Abstract
Actinopterygians (ray-finned fishes) are the most diverse living osteichthyan (bony vertebrate) group, with a rich fossil record. However, details of their earliest history during the middle Palaeozoic (Devonian) ‘Age of Fishes' remains sketchy. This stems from an uneven understanding of anatomy in early actinopterygians, with a few well-known species dominating perceptions of primitive conditions. Here we present an exceptionally preserved ray-finned fish from the Late Devonian (Middle Frasnian, ca 373 Ma) of Pas-de-Calais, northern France. This new genus is represented by a single, three-dimensionally preserved skull. CT scanning reveals the presence of an almost complete braincase along with near-fully articulated mandibular, hyoid and gill arches. The neurocranium differs from the coeval Mimipiscis in displaying a short aortic canal with a distinct posterior notch, long grooves for the lateral dorsal aortae, large vestibular fontanelles and a broad postorbital process. Identification of similar but previously unrecognized features in other Devonian actinopterygians suggests that aspects of braincase anatomy in Mimipiscis are apomorphic, questioning its ubiquity as stand-in for generalized actinopterygian conditions. However, the gill skeleton of the new form broadly corresponds to that of Mimipiscis, and adds to an emerging picture of primitive branchial architecture in crown gnathostomes. The new genus is recovered in a polytomy with Mimiidae and a subset of Devonian and stratigraphically younger actinopterygians, with no support found for a monophyletic grouping of Moythomasia with Mimiidae.
Highlights
Actinopterygians and sarcopterygians together comprise Osteichthyes, representing more than 99% of living vertebrate species richness [1]
The new genus is recovered in a polytomy with Mimiidae and a subset of Devonian and stratigraphically younger actinopterygians, with no support found for a monophyletic grouping of Moythomasia with Mimiidae
With the exception of the heavily compressed Cheirolepis [30], this taxon presents the only Devonian actinopterygian neurocranium reported outside of Australia. Our investigation of this specimen using micro-computed tomography helps to illustrate key aspects of anatomy lost in acid-prepared material of Gogo actinopterygians, most notably the three-dimensional geometry and articulation of the skull and branchial arches
Summary
Actinopterygians (ray-finned fishes) and sarcopterygians (lobe-finned fishes inclusive of tetrapods) together comprise Osteichthyes (bony vertebrates), representing more than 99% of living vertebrate species richness [1]. Detailed understanding of the structure of early actinopterygians, that of the character-rich endoskeleton, has changed little since the exhaustive description and accompanying discussion of acid-prepared material of Mimipiscis and Moythomasia from the Frasnian Gogo Formation of Australia over three decades ago [3,4]. Mimipiscis remains the only Devonian actinopterygian for which both the internal and external structure has been exhaustively described and illustrated It is the default exemplar of primitive ray-fin anatomy With the exception of the heavily compressed Cheirolepis [30], this taxon presents the only Devonian actinopterygian neurocranium reported outside of Australia Our investigation of this specimen using micro-computed tomography (mCT) helps to illustrate key aspects of anatomy lost in acid-prepared material of Gogo actinopterygians, most notably the three-dimensional geometry and articulation of the skull and branchial arches. Bremer support values were calculated using PRAP2 [35]
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