Classifying isolated vertebrate bones to a high level of taxonomic precision can be difficult. Many of Australiaâs Cretaceous terrestrial vertebrate fossil-bearing deposits, for example, produce large numbers of isolated bones and very few associated or articulated skeletons. Identifying these often fragmentary remains beyond high-level taxonomic ranks, such as Ornithopoda or Theropoda, is difficult and those classified to lower taxonomic levels are often debated. The ever-increasing accessibility to 3D-based comparative techniques has allowed palaeontologists to undertake a variety of shape analyses, such as geometric morphometrics, that although powerful and often ideal, require the recognition of diagnostic landmarks and the generation of sufficiently large data sets to detect clusters and accurately describe major components of morphological variation. As a result, such approaches are often outside the scope of basic palaeontological research that aims to simply identify fragmentary specimens. Herein we present a workflow in which pairwise comparisons between fragmentary fossils and better known exemplars are digitally achieved through three-dimensional mapping of their surface profiles and the iterative closest point (ICP) algorithm. To showcase this methodology, we compared a fragmentary theropod ungual (NMV P186153) from Victoria, Australia, identified as a neovenatorid, with the manual unguals of the megaraptoran Australovenator wintonensis (AODF604). We discovered that NMV P186153 was a near identical match to AODF604 manual ungual II-3, differing only in size, which, given their 10â15Ma age difference, suggests stasis in megaraptoran ungual morphology throughout this interval. Although useful, our approach is not free of subjectivity; care must be taken to eliminate the effects of broken and incomplete surfaces and identify the human errors incurred during scaling, such as through replication. Nevertheless, this approach will help to evaluate and identify fragmentary remains, adding a quantitative perspective to an otherwise qualitative endeavour.
=====
Free pdf:
Data archiving statement:
All created files and 3D models in this study are stored under CCâBYâNC license at the custodial threeâdimensional repository of the Museum fÃr Naturkunde Berlin (Germany):
https://doi.org/10.7479/khcz-ar29
The use of surface digitization techniques and methods in palaeontology has increased in the last two decades, mainly due to recent improvements in devices and software. However, many digitization efforts are published only as 3D models, with only a few details on the exact protocols used and sometimes not even indicating how to access these digital data, thus reducing the longâterm reusability of the obtained files. It is important to include this information, as the applied techniques and workflows have significant effects on the final quality of 3D models. We compare 3D meshes created by seven different surface digitization techniques and protocols for a sauropod caudal vertebra and a testudine turtle in a flat slab of rock. These two specimens represent typical examples of objects in vertebrate palaeontology collections, making them a suitable sample for our tests. Besides these quantitative and topological comparisons we also have computed visual perceptual metrics, which aim to predict the visual quality of a 3D model as perceived by a human observer. Our results agree with previous works, confirming that photogrammetry is one of the most suitable options for obtaining high quality 3D models of fossils, producing higher quality meshes than current structured light 3D scanners.