Technical note: A landmark‐based approach to the study of the ear ossicles using ultra‐high‐resolution X‐ray computed tomography data

Previous study of the ear ossicles in Primates has demonstrated that they vary on both functional and phylogenetic bases. Such studies have generally employed two‐dimensional linear measurements rather than three‐dimensional data. The availability of Ultra‐ high‐resolution X‐ray computed tomography (UhrCT) has made it possible to accurately image the ossicles so that broadly accepted methodologies for acquiring and studying morphometric data can be applied. Using UhrCT data also allows for the ossicular chain to be studied in anatomical position, so that it is possible to consider the spatial and size relationships of all three bones. One issue impeding the morphometric study of the ear ossicles is a lack of broadly recognized landmarks. Distinguishing landmarks on the ossicles is difficult in part because there are only two areas of articulation in the ossicular chain, one of which (the malleus/incus articulation) has a complex three‐dimensional form. A measurement error study is presented demonstrating that a suite of 16 landmarks can be precisely located on reconstructions of the ossicles from UhrCT data. Estimates of measurement error showed that most landmarks were highly replicable, with an average CV for associated interlandmark distances of less than 3%. The positions of these landmarks are chosen to reflect not only the overall shape of the bones in the chain and their relative positions, but also functional parameters. This study should provide a basis for further examination of the smallest bones in the body in three dimensions. Am J Phys Anthropol, 2011. © 2011 Wiley‐Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86912/1/21543_ftp.pd

Oldest Skeleton of a Plesiadapiform provides additional evidence for an exclusively arboreal radiation of stem Primates in the Palaeocene

Palaechthonid plesiadapiforms from the Palaeocene of western North America have long been recognized as among the oldest and most primitive euarchontan mammals, a group that includes extant primates, colugos and treeshrews. Despite their relatively sparse fossil record, palaechthonids have played an important role in discussions surrounding adaptive scenarios for primate origins for nearly a half-century. Likewise, palaechthonids have been considered important for understanding relationships among plesiadapiforms, with members of the group proposed as plausible ancestors of Paromomyidae and Microsyopidae. Here, we describe a dentally associated partial skeleton of Torrejonia wilsoni from the early Palaeocene (approx. 62Ma) of New Mexico, which is the oldest known plesiadapiform skeleton and the first Palaechthonid plesiadapiforms from the Palaeocene of western North America have long been recognized as among the oldest and most primitive euarchontan mammals, a group that includes extant primates, colugos and treeshrews. Despite their relatively sparse fossil record, palaechthonids have played an important role in discussions surrounding adaptive scenarios for primate origins for nearly a half-century. Likewise, palaechthonids have been considered important for understanding relationships among plesiadapiforms, with members of the group proposed as plausible ancestors of Paromomyidae and Microsyopidae. Here, we describe a dentally associated partial skeleton of Torrejonia wilsoni from the early Palaeocene (approx. 62Ma) of New Mexico, which is the oldest known plesiadapiform skeleton and the firs

Evolution of arboreality and fossoriality in squirrels and aplodontid rodents: Insights from the semicircular canals of fossil rodents

Reconstructing locomotor behaviour for fossil animals is typically done with postcranial elements. However, for species only known from cranial material, locomotor behaviour is difficult to reconstruct. The semicircular canals (SCCs) in the inner ear provide insight into an animal's locomotor agility. A relationship exists between the size of the SCCs relative to body mass and the jerkiness of an animal's locomotion. Additionally, studies have also demonstrated a relationship between SCC orthogonality and angular head velocity. Here, we employ two metrics for reconstructing locomotor agility, radius of curvature dimensions and SCC orthogonality, in a sample of twelve fossil rodents from the families Ischyromyidae, Sciuridae and Aplodontidae. The method utilizing radius of curvature dimensions provided a reconstruction of fossil rodent locomotor behaviour that is more consistent with previous studies assessing fossil rodent locomotor behaviour compared to the method based on SCC orthogonality. Previous work on ischyromyids suggests that this group displayed a variety of locomotor modes. Members of Paramyinae and Ischyromyinae have relatively smaller SCCs and are reconstructed to be relatively slower compared to members of Reithroparamyinae. Early members of the Sciuroidea clade including the sciurid Cedromus wilsoni and the aplodontid Prosciurus relictus are reconstructed to be more agile than ischyromyids, in the range of extant arboreal squirrels. This reconstruction supports previous inferences that arboreality was likely an ancestral trait for this group. Derived members of Sciuridae and Aplodontidae vary in agility scores. The fossil squirrel Protosciurus cf. rachelae is inferred from postcranial material as arboreal, which is in agreement with its high agility, in the range of extant arboreal squirrels. In contrast, the fossil aplodontid Mesogaulus paniensis has a relatively low agility score, similar to the fossorial Aplodontia rufa, the only living aplodontid rodent. This result is in agreement with its postcranial reconstruction as fossorial and with previous indications that early aplodontids were more arboreal than their burrowing descendants

