Distribution of Vertebrate Ichnotaxa Based on Substrate/Environment

along the Two Camp anticline

Of the twenty tracksites known from within the former Smoky River Coal Mine only a few show evidence of bipedal trackmakers (W2, W3 Bird, W3 Main, W3 Corner, W3 Extension, South Pit Lake, Barrett Wall and 9 Mine West Extension b).  The rest of the tracksites contain the footprints of quadrupeds identified as Tetrapodosaurus borealis (probable nodosaurid ankylosaurs).  This disparity in footprint distribution seems to be related to the nature of the substrate in which the footprints are preserved which in turn was influenced by the nature of the environment at the time.

The tracksites in which T. borealis prints (quadrupeds) are found exclusively are composed of relatively fine-grained sediments which are dark due to organic content within these beds.  Evidence of sedimentary features such as ripple marks are generally absent.  There is often evidence of in situ macro plant remains such as tree stumps with radiating root systems.  These stumps have been observed to range in diameter from a few cm to tens of centimeters (see E2 Pit).

On the tracksites where bipedal prints are dominant over quadrupeds, the sediments are relatively coarser, sometimes showing evidence of cross-bedding in section indicating a higher energy depositional environment.  There are also well-developed ripple marks evident on many of these track-bearing surfaces.  Few macroplant remains have been found in situ, although some fallen trunks have been observed at these sites.

    From these observations of footprint distributions and depositional settings it appears that bipedal animals tended to avoid traversing boggy areas (well-watered, well-vegetated and muddy surfaces) and stuck to the freshly deposited sands with little vegetation, possibly closer to the source of the channel that flooded and brought all of the new sediment to these extensive surfaces.

On the other hand, the quadrupeds (presumed nodosaurids) frequented these boggy areas, perhaps in search for herbaceous plants to eat.  It seems likely from a few other lines of research findings that ankylosaurs preferred an herbaceous diet, being well-equipped with shearing teeth to clip these plants close to the ground and sending them down to be mechanically digested further along the digestive tract. 

Diagram of footprints and palaeoenvironments in the Gates Formation near Grande Cache, by Richard T. McCrea

The quadruped footprints and trackways of T. borealis are present at all of the twenty known tracksites in this area.  They could be considered as a sort of environmental generalist.  The T. borealis footprint makers were well-suited to walking on unstable substrates as seen in a few rare trackways where the individual prints are preserved well enough.

 It is a fact that paleontologists have a difficult time putting flesh on the bones of extinct animals.  Often the result is just an educated guess, based on the interpretation of the function of the particular animal and how large or small it is.  It would be very difficult to be sure how much flesh to put on the hands and feet of an animal from skeletal remains alone.  Since paleontologists desire and are often expected to present extinct animals as close to living biological entities as possible in their interpretation they often use extant animals as models.  For example, if a paleontologist finds the skeleton of a large quadrupedal animal that has been extinct for some time, they might model the  reconstruction after a modern large quadruped, such as an elephant.  In the case of ankylosaurs there are two ways that their feet have been reconstructed.  One way was probably modeled after the feet of elephants, where the flesh of the foot extends to the tips of the digits with a large fleshy pad at the heel.  The other way is where the individual digits are somewhat freer of flesh in that they extend some length from the flesh of the foot and are not enclosed by it.  Quite a lot is implied by this seemingly insignificant aesthetic detail.  The fleshy nature of an elephant's foot is an adaptation which cushions the weight of this large and heavy animal and enable it to walk in an entirely terrestrial habitat.  

 Generally, T. borealis prints are not preserved in such a way as to shed light on this question.  The cause of poor footprint preservation is likely due to the size and weight of the animal and the nature of the substrate it is walking on.  The T. borealis trackmaker was not a small creature, but one that conservatively would have weighed close to two thousand kilograms.  When an animal of this size crosses a freshly deposited (thixotropic) surface the weight of the animal is going to cause quite a bit of displacement of the underlying sediment and can leave behind somewhat distorted prints.  The best substrates to preserve footprints that are the most reflective of the true nature and dimensions of the feet are generally firm ones with not too much water content.  There is a small window of time available for prints to be made on a newly deposited surface; the time between the waters receding enough to permit terrestrial animals to cross and the time before the substrates either become too firm from dewatering to take any further impressions or before the next depositional event.  At the beginning of this footprint cycle the prints are likely to be pretty distorted because the substrates contain too much water for the print to hold its shape, or the substrate may adhere to the soles of the trackmaker's feet distorting the prints.  Only a brief time will exist when the consistency of the substrate is just right for prints that faithfully reflect the trackmaker's skeletal and soft tissue anatomy.  On one surface this point is different for each size class of animal (birds will make better footprint impressions early in the cycle, due to their light weight; larger animals will make better impressions later in the cycle due to their greater weight.  The amount of time a surface is ideal for preserving good impressions is probably pretty short for each size class of trackmaker and it is not to be expected that the majority of prints at any one site would be made during this interval.  However, with the number of T. borealis prints present in this formation reaching a few thousand there is the possibility that some few trackways might have been produced at just the right time.  This is in fact what has been interpreted from a trackway at the W3 Main tracksite.  The prints are quite lightly impressed on the substrate, but are those of a regular size T. borealis trackmaker.  They reveal that there is little flesh on the digits, especially not to the degree that is seen in modern elephants.  The digits of the hands and feet are long, slender and splayed.  It seems likely that the feet were designed to maximize the surface area of the hands and feet, much like snowshoes or skis.  The large surface area of the palms and soles would distribute the weight of the animal very effectively allowing them to walk on unstable substrates such as bogs and coal swamps where herbaceous plants were thriving.  One close modern analogue could be the modern Hippopotamus.