Cranial anatomy of the predatory actinopterygian Brazilichthys macrognathus from the Permian (Cisuralian) Pedra de Fogo Formation, Parnaíba Basin, Brazil

Abstract

Brazilichthys macrognathus is the only named actinopterygian from the Permain (Cisuralian) Pedra de Fogo Formation of northeastern Brazil, where it is represented by a single three-dimensionally preserved but incompletely described skull of unclear systematic placement. We used X-ray computed microtomography (µ-CT) to better document its anatomy and phylogenetic affinities. µ-CT reveals parts of the internal skeleton. We correct errors in original description, including the number of infraorbital bones and the misidentification of the dermosphenotic as sclerotic ossifications. These reinterpretations of external anatomy are joined by new data on internal structure, including the palate, parasphenoid, and branchial and hyoid arches. A maximum parsimony analysis of anatomical data resolves Brazilichthys as a stem actinopterygian, crownward of all Devonian species. This placement is supported by the absence of a dermosphenotic posterior ramus and the presence of opercular process of the hyomandibula. A similar placement is suggested by a Bayesian analysis of this same dataset, although relationships throughout the tree are less resolved. Our results reject previous interpretations of Brazilichthys as a relative of Birgeriidae, a Triassic group consistently placed within the actinopterygian crown. Although Acrolepis is too poorly known to be included in our analysis, we also reject a close relationship between this taxon and Brazilichthys, as their only shared similarities appear to be broadly distributed among early actinopterygians.

A Permian fish reveals widespread distribution of neopterygian-like jaw suspension

“…Bayesian analysis : The exclusion of † Brachydegma from the neopterygian crown group is ratified in our Bayesian analysis ( Appendix 1—figure 6 ), where the neopterygian crown is strongly supported (BPP=0.99). However, relationships outside of the neopterygian crown are volatile (compare, for example, Gardiner and Schaeffer, 1989 ; Gardiner et al, 2005 ; Hurley et al, 2007 ; Giles et al, 2017 ; Argyriou et al, 2018 ; Latimer and Giles, 2018 ; Figueroa et al, 2019 ) and poorly supported; for example, the actinopterygian crown node is not recovered. This picture does not change when † Brachydegma is removed from the matrix.…”

“…This is the earliest example of a condition typically associated with neopterygians ( Patterson, 1975 ; Grande and Bemis, 1998 ; Grande, 2010 ). The posterior parasphenoid stem extending behind the ventral otic fissure clearly differentiates † Brachydegma from most anatomically generalized Paleozoic-early Mesozoic actinopterygians ( Nielsen, 1942 ; Nielsen, 1949 ; Schaeffer and Dalquest, 1978 ; Gardiner, 1984 ; Giles et al, 2015b ; Figueroa et al, 2019 ). The bifurcation of the dorsal aorta into lateral dorsal aortae occurs below the posterior stem of the parasphenoid in † Brachydegma , resembling the condition seen in †saurichthyiforms ( Argyriou et al, 2018 ), conceivably †birgeriids ( Nielsen, 1949 ) and most actinopterans ( Patterson, 1975 ), but not polypterids ( Allis, 1922 ).…”

“…The nearly complete branchial skeleton of † Brachydegma represents one of the best examples known so far from a Paleozoic-early Mesozoic actinopterygian ( Stensiö, 1921 ; Stensiö, 1932 ; Aldinger, 1937 ; Nielsen, 1942 ; Nielsen, 1949 ; Giles et al, 2015b ; Giles et al, 2017 ; Argyriou et al, 2018 ; Figueroa et al, 2019 ). Unlike polypterids ( Allis, 1922 ), † Brachydegma shares five gill arches – with the fifth represented only by a pair of tiny ceratobranchials – and four independent hypobranchial ossifications with most Permian-Triassic actinopterygians ( Nielsen, 1942 ; Nielsen, 1949 ; Giles et al, 2017 ), chondrosteans ( Grande and Bemis, 1991 ; Hilton et al, 2011 ), and most teleosts ( Nelson, 1969 ; Hilton, 2002 ).…”

“…For analyzing the interrelationships of † Brachydegma in a broader osteichthyan context, we modified an already existing, large-scale phylogenetic matrix ( Giles et al, 2017 ; Argyriou et al, 2018 ; Latimer and Giles, 2018 ; Figueroa et al, 2019 ; see Appendix 2 for details). The matrix was edited in Mesquite ( Maddison and Maddison, 2017 ), and the parsimony analyses were performed with ‘New Technology Search’ implemented in TNT ( Goloboff et al, 2008 ; Goloboff and Catalano, 2016 ).…”

“…The Permian is an important link between the stem actinopterygian dominated Devonian-Carboniferous and the neopterygian-rich faunas of Triassic and younger strata. However, the Permian also represents a major knowledge gap for all aspects of actinopterygian biology ( Hurley et al, 2007 ; Friedman and Sallan, 2012 ; Sallan, 2014 ), including endoskeletal structure (for partially preserved examples, see Aldinger, 1937 ; Dunkle, 1946 ; Jessen, 1972 ; Figueroa et al, 2019 ).…”