The inner ear morphology of 80 snake and lizard species, representative of a range of ecologies, is here analysed and compared to that of the fossil stem snake
Dinilysia patagonica
, using three-dimensional geometric morphometrics. Inner ear morphology is linked to phylogeny (we find here a strong phylogenetic signal in the data that can complicate ecological correlations), but also correlated with ecology, with
Dinilysia
resembling certain semi-fossorial forms (
Xenopeltis
and
Cylindrophis
), consistent with previous reports. We here also find striking resemblances between
Dinilysia
and some semi-aquatic snakes, such as
Myron
(Caenophidia, Homalopsidae). Therefore, the inner ear morphology of
Dinilysia
is consistent with semi-aquatic as well as semi-fossorial habits: the most similar forms are either semi-fossorial burrowers with a strong affinity to water (
Xenopeltis
and
Cylindrophis
) or amphibious, intertidal forms which shelter in burrows (
Myron).
Notably,
Dinilysia
does not cluster as closely with snakes with exclusively terrestrial or obligate burrowing habits (e.g. scolecophidians and uropeltids). Moreover, despite the above similarities,
Dinilysia
also occupies a totally unique morphospace, raising issues with linking it with any particular ecological category.