Anatomy and function of expanded ribs in certain edentates and primates.
TL;DR: Anteroposteriorly expanded ribs occur in all three species of myrme-cophagids (Cyclopes didactylus, Tamandua tetradactyla, and Myrmecophaga tridACTyla) and Functionally, expanded ribs may increase the stability of the thorax, which, in turn, increases the stability in the vertebral column.
Abstract: Anteroposteriorly expanded ribs occur in all three species of myrme-cophagids (Cyclopes didactylus, Tamandua tetradactyla, and Myrmecophaga tridactyla). Similar rib expansion occurs in some armadillos (for example, Dasypus novemcinctus, Priodontes giganteas, Euphractus sexcinctus) and in some primates (Arctocebus calabarensis, Perodicticus potto, and Loris tardigradus). In C. didactylus and A. calabarensis rib expansion is so extensive that adjacent ribs imbricate. the epaxial and hypaxial musculature related to the expanded ribs in all three myrmecophagid species is described. the intercostal spaces in T. tetradactyla and M. tridactyla are approximately 50 per cent narrower than in various other mammals with unexpanded ribs; the intercostal musculature of these two species is approximately twice as thick as that in other mammals with unexpanded ribs. the proportion of intercostal musculature in C. didactylus is comparable to that in other mammals. Functionally, expanded ribs may increase the stability of the thorax, which, in turn, increases the stability of the vertebral column. This specialization in T. tetradactyla and M. tridactyla probably relates to their fossorial habitus; the trunk is stabilized during fossorial activity by xenarthrous processes in the lumbar region and by expanded ribs and robust intercostal muscles in the thorax. in strictly arboreal forms such as C. didactylus and A. calabarensis, lumbo-thoracic stability may be employed in methodical arboreal locomotion. C. didactylus, at least, is able to anchor itself with hind feet and tail and extend its trunk and forelimbs to reach an adjacent branch—a feat requiring unusual lumbothoracic stability.
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TL;DR: The discovery of a well-preserved species of fossil sarcopterygian fish from the Late Devonian of Arctic Canada that represents an intermediate between fish with fins and tetrapods with limbs, and provides unique insights into how and in what order important tetrapod characters arose is reported.
Abstract: The relationship of limbed vertebrates (tetrapods) to lobe-finned fish (sarcopterygians) is well established, but the origin of major tetrapod features has remained obscure for lack of fossils that document the sequence of evolutionary changes. Here we report the discovery of a well-preserved species of fossil sarcopterygian fish from the Late Devonian of Arctic Canada that represents an intermediate between fish with fins and tetrapods with limbs, and provides unique insights into how and in what order important tetrapod characters arose. Although the body scales, fin rays, lower jaw and palate are comparable to those in more primitive sarcopterygians, the new species also has a shortened skull roof, a modified ear region, a mobile neck, a functional wrist joint, and other features that presage tetrapod conditions. The morphological features and geological setting of this new animal are suggestive of life in shallow-water, marginal and subaerial habitats.
384 citations
TL;DR: Since hominoid-like features of the wrist joint are found in lorisines, but not in New World monkeys that practice arm-swinging locomotion, these features may have been evolved in both lorisine and large hominoids to enhance wrist mobility for cautious arboreal locomotion involving little or no leaping.
Abstract: In lorisines (Loris, Nycticebus, Perodicticus, Arctocebus), the tip of the ulna is reduced to the dimensions of a styloid process, a new and more proximal ulnar head is developed, and the pisiform is displaced distally away from its primitive contact with the ulna. In someNycticebus, intra-articular tissues separate the ulna from the triquetrum. These traits are not seen in other quadrupedal primates, but they are characteristic of extant hominoids. Among hominoids, these features have been interpreted as adaptations to arm-swinging locomotion. Since hominoid-like features of the wrist joint are found in lorisines, but not in New World monkeys that practice arm-swinging locomotion, these features may have been evolved in both lorisines and large hominoids to enhance wrist mobility for cautious arboreal locomotion involving little or no leaping. Most of the other morphological traits characteristic of modern hominoids can be explained as adaptations to cautious quadrupedalism as well as to brachiation, and may have developed for different reasons in different lineages descended from an unspecialized cautious quadruped resembling Alouatta.
286 citations
TL;DR: This work reanalysed a recent dataset that allied turtles with the lizard–tuatara clade and found that the inclusion of the stem turtle Proganochelys quenstedti and the ‘parareptile’ Eunotosaurus africanus results in a single overriding morphological signal, with turtles outside Diapsida.
