Borealestes, WALDMAN & SAVAGE, 1972

Panciroli, Elsa, Benson, Roger B. J., Fernandez, Vincent, Butler, Richard J., Fraser, Nicholas C., Luo, Zhe-Xi & Walsh, Stig, 2021, New species of mammaliaform and the cranium of Borealestes (Mammaliformes: Docodonta) from the Middle Jurassic of the British Isles, Zoological Journal of the Linnean Society 192 (4), pp. 1323-1362 : 1329-1331

BOREALESTES WALDMAN & SAVAGE, 1972

Type species: Borealestes serendipitus Waldman & Savage, 1972 .

Revised differential diagnosis: Dental formula 4.1.?5.4/4.1.5.5(6). As in other docodontans: possesses an anterior ‘pseudotalonid basin’ formed by cusps a, b and g; retains plesiomorphic mammaliaform trait of attachment of postdentary elements to the dentary; has an eflected angular process (sensu Simpson, 1929; also see: Schultz et al., 2017: figs 2, 3); enlarged medial ridge protuberance (sensu Schultz et al., 2017); and enlarged and pointed upper and lower canines that are twin-rooted. The lower molars of Borealestes are: elongated anteroposteriorly, with labial row of higher cusps arranged in anteroposterior alignment with largest cusp a, and lingual row of smaller cusps with distinctive anterior cusp g and larger posterior cusp c; lower molars have cusps b–a–c in a triangular arrangement. The a–g crest on cusp a is absent (present to variable extent on cusp g) and the a–d crest on cusp a is also absent (but a labially oriented a–d crest is present on cusp d). Borealestes has a distinctive a–c crest. Upper molars of Borealestes : buccolingually wide and anteroposteriorly short; ‘figure 8’ shape, with anteroposteriorly constricted waist; two main buccal cusps, A and C, plus a small cusp B in the buccomesial corner; lingual half of the upper molar has main anterior lingual cusp X; cusp X larger and more prominent than smaller posterior lingual cusp Y; labial cusps connected by a ridge/ ridges anteroposteriorly; transverse ridge extends between the main anterior labial cusp A and the main lingual cusp X. In the lower molars, Borealestes most closely resembles docodontans Krusatodon , Castorocauda and Haldanodon in sharing a larger cusp c than cusp g. It resembles Castorocauda and possibly also Itatodon in having a slightly recurved cusp c. Borealestes is similar to Castorocauda and Docodon in possessing an anterior ‘cingulid’ incorporating cusp e, and cusp e is anteriorly projecting and forms part of the d–df–e interlock with the neighbouring molar, as in Krusatodon and Simpsonodon . Borealestes has a distinct lingual cingulid in the premolars, and a posterior labial cingulid, as seen in most other docodontans. Unlike Simpsonodon , Agilodocodon and Docodon , but like most other docodontans, Borealestes does not have dense creases and pits or other ornamentation on molar enamel surfaces. Borealestes differs from Dobunnodon mussettae in having more elevated b–g crest and c–d crest, and the more lingually positioned cusp e. Borealestes differs from all other docodontans, except Docodon , in having an anterior fovea (sensu Panciroli et al., 2019: fig 1B3) on the upper molars: a concave area anterior to the anterolingual crest. Borealestes differs from Docodon in having the anterior fovea positioned at the anteroposteriorly constricted waist of the upper molars. Borealestes differs from Krusatodon , Agilodocodon , Simpsonodon , Docodon and Haldanodon in having a transversely expanded and anteroposteriorly slightly compressed lingual wing of the upper molar, similar to Docofossor and Dsungarodon . Borealestes resembles Docofossor and Dsungarodon in having more reduced cusps Y and Z on the upper molars, and a larger cusp X.

Referred specimens: Specimens referred to Borealestes sp. but not to species level from the Kilmaluag Formation, Isle of Skye: BRSUG 29007, fragment of right dentary; BRSUG 29008, three fragmentary molars in matrix.