Data from: The European Paromomyidae (Primates, Mammalia): taxonomy, phylogeny, and biogeographic implications

Plesiadapiforms represent the first radiation of Primates, appearing near the Cretaceous-Paleogene boundary. Eleven families of plesiadapiforms are recognized, including the Paromomyidae. Four species of paromomyids from the early Eocene have been reported from Europe: Arcius fuscus, Arcius lapparenti, and Arcius rougieri from France, and Arcius zbyszewskii from Portugal. Other Arcius specimens from the early Eocene are known from Masia de l’Hereuet (Spain), Abbey Wood (England), and Sotteville-sur-Mer (Normandy, France). A cladistic analysis of the European paromomyids has never previously been published. A total of 53 dental characters were analyzed for the four Arcius species and the specimens from Spain, England, and Normandy. The results of a parsimony analysis using TNT agree with previous conceptions of A. zbyszewskii as the most primitive member of the genus. Also consistent with existing hypotheses, Arcius rougieri is positioned as the sister taxon of A. fuscus and A. lapparenti, and the results suggest that the fossil from Normandy is A. zbyszewskii. However, the English fossil pertains to a primitive lineage, rather than grouping with A. lapparenti as had been suggested; as such it is recognized here as a distinct species (Arcius hookeri). The Spanish fossils cluster together with the French species, but do not show the previously proposed special relationship with A. lapparenti, and are sufficiently distinct to be placed in a new species (Arcius ilerdensis). Arcius is recovered as monophyletic, which is consistent with a single migration event from North America to Europe around the earliest Eocene though the Greenland land bridge

New omomyoids (Euprimates, Mammalia) from the late Uintan of southern California, USA, and the question of the extinction of the Paromomyidae (Plesiadapiformes, Primates)

Paromomyidae has been thought to represent the longest-lived group of stem primates (plesiadapiforms), extending from the early Paleocene to late Eocene. We analyzed primate material from the late-middle Eocene of southern California that had initially been ascribed to cf. Phenacolemur shifrae. This material falls at the lowest end of the size range for the family. The Californian specimens also exhibit several dental features that are atypical for paromomyids, such as a strong paraconid on the third lower molar, and differ from early Eocene species of Phenacolemur in lacking a distally expanded distolingual basin on upper molars. This combination of traits is more typical of earlier paromomyids with more plesiomorphic morphologies (e.g., Paromomys) and as such is inconsistent with the late age of these specimens. The purported paromomyids Ph. shifrae and Ignacius mcgrewi known in deposits of similar age are comparably tiny and share many of the characteristics found in the southern California material that distinguish them from typical early Eocene paromomyids. However, these traits are shared with some trogolemurin omomyoid euprimates that are of similar size. We argue that the material from southern California, along with Ph. shifrae and I. mcgrewi, should be transferred to a new genus of trogolemurin omomyoid. Purported European records of paromomyids later than the earliest middle Eocene are reconsidered and found to be non-diagnostic. After the early middle Eocene, only a single tooth of a paromomyid can be confirmed, indicating that the group suffered near-extinction, possibly correlated with the Early Eocene Climatic Optimum

First virtual endocasts of a fossil rodent: <i>Ischyromys typus</i> (Ischyromyidae, Oligocene) and brain evolution in rodents

<p>The evolution of the brain in rodents has rarely been studied from the perspective of the fossil record. Here we describe the first virtual endocast of a fossil rodent, pertaining to <i>Ischyromys typus</i> (ROMV 1007; Orellan North American Land Mammal Age [NALMA], Nebraska), and form comparisons with partial and complete natural endocasts pertaining to the same genus, and with the virtual endocast of a closely related extant rodent (<i>Sciurus carolinensis</i>; AMNH 258346). These data allow us to formulate the first hypotheses informed by the fossil record concerning changes in brain size and shape through time in rodents, and to make comparisons with other euarchontoglirans, including Primates. <i>Ischyromys</i> exhibits several aspects of brain morphology that can be inferred to be primitive, in part based on their presence in plesiadapiform primates (e.g., exposed midbrain), although variation exists within the genus <i>Ischyromys</i> with respect to the visibility of the inferior colliculi. There is some evidence that neocorticalization occurred in rodents through time but to a lesser degree than in Primates. Arboreality might be linked to increases in the encephalization quotient and specializations related to vision in rodents, which contrasts with the situation in Primates. Finally, Oligocene rodents had smaller olfactory bulbs compared with plesiadapiform primates from the Eocene, meaning that olfaction might have been less critical in the early evolution of rodents. These results show that the evolution of the brain in mammals does not always follow the same evolutionary trajectories and demonstrates the importance of considering ecological factors when looking at brain size.</p> <p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at <a href="http://www.tandfonline.com/UJVP" target="_blank">www.tandfonline.com/UJVP</a></p> <p>Citation for this article: Bertrand, O. C., and M. T. Silcox. 2016. First virtual endocasts of a fossil rodent: <i>Ischyromys typus</i> (Ischyromyidae, Oligocene) and brain evolution in rodents. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1096275.</p