paleoenvironment at jones ranch, an early cretaceous sauropod

J. Paleont. Soc. Korea. Vol. 22, No. 1, (2006) : p. 77-89
PALEOENVIRONMENT AT JONES RANCH, AN EARLY
CRETACEOUS SAUROPOD QUARRY IN TEXAS, U.S.A.
1
Dale A. Winkler and Peter J. Rose
1,2
1
Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 USA,
[email protected]
2
Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455
USA, [email protected]
Abstract: Jones Ranch is a unique Early Cretaceous (Aptian-Albian boundary) sauropod dinosaur accumulation
in the Twin Mountains Formation (Trinity Group), Texas, U.S.A. Bones of at least four individuals are found
together with large logs in a fluvial channel deposit. Sedimentary structures, taphonomic considerations and
associated fauna and flora demonstrate that sauropod carcasses and trees decomposed in a seasonally dry stream
bed before being transported to, or rearranged at, their final point of burial. Associated vertebrate fauna includes
terrestrial and fresh water elements including the rare hybodont chondrichthyan Lonchidion anitae and lungfish.
Abundant plant material from the site is referred to the extinct conifer Frenelopsis ramosissima. At Jones Ranch,
sauropods dwelt within a low diversity coniferous forest with trees that reached above 20 meters in height. The
death assemblage and abundant track sites in the Trinity Group suggest gregarious behavior in a sauropod that
occupied both low diversity inland forests drained by ephemeral streams and higher diversity plant communities
along the coast of a shallow sea.
Key words: Paleoenvironment, Jones Ranch, Early Cretaceous, sauropod, Texas
INTRODUCTION
Cretaceous sauropod dinosaurs are uncommon in North America relative to their abundance in Late
Jurassic deposits (Hunt et al., 1994; Maxwell and Cifelli, 2000). The presence of sauropods in the Early
Cretaceous rocks of Texas became well known before the mid-20th century because of the discovery of
impressive trackways (Bird, 1985; Farlow, 1987, Farlow et al., 1989; Winkler, 1997). Despite numerous
track discoveries, the identity of the track-maker has remained elusive for many decades (Langston,
1974). Several incomplete individuals and isolated bones found in Aptian-Albian age sediments in Texas
and Oklahoma were referred to the genus Pleurocoelus (Langston, 1974; Gallup, 1989). That genus was
known from other fragmentary material in the eastern United States. In the 1980’s, a large accumulation
of sauropod bones was discovered at Jones Ranch, Hood County, Texas. No other Texas Early
Cretaceous locality preserves such an abundance of sauropod bones. The first excavation at Jones Ranch
was undertaken in the mid-1980’s, by crews from the University of Texas at Austin. New efforts at the
quarry were begun again as a joint effort of the Fort Worth Museum of Science and History (FWMSH),
Southern Methodist University (SMU), and others in 1993. Gomani et al. (1999) made preliminary comparisons of the sauropod material from Jones Ranch. A brief description of the occurrence was given in
Winkler et al. (2000), in comparison with another Early Cretaceous sauropod locality in Malawi, Africa.
Continued excavation and preparation of bones from Jones Ranch has allowed description of the sauropod material and its attribution to a new genus and species (Rose, in press). This new insight into the sauropod, together with publication of important plant remains from the site (Axsmith and Jacobs, 2005),
and the recent discovery of additional vertebrate fauna, prompt a new examination of the origin and paleoenvironment of this significant dinosaur accumulation.
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SETTING AND DEPOSITIONAL ENVIRONMENT
The Jones Ranch quarry (SMU Loc. v282; FWMSH 93B10) occurs in the Twin Mountains Formation
(Trinity Group). The locality is stratigraphically below the marine Glen Rose Formation, which is the
track-bearing unit (Winkler et al., 2000). A small bed of clean micritic limestone of the Glen Rose
Formation is 9.66 meters above the approximate top of most bones in the quarry (zero vertical datum
used in mapping), and the lowest major fossiliferous marine limestone of the Glen Rose is 12.97 meters
above. At the site, fluvial sediments containing bone correlate with localities that are latest Aptian-early
Albian in age (approximately 112 Ma; Jacobs and Winkler, 1998; Winkler et al., 1989, 1990). This terrestrial site was situated only a few tens of kilometers inland from the paleoshoreline of a shallow marine
shelf and less than 15 km from the well-known track sites formed along the coast in the slightly younger
Glen Rose Formation at Dinosaur Valley State Park. Continued transgression of the sea in North America
flooded the area of Jones Ranch, only a few million years after the bone-accumulation events.
