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F A Jenkins

Publications and source records attributed to F A Jenkins.

17 recordsLinked to original sources
PubMed1997

Haramiyids and Triassic mammalian evolution.

Isolated teeth referred to the family Haramiyidae are among the earliest known fossil evidence of mammals. First discovered in European Late Triassic deposits a century and a half ago, haramiyids have been interpreted as related to multituberculates, a diverse and widespread lineage that occupied a rodent-like niche from the Late Jurassic to the Early Tertiary. Nonetheless, haramiyid relationships have remained enigmatic because the orientation and position of the teeth in the upper or lower jaw could not be determined with certainty; even their mammalian status has been questioned. The discovery of haramiyid dentaries, a maxilla and other skeletal remains in the Upper Triassic of East Greenland reveals haramiyids as highly specialized mammals with a novel pattern of puncture-crushing occlusion that differs dramatically from the grinding or shearing mechanisms of other Early Mesozoic mammals.

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PubMed1993

Mammalian locomotor-respiratory integration: implications for diaphragmatic and pulmonary design.

Diaphragmatic function and intrapulmonary respiratory flow in running mammals were found to differ substantially from the corresponding conditions known in resting mammals. In trotting dogs, orbital oscillations of the diaphragm were driven by inertial displacements of the viscera induced by locomotion. In turn, oscillations of the visceral mass drove pulmonary ventilation independent of diaphragmatic contractions, which primarily served to modulate visceral kinetics. Visceral displacements and loading of the anterior chest wall by the forelimbs are among the factors that contribute to an asynchronous ventilation of the lungs and interlobar gas recycling. Basic features of mammalian respiratory design, including the structure of the diaphragm and lobation of the lungs, appear to reflect the mechanical requirements of locomotor-respiratory integration.

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PubMed1992

An aerodynamic valve in the avian primary bronchus.

The segmentum accelerans in geese is a constriction in the caudal end of the primary bronchus. Experimental evidence suggests that this part of the airway functions as an inspiratory aerodynamic valve, accelerating the incoming airstream past the ventrobronchial openings. The luminal diameter of the segmentum accelerans dilates in the presence of elevated CO2 levels, probably through relaxation of smooth muscle. Physiological control of the segmentum accelerans may permit inspiratory aerodynamic valving to be maintained throughout a wide range of ventilatory flows.

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PubMed1988

A functional analysis of the primary upstroke and downstroke muscles in the domestic pigeon (Columba livia) during flight.

In domestic pigeons (Columba livia), the electrical activity of the major depressor muscle of the wing, the pectoralis (pars thoracicus), beings in late upstroke well before the wing begins its downstroke excursion. The two architecturally distinct heads of the pectoralis, the sternobrachialis and the thoracobrachialis, are differentially recruited during take-off, level flight and landing. In addition to wing depression, the sternobrachialis protracts the humerus and the thoracobrachialis retracts the humerus. At the point of transition from wing upstroke to downstroke, the pectoralis EMG signal typically exhibits a reduction in amplitude. The supracoracoideus, in addition to an expected EMG associated with wing elevation, is co-activated with the pectoralis about 50% of the time.

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PubMed1987

Structure and neural control of the pectoralis in pigeons: implications for flight mechanics.

The pectoralis muscle in pigeons (Columba livia) is composed of two heads (sternobrachialis, thoracobrachialis) that are separately innervated and have different fiber orientations. High-speed film and electromyographic studies of free-flying pigeons reveal that the pectoralis is activated prior to wing depression (the power stroke) and that its two heads are differentially recruited during takeoff, level flight, and landing. The electrical activity patterns of both heads support an interpretation that intramuscular elasticity provides energy storage. The pectoralis is not only the prime wing depressor but is also capable of adjusting the excursion of the wing during different phases of flight.

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PubMed1976

The postcranial skeletons of the Triassic mammals Eozostrodon, Megazostrodon and Erythrotherium.

The purposes of this monograph are to describe the postcranial skeletons of the earliest known mammals, and to probe, in so far as possible by osteological study, biological questions concerning the habits and adaptations of these late Triassic forms. In this context, information on the background of this investigation is useful. Studies of Mesozoic mammals, begun some 150 years ago, are based on rare and fragmentary fossils, principally jaws and teeth. These investigations have yielded a bare outline of some 120 million years of mammalian evolution-about two-thirds of mammalian history. No assessment of the important biological changes occurring during this time can ever be complete, but major advances are possible as new discoveries provide material that is more complete or that represents a previously unknown evolutionary stage. So tenuous is the evidence that at least some concepts are re-evaluated with each discovery. Postcranial anatomy offers especially intriguing prospects for investigation because associated material (that can be positively assigned to a taxon below subclass) has been for the most part unknown, and indeed even dissociated bones are a rarity. Since G.G. Simpson's monographs of 1928 and 1929, progress in the study of Mesozoic mammals has been largely dependent on new finds. A major impetus to renewed investigation came from the discoveries of Mesozoic mammals by Walter Kühne in 1939 and during the immediate post-war years. Kühne first worked on fissures in the Carboniferous limestone quarries at Frome, Somerset, in southwest England where he collected a series of teeth of the problematical form Haramiya and two triconodont teeth which were placed in the genus Eozostrodon (Parrington 1941, 1946). The fissure faunas are generally thought to be of Upper Triassic (Rhaetic) age (Kühne 1946), although Kermack, Musset & Rigney (1973) believe that the evidence is insufficient to determine whether the deposits are Rhaetic or Lower Liassic. After the war Kühne carried his explorations farther west, eventually reaching the quarries at Bridgend in Glamorgan, Wales, where he not only found more triconodont teeth in some quantity (Kühne 1958) but also a symmetrodont tooth (Kühne 1950). Shortly after making these discoveries, Kühne returned to Germany and the work was continued by a team from University College, London, under the leadership of Dr K.A. Kermack.

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PubMed1972

Chimpanzee bipedalism: cineradiographic analysis and implications for the evolution of gait.

Bipedal chimpanzees reorient the pelvis to achieve an upright posture but retain the same pattern of femoral flexion and extension as in quadrupedal walking. Major differences from human gait are the abducted, relatively more flexed excursion of the femur and the timing of pelvic tilt, which raises during the swing phase. The femoral head morphology in the fossil hominid Australopithecus robustus is evidence of an approximately vertical excursion of the femur in contrast to the adducted pattern of modern man and the abducted pattern of chimpanzees.

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PubMed1970

Limb movements in a monotreme (Tachyglossus aculeatus): a cineradiographic analysis.

In a walking echidna the principal movement of the humerus is long-axis rotation. The humerus remains approximately perpendicular to the sagittal plane, but the femur is directed anterolaterally at angles from 35 degrees to 50 degrees . In addition to long-axis rotation, the femur elevates and depresses in an arc which usually varies between 40 degrees and 90 degrees . The femoral angle, the femoral elevation and depression, and the plantar contact of the manus beneath the glenoid are features found also in generalized therians.

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PubMed1967

American Jurassic symmetrodonts and Rhaetic "pantotheres".

The molar morphology of the symmetrodonts Tinodon and Eurylambda from the late Jurassic of North America is virtually identical to that of so-called "pantotheres" from the Rhaetic of Wales. Therefore a primitive symmetrodont molar pattern was probably present in the phylogeny of pantotherian and tribosphenic molars. Occlusion of Tinodon and Eurylambda produced complex wear facets unlike the simple trigon-trigonid shear surfaces of Spalacotherium and Peralestes.

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