MicroCT Analysis of Symphyseal Ontogeny in Archaeolemur

Abstract

Previous analyses of symphyseal fusion in the extinct Malagasy lemurArchaeolemur identified several functional characteristics of joint morphology that vary postnatally (Ravosa and Simons in American Journal of Physical Anthropology 95: [63][64][65][66][67][68][69][70][71][72][73][74][75][76] 1994). To complement that study, we used an imaging technique (microCT) that provides novel data on ontogenetic and local variation in biomineralization along the mandibular symphysis before complete ossification among adult Archaeolemur. Our sample of unfused symphyses comprised juveniles from the 2 earliest postnatal dental ages examined previously. We imaged each specimen (ca.18 μm volume elements) with slices parallel to the coronal plane, i.e., orthogonal to the joint articular surface. In ≤5 labiolingually equidistant joint sites, we collected 40 contiguous slices (18-μm intervals). Each of the 5 joint sites is represented by 1 slice, with biomineralization values sampled at 5 equidistant points along the articular surface and at 3 external cortical bone points. Our analysis of Archaeolemur indicates the presence of ontogenetic increases in bone mineral density accompanying increases in joint size and the number and distribution of symphyseal rugosities. Such postnatal changes are particularly marked for the middle of the joint presumed to lie adjacent to a degrading fibrocartilage pad. In Archaeolemur, labial regions of the symphysis Int J Primatol (ossify earlier and are likewise more biomineralized. Ontogenetic increases in symphyseal biomineralization, overall size, and fusion are consistent with elevated masticatory stresses owing to the postweaning shift to adult-like feeding behaviors. However, the labiolingual pattern of fusion and biomineralization in Archaeolemur appears related more to constraints on synostosis owing to the lingually located vascular supply characteristic of mammalian symphyses.

Pushing the limit: masticatory stress and adaptive plasticity in mammalian craniomandibular joints

“…Intra-and between-group variation in joint structure was assessed via microCT (Wong et al, 1995;Nuzzo et al, 2002;Patel et al, 2003;Stock et al, 2003;Morenko et al, 2004;Nicholson et al, 2006;Ravosa et al, 2007a;Ravosa et al, 2007b). Using a Scanco Medical MicroCT 40 (PA, USA), the microfocus X-ray tube was operated at 70·kV and 57·A, and the beam passed through a 0.13·mm thick beryllium window on the X-ray tube and through a 0.50·mm thick aluminum filter before encountering a sample.…”

“…At each condylar site, was sampled a total of 15 locations: five equidistant points along the articular surface, four equidistant subchondral points and three equidistant points per side along cortical bone of the condylar neck (Fig.·1B). Values of linear attenuation were pooled for each specimen and used to characterize between-group variation in biomineralization or local tissue mineral density along the symphysis and TMJ (Fig.·1) (Nicholson et al, 2006;Ravosa et al, 2007a;Ravosa et al, 2007b In each of five coronal sections, biomineralization levels were evaluated with computer assisted image analysis at five equidistant points along the articular surface (arrows) and four equidistant points along the lateral, superior and inferior cortical bone regions (arrowheads). Symphysis height and width also were quantified (not shown).…”

Bone up: craniomandibular development and hard-tissue biomineralization in neonate mice

“…Better understanding of age-related and regional variation in biomineralization may also aid future studies that seek to more accurately characterize loading environments in living and fossil mammals and connect morphological adaptations to functional behaviors (Ravosa et al, 2007b,c, 2016; Scott et al, 2014b). Our findings likewise have implications for the use of in silico models, such as finite element analysis (FEA), for inferring skull function, especially in extinct animals for which in vivo data cannot be obtained.…”

“…Our findings likewise have implications for the use of in silico models, such as finite element analysis (FEA), for inferring skull function, especially in extinct animals for which in vivo data cannot be obtained. As biomineralization varies within and between masticatory elements and this can affect tissue properties and skeletal performance (Ravosa et al, 2007a– c, 2016; Franks et al, 2017), this challenges FEA model assumptions that cranial tissue is homogenous (Dumont et al, 2005; Strait et al, 2009; Young et al, 2012; Gill et al, 2014). The data presented herein show that biomineralization varies among three distinct skull regions as well as between masticatory and non-masticatory elements, further demonstrating the heterogeneity of cranial elements.…”

Elwyn Simons: Opening windows into Madagascar's past

“…These include lemur systematics and phylogeny (Godfrey et al, ; Jungers et al, ; Simons, Godfrey, Jungers, Chatrath, & Rakotosamimanana, ; Tattersall, Simons, & Vuillaume‐Randriamanantena, ; Vuillaume‐Randriamanantena, Godfrey, Jungers, & Simons, ; Simons et al, ), past versus present biogeography, temporal ranges of subfossil lemurs, community characteristics, and the chronology of extinction of large lemurs and associated fauna (Jungers, Godfrey, Simons, & Chatrath, ; Simons, Burney, Chatrath, Godfrey, Jungers, & Rakotosamimanana, ; Godfrey, Jungers, Reed, Simons, & Chatrath, ; Simons, ; Godfrey, Jungers, Simons, Chatrath, & Rakotosamimana, ; Godfrey, Simons, Jungers, DeBlieux, & Chatrath, ; Muldoon & Simons, ; Muldoon et al, ; Muldoon, Crowley, Godfrey, Rasoamiaramanana, Aronson, Jernvall, Wright, & Simons, ; Muldoon, Crowley, Godfrey, & Simons, ). Other topics are reconstructing the behavior of extinct animals (Jungers, Godfrey, Simons, Wunderlich, Richmond, Chatrath, & Rakotosamimanana, ), subfossil lemur ontogeny, dental microstructure and life history (Ravosa & Simons, ; King, Godfrey, & Simons, ; Schwartz, Samonds, Godfrey, Jungers, & Simons, ; Ravosa, Stock, Simons, & Kunwar, ), dental microwear and occlusal texture analysis (Godfrey, Semprebon, Jungers, Sutherland, Simons, & Solounias, ; Scott, Godfrey, Jungers, Scott, Simons, Teaford, Ungar, & Walker, ), semicircular canal systems (Walker, Ryan, Silcox, Simons, & Spoor, ), hand and foot anatomy (Wunderlich, Simons, & Jungers, ; Jungers, Godfrey, Simons, & Chatrath, ; Hamrick, Simons, & Jungers, ; Jungers, Lemelin, Godfrey, Wunderlich, Burney, Simons, Chatrath, James, & Randria, ), vertebral anatomy (Shapiro, Seiffert, Godfrey, Jungers, Simons, & Randria, ), and field work adventures in Madagascar (Simons, Godfrey, Vuillaume‐Randriamanantena, Chatrath, & Gagnon, ; Simons, ; Simons et al, ). Elwyn's expeditions were responsible for the description and discovery of new species of extinct lemur (Godfrey et al, ; Simons et al, ) and led to the recognition of numerous new skeletal elements of previously known extinct lemurs (Hamrick et al, ), as well as the unearthing of some of the most complete extinct‐lemur skeletons ever found (MacPhee & Simons, ).…”