Basal Jurassic Dinosaur Tracks and Plants

Basal Jurassic Dinosaur Footprints, Fishes, and Plants
Whitmore Point Member of the
Moenave Formation,
St. George, Utah

Squatting Dinosaur Tracks at Johnson’s Farm in St. George,
by Nancy Carruthers, Utah Geological Survey
ugs / utah geology / dinosaurs & fossils / squatting dinosaur

During this Early Jurassic Period, the land in the St. George area was at or near sea level since the Rocky Mountains had not yet begun to form. The location was close to the equator and all of the continents of Earth were attached into the supercontinent of Pangaea. Streams and lakes once covered portions of southern Utah and northern Arizona depositing the many rocks we now see in the St. George area. An enormous freshwater lake called "Lake Dixie" once covered this area during the Early Jurassic period. The St. George Dinosaur Discover Site at Johnson Farm is contained within the Moenave Formation, which consists of sequences of sandstone, mudstone, siltstone, and shale. The Moenave overlies the Upper Triassic Chinle Formation and underlies the Lower Jurassic Kayenta Formation that forms the red cliffs above St. George. Information taken from http://www.sgcity.org/dinotrax/fossilfinds.php

Above photographs courtesy of James I. Kirkland and Paul E. Olsen. Lower painting by Mark Hallett.

Only one other area in North America preserves as many trackways as at the Johnson Farm, but it is located on the opposite side of the United States in Connecticut. The Rocky Hill Dinosaur State Park was created after excavation turned up (uncovered) similar age trackways in an ancient, Early Jurassic lake bed (East Berlin Fm.) in the Hartford basin of Connecticut and Massachusetts. See the East Coast dinosaur park at http://www.dinosaurstatepark.org/.

Early Jurassic footprints and trackways in the East Berlin Fm., Hartford basin, at the Rocky Hill Dinosaur State Park, Connecticut. Courtesy, Paul E. Olsen.

Diorama of the Hartford basin, showing a diverse Late Triassic landscape just before major floral and faunal disruption at the Triassic-Jurassic boundary. Courtesy, Paul E. Olsen. Painting by William Sillin.

Diorama of the Hartford basin, showing an Early Jurassic landscape just after major floral and faunal disruption at the Triassic-Jurassic boundary. Note flood basalts of the Newark Supergroup rift in the foreground. Unlike the Utah deposits, which cover thousands of square miles, the Early Jurassic lakes on the East Coast were restricted to rift basins which were surrounded by high mountains. Courtesy, Paul E. Olsen. Painting by William Sillin.

Above figures from Bick, 2003. Geologic map of the Harrisburg Junction quadrangle, Washington County, Utah. Map 191. Utah Geological Survey.

St. George Dinosaur Discovery Site at Johnson Farm
(Information taken from May 10, 2005, City of St George publication)
http://www.sgcity.org/dinotrax/indexmain.php

“This is the most significant dinosaur tracksite in western North America.”
Dr. James Kirkland, Utah State Paleontologist

In late February 2000, Sheldon Johnson was leveling a hill on his farm property in St. George, Utah when he came across dinosaur tracks preserved as natural fossil casts on the underside of a thick layer of sandstone. That sandstone layer was part of a lake shore that dates back approximately 200 million years ago and formed when an ancient lake covered the land just north and east of St. George Utah. When a large block tipped out of the bucket of Dr. Johnson’s track hoe, he had no idea that he had just uncovered the first trace of an incredible fossil treasure trove –one of the most important discoveries in the history of paleontological research in Lower Jurassic rocks.

After several years of research and continual discovery, the St. George Dinosaur Discovery Site at Johnson Farm has evolved into a world-class dinosaur site that includes the rare combination of fossilized bones and footprints of dinosaurs and many other ancient animals. When you visit, you will:

Walk through a ‘snapshot’ of a lake ecosystem from Early Jurassic time and observe fish, plants, and animal traces made by invertebrates and vertebrates.
See over 2000 tracks made by at least several kinds of dinosaurs, ancient crocodylians, fish, and many other animals.
See the world’s largest collection of ‘swim tracks’ made by dinosaurs swimming in ancient “Lake Dixie”.
See one of only two fossil tracks in the entire world made by a sitting theropod dinosaur.
See the largest single track block in any museum in the world. This block, weighing 52,000 lbs., has fourteen dinosaur trackways across its surface.

St. George Dinosaur Discovery Site at Johnson Farm is a must-see for anyone interested in dinosaurs, geology and the ancient Jurassic world.

“The [St. George] collection is going to be the most important in the world for the researchers working on early Jurassic footprints.”
Gerard Gierlinski, Polish Geological Institute, Warsaw, Poland

Picture Gallery of Tracks, Plants, and Stratigraphy
photographs by Bruce Cornet (April 2004)

View of dinosaur track site from Middleton Black Ridge (Tertiary basalt), looking north. White tent is visible above truck on road, to right of road (E. Riverside Drive).

Above: Outcrops of the Whitmore Point fluvio-lacustrine beds on opposite side of E. Riverside Drive. Middleton Black Ridge in the background. The Fossil Hill Elementary School can be seen behind the outcrops.

