Ben Creisler
Some recent non-dino papers:
Free pdf:
Michael D. Stein, Suzanne J. Hand, Michael Archer, Stephen Wroe & Laura A.B. Wilson (2020)
Quantitatively assessing mekosuchine crocodile locomotion by geometric morphometric and finite element analysis of the forelimb
PeerJ 8:e9349Â
Â
Morphological shifts observed in the fossil record of a lineage potentially indicate concomitant shifts in ecology of that lineage. Mekosuchine crocodiles of Cenozoic Australia display departures from the typical eusuchian body-plan both in the cranium and postcranium. Previous qualitative studies have suggested that these crocodiles had a more terrestrial habitus than extant crocodylians, yet the capacity of mekosuchine locomotion remains to be tested. Limb bone shape, such as diaphyseal cross-section and curvature, has been related to habitual use and locomotory function across a wide variety of taxa. Available specimens of mekosuchine limbs, primarily humeri, are distinctly columnar compared with those of extant crocodylians. Here we apply a quantitative approach to biomechanics in mekosuchine taxa using both geomorphic morphometric and finite element methods to measure bone shape and estimate locomotory stresses in a comparative context. Our results show mekosuchines appear to diverge from extant semi-aquatic saltwater and freshwater crocodiles in cross-sectional geometry of the diaphysis and generate different structural stresses between models that simulate sprawling and high-walk gaits. The extant crocodylians display generally rounded cross-sectional diaphyseal outlines, which may provide preliminary indication of resistance to torsional loads that predominate during sprawling gait, whereas mekosuchine humeri appear to vary between a series of elliptical outlines. Mekosuchine structural stresses are comparatively lower than those of the extant crocodylians and reduce under high-walk gait in some instances. This appears to be a function of bending moments induced by differing configurations of diaphyseal curvature. Additionally, the neutral axis of structural stresses is differently oriented in mekosuchines. This suggests a shift in the focus of biomechanical optimisation, from torsional to axial loadings. Our results lend quantitative support to the terrestrial habitus hypothesis in so far as they suggest that mekosuchine humeri occupied a different morphospace than that associated with the semi-aquatic habit. The exact adaptational trajectory of mekosuchines, however, remains to be fully quantified. Novel forms appear to emerge among mekosuchines during the late Cenozoic. Their adaptational function is considered here; possible applications include navigation of uneven terrain and burrowing.
Free pdf:
https://onlinelibrary.wiley.com/doi/pdf/10.1111/brv.12615Despite many decades of research, the allometric scaling of metabolic rates (MRs) remains poorly understood. Here, we argue that scaling exponents of these allometries do not themselves mirror one universal law of nature but instead statistically approximate the nonâlinearity of the relationship between MR and body mass. This 'statistical' view must be replaced with the lifeâhistory perspective that 'allows' organisms to evolve myriad different life strategies with distinct physiological features. We posit that the hypoallometric allometry of MRs (mass scaling with an exponent smaller than 1) is an indirect outcome of the selective pressure of ecological mortality on allocation 'decisions' that divide resources among growth, reproduction, and the basic metabolic costs of repair and maintenance reflected in the standard or basal metabolic rate (SMR or BMR), which are customarily subjected to allometric analyses. Those 'decisions' form a wealth of lifeâhistory variation that can be defined based on the axis dictated by ecological mortality and the axis governed by the efficiency of energy use. We link this variation as well as hypoallometric scaling to the mechanistic determinants of MR, such as metabolically inert component proportions, internal organ relative size and activity, cell size and cell membrane composition, and muscle contributions to dramatic metabolic shifts between the resting and active states. The multitude of mechanisms determining MR leads us to conclude that the quest for a singleâcause explanation of the mass scaling of MRs is futile. We argue that an explanation based on the theory of lifeâhistory evolution is the best way forward.
===
Christophe Mallet, Guillaume Billet, Alexandra Houssaye & RaphaÃl Cornette (2020)
A first glimpse at the influence of body mass in the morphological integration of the limb long bones: an investigation in modern rhinoceroses.
Journal of Anatomy (advance online publication)
doi:
https://doi.org/10.1111/joa.13232https://onlinelibrary.wiley.com/doi/abs/10.1111/joa.13232The appendicular skeleton of tetrapods is a particularly integrated structure due to the shared developmental origin or similar functional constraints exerted on its elements. Among these constraints, body mass is considered strongly to influence its integration but its effect on shape covariation has rarely been addressed in mammals, especially in heavy taxa. Here, we propose to explore the covariation patterns of the long bones in heavy animals and their link to body mass. We investigate the five modern rhinoceros species, which display an important range of bodyweight. We used a 3D geometric morphometric approach to describe the shape covariation of the six bones composing the stylopodium and zeugopodium both among and within species. Our results indicate that the appendicular skeleton of modern rhinos is a strongly integrated structure. At the interspecific level, the shape covariation is roughly similar between all pairs of bones and mainly concerns the muscular insertions related to powerful flexion and extension movements. The forelimb integration appears higher and more related to body mass than that of the hind limb, suggesting a specialization for weight support. The integration of the stylopodium elements does not seem to relate to body mass in our sample, which suggests a greater effect of shared developmental factors. Conversely, the covariation of the zeugopodium bones seems more associated with body mass, particularly for the radiusâulna pair. The fibula appears poorly integrated with other bones, especially within nonâRhinoceros species, which may represent a case of parcellation due to a functional dissociation between the hind limb bones. The exploration of the integration patterns at the intraspecific level also highlights a more prominent effect of age over individual body mass on shape covariation within C. simum. This study lends support to previous hypotheses indicating a link between high body mass and high integration level.
============
Although still exceedingly rare, the number of known vertebrate-bitten coprolites continues to increase. A vertebrate-bitten coprolite is herein reported from the Piscataway Member of the upper Paleocene Aquia Formation at Liverpool Point, Maryland, U.S.A.The specimen is described, figured, and chemically characterized by means of non-destructive hand-held energy dispersive X-ray fluorescence (HH-XRF). Four roughly parallel and evenly spaced gouges disrupt the surface of this compound ichnofossil. Many more much smaller markings, interpreted as feeding traces by smaller organisms (invertebrates or small actinopterygian fishes?), also ornament its surface. Whereas the studied coprolite can be identified most likely as the fossilized feces of a crocodilian, the identity of the vertebrate(s) that bit it remains unknown.