Abstract
Stem‐group gnathostomes reveal the sequence of character acquisition in the origin of modern jawed vertebrates. The petalichthyids are placoderm‐grade stem‐group gnathostomes known from both isolated skeletal material and rarer articulated specimens of one genus. They are of particular interest because of anatomical resemblances with osteostracans, the jawless sister group of jawed vertebrates. Because of this, they have become central to debates on the relationships of placoderms and the primitive cranial architecture of gnathostomes. However, among petalichthyids, only the braincase of Macropetalichthys has been studied in detail, and the diversity of neurocranial morphology in this group remains poorly documented. Using X‐ray computed microtomography, we investigated the endocranial morphology of Shearsbyaspis oepiki Young, a three‐dimensionally preserved petalichthyid from the Early Devonian of Taemas‐Wee Jasper, Australia. We generated virtual reconstructions of the external endocranial surfaces, orbital walls and cranial endocavity, including canals for major nerves and blood vessels. The neurocranium of Shearsbyaspis resembles that of Macropetalichthys, particularly in the morphology of the brain cavity, nerves and blood vessels. Many characters, including the morphology of the pituitary vein canal and the course of the trigeminal nerve, recall the morphology of osteostracans. Additionally, the presence of a parasphenoid in Shearsbyaspis (previously not known with confidence outside of arthrodires and osteichthyans) raises some questions about current proposals of placoderm paraphyly. Our detailed description of this specimen adds to the known morphological diversity of petalichthyids, and invites critical reappraisal of the phylogenetic relationships of placoderms.
Highlights
PLACODERMS are generally considered to be the outgroup of modern jawed vertebrates (Young 1986; Janvier 1996a, b; Goujet 2001; Goujet & Young 2004; Brazeau 2009) and illuminate key phases in the stepwise evolution of jawed vertebrate anatomy
We have presented the updated cranial morphology of Shearsbyaspis oepiki Young, 1985 based on computed tomography analysis
Given the centrality of this group to questions of placoderm relationships, our new data have shed light on the characters that have been the basis of arguments for placoderm paraphyly
Summary
PLACODERMS are generally considered to be the outgroup of modern jawed vertebrates (Young 1986; Janvier 1996a, b; Goujet 2001; Goujet & Young 2004; Brazeau 2009) and illuminate key phases in the stepwise evolution of jawed vertebrate anatomy. Petalichthyida are non-arthrodiran placoderms that have recently become central in the debate about placoderm relationships and primitive gnathostome anatomical conditions This is due to perceived endocranial similarities with the jawless osteostracans and galeaspids (Janvier 1996a, b; Brazeau 2009; Brazeau & Friedman 2014). This has led to rejections of placoderm monophyly in numerical phylogenetic analyses, resulting in their recovery as a sub-grade of stem-group gnathostomes (Brazeau 2009; Davis et al 2012; Zhu et al 2013, 2016; Dupret et al 2014; Giles et al 2015)
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