Ontogeny of the Postorbital Region in Tarsiers and Other Primates

Micro-computed tomography unveils anatomy of the oldest known plesiadapiform cranium

Palaechthonids are a likely paraphyletic or polyphyletic assemblage of dentally plesiomorphic plesiadapiforms known from the Paleocene of North America. This family is known primarily from isolated dental fossils, but one partial cranium of the palaechthonid Plesiolestes nacimienti (Division of Vertebrate Paleontology, Biodiversity Institute, University of Kansas [KUVP] 9557) exists and was studied a half-century ago to infer aspects of the paleobiology of basal or stem primates. Since then, additional plesiadapiform crania representing several families have been described, but KUVP 9557 remains the best preserved for a palaechthonid and is the geologically oldest known cranial fossil of any plesiadapiform or euarchontan mammal (primates + colugos + treeshrews). Here, for the first time, we scanned the partial cranium of P. nacimienti using micro-computed tomography (μCT) to assess previously described morphology, document novel morphology, and make comparisons with crania of other plesiadapiforms and euarchontans. While several previous cranial descriptions are reaffirmed here (e.g., caudal expansion of the nasals, an intraorbital lacrimal foramen), μCT scan data have clarified that certain previously identified structures (e.g., postorbital process) are not present and have unveiled previously unknown structures (e.g., foramen ovale, optic foramen). Comparisons indicate that the cranial anatomy of P. nacimienti is most like that of non-microsyopid plesiadapiforms and that unambiguous synapomorphies with an extant euarchontan clade are absent. Paleobiological inferences presented here suggest that P. nacimienti was broadly similar to the extant treeshrew Ptilocercus, albeit less insectivorous, which is in line with evolutionary scenarios proposed for the ancestral primatomorphan or the ancestral primate (sensu lato) that emphasize the importance of arboreality and angiosperm products.

A review of ectochondral bone and the role of membranes in shaping endochondral bones of the skull

Bones of the skull are traditionally categorized as derived from either endochondral or intramembranous bone. In our previous work, we have observed the interaction of different tissue types in growth of the skull. We find the dichotomy of intramembranous and endochondral bone to be too restrictive, limiting our interpretation of sources of biological variation. Here, we advocate for the use of the term ectochondral bone to describe bone that originates from an endochondral model but is directed in its subsequent growth by membranes and other fascial attachments. Growth of the alisphenoid and orbitosphenoid are described as two examples of ectochondral bone, influenced in their shape primarily by the surrounding soft tissues. Ectochondral bone may be an ideal mechanism for rapidly evolving new phenotypes. Instead of evolving novelties by altering morphology of the cartilage template, novel features may be formed by ectochondral ossification, a more direct and rapid mode of osteogenesis than that of the cartilage template.

"Ontogenetic Scaling of the Primate Middle Ear"

The study of primate auditory morphology is a significant area of interest for comparative anatomists, given the phylogenetic relationships that link primate hearing and the morphology of these auditory structures. Extensive literature addresses the form-to-function relationship of the auditory system (outer, middle, and inner ear) in primates and, by extension, provides insight into the auditory system of extinct primates and even modern humans. We add to this literature by describing the ontogenetic trajectory of the middle ear cavity and ossicular chain (malleus, incus, and stapes) due to their critical role in relaying auditory stimuli for interpretation. We examined middle ear morphology in neonatal primates and adult primates using a taxonomically broad sample. We focused primarily on nocturnal primate taxa (Daubentonia, Loris, Galago, Aotus, and Tarsier), which are underrepresented in the literature. However, we also included three diurnal taxa (Macaca, Lemur, and Saguinus). Using 3D Slicer, we visualized middle ear structures in three dimensions using conventional micro CT data informed by diffusible iodine-based contrast-enhanced CT (diceCT) data. We illustrated how spatial relationships between otic elements, such as the various epitympanic sinuses of the middle ear and the auditory ossicles, vary throughout ontogeny. Our major findings include that the central tympanic cavity scaled with negative allometry in all taxa and that the accessory cavities scaled with isometry or positive allometry in most taxa. Despite these changes in chamber size, the size of the ear ossicles remained relatively consistent through ontogeny in most taxa. We confirmed our expectation that anthropoids exhibit an increase in the complexity of accessory cavities throughout ontogeny, mirroring the exponential pneumatization of the face in anthropoids. These findings provide an ontogenetic perspective and reveal further functional complexities of the middle ear as a conduit for sound proliferation and as a pressure regulator.

