Internal carotid arterial canal size and scaling in Euarchonta: Re-assessing implications for arterial patency and phylogenetic relationships in early fossil primates

Abstract

a b s t r a c tPrimate 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 nonencephalic 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.

The Central Role of Small Vertical Substrates for the Origin of Grasping in Early Primates

“…However, the genus Tupaia is morphologically and behaviorally different from the more primitive arboreal representative of the group Ptilocercus, often considered as a model of the ancestral primate morphotype [40,41]. As Ptilocercus was not sampled in this study, and due to its morphological similarities to primates [33,60,61], we cannot rule out the possibility that it would behave differently from Tupaia and closer to primates. Finally, our results add evidence on the functional convergence of the foot grasping mechanism between primates and some marsupials [62,63].…”

Fossil skulls reveal that blood flow rate to the brain increased faster than brain volume during human evolution

“…Of course, our analysis relies on the presently untestable assumption that the pattern of cerebral perfusion is the same in humans as it was in ancestral hominins. However, the blood supply to the cerebrum of haplorrhine primates is derived mainly from the ICAs [22], and stapedial arteries that branch from the ICAs in the carotid canal are absent in anthropoid apes [55], including humans [56]. This provides confidence that the pattern of cerebral blood flow was established prior to the divergence of Australopithecus from the common ancestor among the anthropoid apes and that carotid foramen size is a reliable indication of a pattern of increased cerebral perfusion during hominin evolution.…”

Re-Evaluating Cranial Pathways of the Internal Carotid Artery in Notoungulata (Mammalia, Panperissodactyla)

“…However, evidence for this outcome in extinct taxa tends to be ambiguous, usually because there are no useful indicia, pro or con. Small size of the carotid foramen (Boyer et al, 2016) by itself is not an adequate indicium for making a determination of loss, although MacPhee et al (2021, fig. 18) doubtfully wondered if the relatively small size of the exposed piriform fenestra in some typotherians implied such a correlation (see below).…”