![]() ![]() When the effect of size is removed by allometric comparison, the result is not different ( Fig. Obdurodon has a higher angle (a) and lower angle (b) than Ornithorhynchus ( Table 1 and Fig. Mainland platypuses are shown as gray circles, whereas Tasmanian platypuses are shown as black open circles. Scales are log-transformed for linear measurements. Regression lines and 95% prediction intervals of the measurements and angles plotted by GLS and geometric mean in the Ornithorhynchus compared to Obdurodon. Here, we report several skull differences between the two species that would appear to reflect differences in how these two species operated within their respective habitats, and a potentially correlated difference that may have led to loss of teeth in adult Or. However, there has been relatively little attention paid to comparing features that may relate specifically to differences in feeding behaviors and sensory efficiency with modern platypuses. dicksoni has been examined in detail ( 4– 6). However, the difference of sensory efficiency between Ornithorhynchus and Obdurodon is also still unclear. This at least raises the possibility that Obdurodon may have kept its eyes open while foraging for food. However, its eyes show morphological similarities to those of other aquatic and semiaquatic mammals, such that the lens is adapted to underwater vision with a steeply curved posterior surface in relation to the flatter anterior surface ( 8, 9). anatinus swims under water with its eyes shut. The cause of the loss of teeth is still unclear. Therefore, the loss of teeth in Ornithorhynchus is unlikely to be attributable to the lack of their necessity given that commutation of food remains an important component of feeding in the living animal. In contrast, the adult extant platypus still masticates its prey by using horny pads, located in the same position as the cheek teeth of Obdurodon, to crush items being consumed ( 2). Many mammals have lost their teeth as a result of evolution, including echidnas, anteaters, and baleen whales, but none of these edentulous animals actually masticate their prey with their jaw moving ( 7). For example, Ornithorhynchus has no teeth as adults in contrast to Obdurodon, which had fully functional cheek teeth. However, there are also several notable morphological differences. Morphological similarities in bill structure between these species suggest that they may have filled similar ecological niches. anatinus and a seemingly well-developed trigeminal nerve, similar to the extant platypus ( 4– 6). The Miocene monotreme Obdurodon dicksoni has a larger bill than Or. Well-developed electroreceptivity in monotremes is known at least as far back as the early Cretaceous however, there are differences in the extent of elaboration of the feature among members of the ornithorhynchid lineage.Įxtant platypuses ( Ornithorhynchus anatinus) forage at the bottom of streams with their eyes closed, using only electro- and mechanoreception ( 1– 3). ![]() Hence, loss of functional teeth in Ornithorhynchus may possibly have resulted from a shift in foraging behavior and coordinate elaboration of the electroreceptive sensory system. Computed tomography imagery indicates that the enlarged infraorbital canal of Ornithorhynchus restricts the space available for maxillary tooth roots. Whereas bill-focused sensory perception was likely shared among Mesozoic monotremes, the highly developed electrosensory system of Ornithorhynchus may represent an adaptation to foraging in cloudy water. The infraorbital foramen of Obdurodon, through which the maxillary nerve passes sensory data from the bill to the brain, is relatively less developed than that of Ornithorhynchus. ![]() Results indicate that the bill of Obdurodon is more dorsally deflected than that of Ornithorhynchus, suggesting a pelagic foraging behavior in Obdurodon compared to the bottom-feeding behavior in Ornithorhynchus. To estimate differences in foraging, sensory systems, and anatomical divergence between these monotremes, we compared their skull morphologies. We hypothesize that Ornithorhynchus and the Miocene taxon Obdurodon have different sensory capacities, which may have resulted from differences in foraging behavior. The modern platypus, Ornithorhynchus anatinus, has an eye structure similar to aquatic mammals however, platypuses also have a “sixth sense” associated with the bill electro- and mechanoreception that they use without opening their eyes underwater. ![]()
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