A fresh look at Cladarosymblema narrienense, a tetrapodomorph fish (Sarcopterygii: Megalichthyidae) from the Carboniferous of Australia, illuminated via X-ray tomography

• Published in: PeerJ (San Francisco, CA) CA), 2021-12, Vol.9, p.e12597-e12597, Article e12597
• Main Authors: Clement, Alice M, Cloutier, Richard, Lu, Jing, Perilli, Egon, Maksimenko, Anton, Long, John
• Format: Article
• Language: English
• Subjects: Adaptation; Analysis; Anatomy; Axial skeleton; Carboniferous; Computed tomography; Devonian; Endocast; Evolutionary Studies; Fresh water; Genera; Lakes; Megalichthyidae; Morphology; Museums; Palate; Paleontology; Permian; Phylogenetics; Phylogeny; Sarcopterygii; Segmentation; Taxonomy; Tetrapodomorph; Tomography; Zoology
• ISSN: 2167-8359; 2167-8359
• DOI: 10.7717/peerj.12597

The megalichthyids are one of several clades of extinct tetrapodomorph fish that lived throughout the Devonian-Permian periods. They are advanced "osteolepidid-grade" fishes that lived in freshwater swamp and lake environments, with some taxa growing to very large sizes. They bear cosmine-covered bones and a large premaxillary tusk that lies lingually to a row of small teeth. Diagnosis of the family remains controversial with various authors revising it several times in recent works. There are fewer than 10 genera known globally, and only one member definitively identified from Gondwana. Fox et al. 1995 was described from the Lower Carboniferous Raymond Formation in Queensland, Australia, on the basis of several well-preserved specimens. Despite this detailed work, several aspects of its anatomy remain undescribed. Two especially well-preserved 3D fossils of , including the holotype specimen, are scanned using synchrotron or micro-computed tomography (µCT), and 3D modelled using specialist segmentation and visualisation software. New anatomical detail, in particular internal anatomy, is revealed for the first time in this taxon. A novel phylogenetic matrix, adapted from other recent work on tetrapodomorphs, is used to clarify the interrelationships of the megalichthyids and confirm the phylogenetic position of . Never before seen morphological details of the palate, hyoid arch, basibranchial skeleton, pectoral girdle and axial skeleton are revealed and described. Several additional features are confirmed or updated from the original description. Moreover, the first full, virtual cranial endocast of any tetrapodomorph fish is presented and described, giving insight into the early neural adaptations in this group. Phylogenetic analysis confirms the monophyly of the Megalichthyidae with seven genera included ( and ). The position of the megalichthyids as sister group to canowindrids, crownward of "osteolepidids" (e.g., and ), but below "tristichopterids" such as is confirmed, but our findings suggest further work is required to resolve megalichthyid interrelationships.