Intl. J. of Mol. Evol. and Biodivers. 2013, Vol. 3, No.1, 1-4
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on skull characteristics, such as the well-developed
brain, smaller area of the olfactory nerve in the brain,
the orbital bone bridge and formation of bone orbit, as
well as some similar characteristics shared by
primitive prosimians, such as the middle ear structure
similar to lemurs, thumb (toe) separated from other
digits (toe and teeth having premolars).
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The unresolved controversy: Phylogenetic
status
There has been a very big controversy regarding
whether tree shrews should be classified as Insectivora,
Primates, or a standalone order Scandentia in between
(
Young, 2011). In fact, tree shrews are the primitive
type of placental mammals, which share common
characteristics with primates (Martin, 1990). Tree
shrews and primates have many shared features and
characteristics listed in Table1, thus, tree shrews were
classified into Primata. However, tree shrews lack
some common characteristics shared in primates
(
Martin, 1990), and some characteristics of the
primate animals derived from the Primate; some
features are unique to tree shrews, which are not found
in any other mammals (Martin, 1990) (Table 2). Thus,
it can’t be said with certainty that tree shrews are
primate animals (Martin, 1990).
Tree shrews were moved from Insectivora to the
Primates order, because of certain internal similarities
to the latter (for example, similarities in the brain
anatomy, highlighted by Sir Wilfred Le Gros Clark),
and classified as a primitive prosimian.
However, mitochondrial genome analysis showed that
tree shrews had a closer relationship to Lagomorpha than
to Dermoptera or Primates (Xu et al., 2012). Molecular
cytogenetic data supported a Scandentia-Dermoptera
sister clade (Nie et al., 2008; Hallstrom and Janke, 2010).
A recent study by Hallstrom and Janke (2010) grouped
tree shrews into Glires (including rodents and
lagomorphs) based on the 3 000 gene phylogenetic
analysis, suggesting that the tree shrews had a closer
genetic relationship with mouse or rabbit. However, the
Table 1 This table shows supposed shared characteristics between tree-shrews and primates, from Martin (1990)
Context
Shared Similarities
1.
Snout relatively short
2.
Simplified set of turbinal bones
3.
Enlarged, forward-facing orbits
4.
Postorbital bar present
5.
Pattern of bones in medial orbital wall
6.
Well-developed jugal bone with foramen
7.
Enlarged braincase
8.
Inflated auditory bulla containing 'free' ectotympanic ring
9.
Internal carotid pattern (bony tubes)
Skull
10. '
Advanced' form of auditory ossicles
1.
Tooth-comb present at front of lower jaw, linked with a specialized, serrated sublingua
2.
Reduced dental formula
Dentition
3.
Similarities in cheek teeth between tree-shrews and certain primates with relatively primitive cheek teeth
(
e.g.
Tarsius
)
1.
Limbs and digits highly mobile
2.
Numerous details of limb musculature
3.
Osteological similarities in both forelimbs and hindlimbs
Postcranial
morphology
4.
Ridged skin on palms and soles
1.
Olfactory apparatus reduced
2.
Visual apparatus enhanced
3.
Central, avascular area of retina
4.
Neocortex expanded; brain size increased
Brain and
sense organs
5.
Calcarine sulcus present
1.
Penis pendulous; testes scrotal
2.
Discoidal placenta, as in tarsiers and simians
Reproductive
biology
3.
Small litter size; small number of teats
1.
Caecum present
Miscellaneous
2.
Molecular affinities (e.g. albumins)