Chester, Stephen G. B., et al. “Oldestskeleton of a plesiadapiform provides additional evidence for an exclusivelyarboreal radiation of stem primates in the Palaeocene.
” Royal SocietyOpen Science, vol. 4, no. 5, 31 May 2017, doi:10.1098/rsos.170329.The article”Oldest Skeleton of a Plesiadapiform Provides Additional Evidence for anExclusively Arboreal Radiation of Stem Primates in the Palaeocene” published inthe Royal Society Open Science journalassesses the hypothesis that arboreality evolved later in primate evolution byanalyzing postcranial morphology of Palaechthonidae. The authors do this byanalyzing the new partial skeleton of Torrejoniawilsoni.
Dated to be about 62 million years old, Torrejonia wilsoni shows evidence of a mobile shoulder which isthought to be used for arboreality. In addition to a mobile shoulder joint, Torrejonia wilsoni also shows a mobileelbow joint for increased radial rotation, mobile hip, and a mobile anklejoint. Researchers took the data found in the Torrejonia wilsoni skeleton and coded it into a morphological datamatrix that assesses the relationships within Euarchontoglires. From there aphylogenetic tree could be created to show the relation of plesiadapiforms toprimates, and show where arboreality first evolved.For a long timeplesiadapiforms have been considered to be similar to primate based on theirdental anatomy. With more recent discoveries and phylogenetic analysesresearchers are now suggesting that plesiadapiforms are either stem primates orstem members of primatomorpha. With respect to arboreality, dentally associatedpartial skeletons of four plesiadapiform families from the late Paleocene andthe early Eocene indicate that plesiadapiforms were clearly arboreal.
Skeletalremains have indicated that plesiadapiforms had orthograde postures forclinging and climbing vertical structures such as a tree. Looking further intothe partial skeleton of Torrejoniawilsoni, it seems that they did not extend their thighs powerfully. Theyalso had habitually flexed hind limbs and knees, which is not specialized forleaping or terrestrial running, which points towards arboreality. Analysis of Torrejonia wilsoni and other knownplesiadapiforms led to the conclusion that all plesiadapiforms are stemprimates and that primates are a sister taxon to sundatheria which is a taxonthat includes dermoptera and scandentia.
Having primates be a sister taxon to ataxon that includes dermoptera helps explain so many similarities betweenplesiadapiforms and dermoptera, which is often argues to includeplesiadapiforms. Silcox, Mary T., et al. “The evolutionaryradiation of plesiadapiforms.” Evolutionary Anthropology: Issues, News,and Reviews, vol.
26, no. 2, 2017, pp. 74–94., doi:10.
1002/evan.21526.The authors of thearticle “The Evolutionary Radiation of Plesiadapiforms” analyze twelve speciesof plesiaapiforms, their characteristics and diets, and link similarities toprimates. Species that are taken into consideration in this discussion include;Purgatoriidae, Micromomyidae, Picromomyidae, Picrodontidae, Microsyopidae, andToliapinidae, amongst others. Two thoughts are brought to attention in thisarticle, plesiadapiforms are stem primates and second, plesiadapiforms aredistantly related to primates and some families of plesiadapiforms are closerin relation to dermoptera. This article takes the stance that plesiadapiformsrepresent the first evolutionary radiation of primates and considers thebalance of evidence that supports the stem-primate hypothesis. With respect tothe relationship between plesiadapiforms and primates, many characteristicshave been questioned.
One trait that is brought up in discussion is theopposable hallux. The opposable hallux is thought to be a homologous trait toeuprimates. However, some plesiadapids lack a grasping hallux and havedifferences in hallux morphology. This suggests that the ancestor ofplesiadapoids was unlikely to have an opposable big toe. Another characteristicthat is discussed is the presence of a petrosal bulla. It is difficult tointerpret the presence of the bulla in the petrosal region due to the fact thatcranial 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 canbe difficult to compare test results that are based on different datasets.Since there is not one universal dataset to use it leaves data interpretationup to the researcher. By looking at some off the oldest plesiadapiforms, suchas Purgatoriidae and Micromomyids, we see that some plesiadapiforms show toomany primitive and derived traits to be considered primate ancestors.