Astragalar morphology of Afradapis, a large adapiform primate from the earliest late Eocene of Egypt

Abstract

The ∼37 million-year-old Birket Qarun Locality 2 (BQ-2), in the Birket Qarun Formation of Egypt's Fayum Depression, yields evidence for a diverse primate fauna, including the earliest known lorisiforms, parapithecoid anthropoids, and Afradapis longicristatus, a large folivorous adapiform. Phylogenetic analysis has placed Afradapis as a stem strepsirrhine within a clade of caenopithecine adapiforms, contradicting the recently popularized alternative hypothesis aligning adapiforms with haplorhines or anthropoids. We describe an astragalus from BQ-2 (DPC 21445C), attributable to Afradapis on the basis of size and relative abundance. The astragalus is remarkably similar to those of extant lorises, having a low body, no posterior shelf, a broad head and neck. It is like extant strepsirrhines more generally, in having a fibular facet that slopes gently away from the lateral tibial facet, and in having a groove for the tendon of flexor fibularis that is lateral to the tibial facet. Comparisons to a sample of euarchontan astragali show the new fossil to be most similar to those of adapines and lorisids. The astragali of other adapiforms are most similar to those of lemurs, but distinctly different from those of all anthropoids. Our measurements show that in extant strepsirrhines and adapiforms the fibular facet slopes away from the lateral tibial facet at a gradual angle (112-126°), in contrast to the anthropoid fibular facet, which forms a sharper angle (87-101°). Phylogenetic analyses incorporating new information from the astragalus continue to support strepsirrhine affinities for adapiforms under varying models of character evolution.

Evidence for a Grooming Claw in a North American Adapiform Primate: Implications for Anthropoid Origins

“…We think this is unlikely, given previously noted similarities between European adapids and strepsirrhines specifically e.g., [47]. Coding Darwinius as having the strepsirrhine condition for fibular facet slope (Char21), having a grooming claw (Char25), having a laterally placed flexor fibularis groove on the astragalus (Char31), but lacking a posterior trochlear shelf of the astragalus (similar to Afradapis [59]), results in three most parsimonious trees in which the consensus shows a monophyletic strepsirrhine clade where Lemuroidea, Lorisoidea, Darwinius and Notharctus form a polytomy. This is perhaps a reasonable reflection of the current state of knowledge on the inter-relationships among these taxa.…”

Hands of early primates

“…They are often assumed to be part of the crown clade (Wible and Covert, 1987): the former is assigned to haplorhines, whereas the latter is frequently aligned with extant strepsirrhines (Szalay and Delson, 1979;Beard et al, 1988;Dagosto, 1990;Kay et al, 1997Kay et al, , 2004Godinot, 1998;Seiffert et al, 2009). However, debate continues about the validity of the adapiformstrepsirrhine relationship (Franzen et al, 2009;Seiffert et al, 2009;Boyer et al, 2010b;Gingerich et al, 2010;Williams et al, 2010;Gingerich, 2012;Maiolino et al, 2012), as some researchers have suggested anthropoid or haplorhine affinities for adapiforms. Hoffstetter (1977) used the term "Euprimates" in reference to a specific formulation of crown primates that includes omomyiforms and adapiforms as stem-haplorhines and stemstrepsirrhines, respectively.…”

Evolution of postural diversity in primates as reflected by the size and shape of the medial tibial facet of the talus

“…The talus articulates proximally with both the tibia and fibula, two bones that rarely articulate with other (nontalar) tarsals in primates. The complex morphology of the talus, its relatively straightforward functional role in the ankle joint, and its prevalence in the fossil record make it a useful element for both functional and phylogenetic inferences (e.g., Gebo, 1986;Beard et al, 1988;Dagosto, 1988;Gebo, 1988Gebo, , 1993Gebo, , 2011Marivaux et al, 2003;Boyer et al, 2010;Boyer and Seiffert, 2013).…”