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Anne Le MaÃtre, Nicole D. S. Grunstra, Cathrin Pfaff & Philipp Mitteroecker (2020)
Encapsulated within the temporal bone and comprising the smallest elements of the vertebrate skeleton, the ear is key to multiple senses: balance, posture control, gaze stabilization, and hearing. The transformation of the primary jaw joint into the mammalian ear ossicles is one of the most iconic transitions in vertebrate evolution, but the drivers of this complex evolutionary trajectory are not fully understood. We propose a novel hypothesis: The incorporation of the bones of the primary jaw joint into the middle ear has considerably increased the genetic, regulatory, and developmental complexity of the mammalian ear. This increase in the number of genetic and developmental factors may, in turn, have increased the evolutionary degrees of freedom for independent adaptations of the different functional ear units. The simpler ear anatomy in birds and reptiles may be less susceptible to developmental instabilities and disorders than in mammals but also more constrained in its evolution. Despite the tight spatial entanglement of functional ear components, the increased âevolvabilityâ of the mammalian ear may have contributed to the evolutionary success and adaptive diversification of mammals in the vast diversity of ecological and behavioral niches observable today. A brief literature review revealed supporting evidence for this hypothesis.
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Diving as a lifestyle has evolved on multiple occasions when air-breathing terrestrial animals invaded the aquatic realm, and diving performance shapes the ecology and behaviour of all air-breathing aquatic taxa, from small insects to great whales. Using the largest dataset yet assembled, we show that maximum dive duration increases predictably with body mass in both ectotherms and endotherms. Compared to endotherms, ectotherms can remain submerged for longer, but the mass scaling relationship for dive duration is much steeper in endotherms than in ectotherms. These differences in diving allometry can be fully explained by inherent differences between the two groups in their metabolic rate and how metabolism scales with body mass and temperature. Therefore, we suggest that similar constraints on oxygen storage and usage have shaped the evolutionary ecology of diving in all air-breathing animals, irrespective of their evolutionary history and metabolic mode. The steeper scaling relationship between body mass and dive duration in endotherms not only helps explain why the largest extant vertebrate divers are endothermic rather than ectothermic, but also fits well with the emerging consensus that large extinct tetrapod divers (e.g. plesiosaurs, ichthyosaurs and mosasaurs) were endothermic.
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Innovations in foraging behavior can drive morphological diversity by opening up new ways of interacting with the environment, or limit diversity through functional constraints associated with different foraging behaviors. Several classic examples of adaptive radiations in birds show increased variation in ecologically relevant traits. However, these cases primarily focus on geographically narrow adaptive radiations, consider only morphological evolution without a biomechanical approach, or do not investigate tradeoffs with other nonâfocal traits that might be affected by use of different foraging habitats. Here, we use Xâray microâcomputed tomography (microCT), biomechanical modelling, and multivariate comparative methods to explore the interplay between foraging behavior and cranial morphology in kingfishers, a global radiation of birds with variable beaks and foraging behaviors, including the archetypal plungeâdive into water. Our results quantify covariation between the shape of the outer keratin covering (rhamphotheca) and the inner skeletal core of the beak, as well as highlight distinct patterns of morphospace occupation for different foraging behaviors and considerable rate variation among these skull regions. We anticipate these findings will have implications for inferring beak shapes in fossil taxa and inform biomimetic design of novel impactâreducing structures.
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Kiwis (Apterygidae) are an enigmatic family of flightless birds endemic to New Zealand. Apterygidae is made up of a single genus, Apteryx with five species, four of which are characterised as at risk of greater by the New Zealand Department of Conservation. These five species are further separated into two morphologically and genetically distinguishable clades, containing A. haastii, and A. owenii in one and A. rowi, A. mantelli, and A. australis in the other. We reconstructed 17 kiwi mitochondrial genomes from previously published genomic data, nine from A. rowi and eight from A. owenii. Mitochondrial diversity analyses uncovered low levels of genetic diversity consistent with their reduced ranges and conservation concern. We further used one of the assembled A.rowi mitochondrial genomes together with mitochondrial genomes from A. haastii, A. owenii, A. mantelli, and several other individuals from Palaeognathae to estimate the within and between clade divergence times of kiwis. Our study exemplifies how available published data can be used in novel ways to provide new and complementary evolutionary insights to previous studies.
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Also:
Highlights
The geomagnetic polarity time scale for the entire Carnian is established.
Onset of the Mid-Carnian Episode is coeval worldwide.
Newark Basin magnetic polarity series begins at about 20% up in Tuvalian.
The clay horizon in S. Italy correlates to the D siliciclastic pulse in N. Italy.
Abstract
The Stuttgart Formation (traditionally called the Schilfsandstein) in the Germanic Basin (or Central European Basin) is a sand-rich episode representing the Mid-Carnian Episode within the gypsum-rich clayey semi-arid Keuper facies. That regional Mid-Carnian Episode is now recognized to be a manifestation of a significant global disruption of Earth's climate-ocean-biological system during the early Late Triassic. In order to provide an accurate time frame and means for high-resolution correlations among continental and marine records, we obtained a composite magnetostratigraphy spanning the entire Carnian from three boreholes in the Germanic Basin. This composite shows a good consistency with earlier published magnetostratigraphy results from South China and enables the construction of a complete Carnian polarity time scale. The upper Carnian (Tuvalian substage) portion implies that: (1) The lower quarter of the Tuvalian is dominated by a reversed-polarity magnetozone; (2) The termination of the Yangtze Platform is coeval with deposition of the Stuttgart Formation in the Germanic Basin; (3) The radiolarian-rich green-colored clay horizon of the Pignola-2 section in south Italy that is below a volcanic ash bed dated as ca. 231 Ma correlates with the D pulse of siliciclastics in Dibona section of north Italy in the middle of the Tuvalian; (4) The upper three-fourths of the Tuvalian is a major normal-polarity-dominated magnetozone; and this interval correlates with the basal E1-E6 portion of the Newark magnetic polarity reference series according to the favored correlation option; (5) The base of the Arnstadt Formation of the Germanic Basin, which had been traditionally assigned as the Carnian/Norian boundary, instead begins within the lower portion of Newark reversed-polarity zone E8r of earliest Norian according to the favored age model, although pending future verification; (6) The polarity patterns of portions of the upper Carnian from the western Tethys sections of Lower Trench at Silickà Brezovà in Austria and Pizzo Mondello in Italy are verified. This Carnian composite is an enhanced polarity time scale for calibration of other global Carnian successions and events, such as the appearance of the earliest dinosaurs in fossiliferous beds of South America.