Dinosaur Tracking Revolution: New Applied Dimensions
for Geology
Martin Lockley
(and see) (University
of Colorado at Denver, Denver, Colorado, USA)
Ohio Geological Society Meeting, Columbus, Ohio, USA
11 November 1991
Abstract: Although the
traditional dinosaur investigations continue to be important, new important
applications of these studies are evolving. Dinosaur tracks are (now)
equally valuable in paleoecological census studies (community ecology) and the
definition of ichnofaunas, which, in turn, have applications to biostratigraphy
and paleobiogeology. Tracks are also useful in paleoenvironmental studies
pertaining to sediment disturbance (bioturbation), depositional cyclicity, and
sequence stratigraphy. Vertebrate trampling reached its peak in the
Mesozoic when large herds of dinosaurs had significant impact on substrates in
many depositional environments. Previously unrecognized, dinosaur
trampling (dinoturbation) is now known to be widespread and clearly is the
cause of (previously) unexplained reports of load casts and deformed or
contorted beds. Many terrestrial beds are replete with track-bearing
layers. For example, the average vertical spacing of track-bearing levels
in some lacustrine sequences is on the order of only 10-100 cm. Several
formations are now know to contain hundreds of track-bearing levels that
provide useful information of depositional cycles. Regionally extensive
track-bearing layers or units, known as megatracksites, represent trampled
coastal plain tracts on the order of thousands of square miles, and are linked
to sea level fluctuations and sequence-stratigraphic boundaries.
Lecture:
The study of dinosaur tracks has many geological utilities - 1) paleobiological
interpretations (such as behavior, population studies, census data,
biostratigraphy); and 2) paleoenvironmental interpretations (such as shoreline
position, depositional cyclicity, sequence stratigraphy, and “dinoturbation”).
The
preservation of dinosaur tracks includes a dimension not before recognized and
often misinterpreted. A depression of sediment is often found to have
been transmitted to deeper sediment horizons. These are known as undertracks.
Numerous
track sites are known throughout the world. There are a couple of hundred
sites alone in the Colorado Plateau area. There is a very large data
base.
Eleven
main types of dinosaur track morphologies can be recognized. This aspect
will not be discussed extensively in this talk.
Behavior
patterns have been interpreted based on some seemingly unusual tracks. In
Texas (Cretaceous Glen Rose Formation), a set of two tracks supposedly show a
carnivorous dinosaur attacking a brontosaur. The evidence is a missing
theropod footprint in a sequence of tracks showing theropod tracks superimposed
upon brontosaur tracks. The missing footprint supposedly indicated where
the dinosaur leapt into the air, attacking the brontosaur. This scenario
is wrong, as the theropod’s missing footprint was found exactly inside one of
the brontosaur’s tracks. Authentic attack tracks are very rare.
Amphibian tracks are known to approach a carnivorous dinosaur’s tracks, and
then stop, not to be seen again. The Texas tracks seem to indicate a hunting
or stalking behavior, rather than an actual attack.
“Swimming
tracks” are not what they seem to be. The evidence is the presence of
front brontosaur tracks only. The rear tracks are missing from the track
run. The suggestion has been that the dinosaur was swimming, with only
the front legs making contact with the sediment interface. Closer
analysis shows that this is a matter of differential formation and preservation
of undertracks. The front tracks were transmitted to a deeper sediment
depth than the back tracks. It can be shown that these partial tracks are
not developed on the original bedding horizon upon which the dinosuar
walked. The idea of swimming dinosaurs is not considered to be plausible.
Herding
behavior is another interpretation based initially on the Texas tracks.
There is plenty of eivdence that dinosaurs were gregarious. The Texas
tracks are similar to modern tracks of herds. Numerous examples of
parallel trackways exist - up to 50 individuals can be detected. Juveniles
are supposedly protected by the adults by having juveniles walk in the center
of the herd. The original site from which this interpretation was based
shows no proof of that - simply overlapping trackways of juveniles and adults -
no clear pattern can be shown.
Intertrack
spacewidths are fairly regular and even in cases of non-overlapping
tracks. This indicates several individuals moving simultaneously, side by
side, with no overtrampling by subsequent individuals. The tracks tend to
veer to one side or the other together. This veering is further good
evidence of herding - there is no divergence of convergence of these
trackways. The spacing is constant along the entire track length.
Large
vs. small dinosaur tracks - there is a wide range of track sizes, indicating presence
of babies to large adults. In South Korea (Cretaceous Jindong Formation),
there are tracks from dozens of small individuals. They all cross each
other, with a random orientation, as if the baby dinosaurs were milling
around. Definite herding or migratory tracks typically have lots of adult
tracks.
Matching
the foot size to age-size curves is a promising method of estimating the age of
a dinosaur individual based on its tracks. Age-size curves are well
documented, based on skeletal evidence.
