title: Internal carotid arterial canal size and scaling in Euarchonta: Re-assessing implications for arterial patency and phylogenetic relationships in early fossil primates.
abstract: Primate species typically differ from other mammals in having bony canals that enclose the branches of the internal carotid artery (ICA) as they pass through the middle ear. The presence and relative size of these canals varies among major primate clades. As a result, differences in the anatomy of the canals for the promontorial and stapedial branches of the ICA have been cited as evidence of either haplorhine or strepsirrhine affinities among otherwise enigmatic early fossil euprimates. Here we use micro X-ray computed tomography to compile the largest quantitative dataset on ICA canal sizes. The data suggest greater variation of the ICA canals within some groups than has been previously appreciated. For example, Lepilemur and Avahi differ from most other lemuriforms in having a larger promontorial canal than stapedial canal. Furthermore, various lemurids are intraspecifically variable in relative canal size, with the promontorial canal being larger than the stapedial canal in some individuals but not others. In species where the promontorial artery supplies the brain with blood, the size of the promontorial canal is significantly correlated with endocranial volume (ECV). Among species with alternate routes of encephalic blood supply, the promontorial canal is highly reduced relative to ECV, and correlated with both ECV and cranium size. Ancestral state reconstructions incorporating data from fossils suggest that the last common ancestor of living primates had promontorial and stapedial canals that were similar to each other in size and large relative to ECV. We conclude that the plesiomorphic condition for crown primates is to have a patent promontorial artery supplying the brain and a patent stapedial artery for various non-encephalic structures. This inferred ancestral condition is exhibited by treeshrews and most early fossil euprimates, while extant primates exhibit reduction in one canal or another. The only early fossils deviating from this plesiomorphic condition are Adapis parisiensis with a reduced promontorial canal, and Rooneyia and Mahgarita with reduced stapedial canals.
authors.affiliation: Department of Evolutionary Anthropology, Duke University, Durham, NC 27708, USA; Division of Fossil Primates, Duke Lemur Center, Durham, NC 27705, USA. Electronic address: doug.boyer@duke.edu.
authors.forename: Doug M
authors.initials: DM
authors.lastname: Boyer
authors.affiliation: Department of Anthropology, University of Texas, Austin, Austin, TX, USA; Jackson School Museum of Earth History, University of Texas, Austin, Austin, TX, USA.
authors.forename: E Christopher
authors.initials: EC
authors.lastname: Kirk
authors.affiliation: Department of Anthropology, University of Toronto Scarborough, Scarborough, Ontario M1C 1A4, Canada.
authors.forename: Mary T
authors.initials: MT
authors.lastname: Silcox
authors.affiliation: Department of Evolutionary Anthropology, Duke University, Durham, NC 27708, USA; Division of Fossil Primates, Duke Lemur Center, Durham, NC 27705, USA; New York Consortium in Evolutionary Primatology (NYCEP), American Museum of Natural History, New York, NY 10024, USA.
authors.forename: Gregg F
authors.initials: GF
authors.lastname: Gunnell
authors.affiliation: New York Consortium in Evolutionary Primatology (NYCEP), American Museum of Natural History, New York, NY 10024, USA; Department of Anthropology, Hunter College of the City University of New York, 695 Park Avenue, New York, NY 10065, USA.
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Gilbert
authors.affiliation: Department of Evolutionary Anthropology, Duke University, Durham, NC 27708, USA.
authors.forename: Gabriel S
authors.initials: GS
authors.lastname: Yapuncich
authors.affiliation: Department of Anatomy and Neurobiology, Washington University School of Medicine in St. Louis, St. Louis, MO, USA.
authors.forename: Kari L
authors.initials: KL
authors.lastname: Allen
authors.affiliation: Department of Evolutionary Anthropology, Duke University, Durham, NC 27708, USA.
authors.forename: Emma
authors.initials: E
authors.lastname: Welch
authors.affiliation: Department of Vertebrate Paleontology, University of Florida, Florida Museum of Natural History, Gainesville, FL 32611, USA.
authors.forename: Jonathan I
authors.initials: JI
authors.lastname: Bloch
authors.affiliation: Department of Evolutionary Anthropology, Duke University, Durham, NC 27708, USA.
authors.forename: Lauren A
authors.initials: LA
authors.lastname: Gonzales
authors.affiliation: Department of Evolutionary Anthropology, Duke University, Durham, NC 27708, USA.
authors.forename: Richard F
authors.initials: RF
authors.lastname: Kay
authors.affiliation: Division of Fossil Primates, Duke Lemur Center, Durham, NC 27705, USA; Department of Cell and Neurobiology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033, USA.
authors.forename: Erik R
authors.initials: ER
authors.lastname: Seiffert
mesh_terms: D000818:Animals; D002343:Carotid Artery, Internal / Q000033:anatomy & histology*; D004432:Ear, Middle / Q000033:anatomy & histology* / Q000098:blood supply / Q000000981:diagnostic imaging; D005580:Fossils / Q000033:anatomy & histology*; D010802:Phylogeny; D011323:Primates / Q000033:anatomy & histology* / Q000145:classification; D012886:Skull / Q000033:anatomy & histology; D055114:X-Ray Microtomography
keywords: Adapiforms; Allometry; Brain; Internal carotid artery; Omomyiforms; Petrosal; Stapedial artery