A comparative functional analysis of spinous process, transverse process and articular facet morphology of the upper thoracic spine in extant hominids and fossil hominins
Date
2024
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Abstract
This study utilized three focus areas to evaluate and compare the variation in morphology of the spinous processes and articular facets of the upper thoracic spine (T1-T6) in African apes, contemporary humans, Later Stone Age (LSA) humans, baboons and fossil hominins, with the goal of finding and explaining potential functional differences. Further questions included if the spinous process deviations observed were specific to the structure or included changes to articular surface orientation and size (i.e., were changes noted only structural, or were there additional functional changes that occurred simultaneously). A skeletal sample consisting of hominines—chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla), LSA humans and contemporary humans—and baboons (Papio ursinus) were compared to each other (as examples of varying locomotor propensity or lifestyle) and to relevant fossil hominins including Australopithecus africanus (Sts 14), Australopithecus sediba (MH1 and MH2), Australopithecus afarensis (A.L. 288-1) and Australopithecus prometheus (StW 573). A novel process for photography of samples was established and demonstrated to be reproducible. Specimens were photographed and linear and angular measurements taken utilizing ImageJ software and non-metric visual observations. Comparisons of hominines and baboons demonstrated statistically significant differences between species and sex. Spinous process deviations demonstrated a lower than anticipated overall incidence when evaluating the entire vertebrae sample, although the presence within an individual’s sequence of T1-T6 was significantly higher and was more prevalent in males in all groups. Morphological presentations of spinous iv process deviations were classified into five patterns based on the region of deviation identified. Age in the contemporary humans, and locomotor propensity when all selected primates were compared did not appear to have an impact on frequency of occurrence of this feature.
In relation to the comparison to fossil hominins, there did not appear to be sufficient consistency in presentation to allow for this feature to be proposed as a developmental feature in the upper thoracic vertebrae related to bipedalism. The LSA humans sample group demonstrated an increased likelihood of spinous process deviations in the upper thoracic vertebrae and may be related to lifestyle and activity levels that were significantly different to contemporary humans.
In summary, the study found morphometric measurements and spinous process deviation to relate more to species differences and sexual dimorphism than to locomotor differences, with lifestyle and activity levels in humans potentially increasing the frequency of spinous process deviations. It may be concluded that spinous process deviations are a consequence of various related factors that may interact in their development, or in many cases may be the result of random normal variations between individuals. Spinous process deviation in the upper thoracic vertebrae is therefore not a reliable indicator of postural or locomotor behavior in the extant comparative sample or in fossil hominins but may be an indicator of lifestyle and activity levels in humans.
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in Palaeontology to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2022
Keywords
Morphology, Articular facets