Above: Fluvial-deltaic sandstones interbedded with shallow lacustrine mudstones.

Above: Cross section of a Eubrontes footprint (hand pointing), showing that the fine sandstone on top was deposited rapidly soon after dinosaurs had walked on the underlying mudstone, leaving deep footprints while the mudstone was still soft.

Above: Grey and purple shale and mudstone beds, represent shallow lake deposits; the thickly-bedded sandstones represent fluvial sandstones deposited during floods or as prograding deltaic lobes at the edge of a lake.

Above: Fluvial overbank mudstone with burrows, mudcracks, and root casts, deposits above a lake bed (purple).

Above: Possible volcanic ash layer (pinkish white), indicating volcanic activity in the Cordilleran Mobile Belt (West Coast) as rifting intensified along the East Coast, and as a proto-Atlantic Ocean appeared between the North American and African plates during the breakup of Pangaea.

Above: Current ripples indicating flowing water after mudcracks and footprints were formed.

Above: Hopper casts (pseudomorphs after evaporite minerals), indicating that the lake water was brackish to slightly alkaline.

Above: Hopper (= "salt") casts from a lower Portland Fm. lacustrine bed in the Hartford basin, Connecticut and Massachusetts (shown for comparison). Crystal shape indicates that they were probably gypsum crystals that formed within the sediments during deposition. Surface weathering has removed the gypsum, leaving their casts behind.

Above images: Eubrontes and Grallator footprints stored in the temporary specimen shelter. You are looking at the underside of the overlying massive fine sandstone unit that covered the tracks, producing casts of the positive prints on the underlying mudstone.

Above: Slab on exhibit under the tent, showing numerous Grallator and Anchisauripus tracks crisscrossing an exposed lake margin.

Above two images: Eubrontes footprint on exhibit under the tent from different angles. You are looking at the underside of the overlying massive fine sandstone unit that covered the tracks, producing casts of the positive prints on the underlying mudstone.

Above: Radial mudcracks around a Eubrontes track indicate that the footprint came first.

Above three images: Mudcracks deformed by a Eubrontes track indicate that the footprint was made after the mudcracks formed. You are looking at the underside of the overlying massive fine sandstone unit that covered the tracks, producing casts of the positive prints on the underlying mudstone.

Above: Batrachopus-like track, which is thought to have been produced by Stegomus, a crocodilian ancestor.

Above: Reconstruction of trackmaker and foot of the squatting dinosaur. Courtesy, James I. Kirkland.

Above: Andrew Milner lying next to the body impression of a squatting dinosaur. Most distant tail impression is just below his shoe (see close-up below). Milner can barely stretch the distance from the tip of the dinosaur's tail to where its front feet made an impression. The head and neck of the dinosaur would have extended even further above Milner's hand.

Above: Zoom out picture showing trackway with body impression where dinosaur stopped and squatted. A tail impression is located just to the right of the quarter used for scale.

Above: Body impression labeled, courtesy Utah Geological Survey and Nancy Carruthers. Details and close-ups below.

Above figure 7.9 from Schlische, R.W. and Olsen, P.E., 1989. Geology of the Hartford and Deerfield Basins. In Olsen, P.E. and Gore, P.J.W. (eds.), Tectonic, Depositional, and Paleoecological History of Early Mesozoic Rift Basins, Eastern North America. 28th International Geological Congress, Field Trip Guidebook T351: 107-118.

Above: Claw gouge marks made by swimming dinosaurs as water depth became too high for them to walk on the lake bottom. You are looking at the underside of the overlying massive fine sandstone unit that covered the tracks, producing casts of the positive prints on the underlying mudstone.

Above: Dozens of parallel claw marks might indicate that these dinosaurs swam together in migrating herds, crossing shallow narrows in the lake. You are looking at the underside of the overlying massive fine sandstone unit that covered the tracks, producing casts of the positive prints on the underlying mudstone.

Above: Invertebrate trackway (crayfish or insect) from a dinosaur footprint layer (just above quarter).

Above: Semionotus kanabensis, a holostean fish from one of the deeper lake units in the Whitmore Point Member.

Above: Semionotus sp. (for comparison) from the basal Jurassic Bluff Head locality, Shuttle Meadow Formation of the Hartford basin, Connecticut. For more information, go to http://www.monmouth.com/~bcornet/blufhead.htm. This specimen was prepared by B. Cornet.

Above: Semionotus elegans (for comparison) from the basal Jurassic Bluff Head locality, Shuttle Meadow Formation of the Hartford basin, Connecticut. For more information, go to http://www.monmouth.com/~bcornet/blufhead.htm. This specimen was found and prepared by B. Cornet.

Above: Cliff exposure of Moenave Formation directly beneath plateau where dinosaur track beds are located. Channel sandstones containing plant fossils are located at the top of this exposure (in foreground). The buff-colored sandstone unit thickens behind the photographer, where a point bar sequence is exposed.

Above: Channel sandstones (point bar) displaying cross bedding. Sandstone beds thicken (become more massive) to the right of this photograph, where plant fossils were found.