Dinosaur bones are preserved at the site in cross-bedded sands intercalated with beds and lenses of
muddy sand, clays and mud (Figs. 1 and 2). The scoured bases of cross-bedded sand units are cut into
fine-grained ripple cross-bedded sand and tabular muddy sands. Similar sands with small scale cross-
Fig. 1. Outcrop at north wall of quarry (March,
1999). Weathered face has been scraped to reveal cross-bedded sands and muds at and just
above the main bone producing level. Lenses
near the base are sandy mud rich in charcoal
and dark organic debris. Scale in centimeters
(left edge).
Winker and Rose - Paleoenvironment at Jones Ranch
79
bedding, and tabular muddy sands with horizons of abundant small calcareous nodules, overlie the
bone-bearing units. The overlying muddy sands contain petrified wood, locally in abundance, but no
traces of bone have been found. Abundant plant debris and petrified wood occurs in conjunction with
large sauropod bones and fragments. Some of the large logs have preserved lengths approaching six
meters. Cones, branches, stems, charcoal (fusain), and macerated plant debris occur concentrated within
specific sandy and muddy lenses within the cross-bedded sands (Fig. 3). A distinctive, clay-clast rich,
well cemented, sand forms much of the base of the accumulation and has an irregular scoured base that
varies over at least one meter vertically. The clay clasts within the basal bone-bearing sand are clearly
clay rip-ups. Lithic cobbles occur in parts of this basal sand. Bones occur throughout approximately 1.5
m of vertical section across the width of the quarry, in part because the base of the bone-bearing units is
low in the south and west edge of the quarry and rises toward the northeast. In the central quarry area,
bones were found in stacked assemblages at least one meter thick. Much bone and plant material is associated with disjunct irregular lenses of dense calcite-cemented sandstone concretion. Pyrite nodules and
iron-rich concretions of small size occur throughout the deposit. Significant for the interpretation of the
sediments are units of clay and, commonly, plant debris that fill and drape scoured horizons within the
bone-bearing sands. Overlying cross-bedded sands commonly scour and cut out these muddy units along
irregular bases.
The scale of the cross-beds and the ability of currents that formed them to carry large clasts such as
portions of sauropod dinosaur carcasses and large logs contrasts markedly with the quiet water clay
drapes with plant debris layers that are intercalated. The scoured base of the cross-bedded sands indicates
a river channel setting for the bone accumulation with overlying off-channel sands and muds above and
below. In addition, the fine-grained sands overlying the bone-bearing sands show evidence of calcareous
nodule horizons. These nodules are interpreted as products of pedogenesis in a semi-arid climate. All of
these factors indicate a depositional system that was seasonally dry and a river environment subject to
Fig. 2. Measured section at south end of sacral block (9 m south and 1 m east of datum). Bone occurs throughout the
section beginning with the densely cemented clay clast rich sandstone. Plant material and charcoal are abundant in the
upper sand unit of this section. Cemented cross-bedded sand with bone at least 50 cm thick was removed from above
this section.
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Fig. 3. Male cone of Frenelopsis ramosissima in situ in matrix. Scale bar is 3 cm.
flashy intermittent flow with periods of slack water in the channel such as is found in ephemeral streams
(McKee et al. 1967, Williams, 1971; Stear, 1985). The channel in which the bone accumulation occurs
was a least a few 10’s of meters wide.
TAPHONOMY
Bones occur throughout an area greater than 400 square meters, but most are concentrated within
about 300 square meters (Fig. 4). Approximately 150 field jackets were removed from the site through
2005 and these include many large blocks with multiple bones (Fig. 5). To date, nearly 40 large blocks
and many small jackets have been prepared. Field jackets weighed up to approximately 10 metric tons,
the largest containing a sacrum, pelvic girdle, hindlimb, and some dorsal vertebrae (among other bones).
Petrified logs are distributed more widely across the quarry, but are commonly intermixed with bones.
During more than a decade of excavation, all of the large vertebrate elements recovered and the vast majority of the small fragments could only be attributed to sauropod dinosaurs (Winkler et al., 2000). That
is still true for the central quarry area, but see section on new discoveries. The large vertebrate assemblage appears to be monospecific.