Maxilla of Siamopithecus eocaenus (Anthropoidea, Primates) from the Paleogene of Krabi, Thailand, and its taxonomic status

Detailed descriptions of the maxillae of Siamopithecus eocaenus, discovered from the latest Eocene/earliest Oligocene lignite mine in the Krabi basin of Peninsular Thailand, are presented. They include the morphology of P3-M3, the palate, a partial orbital region, and the zygomatic root. The specimen exhibits distinctive dental features including a single-rooted P2 alveolus, a protocone on the P3 and P4, and a true hypocone on the upper molars, indicating its derived anthropoid dentition. Comparative studies and virtual reconstructions of the facial anatomy reveal close affinities with diurnal anthropoids rather than the notharctid strepsirrhines. The reconstructed facial morphology of Siamopithecus displays a short and subvertically oriented face, significant orbital convergence (72.1°), and frontation (81.6°), distinguishing it from both fossil and extant strepsirrhines. Moreover, the presence of a thin bony lamina extending from the distal part of the upper preserved area of the zygomatic suggests partial or complete postorbital closure. Phylogenetic analyses suggest an affiliation with amphipithecids, but recent morphological observations challenge this, leading to the proposal of an elevated family-group ranking, Siamopithecidae. Comparison with the most primitive known Afro-Arabian propliopithecid, the Taqah propliopithecid from Oman, reveals similarities in their molar structure but differences in dental formula (retention of P2) and premolar structure. The abrupt appearance of propliopithecids in the early Oligocene of Afro-Arabia, without a local ancestor, contributes to the debate on whether catarrhine origins were in Asia or Africa. However, alternative views, based on sister-group relationships with oligopithecids, support an African origin of propliopithecids from an undocumented Afro-Arabian region. This research provides new insights into the evolutionary history of early anthropoids, suggesting a complex biogeographical scenario involving both Asian and African lineages.

Virtual endocast of late Paleocene Niptomomys (Microsyopidae, Euarchonta) and early primate brain evolution

Paleogene microsyopid plesiadapiforms are among the oldest euarchontans known from relatively complete crania. While cranial endocasts are known for larger-bodied Eocene microsyopine microsyopids, this study documents the first virtual endocast for the more diminutive uintasoricine microsyopids, derived from a specimen of Niptomomys cf. Niptomomys doreenae (USNM 530198) from the late Paleocene of Wyoming. Size estimates of smaller-bodied uintasoricines are similar to those inferred for the common ancestor of Primates, so the virtual endocast of Niptomomys may provide a useful model to study early primate brain evolution. Due to the broken and telescoped nature of the neurocranium of USNM 530198, a μCT scan of the specimen was used to create a 3D model of multiple bone fragments that were then independently isolated, repositioned, and merged to form a cranial reconstruction from which a virtual endocast was extracted. The virtual endocast of Niptomomys has visible caudal colliculi, suggesting less caudal expansion of the cerebrum compared to that of euprimates, but similar to that of several other plesiadapiforms. The part of the endocast representing the olfactory bulbs is larger relative to overall endocast volume in Niptomomys (8.61%) than that of other known plesiadapiforms (∼5%) or euprimates (<3.5%). The petrosal lobules (associated with visual stabilization) are relatively large for a Paleocene placental mammal (1.66%). The encephalization quotient of Niptomomys is relatively high (range = 0.35-0.85) compared to that of Microsyops (range = 0.32-0.52), with the upper estimates in the range of values calculated for early euprimates. However, this contrast likely relates in part to the small size of the taxon, and is not associated with evidence of neocortical expansion. These findings are consistent with a model of shifting emphasis in primate evolution toward functions of the cerebrum and away from olfaction with the origin of euprimates.

Evolution of the Jugal/Zygomatic Bones

This issue of the Anatomical Record is the second of a two-volume set on the zygoma (also called the cheek bone, the zygomatic bone, the malar, or the jugal, the latter term being used in vertebrates other than mammals). The zygoma is an important component of the craniofacial skeleton, in which the zygoma is a connection between the midfacial and the cranial skeletons; has a functional role as the origin of one of the masticatory muscles, the masseter muscle, and several facial muscles; has been considered as an essential buttress of the facial skeleton for resisting masticatory forces; and has importance for determining phylogenetic relationships. In humans, the zygoma is also of aesthetic significance for facial appearance, and its restoration following trauma has resulted in a large clinical literature. In this second half of the special issue on the zygoma, a series of papers discuss studies related to evolution of the zygoma and related parts of the craniofacial skeleton throughout the vertebrates, and in particular in human evolution. There are also a series of articles discussing variation of the zygoma in modern humans. This article is an overview in which we discuss the primary findings of these studies and some of their implications. Anat Rec, 300:12-15, 2017. © 2016 Wiley Periodicals, Inc.

Development, Structure, and Function of the Zygomatic Bones: What is New and Why Do We Care?

This issue of The Anatomical Record is the first of a two-volume set on the zygoma (also called the cheek bone, the zygomatic bone, the malar, or the jugal, the latter term being used in vertebrates other than mammals). The zygoma is an important component of the craniofacial skeleton, in which the zygoma is a connection between the midfacial and the cranial skeletons; has a functional role as the origin of one of the masticatory muscles, the masseter muscle, and several facial muscles; has been considered as an essential buttress of the facial skeleton for resisting masticatory forces; and has importance for determining phylogenetic relationships. In humans, the zygoma is also of aesthetic significance for facial appearance, and its restoration following trauma has resulted in a large clinical literature. In this first volume of this Special Issue, a wide ranging series of papers discuss studies related to issues of development, structure, and function of the zygoma and closely related parts of the craniofacial skeleton in mammals, and in particular primates. This Introductory article provides an overview in which we discuss the primary findings of these studies and some of their implications. The second volume, which will be published as the January 2017 issue of The Anatomical Record, will focus on variation and evolution of the zygoma throughout the vertebrates. Anat Rec, 299:1611-1615, 2016. © 2016 Wiley Periodicals, Inc.