Abstract: The origin of turtles is one of the most contentious issues in systematics with three currently viable hypotheses: turtles as the extant sister to (i) the crocodile–bird clade, (ii) the lizard–tuatara clade, or (iii) Diapsida (a clade composed of (i) and (ii)). We reanalysed a recent dataset that allied turtles with the lizard–tuatara clade and found that the inclusion of the stem turtle Proganochelys quenstedti and the ‘parareptile’ Eunotosaurus africanus results in a single overriding morphological signal, with turtles outside Diapsida. This result reflects the importance of transitional fossils when long branches separate crown clades, and highlights unexplored issues such as the role of topological congruence when using fossils to calibrate molecular clocks.
130 citations
TL;DR: A model of shell assembly that makes predictions for the as-yet unestablished history of the turtle stem group is built on by integrating novel data for Eunotosaurus africanus, which is congruent with molecular-based divergence estimates for the lineage, and remains viable whether turtles originated inside or outside crown Diapsida.
107 citations
Cites background from "Anatomy and function of expanded ri..."
...This differs from other amniote ribs, in which intercostal muscles extend between the ribs, with no muscles inserting on the ventral face of the rib [42, 43] (Figure S2)....
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TL;DR: It is proposed that Mayulestes and Pucadelphys represent an ancestral morphotype suggesting that the generalized type of locomotion of Paleocene marsupials was partly terrestrial with some climbing ability.
Abstract: In this study, the axial skeletons of two Early Paleocene marsupials, Mayulestes ferox and Pucadelphys andinus, were analyzed functionally and compared to that of six South American and three Australian species of extant marsupials. In the case of the South American opossums, myological data of the epaxial musculature were collected and analyzed and osteological-myological associations were related to locomotor behavior. Various features of the vertebral column that relate to diet or to locomotor or postural patterns were pointed out. These features include: the craniocaudal development of the neural process of the axis; the position of the anticlinal vertebra; the morphology of the neural processes of the thoracolumbar vertebrae (orientation, length, and craniocaudal width); the length, orientation, and curvature of the transverse processes of the lumbar vertebrae; and the length and robustness of the caudal vertebrae. In both fossil forms the vertebral column is mobile and allows a great range of flexion and extension of the spine, more so than in most of the living didelphids. It is emphasized here that the analysis of the axial skeleton complements and improves the conclusions provided by the forelimb and hindlimb analyses. It is proposed that Mayulestes and Pucadelphys represent an ancestral morphotype suggesting that the generalized type of locomotion of Paleocene marsupials was partly terrestrial with some climbing ability. J. Morphol. 255:279–300, 2003. © 2003 Wiley-Liss, Inc.
99 citations
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TL;DR: There is a great discrepancy between the force that can theoretically be applied to the spine if the role of intracavitary pressures is ignored and theforce that can be tolerated experimentally by the isolated ligamentous human spine.
Abstract: There is a great discrepancy between the force that can theoretically be applied to the spine if the role of intracavitary pressures is ignored and the force that can be tolerated experimentally by the isolated ligamentous human spine.In estimating that a force of approximately 2,071 pounds is impos
438 citations
TL;DR: The investigations reported here represent an effort to explore the possibilities of obtaining quantitative data on the mechanical properties of the lumbosacral spine by applying to fresh autopsy specimens of the spine testing techniques used in civil or mechanical engineering.
Abstract: The investigations reported here represent an effort to explore the possibilities of obtaining quantitative data on the mechanical properties of the lumbosacral spine by applying to fresh autopsy specimens of the spine testing techniques used in civil or mechanical engineering. Few tests have been made and the data are insufficient. However, the findings to date are of interest and encourage us to continue these investigations.
With the exception of the markedly osteoporotic spines which failed at relatively low axial loads, the ultimate axial compressive load for the lumbar discs teste ranged from 1,000 to 1,300 pounds. Stiffness values for these discs ranged from 470 to 8,250 pounds per inch initially but increased to 12,000 to 20,000 pounds per inch after the applied load reached 200 to 400 pounds. Failure under axial compression took place in the vertebral end-plates even when well developed ruptures of the annulus were present. Failure of the plate ranged from imperceptible cracks to more or less complete collapse of the endplate depending on the condition of the bone (presence or absence of osteoporosis) and on the size of the load applied.
Under the experimental conditions employed here, which involved a gradually increasing load over periods ranging from ten to thirty minutes, the volumes of five discs decreased under axial compression by from 1.0 to 2.5 cubic centimeters before failure occurred due to fracture of one of the vertebral end-plates and collapse of the underlying vertebral body. This decrease in volume is thought to be due both to collapse of the fissures and spaces ordinarily present within all adult discs and to the passage of fluid across the vertebral end-plates into the medullary spaces of the adjacent vertebral bodies.
Tests of the tensile strength of various portions of two intervertebral discs revealed that the tensile strength ranged from 0 to 700 pounds per square inch. In general, the weakest areas in both discs were the central and lateral portions. The tensile strength of three specimens of ligamentum flavum ranged from 226 to 373 pounds per square inch.