Most bones occur within close proximity to others (Fig. 4). None of the sauropod specimens is fully
articulated. However, segments of articulated elements are common and include bones from all regions
of the body: strings of cervical, dorsal and caudal vertebrae, shoulder girdle, pelvic girdle and sacrum,
parts of limbs, and metapodials (Fig. 6). Six or seven cervical vertebrae belonging to one individual occurred in separated strings with scattered vertebrae nearby (Fig. 7). Of these, four cervicals in series occurred in two articulated strings twisted 180° from their natural position. Near the center of the quarry
large limb and girdle elements (and smaller elements) occurred in layers stacked two and three bones
deep. Some bones and logs were found with their long axis tilted from the horizontal. One scapula was
Winker and Rose - Paleoenvironment at Jones Ranch
81
Fig. 4. Quarry map of Jones Ranch (SMU loc. v282; FWMSH 93B10) showing selected sauropod bone elements (gray)
and larger petrified logs (black). Grid is 2 m square. Positions were measured from the horizontal datum point shown
in the left center. Beds with microvertebrate concentrations were first discovered at the point indicated in the northeast,
and these beds continue on to the northeast along with fragmentary sauropod remains. Outline map shows Jones Ranch
within Texas, USA.
Fig. 5. Jones Ranch quarry showing field jackets near the northern edge of excavations, Nov. 1996. Some jackets encased
concretionary masses with numerous bones enclosed. Scale in centimeters on the left of card.
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Fig. 6. Caudal vertebrae in series in quarry at Jones Ranch (blocks JR 95-18, -19). Arrows highlight individual vertebrae
in semi-articulation. Scale in centimeters on bottom.
Fig. 7. Two articulated cervical vertebrae with associated cervical rib projecting down at 90o from axis of centra (field
block JR 98-7). These are part of 6 or 7 cervical vertebrae from one individual found disarrayed but in close proximity.
Scale in centimeters on the right.
tipped against a large log, and one elongate cervical vertebra associated with the set described previously
was buried on end. Many bones were fragmented from post-depositional processes, but others were
clearly fractured and transported before burial. In the upper bone-bearing levels of the cross-bedded
sands, bone fragments are common and scour surfaces show that some previously deposited elements
were damaged during floods that postdated their burial.
Winker and Rose - Paleoenvironment at Jones Ranch
83
Wood at the site ranges from centimeter size fragments to logs as long as six meters. Many of the
specimens appear to have been weathered before and fragmented during transport. One large tree was
represented by a stump with an attached root mass, with the root mass more than 2.5 m wide at its base.
Its trunk was scarcely 3 meters in preserved length with its top roughly fractured. Neither logs nor long
bones appear to have a preferred long axis orientation.
Most of the sauropod bones do not show evidence of long distance transport, but from their distribution it is clear that many were moved or at least disturbed from the site where the individual died.
There is no obvious evidence of scavenging. Some of the densest concentrations of bone are encased by
the concretionary masses already noted. This style of preservation also characterized the more articulated
remains of a large specimen of the theropod Acrocanthosaurus atokensis from central Texas, also in the
Twin Mountains Formation (Harris, 1998). The non-woody (non-silicified), three-dimensional plant remains (i.e., cones) were all found in the concretionary masses. Although the sauropod remains are not
fully articulated, numerous body segments that are articulated, or nearly articulated, indicate that connective tissue remained on the skeletons when they were buried. Partial decomposition of the carcasses
must have occurred after death, but before the burial event. Because individual bones are not significantly surface weathered (with some exceptions), the length of time of exposure of the carcasses after
death but before burial must have been months and not years (Behrensmeyer, 1978; Weigelt, 1989). One
or more flood events must have rearranged or brought bones to the point of burial because of the bones
stacked arrangement. Subsequent flooding events scoured and reworked some of the already buried
elements. All sizes of bones, and both dense and delicate elements, are preserved indicating no sorting of
the remains. The quarry is not a lag assemblage, but is compatible with Behrensmeyer’s (1988) channel
fill mode of accumulation. A high energy event was required to move the large bones and logs to their
burial site. Large logs may have been a significant factor in blocking continued flow and in helping to aggregate the assemblage of bones.
At least seven femora have been identified thus far representing a minimum of four individuals (Rose,
in press). Most of the sauropod elements are comparable in size, although one individual is slightly
smaller. All of the sauropod bones from the quarry can be attributed to the same species based on their
close association and because duplicate elements do not show appreciable variation in size, proportion,
or morphology beyond what can be expected from individual variation (Rose, in press). More than one
depositional event may have moved some of the bones, but most of these individuals must have perished
in close temporal and spatial proximity. A behavioral mechanism is strongly implied to explain such assemblages of single species (Coria, 1994). Trackways in Texas have been interpreted as evidence of gregarious behavior in Early Cretaceous sauropods (Farlow, 1987). Monospecific bonebeds imply not only
gregarious behavior but also a common mode of death. As yet, the cause of death for the Jones Ranch
sauropods is not apparent, and customary explanations such as being overwhelmed by floodwaters, or
concentrating at water holes due to drought must suffice. One intriguing possibility suggested by the
abundance of charcoal with the plant debris is forest fire.
