The role of bioturbation-driven substrate disturbance in the Mesozoic brachiopod decline

Brachiopods dominated the seafloor as a primary member of the Paleozoic fauna. Despite the devastating effects of the end-Permian extinction, the group recovered during the early Mesozoic only to gradually decline from the Jurassic to today. This decline likely had multiple causes, including increased predation and bioturbation-driven substrate disruption, but the role of changing substrate is not well understood. Given the importance of substrate for extant brachiopod habitat, we documented Mesozoic–Cenozoic lithologic preferences and morphological changes to assess how decreasing firm-substrate habitat may have contributed to the brachiopod decline. Compared with bivalves, Mesozoic brachiopods occurred more frequently and were disproportionately abundant in carbonate lithologies. Although patterns in glauconitic or ferruginous sediments are equivocal, brachiopods became more abundant in coarser-grained carbonates and less abundant in fine-grained siliciclastics. During the Jurassic, brachiopod species rarely had abraded beaks but tended to be more convex with a high beak, potentially consistent with a non-analogue lifestyle resting on the seafloor. However, those highly convex morphotypes largely disappeared by the Cenozoic, when more terebratulides had abraded beaks, suggesting closer attachment to hard substrates. Rhynchonellides disproportionately declined to become a minor component of Cenozoic faunas, perhaps because of less pronounced morphological shifts. Trends in lithologic preferences and morphology are consistent with bioturbation-driven substrate disruption, with brachiopods initially using firmer carbonate sediments as refugia before adapting to live primarily attached to hard surfaces. This progressive habitat restriction likely played a role in the final brachiopod decline, as bioturbating ecosystem engineers transformed benthic habitats in the Mesozoic and Cenozoic.

Material remodeling and unconventional gaits facilitate locomotion of a robophysical rover over granular terrain

Autonomous robots and vehicles must occasionally recover from locomotion failure in loosely consolidated granular terrain. Recent mobility challenges led NASA Johnson Space Center to develop a prototype robotic lunar rover Resource Prospector 15 (RP15) capable of wheeled, legged, and crawling behavior. To systematically understand the terradynamic performance of such a device, we developed a scaled-down rover robot and studied its locomotion on slopes of dry and wet granular media. Addition of a cyclic-legged gait to the robot’s wheel spinning action changes the robot dynamics from that of a wheeled vehicle to a locomotor paddling through frictional fluid. Granular drag force measurements and modified resistive force theory facilitate modeling of such dynamics. A peculiar gait strategy that agitates and cyclically reflows grains under the robot allows it to “swim” up loosely consolidated hills. Whereas substrate disturbance typically hinders locomotion in granular media, the multimode design of RP15 and a diversity of possible gaits facilitate formation of self-organized localized frictional fluids that enable effective robust transport.

Spatial patterns in lotic invertebrate community composition: is substrate disturbance actually important?

Twenty-five forest streams were sampled in August 1994 in Te Urewera National Park, New Zealand, to examine the effect of substrate disturbance on invertebrate community structure. Stream size, flow permanence, and riparian cover were more influential than substrate disturbance in affecting invertebrate composition. Three community types were distinguishable based on these three factors: small (<1 m wide), intermittent streams were dominated by Chironomidae; larger (12–15 m wide), open streams were dominated by Chironomidae, Plecoptera, and Ephemeroptera; and intermediate-sized (1–10 m wide) streams with continuous riparian cover were dominated by mayflies and caddisflies. Periphyton biomass was negatively affected by substrate disturbance but not to the same degree as reported by others studying unshaded streams. This may explain why the influence of substrate disturbance on community composition was less than that of stream size, flow permanence, and riparian cover. The key effect of substrate disturbance on postdisturbance community composition in these light-limited New Zealand streams appears to be the removal of animals rather than food loss. Thus, differences between communities that experience high flows and those that do not are far less than they might be in unshaded streams in which the food base is more severely affected by substrate disturbance.