Dinosaur Tracking Revolution: New Applied Dimensions for Geology
Martin Lockley (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.