Survey Monograph 27,556 p. CONCLUSIONS Special thanks are due to former landowner Eugster, H. P., 1980, Geochemistry of evaporitic lacustrine deposits: Annual Chris Stineman and Mr. and Mrs. C. Hughes for Review of Earth and Planetary Sciences, v. 8, p. 35-63. Eugster, H. P., and Hardie, L. A., 1975, Sedimentation in an ancient playa lake The Lower Morrison Formation of southeast- allowing us continued access to the Purgatoire complex: The Wilkins Peak Member of the Green River Formation of Wyoming: Geological Society of America Bulletin, v. 86, p. 319-334. ern Colorado indicates a lacustrine system site and to Larry Patterson, who allowed us ac- Folk, R. L-, 1974, Petrology of sedimentary rocks: Austin, Texas, Hemphill which was probably larger than any previously cess to the Fort Collins site. Frank Frazier first Publishing Co., 184 p. Frazier, F., Houck, K., Lockley, M. G., Prince, N., Vest, W., and Coringrato, postulated for the formation. Support for such brought the site to our attention and participated V., 1983, Interpretations of some depositional environments and paleo- ecology of a pan of the Morrison Formation in SE Colorado: Geologi- an interpretation comes from the distribution of in preliminary research. Dave Kuntz of the Col- cal Society of America Abstracts with Programs, v. 15, p. 333. lacustrine facies; the physical sedimentary struc- orado Department of Natural Resources worked Galton, P. M., and Jensen, J. A., 1978, Remains of Ornithopod dinosaurs from the Lower Cretaceous of North America: Brigham Young University tures, including ripple marks; and the nature of with us to try to assure future protection for the Geological Studies, v. 25, p. 1-10. Gilbert, G. K., 1896, Underground waten of the Arkansas Valley in eastern the fauna, which includes abundant prosobranch Purgatoire site. The project was sponsored in Colorado: U.S. Geological Survey 17th Annual Report, pt. 2, snails and fish remains. part by a grant from the University of Colorado p.553-601. Ginsburg, L, Lapparent, A. F., Loiret, B., and Taquet, P., 1966, Empreintes de pas Dinosaur trackways, particularly those at the at Denver, a Sigma Xi grant to N. K. Prince, and de Vertebres tetrapodes dans les series continentales a l'ouest d'Agades (Republique du Niger): Academie des Sceances de Paris Comptes Ren- Purgatoire site, demonstrate the usefulness of by contributions raised by the CU Denver dino- dus, v. 263, p. 28-31. footprints in the paleoenvironmental interpreta- saur research team. Channel 9 (KUSA) and Grande, L-, 1980, Paleontology of the Green River Formation, with a review of the fish fauna: Wyoming Geological Survey Bulletin, no. 63, 333 p. tion of shoreline configuration and paleowater Don Eicher also provided aerial support for Harman, W. N., 1974, Snails (Mollusca: Gastropoda), in Hart, C. W., Jr., and Fuller, S.L.H., Pollution ecology of fresh water invertebrates: New tables, as well as in the taphonomic process (di- photography. York, Academic Press, 389 p. noturbation). Morrison tracks also provide Hatcher, J. B., 1903, Osteology of Hapiocanthosaurus, with description of a new species and remarks on the probable habits of the Sauropods many insights into dinosaur biology by provid- and the age and origin of the Ailantosaurus beds: Carnegie Museum Memoir 2(1). ing information on sauropod anatomy (claw REFERENCES CITED Heaton, R. L., 1939, Contribution to Jurassic stratigraphy of the Rocky Moun- posture), gait (speed and trackway width), popu- tain region: American Association of Petroleum Geologists Bulletin, Albritton, C. G, Jr., 1942, Dinosaur tracks near Comanche, Texas: Field and v. 23, p. 1153-1177. lation dynamics (size frequency), and social be- Lab., v. 10, p. 161-181. Hendricks, A., 1980, Die saurierfahrte von Münchehagen bei Rohburg-Loccum Alexander, R. 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