A TINY DEER WITH BIG IMPLICATIONS: REEVALUATING THE EVOLUTIONARY RELATIONSHIPS OF LEPTOMERYX ELISSAE (ARTIODACTYLA, LEPTOMERYCIDAE) BASED ON NEW CRANIAL MATERIAL

A leptomerycid skull was collected at Badlands National Park in 2016 from the earliest Whitneyan “Leptauchenia beds” (Early Oligocene, ~32 Ma) of the Poleslide Member, Brule Formation. This specimen (BADL 64015) is the first skull referable to Leptomeryx elissae and is by far the most complete specimen of the species to date. The skull is nearly complete, lacking only the premaxilla, and retains the associated dentaries. Prior to BADL 64015, only isolated dentaries had been referred to L. elissae, all from the late Orellan of the Brule Formation in northwestern Nebraska.

Previous literature reports that L. elissae differs from the much more common Leptomeryx evansi in that it has smooth tooth enamel, lacks the Palaeomeryx-fold on the lower molars, has a rounded and evident entoconulid on m3, and is smaller in size. With measurements and diagnostic characters, it is clear that BADL 64015 exhibits significant deviation from L. evansi beyond the differences in the lower teeth.

BADL 64015 exhibits a shift towards brachycephaly, a shortening of the cranium. The weakly developed postorbital constriction and reduced length of the sagittal crest demonstrate that BADL 64015 is more similar to the Hemingfordian (early Miocene) leptomerycid, Pseudoparablastomeryx, than it is to Leptomeryx.This suggests L. elissae cannot be attributed to the genus Leptomeryx. However, in BADL 64015 characters of the premolars are more similar to Leptomeryx than Pseudoparablastomeryx, with some minor differences including the shortening of P2-3. An enigmatic Arikareean (Oligocene to early Miocene) leptomerycid, Pronodens, also demonstrates a shortening of the anterior dentary, but has never been associated with cranial material. The holotype for Pronodens consists of only extremely worn lower teeth, proving near impossible to draw any meaningful comparisons between Pronodens and other leptomerycids.

Based on the combination of cranial and dental characteristics, L. elissae should thus be repositioned into a new unnamed genus intermediate between Leptomeryx and Pseudoparablastomeryx. The evolutionary relationships among the Leptomerycidae will remain uncertain until additional specimens can be collected that provide unambiguous characters to clarify the relationships between these genera.