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University of Bristol
EARTH
SCIENCES

Posture and Stance


Tetrapod Stance

Bipeds and Quadrupeds

Cursorial Adaptations

Diversity of Locomtion

The relevance of stance to tracks


Tetrapod Stance

Classically, three distinct types of tetrapod stance are recognised. In 1972, Charig published a paper separating tetrapod locomtion into three distinct groups, known as sprawling, semi-erect and fully-erect.

Tetrapod Stance

Clearly these different stances would result in differences in footprint morphologies and trackway widths. By examining the fossil footprints and trackways of animals, scientists hope to be able to map the progress of dinosaur locomotory evolution. Generally it has been accepted that evolution has followed the path of sprawling gait evolving into semi-erect and then to a fully-erect stance.

Fully-erect stance is also linked with the evolution of endothermy, another derived character of more modern tetrapods, such as birds and mammals (Bakker, 1988).

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Bipeds versus Quadrupeds

Dinosaurs can also be described as being bipedal (walking on two hindfeet) or quadrupedal (walking on all four limbs). Bipedal tracks are usually distinguishable from the tracks of quadrupeds. However, the foreprints of quadrupeds can be lost and therefore misinterpretted as bipedal tracks. This is often because the forefeet were smaller and made less impact on the substrate than the main load-bearers, the hindfeet. It can also be due to the hindprints being placed over the foreprints, and thus obscuring them. (Thulborn, 1990).

Many habitually bipedal dinosaurs were capable of quadrupedal locomotion. There is also debated evidence to suggest that some quadrupeds were able to rise up onto their tails and hindfeet, into a tripodal posture (as demonstrated by the brontosaur at the beginning of Jurassic park). Tracks and trackways form a part of this debate, alongside skeletal reconstructions.

Example Tracks - Silhouettes

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Cursorial Adaptations

We can suggest the possible cursorial adaptations of an animal based on evidence accumulated from body fossils and representative trackways.

Graviportal

"One whose limb structure is primarily designed to meet the problems of bearing weight..rarely have cursorial adaptations." (Coombs, 1978).

Graviportal posture

Photograph contributed by O.Rauhut

Mediportal

"..with limbs primarily designed to meet the problems of bearing weight but with some elements of cursorial design, commonly retained from a more cursorial ancestor." (Coombs, 1978).

Subcursorial

"..has moderate development of most cursorial adaptations and little or no limb modification for bearing weight." (Coombs, 1978).

Cursorial

"..has an extensive development of cursorial adaptations. Most long-distance runners are fully cursorial." (Coombs, 1978)

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Diversity of Locomotion

Recently, the classical view presented above has been questioned, especially by Steven Gatesy and colleagues (1996, 1997). Many species that are classically regarded as sprawlers, lizards for example, are often capable of both semi-erect and bipedal stances also. Crocodiles are regarded as semi-erect movers. However, in some conditions crocodiles will sprawl with their bellies on the ground or even gallop bipedally (David Gower, pers.comm.). Many species have a suite of locomotory behaviours.

Rather than describing an animals favoured style of locomtion, we can discuss the numbers of different modules of locomotion that they employ (Gatesy & Dial, 1996; Gatesy & Middleton, 1997).

Under different circumstances, different mechanical and energetic advantages will favour one of a number of postures (Sereno, 1991).

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How does this relate to dinosaur tracks or trackways?

If a single animal can move in a number of different ways, then they will leave tracks in the fossil record that may be attributed to different ichnospecies. This may create particular problems for ichnotaxonomists. The different stances described above will affect the way the foot impacts and interacts with the substrate. Clearly track widths would vary depending upon the stance used. In addition, other trace features would be apparent. If a sprawling posture was employed we might expect to see body or tail marks more clearly defined along the midline of a trackway.

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References

Charig, A.J. (1972) The evolution of the archosaur pelvis and hind-limb: an explanation in functional terms. In Studies in Vertebrate Evolution (eds K.A.Joysey and T.S.Kemp). Oliver & Boyd, Edinburgh, pp.121-55.

Coombs, W.P. (1978) Theoretical aspects of cursorial adaptations in Dinosaurs. The Quartely Review of Biology. Vol.53, pp.393-417.

Gatesy, S.M., Dial, K.P. (1996) Locomotor modules and the evolution of avian flight. Evolution 50: 331-340.

Gatesy, S.M., Middleton, K.M. (1997) Bipedalism, flight, and the evolution of theropod locomotor diversity. Journal of Vertebrate Palaeontology 17:308-315.

Paul, G.S. (1987) The science and art of restoring the life appearance of dinosaurs and their relatives. A Rigorous How-to Guide. in Dinosaur Past & Present Volume II pp.4-49. University of Washington Press, Seattle.

Sereno, P.C. (1991) Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Palaeontology.11(Supplement), pp.1-53.

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


Trackways

Behaviour


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