Comparative study of Moroto vertebral specimens

The hypodigm of Morotopithecus bishopi includes several vertebral specimens from Moroto II in addition to a scapular fragment, and femoral and craniodental specimens. The Moroto vertebral specimens include UMP 67.28, which is a well-preserved lumbar vertebra. Based on the derived morphological traits in UMP 67.28, together with evidence from other postcranial elements, it has been claimed that certain aspects of the modern hominoid body plan appeared in the hominoid lineage by as early as 20 Ma. Other vertebral specimens from the site are not well-preserved and have not been described in detail. This article provides the first detailed description of these specimen with an emphasis on a lumbar vertebral body UMP 68.06. Results confirm the existing interpretations that M. bishopi had a more dorsostable lumbar column compared to other African Miocene hominoids, such as Proconsul nyanzae/heseloni or Nacholapithecus kerioi. The vertebral body is craniocaudally short and the median ventral keel is absent through the lumbar column. However, M. bishopi might have had a similar number segments as inferred for P. nyanzae (6-7) if UMP 68.06 and UMP 67.28 are associated. Likewise, the ventral wedging of UMP 68.06 may suggest that M. bishopi had more lumbar vertebrae than extant great apes. The origin of the transverse process relative to the vertebral body is variable by level among the Moroto specimens. Thus, if these specimens derive from a single taxon, this may suggest considerable variability in this feature that would advise caution when using this feature to draw taxonomic or functional conclusions.