June 15, 2013 | 92
Macroscelideans – the elephant shrews or sengis – are an exclusively African group of animalivorous placental mammals, famous for their long, mobile snouts [adjacent image of a rhynchocyonine sengi by Joey Makalintal]. They have long tails, proportionally elongate limbs, and range in size from 10 to 30 cm, and from 50g to over 500g. Digital reduction and elongate, often fused, distal limb segments are in keeping with often saltatorial habits: sengis are speedy runners, but also adept at leaping when avoiding obstacles and predators. Limb bone fragments of fossil taxa are similar to the corresponding parts of living taxa and suggest that fossil forms were similar in locomotor abilities. Close relatives of living species (some belonging to the same genera, such as Rhynchocyon) are known from the Miocene onwards, with molecular clock data indicating that aridification events in Africa drove diversification events within the crown-clade (Douady et al. 2003).
Paleogene macroscelideans were more diverse but none are represented by especially good remains. The oldest taxa belong to the Middle and Late Eocene group Herodotiinae, recognisable due to molariform upper fourth premolars, a wide mesial cingulum on the upper molars and other dental characters. Herodotiines appear to be the sister-group to a clade that contains all other macroscelideans. Within this latter clade, Metoldobotes from the Oligocene seems to be sister-taxon to a Rhynchocyoninae + Myohyracinae + Macroscelidinae clade (Tabuce et al. 2001). Metoldobotes is the biggest fossil macroscelidean yet reported, but even it was only slightly larger than the biggest living species (Patterson 1965). Metoldobotes resembles living sengis in tooth form but has more bulbous tooth cusps. Its enlarged, procumbent lower third incisor possesses a lingual groove of unknown function (to my knowledge, no-one has yet suggested that it was a venomous sengi that used that groove to deliver venom! Tooth grooves are widespread in mammals and other tetrapods and are frequently unrelated to the production or presence of venom).
Among the more interesting of fossil sengis is Mylomygale of Pliocene Africa. This taxon has large, complex, strikingly rodent-like cheek teeth and was almost certainly a rodent-like herbivore. The Miocene myohyracines (like Myohyrax, illustrated above) were superficially hyrax-like and were initially thought to be hyraxes. Like hyraxes, their molariform teeth indicate that they were browsing herbivores though, again, the biggest taxa were probably only slightly larger than the largest living sengis.
Traditional anatomical hypotheses linked macroscelideans with Archonta, Eulipotyphla or Glires. These proposed relationships were always problematic in view of the vaguely ‘ungulate-like’ form of macroscelidean teeth and limb bones (Simons et al. 1991). Their current inclusion within Afrotheria and close to various Paleogene taxa traditionally classified as condylarths does make more sense.
For previous Tet Zoo articles on afrotherians, see…
Refs – -
Douady, C. J., Catzeflis, F., Raman, J., Springer, M. S. & Stanhope, M. J. 2003. The Sahara as a vicariant agent, and the role of Miocene climatic events, in the diversification of the mammalian order Macroscelidea (elephant shrews). Proceedings of the National Academy of Sciences 100, 8325-8330.
Patterson, B. 1965. The fossil elephant shrews (family Macroscelididae). Bulletin of the Museum of Comparative Zoology, Harvard University 133, 295-335.
Simons, E. L., Holroyd, P. A. & Bown, T. M. 1991. Early tertiary elephant-shrews from Egypt and the origin of the Macroscelidea. Proceedings of the National Academy of Sciences 88, 9734-9737.
Tabuce, R., Coiffait, B., Coiffait, P.-E., Mahboubi, M. & Jaeger, J.-J. 2001. A new genus of Macroscelidea (Mammalia) from the Eocene of Algeria: a possible origin for elephant-shrews. Journal of Vertebrate Paleontology 21, 535-546.