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. 2018 May 16;5(5):171727.
doi: 10.1098/rsos.171727. eCollection 2018 May.

A primitive actinopterygian braincase from the Tournaisian of Nova Scotia

Conrad D Wilson  1   2 , Jason D Pardo  2   3 , Jason S Anderson  2   3
Affiliations

A primitive actinopterygian braincase from the Tournaisian of Nova Scotia

Conrad D Wilson et al. R Soc Open Sci. .

Abstract

The vertebrate fossil record of the earliest Carboniferous is notoriously poorly sampled, obscuring a critical interval in vertebrate evolution and diversity. Recent studies of diversity across the Devonian-Carboniferous boundary have proposed a vertebrate mass extinction at the end-Devonian, and recent phylogenies suggest that the origin of the actinopterygian crown may have occurred in the earliest Carboniferous, as part of a broader recovery fauna. However, the data necessary to test this are limited. Here, we describe a partial actinopterygian skull, including diagnostic elements of the posterior braincase, from the Tournaisian Horton Bluff Formation of Blue Beach, Nova Scotia. The braincase surprisingly shows a confluence of characters common in Devonian taxa but absent in Mississippian forms, such as an open spiracular groove; lateral dorsal aortae that pass through open broadly separated, parallel grooves in the ventral otoccipital region, posterior to the articulation of the first infrapharyngobranchial and an intertemporal-supratemporal complex. Phylogenetic analysis places it deep within the actinopterygian stem, among Devonian moythomasiids and mimiids, suggesting more phylogenetically inclusive survivorship of stem group actinopterygians across the end-Devonian mass extinction. With a high lineage survivorship in tetrapods and lungfish across the Devonian-Carboniferous boundary and high vertebrate diversity at Tournaisian localities, this hints at a more gradual turnover between Devonian and Carboniferous vertebrate faunas.

Keywords: Actinopterygii; Romer's Gap; Tournaisian; braincase.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1.
Figure 1.
Avonichthys manskyi gen. et sp. nov. Photograph and interpretive drawing of specimen in dorsal view. Scale bar = 10 mm. br.psp, broken dorsal surface of parasphenoid; dsph, dermosphenotic; ?esc, indeterminate extrascapular; f, frontal; hm, hyomandibular; ifc, infraorbital canal; it, intertemporal; mx, maxilla; nIII, foramen for oculomotor nerve; p, parietal; por, post-orbital process; st, supratemporal.
Figure 2.
Figure 2.
Avonichthys manskyi gen. et sp. nov. Photograph and interpretive drawing of dermal skull roof in dorsal view. Scale bar = 5 mm. dsph, dermosphenotic; ?esc, indeterminate extrascapular; f, frontal; hm, hyomandibular; ifc, infraorbital canal; it, intertemporal; p, parietal; ?pinf, pineal foramen; st, supratemporal.
Figure 3.
Figure 3.
Avonichthys manskyi gen. et sp. nov. Photograph and interpretive drawing of specimen in lateral view. Scale bar = 10 mm. aip1, articulation for the first infrapharygobranchial; bo, basioccipital; br.psp, broken dorsal surface of parasphenoid; dsph, dermosphenotic; ?esc, indeterminate extrascapular; f, frontal; hm, hyomandibular; ios, interorbital septum; it, intertemporal; mx, maxilla; nIII, foramen for oculomotor nerve; nVIIhm, groove for the hyomandibular branch of facial nerve; nVIIim, foramen for the internal mandibular branch of the facial nerve; p, parietal; por, post-orbital process; pq, palatoquadrate; psp, parasphenoid; st, supratemporal.
Figure 4.
Figure 4.
Avonichthys manskyi gen. et sp. nov. Photograph and interpretive drawing of specimen in ventral view. Scale bar = 10 mm. aip1, articulation for the first infrapharygobranchial; bo, basioccipital; csp, craniospinal process; focn, foramen for the occipital nerve; fv, ventral fissure; lda, lateral dorsal aorta; nIXst, foramen for the supratemporal branch of glossopharyngeal nerve; oof, otic-occipital fissure; pdp, parasphenoid denticle plate; psp, parasphenoid; pt, pterygoid; sg, spiracular groove; vfon, vestibular fontanelle.
Figure 5.
Figure 5.
Avonichthys manskyi gen. et sp. nov. Photograph and interpretive line drawing of otic-occipital region in ventral view. Scale bar = 5 mm. aip1, articulation for the first infrapharygobranchial; asc.pr, ascending process of the parasphenoid; bo, basioccipital; csp, craniospinal process; focn, foramen for the occipital nerve; fv, ventral fissure; lda, lateral dorsal aorta; nIXst, foramen for the supratemporal branch of glossopharyngeal nerve; nX, widening for the passage of the vagus nerve; psp, parasphenoid; sg, spiracular groove; vfon, vestibular fontanelle.
Figure 6.
Figure 6.
Avonichthys manskyi gen. et sp. nov. Photograph and interpretive line drawing of the parasphenoid in right (top) and left (bottom) lateral view. Scale bar = 10 mm. aip1, articulation for the first infrapharygobranchial; asc.pr, ascending process of the parasphenoid; ?br.asc.pr, broken component of the ascending process; f, frontal; fv, ventral fissure; it, intertemporal; lda, lateral dorsal aorta; psp, parasphenoid; sg, spiracular groove.
Figure 7.
Figure 7.
Internal structure of the braincase of Avonichthys manskyi gen. et sp. nov. rendered from µCT. Scale bar = 5 mm. (a) Volume rendering of specimen in ventral view. (b) Transparent µCT rendering of specimen in ventral view with internal canals segmented. da, dorsal aorta; epa, efferent pseudobranchial artery; ic, internal carotid; lda, lateral dorsal aorta.
Figure 8.
Figure 8.
Phylogenetic and temporal placement of Avonichthys manskyi gen. et sp. nov. Adams consensus of 138 064 trees. Tree length = 1325 steps, CI = 0.223 and RI = 0.641. Taxon ages derived from the literature (electronic supplementary material, S4).
See this image and copyright information in PMC

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