University of Bristol

Describing fossil footprints & Ichnotaxonomy

"Trace fossils can contribute very little to the study of dinosaur phylogeny."
(Thulborn, 1990).

Describing footprints


Describing Tracks

Tracks can be described using a number of features:

Some useful definitions

Toes and hand digits are often described with Roman numerals (I to V), with 'I' representing the hallux, or pollex and 'V' representing the outermost digit. Many tridactyl prints represent dinosaurs with fingers II, III and IV. Tetradactyl dinosaurs usually have digits I to IV rather than II to V or any other combination.

The size of digits is important in describing tracks. The largest digit is often digit III and is described as the principle digit.

The shape of the feet, particularly the morphology of the nodes and any oddly shaped digit, may be vital clues in detecting the type of trackmaker. Isolated or shallow node traces may indicate that the dinosaur was walking lightly. Equally it might suggest that the substrate it walked upon was only just pliable enough for footprints to leave their mark. The posture of the dinosaur may be inferred to some degree; we may be able to deduce whether the animal walked in a digitigrade mode (on its toes), or a plantigrade fashion (on its feet).

Many dinosaur tracks have a nearlysymmetrical pattern. The angles between digits (interdigital angle), and between the two outermost digits (total divarification) may be useful tools in categorising types of footprint. Such measurements must be treated with a degree of scepticism though, since there are a number of reasons, other than phylogenetic ones, that might explain changes in interdigital angles. (Thulborn, 1990).

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Dinosaur footprints - Photographs contributed by Phil Manning

Ichnotaxonomy is a system of scientific classification used to identify tracks and traces of ancient animals. It is "generally regarded that modern traces should not be named" (Simon Braddy, pers.comm.). Ichnites include vertebrate footprints, invertebrate burrows, fossil eggs and various other traces resulting from biological activity.

Ichnotaxonomy should not be confused with normal Linnean taxonomy which is used in reconstructing evolutionary family trees, or phylogenies. Evidence from trace fossils can be used to influence the reconstruction of phylogenetic trees, or to suggest a more likely tree from a choice of several. This is because tracks and traces "may extend stratigraphic ranges of groups to fit the fossil record better" (Simon Braddy, pers.comm.).

Ichnotaxonomy is not useful for making phylogenetic reconstructions; it is useful for describing and categorising types of trace. Edward Hitchcock, an emminent ichnologist (trace fossil scientist) from the last century was the the first to use a the Linnean system of classification for trace fossils. Ichnites are given an ichnogenus (e.g. Eubrontes) and an ichnospecies name (e.g. glenrosensis), just as whole organisms are given a genus name (e.g. Homo) and a species name (e.g. sapiens). (Thulborn, 1990).

Ultimately, the distinctions between ichnospecies and ichnogenera are arbitrary. However, an ichnospecies is usually considered to be a "discrete morphological trace" and an ichnogenus "a grouping of similar ichnospecies" (Simon Braddy, pers.comm.).

Unfortunately, "one person's ichnogenus is another person's ichnospecies. One organism can create many ichnotaxa and conversely, many organisms can create one ichnotaxa" (Simon Braddy, pers.comm.).

Therefore, ichnotaxonomy is only really useful in that it provides a formal way of classifying trace fossils.

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Thulborn, R.A. (1990) Dinosaur Traces.Chapman & Hall, London.

Track Taphonomy


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