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Chester, Stephen G. B., et al. “Oldest
skeleton of a plesiadapiform provides additional evidence for an exclusively
arboreal radiation of stem primates in the Palaeocene.” Royal Society
Open Science, vol. 4, no. 5, 31 May 2017, doi:10.1098/rsos.170329.

The article
“Oldest Skeleton of a Plesiadapiform Provides Additional Evidence for an
Exclusively Arboreal Radiation of Stem Primates in the Palaeocene” published in
the Royal Society Open Science journal
assesses the hypothesis that arboreality evolved later in primate evolution by
analyzing postcranial morphology of Palaechthonidae. The authors do this by
analyzing the new partial skeleton of Torrejonia
wilsoni. Dated to be about 62 million years old, Torrejonia wilsoni shows evidence of a mobile shoulder which is
thought to be used for arboreality. In addition to a mobile shoulder joint, Torrejonia wilsoni also shows a mobile
elbow joint for increased radial rotation, mobile hip, and a mobile ankle
joint. Researchers took the data found in the Torrejonia wilsoni skeleton and coded it into a morphological data
matrix that assesses the relationships within Euarchontoglires. From there a
phylogenetic tree could be created to show the relation of plesiadapiforms to
primates, and show where arboreality first evolved.

For a long time
plesiadapiforms have been considered to be similar to primate based on their
dental anatomy. With more recent discoveries and phylogenetic analyses
researchers are now suggesting that plesiadapiforms are either stem primates or
stem members of primatomorpha. With respect to arboreality, dentally associated
partial skeletons of four plesiadapiform families from the late Paleocene and
the early Eocene indicate that plesiadapiforms were clearly arboreal. Skeletal
remains have indicated that plesiadapiforms had orthograde postures for
clinging and climbing vertical structures such as a tree. Looking further into
the partial skeleton of Torrejonia
wilsoni, it seems that they did not extend their thighs powerfully. They
also had habitually flexed hind limbs and knees, which is not specialized for
leaping or terrestrial running, which points towards arboreality. Analysis of Torrejonia wilsoni and other known
plesiadapiforms led to the conclusion that all plesiadapiforms are stem
primates and that primates are a sister taxon to sundatheria which is a taxon
that includes dermoptera and scandentia. Having primates be a sister taxon to a
taxon that includes dermoptera helps explain so many similarities between
plesiadapiforms and dermoptera, which is often argues to include


Silcox, Mary T., et al. “The evolutionary
radiation of plesiadapiforms.” Evolutionary Anthropology: Issues, News,
and Reviews, vol. 26, no. 2, 2017, pp. 74–94., doi:10.1002/evan.21526.

The authors of the
article “The Evolutionary Radiation of Plesiadapiforms” analyze twelve species
of plesiaapiforms, their characteristics and diets, and link similarities to
primates. Species that are taken into consideration in this discussion include;
Purgatoriidae, Micromomyidae, Picromomyidae, Picrodontidae, Microsyopidae, and
Toliapinidae, amongst others. Two thoughts are brought to attention in this
article, plesiadapiforms are stem primates and second, plesiadapiforms are
distantly related to primates and some families of plesiadapiforms are closer
in relation to dermoptera. This article takes the stance that plesiadapiforms
represent the first evolutionary radiation of primates and considers the
balance of evidence that supports the stem-primate hypothesis.

With respect to
the relationship between plesiadapiforms and primates, many characteristics
have been questioned. One trait that is brought up in discussion is the
opposable hallux. The opposable hallux is thought to be a homologous trait to
euprimates. However, some plesiadapids lack a grasping hallux and have
differences in hallux morphology. This suggests that the ancestor of
plesiadapoids was unlikely to have an opposable big toe. Another characteristic
that is discussed is the presence of a petrosal bulla. It is difficult to
interpret the presence of the bulla in the petrosal region due to the fact that
cranial sutures can be destroyed in development and can tamper data in adult specimens.
Another issue in determining primate origins that is addressed is that it can
be difficult to compare test results that are based on different datasets.
Since there is not one universal dataset to use it leaves data interpretation
up to the researcher. By looking at some off the oldest plesiadapiforms, such
as Purgatoriidae and Micromomyids, we see that some plesiadapiforms show too
many primitive and derived traits to be considered primate ancestors. 

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