New basicrania of Paleocene‐Eocene Ignacius: Re‐evaluation of the Plesiadapiform‐Dermopteran link

Abstract

Plesiadapiformes has long been considered to be an archaic group of Primates. Discovery of a paromomyid plesiadapiform skull and independent analysis of referred postcrania have led investigators to conclude that Plesiadapiformes shares a closer relationship to extant flying lemurs (Dermoptera) than to Primates (= Euprimates of Hoffstetter [1977] Bull Mem Soc Anthropol Paris Ser 13 4:327-346). Despite challenges to this interpretation, the plesiadapiform-dermopteran relationship has gained currency in recent years. Here we show that newly discovered crania of Ignacius graybullianus, preserving previously undocumented portions of the ear, are more similar to primates than to dermopterans. New specimens confirm that paromomyids lacked the petrosal bulla of primates. However, these new specimens also demonstrate that paromomyids likely had: 1) a small promontorial branch of the internal carotid artery; 2) a lateral route for the internal carotid nerves crossing the promontorium; and 3) a ring-like ectotympanic with an annular bridge. This pattern is similar to primitive primates and fundamentally different from dermopterans, which have: 1) no internal carotid artery; 2) internal carotid nerves that take a more medial route; and 3) no annular bridge. Recognition of some primate-like traits, documented here by new evidence, indicates that Paromomyidae is likely to be more closely related to other Paleogene Plesiadapiformes and Eocene Primates than to extant Dermoptera. In view of these findings, a link between paromomyids and extant dermopterans ("Eudermoptera") is not convincingly supported by a single characteristic of the basicranium.

New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates

“…New cranial specimens of C. simpsoni (28) and D. szalayi and new data on paromomyids (9,20) demonstrate that plesiadapiforms are also characterized by a previously unappreciated diversity of cranial form (see SI Text, Part 4). Variation among scandentians has rarely been considered in studies of mammalian phylogenetics, with Tupaia typically being used to represent the clade (4), even though Ptilocercus is widely considered to be the most plesiomorphic living tree shrew (17,19,29,30).…”

“…Anatomical observations of an exceptionally well preserved cranium of a paromomyid (10) seemed to independently support a plesiadapiform-dermopteran link, leading to the widespread acceptance of this phylogenetic hypothesis. The evidence supporting this interpretation has been questioned (7,9,13,15,16,19,20), but no previous study has evaluated the plesiadapiformdermopteran link by using cranial, postcranial, and dental evidence, including new data on the most plesiomorphic tree shrew, Ptilocercus, and from recently discovered plesiadapiform skeletons. Here, we describe two new Paleocene plesiadapiform species and demonstrate that, when viewed in an appropriate phylogenetic context, anatomical diversity among plesiadapiforms documents a gradual acquisition of traits leading to the first appearance of Euprimates.…”

Skull of the Eocene primate Omomys carteri from western North America

“…Medially positioned posterior carotid foramina are also found in Tetonius, Necrolemur, Plesiadapis, living lorises, and tree shrews. However, this condition is not viewed as a primitive trait since some Paleocene fossils such as Ignacius and Phenacolemur exhibit a posterolateral position for the posterior internal carotid foramen (Bloch & Silcox, 2001;Gingerich, 1976;Wible, 1993).…”

“…Considerable research has been undertaken to document the presence, absence, and position of these branches in primates (Gregory, 1920;Klaauw, 1931;Saban, 1963;Russell, 1964;McKenna, 1966;Bugge, 1972;Szalay & Katz, 1973;Cartmill, 1975;Archibald, 1997;MacPhee, 1981;Wible, 1983Wible, , 1984Wible, , 1993MacPhee & Cartmill, 1986;Simons & Rasmussen, 1989;Ross, 1994;Beard & MacPhee, 1994;Norris & Harrison, 1998;Ross & Covert, 2000;Bloch & Silcox, 2001;Silcox, 2003). Bony canals or grooves trace the pathways of these arterial branches along the surface of the promontorium.…”

The origin and evolution of mammals

“…are part of the super-order Euarchontoglires is not doubted, but within it they could be stem-group Primates (Bloch and Silcox 2001;Bloch and Boyer 2003a,b), basal members of the Dermoptera (Beard 1993), or have no sister-group relationship with anyone of the other euarchontogliran orders (Kirk et al 2003).…”