Functional morphology of synarthrial articulations in the crinoid stem

Synarthrial fulcral ridges are found in crinoid columnals from the mid-Ordovician to the present and in all four subclasses. Similar articulations did not become common in the cirri until the Mesozoic. Synarthrial stem articulations fall into two broad groups. Type I articulations have a fulcral ridge in the centre of the articular facet. In elliptical ossicles this ridge corresponds to either the long (IA) or short (IB) axis of the facet. Although both are functionally similar, Type IA ossicles are more common. Type II articulations have an excentric fulcral ridge, parallel to either the long (IIA) or short (IIB) axis of the articular facet. Type IIA articulations are found in crinoid stems capable of coiling. Type II articulations are particularly common in the cirri of articulates and are well adapted for attachment to hard and soft substrates. Columnals with Type I articulations often have divergent fulcra, giving the stem flexibility in all directions, but this feature is not seen in cirri or in coiled stems, where it would impair normal functions. Only the cirri of isocrinids and comatulids are muscular, so the movement of columns with fulcra must be passive.     

Functional morphology of the cephalopod buccal mass: a novel joint type

The arrangement of the musculature and connective tissues of the buccal mass of the coleoid cephalopods Octopus bimaculoides, Sepia officinalis, and Loliguncula brevis was examined using dissection and histology. Serial sections in three mutually perpendicular planes were used to identify the muscles and connective tissues responsible for beak movements and stability and to describe their morphology and fiber trajectories. Four major beak muscles were identified: the anterior, posterior, superior, and lateral mandibular muscles. The anterior, posterior, and superior mandibular muscles connect the upper beak and the lower beak. Although the lateral mandibular muscles originate on the upper beak, they do not connect to the lower beak and instead insert on a connective tissue sheath surrounding the buccal mass. Examination of the fibers of the lateral mandibular muscles reveals that they have the organization of a muscular hydrostat, with muscle fibers oriented in three mutually perpendicular orientations. Although the beaks are capable of complex opening, closing, and shearing movements, they do not contact one another and are instead connected only by the musculature of the buccal mass. Based on the morphological analysis and observations of freshly dissected beaks undergoing the stereotyped bite cycle, the functional role of the beak muscles is hypothesized. The anterior and superior mandibular muscles are likely responsible for beak closing and shearing movements. The posterior mandibular muscle is likely also involved in beak closing, but may act synergistically with the lateral mandibular muscles to open the beaks. The lateral mandibular muscles may use a muscular-hydrostatic mechanism to control the location of the pivot between the beaks and to generate the force required for beak opening. The lack of contact between the beaks and the morphology of the lateral mandibular muscles suggests that the buccal mass of coleoid cephalopods may represent a previously unexamined flexible joint mechanism. The term "muscle articulation" is proposed here to denote the importance of the musculature in the function of such a joint.     

A finite element evaluation of the moment arm hypothesis for altered vertebral shear failure force

The mechanism of vertebral shear failure is likely a bending moment generated about the pars interarticularis by facet contact, and the moment arm length (MAL) between the centroid of facet contact and the location of pars interarticularis failure has been hypothesised to be an influential modulator of shear failure force. To quantitatively evaluate this hypothesis, anterior shear of C3 over C4 was simulated in a finite element model of the porcine C3–C4 vertebral joint with each combination of five compressive force magnitudes (0–60% of estimated compressive failure force) and three postures (flexed, neutral and extended). Bilateral locations of peak stress within C3's pars interarticularis were identified along with the centroids of contact force on the inferior facets. These measurements were used to calculate the MAL of facet contact force. Changes in MAL were also related to shear failure forces measured from similar in vitro tests. Flexed and extended vertebral postures respectively increased and decreased the MAL by 6.6% and 4.8%. The MAL decreased by only 2.6% from the smallest to the largest compressive force. Furthermore, altered MAL explained 70% of the variance in measured shear failure force from comparable in vitro testing with larger MALs being associated with lower shear failure forces. Our results confirmed that the MAL is indeed a significant modulator of vertebral shear failure force. Considering spine flexion is necessary when assessing low-back shear injury potential because of the association between altered facet articulation and lower vertebral shear failure tolerance.     

The wing base of the palaeodictyopteran genus Dunbaria Tillyard: Where are we now?

The structure of insect wing articulation is considered as reliable source of high level characters for phylogenetic analyses. However, the correct identification of homologous structures among the main groups of Pterygota is a hotly debated issue. Therefore, the reconstruction of the wing bases in Paleozoic extinct relatives is of great interest, but at the same time it should be treated with extreme caution due to distortions caused by taphonomic effects. The present study is focused on the wing base in Dunbaria (Spilapteridae). The articulation in Dunbaria quinquefasciata is mainly formed by a prominent upright axillary plate while the humeral plate is markedly reduced. Due to unique preservation of surface relief of the axillary plate, its composition shows a detailed pattern of three fused axillary sclerites and presumable position of the sclerite 3Ax. The obtained structures were compared among Spilapteridae and to other palaeodictyopterans Ostrava nigra (Homoiopteridae) and Namuroningxia elegans (Namuroningxiidae). The comparative study uncovered two patterns of 3Ax in Dunbaria and Namuroningxia, which correspond to their different suprafamilial classification. In contrast to previous studies these new results reveal the homologous structural elements in the wing base between Paleozoic Palaeodictyoptera and their extant relatives of Ephemeroptera, Odonata and Neoptera.     

