What is a pore-canal system?

• The pore-canal system is part of the defining ‘comsine’ structure of early sarcopterygians. It lies in the upper dentinous region of the dermal skeleton and has abundant external openings connected by horizontal ‘Maschenkanäle' to a deeper canal system in the thickened spongia layer – the ‘Unter-Maschenkanäle'. A similar horizontal canal system is also present in early actinopterygians but is not generally considered homologous. We generated 3D reconstructions of scale canals in three early osteichthyans: Lophosteus, Andreolepis, and Psarolepis, the latter being a basal sarcopterygian. Well-developed horizontal canal-systems were found in all of the sampled taxa, although their morphology was more regular in Psarolepis. In addition, Psarolepis possesses a less regular canal system at slightly deeper levels within the bony tissues, which might correspond to the ‘Unter-Maschenkanäle' of crown sarcopterygians (e.g. Porolepis). Conversely, the dentinal canals in Psarolepis appear to arise from both the lower canal system and horizontal ‘Maschenkanäle'. This feature represents a potential link between actinopterygians and sarcopterygians, thus rendering the horizontal canal system (probably part of the vascularization of the scale) potentially homologous across early osteichthyans.

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