Two discs (the fourth lumbar) were subjected to combined axial loading and bending while the expansion and contraction of the discs were recorded at various points about the circumference of the annulus. During bending the discs expanded on the concave side and contracted on the convex side of the curve, while at points located at 90 degrees to the plane of motion little or no expansion or contraction occurred. The only exceptions to this general pattern of behavior were in the posterolateral portions of the discs, where annular ruptures are most frequently found. The greatest expansion and contraction occurred On the anterior aspect of these discs during straight flexion and extension.
One fatigue test was performed in which cyclic bending and compression stresses were imposed on a single disc. For technical reasons the test was not satisfactory. It is of interest, however, that rapid failure of the annulus occurred under the conditions of this experiment characterized by a horizontal tear of all but the most peripheral fibers of the annulus. The resulting picture was not unlike that seen in some extensively degenerated dises.
The findings of particular interest in the work presented here are as follows:
1. Under axial compressive stress, failure of the disc complex invariably took place in the cartilaginous plate. In the few specimens tested the type of failure varied according to the condition of the bone rather than according to the condition of the disc. In the spines from younger individuals small cracks occurred, while in the old spines with osteoporosis, there was more or less total collapse of the plate. The similarity of the small cracks produced in these specimens to those found in the autopsy material studied by Beadle and by Coventry, Ghormley, and Kernohan was quite striking. 13, 27 have been inclined to assume that these defects were of little clinical significance due to their small size. However, the findings presented here suggest that such an assumption may not be valid. Failures of the end-plate may play a role in the causation of pain in some back injuries when the roentgenograms show no abnormality. In fact, Hirsch 23 reported one case in which failure of the plate was demonstrated in a man in whom sudden low-back pain developed after a strenuous effort to right an overturned car. However, the special technique of tomography was required to visualize the defect. The relief of back pain resulting from the intradiscal injection of a local anaesthetic or the anti-inflammatory hormone, hydrocortisone, might be explained by this mechanism also.
2. In these tests failure of the annulus fibrosus occurred only as the result of extremely rapid cyclic bending combined with mild axial compression. However, no protrusion of disc material was produced in this short period but rather a linear horizontal tear through all but the most peripheral fibers which still remained intact. In the one specimen in which failure was induced by flexion, the inferior posterior margin of the superior vertebra was avulsed, but the annulus remained intact.
3. It is commonly stated that the nucleus pulposus is the structure by which stresses are distributed uniformly to the annulus fibrosus and cartilaginous plates 40. It is furthermore stated in the literature that the nucleus pulposus moves toward the convex side of the curve when the spine bends to either side or forward and backward 36. The behavior of the two specimens subjected to combined axial loading and bending in this investigation did not seem consistent with this concept. The annulus invariably bulged on the concave side, apparently as the result of compression between the opposing vertebral surfaces. The thickest and strongest portion of the annulus, that is its anterior third, bulged the most. This would be more consistent with direct compression of the annulus than with bulging caused by deformation of the nucleus pulposus or displacement of the nucleus toward the opposite side. If there were a significant amount of displacement of the nucleus toward the convex side, it would not be anticipated that the annulus would retract on this side, as was observed in these specimens. Therefore during bending the annulus in these rather aged discs would appear to be subjected primarily to direct compression rather than to hoop tension as would be the case if its only function were to resist deformation of the nucleus pulposus. Friberg, as the result of his studies of fresh specimens of the spine, also concluded that the annulus had a weight-bearing function.
This behavior of the disc may well vary with age. The more fluid nucleus of the child may displace more during bending than the relatively fibrotic and fragmented nucleus of the adult.
4. Under axial compressive loads the volume losses of the intervertebral discs before failure in the four specimens tested were 1.0, 1.5, 2.5, and 2.5 cubic centimeters, respectively, during time intervals ranging from ten to thirty minutes. Previous clinical observations on the disappearance of water-soluble radiopaque solutions after injection into the disc space11 have suggested that fluid transfer across the cartilaginous plates occurs quite rapidly during life even in the recumbent position when compression forces are minimal. It remains to be determined how such factors as age and degeneration of the cartilaginous plates effect. the rate and amount of this transfer. Degenerative changes which occur within the disc with advancing age may be related in some way to alterations in this fluid transfer.
The experimental methods reported here are thought to show sufficient promise to warrant a more extensive testing program on specimens from different age groups and in sufficient numbers to obtain data of statistical significance.
Furthermore, other tests would be desirable. For instance, impact tests might furnish quantitative data on the mechanism of injuries associated with falls and other sudden movements. The effect of other types of stress including shear and axial torsion would also be of interest.
414 citations