Robotic simulation of identical athletic-task kinematics on cadaveric limbs exhibits a lack of differences in knee mechanics between contralateral pairs

Limb asymmetry is a known factor for increased ACL injury risk. These asymmetries are normally observed during in vivo testing. Prior studies have developed in vitro testing methodologies driven by in vivo kinematics to investigate knee mechanics relative to ACL injury. The objective of this study was to determine if mechanical side-to-side asymmetries persist in contralateral pairs during in vitro simulation testing. In vivo kinematics were recorded for male and female drop vertical jump and sidestep cutting tasks. The recorded kinematics were used to robotically simulate the motions on 7 contralateral pairs of cadaveric lower extremities specimens. ACL and MCL force, torque, and strains were recorded and analyzed for differences between contralateral pairs. There was a general lack of mechanical differences between limb sides. Adduction peak torque for the male sidestep cut movement was significantly different between limb sides (p=0.04). However, this is consistent with ACL injury mechanics in that movement in the frontal plane (abduction/adduction) increases injury risk and it is possible loading differences in this plane may have resulted from tolerances within the setup process. The findings of this study indicate that contralateral knee joints were representative of each other during biomechanical in vitro tests. In future cadaveric robotic simulations, contralateral limbs can be used interchangeably. In addition, direct comparisons of the structural behaviors of isolated conditions for contralateral knee joints can be performed.     

Articulated Knowledges: Environmental Forms after Universality's Demise

Critiques of universalism require rethinking when confronted with environmental political arenas, in which the very concepts of "universality" and "particularity" are in a constant process of self-conscious deployment. In this article, I attend to the analytic and political implications of such deployments in a recent incinerator controversy in Hong Kong. I suggest that an aesthetic of local appropriateness and its formal requirement of simultaneous universal and particular truth value normalize the politics of environmental expertise such that the only legible form for counterknowledge is one of articulated knowledges. To understand how the knowledges emergent in an NGO–village collaboration were scaled, linked, and mobilized, I analyze a translation of expert knowledge and the event's metapragmatic effects. A subsequent account of unarticulated knowledges emphasizes the political-economic conditions that limit whose knowledges can count as particular in articulations of counterexpertise.     

The first rib of hominoids

Homo sapiens is unique among extant hominoids in displaying a univertebral articular pattern for the first rib; that is, the head of the first rib articulates only with the body of the first thoracic vertebra. All other hominoids, indeed virtually all other mammals, display a bivertebral pattern; that is, the head of the first rib articulates with the bodies of both the seventh cervical and the first thoracic vertebrae, as well as the intervening disk. Two fossil hominid partial first ribs, A.L. 288-lax and A.L. 333-118, show that the univertebral pattern was fully established in the hominid lineage by the appearance of Australopithecus afarensis. Four hypotheses, based in functional anatomy, can be postulated for the evolution of the univertebral pattern: (1), it increases the volume (via increased length) of the neck, which could, in turn, compensate for the functional loss of the laryngeal sac systems in hominid vocalization; (2), it is a consequence of the more barrel-shaped thorax in hominids; (3), it is a consequence of functional modifications in the hominid shoulder girdle; and/or (4), it is a consequence of modifications in hominid first rib motion while breathing in an upright stance. Fossil evidence supports all but the first hypothesis, and most strongly supports the third. However, evidence for the first hypothesis does suggest that the evolution of descent of the upper respiratory system in the hominid lineage may have been permitted by the presence of the univertebral pattern, while the reverse is probably not true. Furthermore, fossil evidence for the third hypothesis shows that, by the appearance of A. afarensis, the hominid upper limb had been freed from locomotor constraints, which concomitantly confirms full adaptation to upright posture. Thus, because of their potential relationship with upright posture, the two remaining hypotheses (i.e., "thoracic shape" and "first rib movement during breathing") also have support from the fossil evidence.     

The skull of Morganucodon

Morganucodon is a triconodont (atherian) mammal from the Lower Jurassic. Two species are described: M. oehleri from China and M. watsoni from Wales. The skull in M. walsoni is 26 mm long; M. oehleri is slightly larger. The dentition is differentiated functionally into incisors, canines, premolars and molars. The pineal foramen is closed. The prefrontals, postfrontals and postorbitals are lost. Septomaxilla, quadratojugal, tabular and pterygoid flanges are retained. The bony external nares are unpaired. The nasal cavity had the mammalian complement of turbinals. The posterior palate has ridges and troughs similar to those in tritylodonts, triconodonts and multituberculates. The alisphenoid ascending process is narrow and is not in contact with the anterior lamina of the petrosal, lying lateral to it. There is a cavum epiptericum, as in late therapsids. The anterior lamina forms the lateral braincase wall, perforated by the foramina pseudovale and pseudorotundum. There is a squamosal-dentary articulation, but the reptilian jaw joint is retained. The ear resembles that in later therapsids, with the tympanum in the lower jaw. The small quadrate was moveable, buttressed medially be a large stapes. Sound conduction from the tympanum was via articular, quadrate and stapes. The systematic position of Morganucodon is discussed.