elephant shrews (Macroscelidea) and tree shrews (Scandentia) have been phylogenetically
problematic since they were first recognised as taxonomic groups. First
included in the Insectivora, (as the families Macroscelididae and Tupaiidae
respectively), by Wagner in 1855, they were later assigned their own suborder
within the Insectivora, named Menotyphla, by Haekel in 1866 (quoted in Butler
1972). Also included in this suborder was the flying-lemur (Cyanocephalus),
although this was soon removed and promoted to ordinal status (Dermoptera).
Other authors have regarded the Menotyphla as its own order (e.g., Gregory
1910). Although now widely accepted as belonging to their own separate orders,
(Macroscelidea and Scandentia), the phylogenetic positions of elephant shrews
and tree shrews are still disputed.
tree shrews have been a source of particularly fierce debate among taxonomists,
as "few other mammalian families are so difficult to classify" (Nowak 1991).
Resembling long-snouted squirrels, tree shrews possess many primitive mammalian
features that not only prompt suggestions of a close relationship with Lipotyphla,
but also that they may closely resemble the common ancestor of placental
mammals. However, they also have several derived features that are only
otherwise found in primates. This has been the source of many of the arguments
regarding their classification, as the possibility of tree shrews and primates
sharing a common ancestor is highly controversial. The 'primate-like' features
of tree shrews include a relatively large cranium, a permanent scrotum,
orbits that are completely encircled by bone, and the remarkable resemblance
of the carotid and subclavian arteries to those of humans (Nowak 1991).
Historically, many authors have acknowledged these shared derived characters
by assigning close relationships between tree shrews and primates. For example,
Simpson (1945) went as far as recognising the Tupaiidae as being an infraorder
of the prosimians. For some time this arrangement was widely accepted, and
indeed was still used by Mahe in 1976. >>see Fleagle for little more info
here<< Since that time, the tree shrews have been subjected to a variety
of phylogenetic positions, ranging from re-inclusion within Insectivora
(Campbell 1974), to placement at the "bottom rung of the primate ladder"
(Sorenson 1970, quoted in Nowak 1991).
the benefit of molecular data, it appears that the Tupaiidae are not so
directly related to the primates, or the Insectivora, but instead are now
accepted as belonging to their own order, Scandentia. According to many
recent studies Scandentia are currently thought to be a sister group of
the Dermoptera, Primates, and possibly Chiroptera in a superorder traditionally
known as Archonta (Waddell et al 1999a, Novacek 1992b, Kupferman 1999, McKenna
1975, Yates 1984). A recent study however, by Allard et al (1996), suggests
tentative support for archontan polyphyly rather than monophyly, and as
Kupferman (1999) reviews, the support for the monophyly of Archonta comes
primarily from morphological studies (e.g., Novacek 1994), whereas molecular
studies however, reject archontan monophyly (e.g., Emerson et al 1999).
The exact position of Scandentia among other placental orders is still unresolved,
and if eventually its position as a sister group to Primates and Dermoptera
is convincingly rejected, a Scandentia and Lagomorpha grouping, or a Scandentia
and Dermoptera grouping are highly likely alternatives (Kupferman 1999).
phylogenetic position of elephant shrews is also far from determined, although
like the Tupaiidae, the Macroscelididae are now generally recognised as
being separate from the Insectivora. This traditional association with the
Insectivora (e.g., Gregory 1910) seems to be largely a function of their
shared primitive resemblance rather than a phylogenetic relationship. Assigned
to their own order, Macroscelidea, (as first proposed by Butler in 1956),
this group consists of 15 species of small, very long-snouted mammals which
are all endemic to Africa. Stanhope et al (1998) included them within his
all-African superordinal clade, Afrotheria, based on the congruence of a
variety of molecular investigations. Other studies are less definite about
the position of Macroscelidea.
(1992a) agrees that the suborder Menotyphla is obsolescent, but that "recent
morphological and molecular studies spread them [Macroscelidea] over the
mammalian map", providing contradictory evidence for several classification
schemes. The first proper indication of the position or relationship of
Macroscelidea with other eutherian orders based on molecular data was proposed
by de Jong et al in 1993. Eye lens crystallin sequences were identical for
Elephantulus rufescens (elephant shrew) and Procavia capensis (hyrax). This
groups the elephant-shrews closest to the paenungulates and aardvark, and
adds support to Macroscelidea's inclusion in Stanhope et al's Afrotheria
- a group that has now been independently and unanimously united by all
available sequence data.