The superfamily Sauropodomorpha comprised some of the largest terrestrial herbivorous vertebrates that have ever walked on this planet. The study of these fascinating beings has been at the forefront of palaeontological research throughout the years. Especially in the last 20 years, due to the advent of modern techniques and technologies, significant steps have been made towards our understanding of their physiology. One of the sauropodomorphs’ central morphological characteristics is the expression of postcranial skeletal pneumaticity (PSP), a condition that is also expressed in the other superfamily of the order Saurischia, the Theropoda, as well as in their extant relatives, the Avians. This condition remodels the vertebrae and, occasionally the girdles and appendicular elements, producing perforations, depressions, excavations, cavities and internal chambers. This is a result of bone invasion and resorption from the development of the lung-air sac diverticula of the respiratory system. The various forms of this expression have been addressed across all subfamilies of sauropodomorphs mostly from an evolutionary aspect and, recently, from a developmental scope too. The resulting hypotheses state that PSP may have acted as a mechanism for weight reduction, allowing for sauropods to attain large sizes without having to suffer analogous gravitational constraints from equally attained masses. Any possible associations, though, between the expression of pneumaticity and body size have not been put under test. Through this study, a method of quantification and categorisation of PSP, and therefore, classification of the sauropodomorphs which express it, is created from the data retrieved from 61 taxa across all subfamilies, permitting us not only to trace any correlation between PSP and metric size data (body mass and body length) but also to visualise the evolution of PSP throughout Sauropodomorpha. This classification scheme from highest to lowest expression of PSP, ‘Alpha’, ‘Beta’, ‘Gamma’, ‘Delta’, and ‘Epsilon’ stems from the numerical estimate of pneumaticity in terms of a percentage, called the Pneumaticity Degree Index (PDI%). The revised scheme, Pneumaticity Degree Index (PDI%), takes account of the number of vertebral elements that are pneumatized in a single vertebra, the nature of pneumaticity traits, as well as the intensity of pneumatization in different body regions of interest (e.g. vertebral column), resulting in an integral and comprehensive measure of PSP. The proposed method ranks each pneumaticity trait with a value from 1 to 5 with 1 (100%) representing the most invasive unambiguous trait (e.g. camellae) and 5 (~ 10%) the least invasive and most ambiguous trait (i.e. fossa). By adding and dividing by their number all of the observed traits of every available vertebra of a region we retrieve a decimal numeric outcome and this outcome is translated to a percentage. The total average pneumatization of any taxon is calculated by retrieving the total average of the pneumaticity from all available body regions. In this study, only vertebral and pelvic elements were used for the retrieval of pneumaticity data since they are the most frequently exhibiting pneumatic traits in comparison to pectoral and appendicular elements. Results show that the total average PDI% range of basal Sauropodomorpha is 0%-59%, of the non-neosauropod Eusauropoda is 23%-90%, of Diplodocoidea is 45%-73%, of Macronaria is 45%-92% and of Somphospondyli is 46%-94%. The most pneumatized vertebral landmarks are the centrum, neural arch and neural spine. The vertebral regions most commonly pneumatized are the cervicals and dorsals. No genus or subfamily of sauropodomorphs ever reaches 100% pneumatization in all vertebral regions. Furthermore, PSP is not always positively correlated with mass, rather its expression is mostly correlated with length. Taxa with low or high masses may exhibit either low or high PDI’s. In addition, increasing progression of the extent and expression of PSP occurs mostly on a subfamilial level and less throughout the entire lineage. Finally, modelling calculations result in an Ornstein-Ulenbeck with an early burst progress of the expression of pneumatisation in sauropodomorphs. After that, the expression proceeds relatively steadily throughout the entire superfamily. The biotic causes of PSP are still unclear, as it could be an artefact of inheritance and genetic drift throughout speciation events. The pneumatization degree index (PDI%) is a means of quantifying and categorising pneumatization in any archosaurian taxon that is faster and cheaper, though less accurate, than CT scanning. It is more precise than the Pneumaticity Index since it integrates a wider selection of the qualities and osteological characteristics of pneumaticity we want to measure, provided that the bone under study is at least 50% free of matrix.

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