The Middle Jurassic docodont Borealestes serendipitus was the first Mesozoic mammal found in Scotland over 40 years ago. Its affinities and morphology have remained poorly understood. Although multiple dentary fragments and isolated teeth have been recovered from Scotland and England, they have not yet been described in sufficient detail. We report new, more complete specimens collected during recent field work on Skye, Scotland, combined with previously collected material. This includes upper and lower dentition and an almost complete right dentary. We present an updated description and diagnosis of the genus Borealestes, based on high-resolution micro-computed tomography (micro-CT) and synchrotron scans. We identify seven key features that distinguish Borealestes from other docodonts, including a pronounced a–c crest, absence of the a–g crest on cusp a, an anterior fovea at the buccolingual midpoint of the upper molar, and the convergence of the Meckel’s groove with the ventral margin of the mandible. We also present a revised diagnosis for the second species, B. mussettae. Our phylogenetic analysis supports a clade formed by Borealestes, Haldanodon, Docofossor, and Docodon. Ontogenetic variation in the mandibular morphology of Borealestes is similar to that seen in Docodon and Haldanodon, with the delayed emergence of the ultimate lower molar, the shift of the last molar to the front of the coronoid process, and a posterior shift of the Meckel’s sulcus in successively older individuals. This supports a distinctive growth pattern in the clade including Borealestes and Docodon, one that may be present in Docodonta as a whole.
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