Abstract
AbstractWe describe, in unprecedented detail, the petrosals and stapes of the docodont Borealestes from the Middle Jurassic of Scotland, using high resolution μCT and phase‐contrast synchrotron imaging. We describe the inner ear endocast and the vascularized interior structure of the petrosal, and provide the first endocranial view of a docodontan petrosal. Our study confirms some similarities in petrosal and stapedial morphology with the better known Haldanodon of the Late Jurassic of Portugal, including: (1) the degree of curvature of the cochlea; (2) multiple features related to the highly pneumatized paroccipital region; (3) the shape of lateral trough, the fossa of the M. tensor tympani, and the ridge on the promontorium; (4) the round shape of the fenestra vestibuli; and (5) overall morphology of the stapes. But Borealestes differs from Haldanodon in having a bony ridge that separates the tympanic opening of the prootic canal, the secondary facial foramen and the hiatus Fallopii, from the fenestra vestibuli. We identify two new vascular structures: the anterior and posterior trans‐cochlear sinuses, which traverse the pars cochlearis around the cochlear nerve (VIII). These trans‐cochlear sinuses have not been observed in previous docodont specimens, and could be an autapomorphy of Borealestes, or apomorphic for this clade. We also establish the anatomical relationship of the circum‐promontorium plexus to the inner endocast. The high quality of our scans has made these structures visible for the first time.
Highlights
DOCODONTA comprise an extinct branch of Mammaliaformes that falls outside crown Mammalia, but are closer to crown mammals than Sinoconodon, morganucodontans, haramiyidans and kuehneotheriids (Wible & Hopson 1993; Luo et al 2002; Kielan-Jaworowska et al 2004; Martin 2005; Luo et al 2015a)
PANCIROLI ET AL.: PETROSAL AND STAPES OF BOREALESTES 3 structure, and in the paroccipital and mastoid regions. All of these structures are consistent with those identified in the petrosals of the docodont Haldanodon (Ruf et al 2013)
We describe each of the petrosal structures by referring to both petrosals
Summary
DOCODONTA comprise an extinct branch of Mammaliaformes that falls outside crown Mammalia, but are closer to crown mammals than Sinoconodon, morganucodontans, haramiyidans and kuehneotheriids (Wible & Hopson 1993; Luo et al 2002; Kielan-Jaworowska et al 2004; Martin 2005; Luo et al 2015a). Docodonts possess complex cusps and crests on their molars, creating shearing and crushing surfaces, providing many characters that distinguish them from most contemporaneous stem mammaliaform families (Averianov & Lopatin 2006; Luo & Martin 2007; Luo et al 2015b). Their derived molar morphology (Kermack et al 1987; Sigogneau-Russell 2003; Meng et al 2015; Schultz et al 2017a) and very distinctive differences in postcranial skeletons are likely to have contributed to their wide ecological diversity during the Jurassic (Ji et al 2006; Luo 2007; Meng et al 2015). We describe the petrosals of Borealestes, expanding the information on the cranial morphology of docodonts
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