Meiofaunal organisms are indirectly influenced by the activity of benthic macroinvertebrates within the sediment, which plays a role in modifying physical and chemical characteristics of the habitat. The association of meiofaunal organisms and macroburrows is well known in modern environments, but the record of this relationship in the geological record is still incipient. This study documents diminutive burrows (Helminthoidichnites tenuis) associated with the surface of macroburrows (Palaeophycus tubularis) in Early Permian deposits. The cylindrical shape and meandering to loop trajectory of the diminutive burrows indicate that they were produced by small free‐living meiofaunal nematodes. Apparently, P. tubularis (open burrow) constituted a favourable microhabitat for nematodes, providing the following: (1) protection against erosive processes and meiofauna predators; (2) oxygen access to more in‐depth layers within the sediment; (3) temperature stabilization; and (4) food supply due to mucus impregnation in the macroburrow walls by the Palaeophycus tracemaker. The association between H. tenuis and P. tubularis constitutes the first fossil record of a symbiotic relationship between meiofaunal nematodes and macrobenthic organisms (polychaetes). It also suggests that ecological strategies such as mutualism or commensalism, which are common between extant nematodes and macrobenthic invertebrates, were available in the behavioural programme of these organisms since the Early Permian.

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Ostracods are common yet understudied prey item in the fossil record. We document drill holes in Paleocene (Danian) ostracods from central Argentina using 9025 specimens representing 66 species. While the percentage of drilled specimens at assemblage-level is only 2.3%, considerable variation exists within species (0.3-25%), suggesting prey preference by the drillers. This preference is not determined by abundance because no significant correlation is found between species abundance and drilling percentages. Seven methods were used, some of which are new, to quantify drilling percentages for the abundant and commonly drilled Togoina argentinensis. The resulting range, from 9.9% to 14.6%, suggests that drilling percentages are fairly insensitive to the method used, implying that comparisons across studies are possible. This information, combined with data from the literature, suggests the Cretaceous-Palaeogene mass extinction may have had a limited effect on drilling intensity in ostracods. The cylindrical (Oichnus simplex) and parabolic (O. paraboloides) drill holes from Argentina may have been caused primarily by naticid and possibly muricid gastropods. Two oval drill holes (O. ovalis) are morphologically similar to octopod drill holes, but their small size, the fact that extant octopods are not known to drill ostracods and the absence of such holes in co-occurring gastropods, preclude ascription to a predator clade. Drill holes are located preferentially in the median and dorsal regions, where most soft tissue, including the adductor muscle, is located. Drilled specimens are statistically taller than non-drilled specimens for T. argentinensis and larger predators selected larger ostracods. The drilling percentage does not significantly differ in ornamented ostracods.

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This paper presents the results of a paleoenvironmental study of two Hauterivian–Aptian adjacent sections (Transnordestina A/B) of the Iguatu Basin using ostracods and aided by X-ray fluorescence (XRF), total organic carbon (TOC), total sulfur (S), and spectral analyses. Cluster analysis divided the 10 genera found into two main groups: one composed of Alicenula-Pattersoncypris?-Brasacypris-Hastacypris-Ilyocypris?, and the second composed of Cypridea-Looneyellopsis-Rhinocypris?. The first group is interpreted as representing paleoenvironments with permanent waterbodies, such as lakes, and the second as being indicative of ephemeral settings, such as temporary pools in an inundation plain. XRF analysis using the positive peaks of Ca/Ti and Ca/ΣTi, Fe, Al ratios show a few dry periods, particularly in the lower and middle part of Transnordestina A, between 0 to 175 m and 385 to 475 m, which also display a small number of ostracods. Total S shows several high peaks which might be indicative of gypsum deposition during droughts. Spectral analysis of molar Ti/Al ratio shows two intervals with different sedimentation rates, between 0 and 233 m, and 233 to 836 m. For the first interval, six 100 ka eccentricity cycles were identified; the low number of ostracods for the interval and highly variable Ca/Ti and Ca/ΣTi, Fe, Al values indicate a different sedimentation rate in a drier environment. For the second interval, ten 405 ka cycles were identified; its higher ostracod count and more stable Ca/Ti and Ca/ΣTi, Fe, Al values could be indicative of increasing humidity.

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