The internal carotid artery (ICA) is one of the major vessels in the cranial circulation. Characters concerning the ICA, such as its course in the auditory region, have been employed frequently in phylogenetic analyses of mammals, including extinct taxa. In lagomorphs, however, our knowledge on vascular features of the auditory region has been based predominantly on living species, mostly on the European rabbit. We present the first survey on 11 out of 12 extant genera and key fossil taxa such as stem lagomorphs and early crown representatives (Archaeolagus and Prolagus). The ICA pattern shows a modified transpromontorial course in stem taxa (Litolagus, Megalagus and Palaeolagus) and Archaeolagus, which we propose as the ancestral character state for Lagomorpha, similar to that for the earliest rodents, plesiadapids and scandentians. The ICA pattern in leporids is perbullar but shows structural similarities to stem taxa, whereas the extrabullar ICA course in Ochotona is apparently a highly derived condition. Prolagus shows a mixed character state between leporids and Ochotona in its ICA route. The persistence of the transpromontorial ICA course and similarities in the carotid canal structure among stem taxa and crown leporids support morphological conservatism in Lagomorpha, in contrast to their sister clade Rodentia.

The auditory region of seven fossil and 29 extant lagomorph specimens has been studied, using the µCT scans and virtual 3D reconstructions performed in Avizo 9.0.1 (Thermo Fischer Scientific 1995–2019) and VG Studio MAX 2.2 (Volume Graphics GmbH, Heidelberg, Germany) software. Detailed information on the species sample and scan parameters are given in Table S1 (Ruf et al. 2023).

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