Many localities in the Trinity Group produce an abundance of small vertebrate fossils, notably sharks
and rays, bony fishes, amphibians, turtles, lizards, and crocodylomorphs as well as dinosaur teeth and occasionally mammals (Winkler et al., 2000). However, for more than a decade of excavation at Jones
Ranch, the only vertebrate remains recovered in the quarry besides sauropod bones were a few fragmentary crocodylomorph scutes and fish scales. This was despite attempts to screen-wash promising
looking fine-grained sediment for microfauna. In late 2003, a lungfish toothplate was recovered in dark
muddy sands in the northeast quarry area. Samples taken for screen-washing are producing a moderate
vertebrate microfauna. The productive units are plant and charcoal rich, gray muddy sands and clays associated with orange mottled muddy sands. These muddy sands are less than 20 cm thick and sit upon an
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irregular scoured base of cleaner buff sands. They extend at least 5 meters across the northeast area of the
quarry. Microfaunal sampling is continuing. Isolated or fragmentary larger bones (sauropod) are found
associated with the microfauna. The fauna resembles other Trinity Group sites in composition, especially
those removed from the marine environment. The bones and teeth exhibit considerable preburial breakage, in addition to that induced by the washing process. This is apparent from in situ bone fragments and
small elements still surrounded by matrix in the wash samples. Fossils range in size from fragments tens
of centimeters long to teeth little more than one millimeter in diameter. Vertebrate remains as well as
small plant debris and charcoal fragments may have been concentrated as lags in troughs during low or
receding flows. Fine-grained deposits such as these are common throughout the quarry filling and mantling irregular scour surfaces, although most contain only plant debris and charcoal. It is apparent that all
these elements were buried in slack water deposits in a river channel during periods of no flow. Standing
water was commonly muddy and filled with plant debris.
FLORA AND FAUNA
Fossil plants from the Trinity Group demonstrate that angiosperms were present in the flora of Texas,
but not yet abundant (Kessler, 1968; Jacobs, 1989). Ferns, cycads, bennettitaleans, and conifers have
been recognized (Fontaine, 1893; Jacobs, 1989). Axsmith and Jacobs (2005) have referred the cones,
branches and cuticles found at Jones Ranch to the conifer Frenelopsis ramosissima. Male cones found at
the site contain Classopollis-type pollen. This together with the wood anatomy of the petrified logs all
support attribution of Frenelopsis ramosissima to the family of extinct conifers Cheirolepidiaceae. Thus,
the vast majority of the abundant plant remains at Jones Ranch, and all that are identifiable, can be referred to a single conifer species. Previous work on the paleoecology of Frenelopsis and Pseudofrenelopsis
(both members of the family Cheirolepidiaceae and known from Early Cretaceous deposits in the eastern
U.S. and England) has suggested that many species bore adaptations for water stressed (xeric) or halophytic environments (Upchurch and Doyle, 1981). It is common for members of this family to occur in
low diversity assemblages (Gomez et al., 2001). Upchurch and Doyle (1981) however, noted that in the
eastern U.S., Frenelopsis ramosissima occurs in a diverse mesic assemblage. Others had reconstructed
this species as a small shrub (Axsmith and Jacobs, 2005). Evidence from Jones Ranch now supports the
interpretation of Frenelopsis ramosissima as a large tree reaching over 20 m in height and living in low
diversity or monospecific stands under a semi-arid climate near the coast (Axsmith and Jacobs, 2005).
The sauropod dinosaurs found at Jones Ranch inhabited a coastal region dominated by ferns, multiple
conifers, cycads, and cycadeoids, but they also dwelt in low diversity conifer forests inland.
