An exceptionally preserved Late Devonian actinopterygian provides a new model for primitive cranial anatomy in ray-finned fishes

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.

A critical appraisal of appendage disparity and homology in fishes

“…The presence of a single dorsal fin has been considered as an actinopterygian synapomorphy (Cloutier & Arratia, 2004). However, since its description, phylogenetic investigations have placed Dialipina sometimes as a stem actinopterygian (Giles, Darras, Clement, Blieck, & Friedman, 2015;Long et al, 2015;Schultze & Cumbaa, 2001;Taverne, 1997;Zhu & Schultze, 2001;Zhu, Yu, Wang, Zhao, & Jia, 2006;, and other times as a stem osteichthyan (Brazeau, 2009;Brazeau & de Winter, 2015;Burrow et al, 2016;Davis et al, 2012;Dupret et al, 2014;Friedman, 2007;Lu, Giles, Friedman, den Blaauwen, & Zhu, 2016;Qiao et al, 2016). However, since its description, phylogenetic investigations have placed Dialipina sometimes as a stem actinopterygian (Giles, Darras, Clement, Blieck, & Friedman, 2015;Long et al, 2015;Schultze & Cumbaa, 2001;Taverne, 1997;Zhu & Schultze, 2001;Zhu, Yu, Wang, Zhao, & Jia, 2006;, and other times as a stem osteichthyan (Brazeau, 2009;Brazeau & de Winter, 2015;Burrow et al, 2016;Davis et al, 2012;Dupret et al, 2014;Friedman, 2007;Lu, Giles, Friedman, den Blaauwen, & Zhu, 2016;Qiao et al, 2016).…”

A new actinopterygian from the Late Devonian Gogo Formation, Western Australia

“…This represents only the third well‐preserved three‐dimensional neurocranium of a Devonian actinopterygian to be made available for detailed digital imaging and analysis, along with the Mimipiscis toombsi specimen NHMUK PV P.53247 (Giles & Friedman 2014) and the holotype of Raynerius splendens (Giles et al . ).…”

“…nov. C–D, Raynerius splendens , Frasnian of northern France (from Giles et al . ). E–F, Mimipiscis toombsi , Frasnian of Western Australia (from Giles & Friedman 2014).…”

“…This differs from the condition in the majority of Devonian actinopterygians where the skull roof is typically ornamented with linear or vermiform ridges (Gardiner ; Long ; Choo ; Giles et al . ).…”

“…To test the phylogenetic position of Pickeringius , an analysis was conducted by coding the new taxon into the matrix of Giles et al . () that accompanied the description of Raynerius . Several codings were updated regarding Entelognathus (chars 3, 5, 64, 117–124, 151, 152, 154–157) and Onychodus (chars 152, 154).…”