CT investigations of Australian Devonian fossil fishes, and the application of 3D segmentation and modelling in vertebrate morphology

Abstract

This thesis by compilation covers four publications, which together demonstrate and apply computed tomography (CT) data, three-dimensional (3D) segmentation, and 3D printing, for the non-destructive high-resolution detection of internal structure in early vertebrate fossils. Paper 1 (published 2019) studied the tetrapodomorph fish, Gogonasus, a three dimensionally well-preserved acid etched Devonian sarcopterygian. 3D modelling and printing were used for the reconstruction of its shoulder girdle and opercular series. A close fit of the opercular series against the upper bones of the shoulder girdle required a more horizontally aligned anocleithrum, supracleithrum and post-temporal than in previous reconstructions. Subopercular bone 2 is partly covered the clavicle, and the ascending process of the clavicle, and the ventral process of the anocleithrum, do not fit closely inside the cleithrum, suggesting they may have functioned for ligamentous attachment. A rugose area on the anocleithral process has a similar relative position to muscle ligament attachements in the shoulder girdle of various living actinopterygians. The manipulation of 3D models demonstrates testing of morphological fit for extremely fragile acid-etched bones. Paper 2 (published 2017) dealt with an articulated Devonian placoderm. Micro CT, 3D segmentation, and enlarged 3D models of a buchanosteid arthrodire, demonstrated a double ethmoid and a palatobasal connection for the palatoquadrate, but no otic connection. A separately ossified cartilage behind the mandibular joint is comparable to the interhyal of osteichthyans. Two articular facets on the braincase associated with the hyomandibular nerve foramen supported a possible epihyal element and a separate opercular cartilage. Reassembling 3D printouts demonstrates the limits of jaw kinetics. New details on the hyoid arch will help to reformulate characters that are key in the heated debate of placoderm monophyly or paraphyly. Paper 3 (published 2019) describes the detailed morphology of the three gnathal elements of the same specimen as on the previous paper, giving insights into the morphology and organization of the dentition in arthrodires. Arthrodire placoderms, as a possible sister group of Chinese maxillate placoderms plus crown gnathostomes, provide important information regarding early evolution of jaws and teeth. In displaying numerous denticle rows, the gnathal element morphology is different from the much-reduced denticulation of higher brachythoracid arthrodires. Ossification centres are anterolateral on the anterior supragnathal (attached to the braincase), anteromesial on the posterior supragnathal (attached to the palatoquadrate), and in the central part of the biting portion of the infragnathal (attached to the meckelian cartilage). The infragnathal shows no evidence of two ossification centres, as has been interpreted for more advanced arthrodires. The new evidence gives insights into the primitive arthrodire condition for comparison with the dermal jaw bones of Chinese 'maxillate' placoderms that have been homologised with the premaxilla, maxilla, and dentary of osteichthyans. Paper 4 (published 2020) summarizes the relationship between different modules of Drishti, an open-source volume exploration, rendering and three-dimensional segmentation software program that was used in the previous three published papers. A new version (Drishti v2.7) is presented, with a new tool for thresholding volume data (i.e. gradient thresholding). A protocol is introduced for performing three-dimensional segmentation using the new 3D Freeform Painter tool. In Drishti Paint, these new tools and workflow enable more accurate and precise digital reconstruction, 3D modelling and three-dimensional printing/modelling results. Scan data from the buchanosteus arthrodire (Paper 2 and 3) was used as a case study but published procedure is widely applicable in biological, medical and industrial research.