Oldest known euarchontan tarsals and affinities of Paleocene Purgatorius to Primates

Abstract

Earliest Paleocene Purgatorius often is regarded as the geologically oldest primate, but it has been known only from fossilized dentitions since it was first described half a century ago. The dentition of Purgatorius is more primitive than those of all known living and fossil primates, leading some researchers to suggest that it lies near the ancestry of all other primates; however, others have questioned its affinities to primates or even to placental mammals. Here we report the first (to our knowledge) nondental remains (tarsal bones) attributed to Purgatorius from the same earliest Paleocene deposits that have yielded numerous fossil dentitions of this poorly known mammal. Three independent phylogenetic analyses that incorporate new data from these fossils support primate affinities of Purgatorius among euarchontan mammals (primates, treeshrews, and colugos). Astragali and calcanei attributed to Purgatorius indicate a mobile ankle typical of arboreal euarchontan mammals generally and of Paleocene and Eocene plesiadapiforms specifically and provide the earliest fossil evidence of arboreality in primates and other euarchontan mammals. Postcranial specializations for arboreality in the earliest primates likely played a key role in the evolutionary success of this mammalian radiation in the Paleocene.

Mammalian distal humerus fossils from eastern Montana, USA with implications for the Cretaceous-Paleogene mass extinction and the adaptive radiation of placentals

“…because this genus is the most abundant plesiadapiform in our Pu3 assemblages (Clemens, 2002(Clemens, , 2004. This is consistent with the presence of teeth and isolated tarsals that have been found at this locality and attributed to Purgatorius (Clemens, 2002(Clemens, , 2004Chester et al, 2015). Distal humerus morphotype EuB (Figure 13).…”

“…Likewise, the late Puercan ?plesiadapiform primate (EuA) has many humerus features in common with primates and Ptilocercus that imply an arboreal mode of locomotion; e.g., a wide entepicondyle for attachment of strong grasping muscles, a nearly spherical capitulum and separation of the capitulum and trochlea, both allowing forearm rotation, and weight-bearing humeroulnar joint (e.g., Szalay et al, 1975;Szalay and Dagosto, 1980;Szalay and Lucas, 1996;Sargis, 2002;Bloch et al, 2007;Boyer et al, 2010). This interpretation is consistent with recent analysis of tarsal fossils from the same assemblage that were attributed to the plesiadapiform Purgatorius (Chester et al, 2015). The large archaic ungulate (EuE) has broad epicondyles, unlike those found in cursorial mammals (e.g., Rose, 1987;Polly, 2007;Argot, 2010), and an elbow joint that does not restrict rotational movement on the posterior trochlea.…”

“…Research to date on K-Pg mammalian postcrania has mostly focused on a narrow taxonomic scope (i.e., multituberculates, plesiadapiform primates; Deischl, 1964;Krause and Jenkins, 1983;Szalay, 1994;Borths and Hunter, 2008;Chester et al, 2015) or on a single assemblage (e.g., the Bug Creek Anthills; Deischl, 1964;Sloan and Van Valen, 1965;Szalay and Decker, 1975). More temporally and taxonomically comprehensive research has been limited by low sample sizes of postcranial fossils, which is likely a function of the rarity of these elements (and extreme rarity of skeletons) relative to the thousands of mammalian teeth known from these deposits (e.g., Sloan and Van Valen, 1965;Archibald, 1982;Lofgren, 1995;Clemens, 2002;DeBey and Wilson, 2014;.…”

Cranial anatomy of Paleogene Micromomyidae and implications for early primate evolution

“…It is also understood that the closest living relatives of primates are treeshrews (Scandentia) and colugos (Dermoptera), together classified as Euarchonta (e.g., O'Leary et al, 2013), and that the Paleocene euarchontan fossil record is wellrepresented by a diversity of fossil taxa traditionally classified in the "Plesiadapiformes" (see Silcox and Gunnell, 2008). What is controversial is whether some or all plesiadapiforms represent an early fossil record of Dermoptera (e.g., Beard, 1990Beard, , 1993aKay et al, 1992;Ni et al, 2009), Primates (Bloch and Boyer, 2002;, stem Primatomorpha , or are perhaps more distantly related to these modern orders (e.g., Wible et al, 2009; but see Chester et al, 2015). While the controversy is largely driven by an understanding that some plesiadapiforms lack certain diagnostic characteristics of crown Primates (see Martin, 1968Martin, , 1972Cartmill, 1972Cartmill, , 1974, until relatively recently most of the diversity for this group has been represented by only fragmentary dentitions with very few associated crania or postcrania.…”

Paleoenvironmental and Sea‐Level Change

“…During the warm Palaeocene and Eocene, broadleaf forests were common in high latitudes far from the tropics, including the regions from which early primates (or primate-like mammals) have been recovered. The early Palaeocene plesiadapiform Purgatorius has tarsal (foot) bones with adaptations to arboreality and is found at a time when angiosperm (flowering) plants (which include broadleaf trees) were prevalent in the North American west (Chester et al 2015). Detailed study of mammalian communities and fossil plants from north west Europe at the end of the Palaeocene and the beginning of the Eocene indicates the presence in Europe of warm-loving, dense, evergreen forest with a thick understory that gave way to forests with much less ground cover (Hooker and Collinson 2012).…”