Initial processing for microvertebrates has added to the understanding of the paleoenvironment at
Jones Ranch (Table 1, Figure 8). Lungfish are rare in the Trinity Group of Texas, so the occurrence at
Jones Ranch is unexpected. Specimens of Ceratodus had been recovered only from the intensely sampled mammal localities Butler Farm and Greenwood Canyon in northern Texas. The fauna at both these
localities contains no marine elements. Likewise, the Jones Ranch microfauna lacks lamniform chondrichthyans and diverse pycnodonts, which are ubiquitous in many marine and marginal marine localities
at this time. Paluxy Church is the nearest well sampled locality (SMU Loc. v138) to Jones Ranch, is only
a few kilometers basinward, and is also in the Twin Mountains Formation. The fauna there has terrestrial
elements (frogs, mammals) as well as more marine elements like diverse pycnodonts (Winkler et al.,
1990). Like Paluxy Church, but unlike the northern mammal localities, Jones Ranch contains the hybodont shark Polyacrodus aff. P. parvidens (Fig. 8), instead of Hybodus butleri. However, the small hybodont Lonchidion anitae is found only at Butler Farm and Greenwood Canyon in addition to the new oc-
Winker and Rose - Paleoenvironment at Jones Ranch
85
Table 1. Vertebrate microfauna from Jones Ranch, NE Quarry.
Chondrichthyes
Polyacrodus aff. P. parvidens (abunadant)
Lonchidion anitae (uncommon)
Osteichthyes
Amiiform indet. (abundant teeth)
Lepidotes sp. (teeth and compatible scales)
Pycnodontiformes
?Palaeobalistum sp. (isolated teeth, dermal armor, partial vomer; compatible scales)
Sarcopterygii
Dipnoi
Ceratodus sp. (rare)
Amphibia
Anura indet. (skull elements, postcrania)
Reptilia
Crocodyliformes
At least 2 distinct tooth morphs (abundant teeth, scutes)
Dinosauria
Theropoda
Dromaeosauridae
Indet. large theropod
Sauropoda
New genus and species
currence at Jones Ranch. Lonchidion sp. and Polyacrodus parvidens are reported together in the AptianCenomanian Cedar Mountain terrestrial fauna of Utah (Cifelli et al., 1997), and most species of
Lonchidion are associated with non-marine habitats (Rees and Underwood, 2002). Other fish remains at
Jones Ranch are compatible with a fresh water environment. Density of sampling may be at issue, but
turtles, which are abundant in most Trinity Group faunas, have not yet been identified at Jones Ranch. At
least two distinctive crocodyliforms are present based upon teeth, one with laterally compressed bladelike teeth (at least in the posterior dentition) and another with teeth of the typical conical form (Fig. 8).
Sauropod and theropod teeth have been obtained from the screen washed samples as well as amphibians,
highlighting the terrestrial nature of the fauna. Missing, but expected are lizards and mammals.
Sauropods are not abundant in the Early Cretaceous of North America, but a number of new taxa have
been named in recent years based on fragmentary material (e.g., Tidwell et al., 1999, 2001; Wedel et al.,
2000). In the past, one name traditionally attached to partial sauropod remains from Early Cretaceous
sites in North America, including Texas, has been Pleurocoelus (Langston, 1974; Gallup, 1989). Rose (in
press) argues that the Jones Ranch material is distinct from both the original fragmentary material referred to Pleurocoelus from the east coast (Carpenter and Tidwell, 2005 have now referred this material
to Astrodon), and from vertebrae named Sauroposeidon proteles (Wedel et al., 2000) from the Antlers
Formation (Trinity Group) of Oklahoma. Based on size and morphological variation, all of the sauropod
bones at Jones Ranch can be attributed to the same species (Rose, in press). Like Sauroposeidon, the sauropod at Jones Ranch is a titanosauriform with affinities to Brachiosaurus (Rose, in press). It is less derived than the titanosaurid sauropod (Alamosaurus) that is found in latest Cretaceous faunas in North
America. Although not as large as Sauroposeidon (Wedel et al., 2000), the individuals from Jones Ranch
approached 20 m in body length.
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Fig. 8. SEM images of microfauna from Jones Ranch. A, Polyacrodus parvidens, tooth (reversed) Scale bar = 200 µm;
B, amiiform tooth, scale bar = 100 µm; C = crocodyliiform tooth, scale bar = 100 µm; D = anuran maxilla, medial view,
scale bar = 200 µm.
SUMMARY AND CONCLUSIONS
The Jones Ranch locality in Texas, U.S.A., is one of the richest accumulations of sauropod bones in
the Early Cretaceous of North America. It is the first site in Texas to produce abundant remains of the dinosaur that is presumed to have made the famous tracks at Glen Rose, Texas. Fluvial sediments of the
Twin Mountains Formation (Trinity Group) at Jones Ranch correlate to the base of the Glen Rose
Formation farther basinward. The age of the site is latest Aptian to earliest Albian, but more likely the
latter. Dinosaur bones at Jones Ranch are buried in cross-bedded sands with lenses of muddy sand and
Winker and Rose - Paleoenvironment at Jones Ranch
87
mud rich in plant material. Large logs were emplaced simultaneously with the bone assemblage.
