Membranous Support for Eyes of Strepsirrhine Primates and Fruit Bats

Abstract

Living primates have relatively large eyes and support orbital tissues with a postorbital bar (POB) and/or septum. Some mammals with large eyes lack a POB, and presumably rely on soft tissues. Here, we examined the orbits of four species of strepsirrhine primates (Galagidae, Cheirogaleidae) and three species of fruit bats (Pteropodidae). Microdissection and light microscopy were employed to identify support structures of the orbit. In bats and primates, there are two layers of fascial sheets that border the eye laterally. The outer membrane is the most superficial layer of deep fascia, and has connections to the POB in primates. In fruit bats, which lacked a POB or analogous ligament, the deep fascia is reinforced by transverse ligaments. Bats and primates have a deeper membrane supporting the eye, identified as the periorbita (PA) based on the presence of elastic fibers and smooth muscle. The PA merges with periostea deep within the orbit, but has no periosteal attachment to the POB of primates. These findings demonstrate that relatively big eyes can be supported primarily with fibrous connective tissues as well as the PA, in absence of a POB or ligament. The well-developed smooth muscle component within the PA of fruit bats likely helps to protrude the eye, maintaining a more convergent eye orientation, with greater overlap of the visual fields. The possibility should be considered that early euprimates, and even stem primates that may have lacked a POB, also had more convergent eyes than indicated by osseous measurements of orbital orientation.

Fissures, folds, and scrolls: The ontogenetic basis for complexity of the nasal cavity in a fruit bat (Rousettus leschenaultii)

“…In primates, the convergent eyes are known to be associated with medial orbital resorption in small‐bodied species, which can inhibit the expansion of different regions of the nasal fossa (Smith et al, 2014). Although pteropodid bats do have relatively large, forward‐facing eyes, they are also ectopic (extrude out of the skeletal boundaries of the orbit—Harvey et al, 2016), so the eyes do not likely constrain interorbital expansion. Still, such “trade‐offs” are unaddressed by our current sample due to the lack of a full postnatal cross‐sectional age series.…”

Eye Size and Set in Small‐Bodied Fossil Primates: A Three‐Dimensional Method

“…The following species were dissected: Galago moholi, Otolemur garnettii, Lemur catta and Tarsius syrichta. The fruit bat Cynopterus sphinx was dissected in a separate study by Harvey et al (2016), and was useful for assessing eye diameter and position in the present study. The same specimen was dissected further as reported here, removing additional loose connective tissue and fat to expose the greatest diameter of the eye for caliper measurement.…”

“…This adult specimen was CT-scanned and dissected for a different study examining soft-tissue support for the eye (Harvey et al, 2016). Figure 15 presents the skull and virtual eye of Cynopterus (Fig.…”

“…It shows that the position of the eye as revealed by dissection is consistent with the virtual reconstruction of eye position. Harvey et al (2016) describe how the degree of orbital convergence is largely explained by soft tissue support, including fascia (already dissected away) and the skin, which is pulled anteriorly to expose the ectopic eye (Fig. 15B).…”

Evolution of the Jugal/Zygomatic Bones

“…Many of the articles in the first volume (December 2016) at least in part included topics that shed light on the evolutionary history of the zygoma, with concerns relating to evolutionary adaptation and function (Dechow and Wang, ). Those articles discussed how development and genetics related to our understanding of evolution in the craniofacial region (Heuzé et al, ), the implications of developmental patterns in perinatal postorbital anatomy in 21 genera of primates (DeLeon et al, ), the vascularization of the zygomatic arch (Herring and Ochareon, ), the effects of hard and soft diet on the morphology of the zygomatic region (Franks et al, ), structure and function of the orbit in some modern and fossil primates (Harvey et al, ; Rosenberger et al, ), the histological structure of soft tissues overlying the zygomatic and facial region (Burrows et al, ), the functional implications of zygomatic structure in primates (Edmonds, ), the structure and elastic properties of the facial skeleton including the zygoma (Gharpure et al, , Pryor MacIntosh et al, ), implications of sutural variation in the zygoma of primates (Wang and Dechow, ), the significance of variation in the shape of the zygomatic arch explored with finite element modeling (Smith and Grosse, ), and a biomechanical assessment of the utility of a pillar and buttress model for evaluating zygomatic function (Prado et al, ).…”