Arthropod trackways Arthropod Locomotion

Arthropods are a highly diverse group of invertebrates and use many forms of locomotion including walking, swimming and flying. This site is concerned with their walking techniques as it is these which produce the fossilised trackways we see in the rock record. Depending on their life habits, arthropods will walk either sub-aerially, that is on terrestrial sediments, or sub-aqueously, or underwater; some species will do both.

The stride of an arthropod can be thought of as the distance between successive footfalls of the same leg, and is made up of the propulsive backstroke (remotor), and the recovery forward stoke (promotor). During the remotor the foot is in contact with the substrate, and during the promotor it is raised, one promotor/remotor sequence is known as a step cycle. The remotor can be worked out from measurements of the stride from a trackway, and knowledge of the arthropod producer's morphology. Once this is known a promotor (p): remotor (r) ratio is calculated as a ratio out of ten, this is known as the gait ratio, and can tell us a lot about the walking techniques employed by a particular arthropod.

For example a high geared gait such as 7:3 means that only 30% of the animal's legs were in contact with the ground at any one time, this indicates a fast speed. Lower geared gaits indicate a slower speed or some obstruction to movement, for example pushing through vegetation. All arthropods have a preferred gait but will change their gait patterns to suit the substrate they are walking on, whether they are walking sub-aerially or sub-aqueously etc. A giant myriapod (millipede) 1 metre long from the Carboniferous Period of Scotland was found to be employing a gait of 5.5:4.5 using measurements of a trackway it produced2

The three gait parameters:
  • Gait ratio is the relative time that a particular leg (e.g leg I) is off the ground compared to on the ground, the p:r ratio.
  • Opposite phase difference is the difference in time between the movement of legs on opposite sides of the body. If this insect moved leg I on both sides of the body at the same time, it is said to be moving in-phase, and the opposite phase difference would be 0.
  • Successive phase difference is the relative time difference between moving legs on the same side of the body. If one step cycle takes 0.5 seconds, and leg II is 0.1 seconds behind leg III in its movement, the successive phase difference is 0.2.
The three gait parameters are used to define a particular walking techinque.

As well as the gait ratio, an arthropod's walking style is defined by its opposite and successive phase differences, or the time difference between moving legs on opposite and the same sides of the body. Arthropods walking in phase move the same leg on each side of the body at the same time. This walking technique is less stable than out-of-phase walking, and is generally practised by arthropods walking sub-aqueously.
Fossilised trackways sometimes display a switch by the producer from an in-phase to an out-of-phase technique. For example a eurypterid trackway (eurypterids are an extinct group of chelicerates) from the Carboniferous of Wales demonstrates just this kind of switch from in-phase to out-of-phase walking by the producer. This has been interpreted as the animal changing from a swimming to a walking style of locomotion as it emerged from shallow water.3

As a consequence of this kind of interpretation, the appearance of out of phase trackways in the fossil record is taken as good evidence for the terrestrialisation of land by arthropods 4.