Sediments at the site indicate a semi-arid climate because of the presence of clay rip-up clasts, mud and
plant debris filling scour holes and mantling scour surfaces, and calcareous soil nodules. At least four individuals of one large titanosauriform sauropod are buried at the site and they represent a new taxon.
Sauropod bones are found semi-articulated or isolated indicating some (but not total) decomposition of
the bodies prior to emplacement at the site. A common mode of death and a behavioral mechanism of aggregation are indicated by the monospecific assemblage. Both logs and bones were moved in the depositional event that buried them within a seasonally dry channel that they must have occupied before
emplacement. Transport distance was not great for the dinosaurs, but may have been longer for the logs.
Powerful transport currents at Jones Ranch were interspersed with periods of slack or standing water.
Plants, but few other vertebrate remains, were found with the central mass of sauropod bones. However,
microvertebrates are known from the northeast section of the quarry. The fauna includes some rare elements, such as the hybodont Lonchidion anitae and the lungfish Ceratodus, that occur in only two other
localities in the Trinity Group of Texas, both of which are terrestrial. Other elements also indicate a terrestrial fauna. A locality only a few kilometers basinward contains terrestrial as well as possible marine
forms such as diverse pycnodont fishes. Small plant fossils and microvertebrates at Jones Ranch were deposited within or near the concentration of sauropod bones and logs, but the accumulation of these small
remains was brought about by waning currents and in slack water, after ephemeral flooding events. The
environment around the Jones Ranch site was a low diversity forest dominated by the extinct conifer
Frenelopsis ramosissima. These were large trees exceeding 20 meters in height (Axsmith and Jacobs,
2005). Sauropods undoubtedly occupied this inland forest as well as more diverse plant assemblages near
the coastline only ten to twenty kilometers to the west.
ACKNOWLEDGMENTS
None of the work at Jones Ranch would have been possible without the tireless support of Bill and
Decie Jones. Collection and logistics were made possible by Jim Diffily and the staff and volunteers of
the Ft. Worth Museum of Science and History, and, Louis and Bonnie Jacobs, Kent Newman, and staff
and students of the Shuler Museum at Southern Methodist University. We especially thank Bill Lowe,
Alice Murphy, Steven Cohen, Wendy Gates, Laurel and John Wilson, Arlene Pike, and Dr. Phil Murry,
Dr. Jeff Pittman, and Dr. Wann Langston. This work was supported by NSF EAR-9206691 to DAW.
미국 텍사스 전기 백악기 용각류 화석지인 Jones Ranch의 고환경
1
Dale A. Winker and Peter J. Rose
1,2
1
Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 USA,
[email protected]
2
Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455 USA,
[email protected]
요 약: Jones Ranch는 미국 텍사스 Twin Mountains층 (Trinity 층군)의 전기 백악기 (Aptian-Albian 경계) 용각류 공
룡들이 함께 묻혀있는 독특한 화석지다. 최소한 4개체의 뼈들이 하성 퇴적물 속에 커다란 나무 둥치들과 함께 묻혀있
다. 퇴적구조, 화석화과정, 함께 산출되는 동식물 화석군을 보면 계절적으로 건기 때 용각류 시체와 나무들이 함께 썩
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은 후 운반되어 재배치되어 현재 위치에 온 것으로 해석된다. 함께 산출되는 척추동물화석들은 육성환경과 민물에 사
는 hybodont chondrichthyan Lonchidion anitae 와 폐어를 포함한다. 본 화석지에 풍부한 식물화석은 멸종된 침엽수
인 Frenelopsis ramosissima 이다. 그 당시 Jones Ranch에는 용각류들이 생물의 다양성이 그리 높지 않은 20 미터 높
이에 이르는 침엽수림에 살고 있었다. Trinity 층군에서 산출되는 풍부한 발자국화석과 이들 용각류 화석들은 용각류
들이 얕은 해안을 따라 발달한 높은 다양성을 가지는 숲과 계절적으로 가문 다양성이 낮은 내륙 숲속 모두에 군집을 이
루며 살았다는 것을 암시한다.
주요어: 고환경, Jones Ranch, 전기 백악기, 용각류, 텍사스
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