Cavicchini, I. | Search bioRxiv

bioRxiv2024

A Portrait of a Young Fish: Redescription of Pteronisculus gunnari (Nielsen, 1942) from a juvenile specimen from the Early Triassic of East Greenland, with implications for ontogenetic development in early actinopterygians

The Early-Middle Triassic actinopterygian genus Pteronisculus (White, 1933) is part of the Triassic Early Fish Fauna (TEFF), a cosmopolitan group of taxa that thrived in the aftermath of the end-Permian mass extinction. Pteronisculus is considered an important non-neopterygian outgroup taxon in many works dealing with the interrelationships of early crown actinopterygians, but the phylogenetic relationships of many of TEFF genera are debated, with the topology of the lineages giving rise to crown actinopterygians consequently unclear. This is despite exceptional, three-dimensionally preservation of an abundance of fossils associated with TEFF fishes. P. gunnari, from the Induan (Early Triassic) Kap Stosch Formation, East Greenland, is known in less detail than other species of the genus. Here, we use X-ray micro-computed tomography to comprehensively redescribe the three-dimensionally preserved holotype of P. gunnari, including a detailed description of the internal anatomy. The specimen shows morphological features previously undescribed for the genus, including paired premaxillae, medially-directed teeth on the palate, canals for the buccohypophysial canal and internal carotids piercing the parasphenoid, and numerous parotic toothplates. Scale covering is complete, but the braincase and palatoquadrate are ossified as multiple elements, indicating that the specimen was not fully mature and allowing new insights into ossification patterns and ontogenetic development of non-neopterygian actinopterygians. These new anatomical data enrich our understanding of both the morphological complexity and the interrelationships of actinopterygians from the Triassic Early Fish Fauna.

paleontology↗

bioRxiv2024

Investigating the impact of X-rays on decay: X-ray computed tomography as a non-invasive visualisation technique for sediment-based decay experiments

Decay experiments are ever increasing in complexity to better understand taphonomic processes. However, adding new variables, such as sediment, can create methodological biases, such as artificial anatomical character loss during exhumation. Non-invasive in situ imaging techniques such as X-ray computed tomography (XCT scanning) could mitigate this, but the consequences of exposing carcasses to X-rays are not fully understood, and evidence regarding the impact of X-rays on internal microbial faunas that drive decay is conflicting. Here, we test whether XCT scanning impacts the decay of Danio rerio carcasses within a substrate. Our control experiments show that quartz sand sediment physically stabilises the carcass throughout decay and the sequence of anatomical character loss remains constant, however, both the onset and rate of decay of soft tissues are initially accelerated. Our XCT data show that exposure to X-rays does not cause a deviation from the normal sequence of decay, validating XCT as a non-destructive visualisation method for decay experiments. Furthermore, when accompanied with traditional exhumation and dissection, XCT provides decay data with higher accuracy of character analysis than traditional methods, and allows novel quantitative techniques to monitor physical changes in the decaying carcass (e.g., total volume, build-up of gases, collapse of the body cavity etc.). We also underline limitations with the technique, but our experiment acts as an important stepping stone for progression toward non-invasive designs of decay experiments.

paleontology↗