Reconstruction
of dinosaur communities (paleoecology) can be done based on census data from
different tracks. In eastern Utah (Jurassic beds), there are no bones,
only tracks. A qualitative reconstruction shows that the original
community consisted, at least, of dinosaurs, crocodilians, mammal-like
reptiles, lizards, and scorpions. Even invertebrate tracks can be
identified on the same bedding plane with dinosaur tracks. By
reconstructing communities, community evolution can be traced. Different
types of communities existed at different times. For example, a
sauropod-theropod fauna is known from limestone beds only (carbonate lakes or
carbonate shorelines). This assemblage is found throughout the
Mesozoic. Another example is an ornithopod fauna. Different tracks
are specific to certain environments. Some are found only around playas,
deserts, and alkaline or saline lakes.
Surprisingly,
a biostratigraphic sequence can be established based on lots of tracks
throughout the world. The compiled column shows that certain track
assemblages occurred at different times. Usefulness of such a
biostratigraphy can be seen on the Korean Peninsula. Only two or three
dinosaur skeletal occurrences are known. However, there are over 500
track levels known. The precise stratigraphic level can be determined
using dinosaur track biostratigraphy.
A
popular question has been whether or not the dinosaurs were already dying out
before the K-T event. There had been no known living dinosaurs close to
the boundary. The closest skeletal evidence was 3 meters below the
boundary clay. Track workers have found hadrosaur (duck-billed dinosaur)
tracks only 30 cm below the K-T boundary. This makes the youngest known
living dinosaur, and eliminates the “3-meter gap” that has plagued the K-T
extinction discussion. It had been interpreted as a dinosaur die-out up
to several hundred thousand years before the K-T event. Now, the gap is
reduced to only 30 cm.
Many
tracks were made in specific environments (lake shores, coastal plains,
etc.). The tracks can be used to pinpoint shorelines precisely.
Walking animals tend to follow the shoreline. This is known to be true in
modern animal behavior. The tracks get preserved on the mudflats along
the periphery of the lake. Lots of tracks show back and forth movement,
to and from the shoreline.
Bird
tracks can be used as water depth indicators. Bird tracks are actually
quite common. They are used to pinpoint Tertiary lake shorelines.
South American borax exploration has utilized bird tracks to find lake
deposits.
“Dinoturbation”,
or extensive dinosaur trampling, is known in the Colorado Plateau. Some
of the tracks are 80 cm deep. This disturbs significant parts of some
sediment packages. It is hard to quantify dinosaur trampling, usually
referred to as a “mess”. The attempt to measure trampling is to measure
the % of the bed surface that is disturbed. There are a few cases of
clams in a sand substrate being smashed and killed by a walking dinosaur.
It can be shown that the dinosaur actually killed the clams. About 6
clams were killed per footstep. A significant trampling impact.
Some trampled horizons are very extensive. Plotting the frequency of
trampled horizons on a time scale shows peaks at the end of the Mesozoic and
Cenozoic. This may be a preservational artifact, or due to the higher
abundances of larger animals at those times. The beginning of the
Cenozoic has very few trampled horizons, because all the large animals are
gone. Very large herds probably trampled many of these beds.
Trampling horizons are fairly common and are often overlooked.
Megatracksites are
extensive stretches of a trampled horizon. Near Arches National Monument,
in eastern Utah (Jurassic Entrada Formation), there are lots of known tracksites.
The study area of 300 km2 has no skeletal remains but lots of
extensive track horizons. A very thin bed (<1 m thick) contains just
tracks at a certain level, and can be traced for miles and miles.
Estimates of up to billions of tracks preserved over the entire stretch.
Individual tracks are extremely numerous on this horizon. It probably
represents regular dinosaur populations making tacks over a long period of
time, related to a sequence stratigraphic boundary.
The
“dinosaur freeway” (“Cretaceous 470”) is present in the Cretaceous Dakota
Formation along the Front Range of Colorado. It is along an old western
sea shore. The tracks are all in a thin unit (“sequence 3”, a few tens of
meters thick) near the top of the Dakota and are confined to the middle and
upper part of sequence 3. Coastal plain deposits are associated with this
dinosaur freeway, or megatracksite.
The
Glen Rose Formation (Texas) tracks are known at sequence boundaries,
also. The Texas megatracksite is a couple of hundred thousand square
kilometers large, representing a large stretch of coastal plain.
Predictions
using tracks:
1)
tracks are random in desert playa settings
2)
tracks are stacked along lake edges
3)
extensive tracts (megatracksites) will be found along coastal plains
Very,
very few known tail drag marks. Ninety-nine point nine percent of tracks
lack tail lines. Maybe only the tip of the tail may have brushed the
sediment surface. If so, it was not preserved in most cases.
Estimates
are that the average pressure exerted downward by an average sauropod is 40
tons per square meter, or 20 tons per half-square-meter, which is the average
area of a typical footprint. The front footpads have less surface area,
and will sink down further into the sediment (going back to the “swimming
tracks”). On a wet substrate, there may be 2-3 feet of sinking, which
isn’t a concern for an animal with a 12 foot high leg.
Only a few rare tracks
show evidence of running. In carnivorous theropod forms, the estimate is
25 mph.