Andrew Milern lifting slabs of channel sandstone containing stems, leaves, and cones of conifers.

Above: Conifer branch with leaves (Brachyphyllum sp.) found in a light brown siltstone-fine sandstone.

Above: Stem or branch of a plant with a wide pith and a thin outer ring of wood (black).

Above: Plant debris deposited in a point-bar fine sandstone, illustrating Pagiophyllum branches/leaves, tree bark, twigs, and wood.

Above: Various types of conifer foliage from the Early Jurassic of North America. Two types of form-genera are illustrated: Brachyphyllum sp. (12); Pagiophyllum spp. (4-11). Pagiophyllum in figure 9 was found attached to cones of the Cheirolepidaceae. The microphyllous Brachyphyllum sp. 12 comes from the oldest (Feltville Fm.) Jurassic strata in the Newark basin, PA-NJ-NY, while the macrophyllous Pagiophyllum sp. 9 with the longest leaves comes from the youngest (Portland Fm.) fossiliferous strata in the Hartford basin, CT-MA, indicating a decrease in aridity and/or temperature through the Hettangian and Sinemurian on the East Coast following a mass extinction at the Triassic-Jurassic boundary. The Whitmore Point Mbr. conifer leaves fall in between these two extremes. Pagiophyllum sp. 4 resembles foliage of the extant Monkey Puzzle Tree from Australia (Araucariaceae). For more information on Early Jurassic conifers, go to http://bcornet.homestead.com/files/Conanthus/conanthus.htm.

Above: Location where the dinosaur trackway was found with one footprint containing the leaf fossil. Exfoliation from rain severely damaged the footprints and leaf fossil before they could be coated with preservative.

The leaf impression was found inside a dinosaur track impression. The fossil was photographed and casts made of it before it was destroyed. The picture below appeared in the Utah Geological Survey publication, Survey Notes (Kirkland, et al., 2002).

Above images and close-ups courtesy of James I. Kirkland, Utah Geological Survey.

This leaf impression resembles an extant Nuphar leaf (Nymphaeaceae), complete with petiole and impressions of radiating veins.

Above: Nuphar polysepala from Wyoming. Photo by Barre Hellquist. Note footprint impressions in soft mud and how a lowering of lake level exposes the footprints and plants to drying and potential fossilization, if additional mud is deposited on top of the dried leaves and prints in a subsequent flood or rapid rise in lake level during a heavy rain.

Above: A cast was made of the specimen before it was destroyed by rain. The outline of the dinosaur track is shown; it was within this track that the leaf impression was preserved. The leaf was either 1) washed into the track after it was made, or 2) it was sticking to the bottom of the dinosaur's foot until this step, where the leaf came off the foot and stayed with the imprint. The second explanation would explain the damage to the leaf lamina and squashed (splayed) appearance of the petiole (more flattened under the "heal"). It would also mean that the dinosaur trekked through a patch of these water plants before coming out of the water onto the muddy bank of Lake Dixie.

The dinosaur trackway shows the direction from which the theropod traveled before the leaf fell off the bottom of its foot. The leaf was squashed and distorted (petiole splayed out) enough to indicate that it had traveled with the dinosaur for a good number of steps (stuck on by mud and/or between the dinosaur's toes). The relatively good condition of the fossil impression after having been stepped on however many times between the lily patch and where the detached leaf ended up speaks volumes on its durability (ruggedness and laminar thickness). Nuphar leaves are equally durable (thick), making it an excellent example for comparison.

Retracing the path of the theropod places it across E. Riverside Dr. in a quarried area where dinosaur bones have been found, a sauropod trackway, many theropod tracks, and what Andrew Milner pointed out as possible plant root casts. Those "roots" resemble thick rhizomes (horizontal in situ stems) spreading through the lakebed siltstone and mudstone layers. If they represent a colony of water lilies, by excavating that layer, additional leaves (attached) and even reproductive structures and seed pods might be found.

Above: A block of siltstone and overlying darker mudstone contains a cast of a rhizome (horizontal stem) bearing dozens of small rootlets. This specimen came from the rock layers below the sauropod trackway. The light gray aureole around the rhizome was caused by the reducing effect of decay, while the red cast represents infilled red clay after the rhizome decayed. At least one upwards branch might represent the base of a leaf petiole (the rest of the leaf may be hidden in this block or in its counterpart). Only through additional excavation and preparation of specimens such as this will it be determined if more of this plant is preserved in that unit.

Above: Close-up of another block of mudstone containing the cast of a rhizome with rootlets, and a possible attached petiole base.

If this fossil belongs to the Nymphaeaceae, it would not only confirm the basal position of the Nymphaeaceae based on molecular evidence, but also extend crown-group angiosperms back at least to the basal Jurassic. Additional specimens, along with reproductive structures are needed to prove the fossil is that of an angiosperm. Nevertheless, it establishes that this plant existed; if it was preserved once, it will probably be preserved elsewhere in these deposits. See why.htm.

This page was first created by Bruce Cornet on 10 May 2005.
This page was last modified on 11/26/2014.