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The ecological structure of ancient marine communities is impacted by the environmental gradients controlling assemblage compositions and the heterogeneous distribution of sediment types. Closely spaced, replicate sampling of fauna has been suggested to mitigate the effects of such heterogeneity and improve gradient analyses, but this technique has rarely been combined with similar sampling of lithologic data. This study analyses lithological and faunal data to determine the environmental gradients controlling the composition of Mississippian fossil assemblages of the lower Madison Group in Montana. Eighty-one lithological and faunal samples were collected from four stratigraphic columns in Montana, which represent the deep-subtidal, foreshoal, and ooid-shoal depositional environments within one third-order depositional sequence. Cluster analysis identifies three distinct lithological associations across all depositional environments—crinoid-dominated carbonates, peloidal-crinoidal carbonates, and micritic-crinoidal carbonates. Cluster analysis and nonmetric multidimensional scaling (NMS) identifies a highly diverse brachiopod biofacies and a solitary coral-dominated biofacies along an onshore-offshore gradient. Carbonate point count data and orientation of solitary corals indicate that substrate and wave energy are two potential variables that covary with the onshore-offshore gradient. Overlaying lithological information on the NMS indicates a secondary gradient reflecting oxygen that is expressed by increasing bioturbation and gradation from brown to dark gray carbonates to medium-light gray carbonates. Taken together, these findings demonstrates how combining closely spaced, replicate sampling of lithologic and faunal data enhances multivariate analyses by uncovering underlying environmental gradients that control the variation in fossil assemblages.
Plio?-Pleistocene outcrops located at the southwestern edge of the Guadalquivir Basin in the area of Lepe (Huelva, Spain) provide an interesting example for studying the contemporaneity of traces with the rocks that contain them. Two different types of cells compatible with the ichnogenera Celliforma (Type 1) and Palmiraichnus (Type 2) were found in these outcrops. Their walls were constructed with the same material as the matrix and our first research in the area showed no extant bees producing them suggesting that they were coeval with the trace-bearing rocks. The case of the “Palmiraichnus-like” Type 2 cells was misleading because of its similarity with Palmiraichnus described from the region in the Canary Islands and Balearic Archipelago (Spain). Two determining features were vital in clarifying this first appearance. In the Palmiraichnus-like cells we found remains of a larval cocoon in one cell that could be dated by C14, giving a modern age. In the Celliforma-like cells more field research in the area allow us to observe extant bees nesting in these rocks in autumn. Ichnological literature show a few cases of asynchronies involving extant traces found mostly in Paleozoic and Mesozoic rocks. In contrast, the case presented herein indicates the time gap between the bearing rocks and the Lepe traces was shorter (ca. 12 ky–2.6 My), enhancing the similarity of traces and rocks and thus their potential coevalness. This case may serve as a warning about other potential examples in the fossil record in which relatively short asynchronies between traces and paleosols exist.
The insect trace fossil Laetolichnus kwekai, which is composed of a small chamber extending to slender cylinders at each end, was tentatively included in the ichnofamily Krausichnidae as termite nests. New evidence presented here provides information to validate these inferences. A more complex structure formed by interconnected Laetolichnus was recently found in the same Pliocene deposits (Laetoli, Tanzania) as the isolated specimens reported previously. Our study confirms inclusion of Laetolichnus in Krausichnidae and supports the inference that it represents a nest of a social insect. Neoichnological field studies in the coastal dunes of Buenos Aires Province, Argentina, were undertaken to refine further the nature of these affinities. Survey of the dune surface revealed abundant loose fragments of termite nests of a size and shape comparable to that of L. kwekai. The fragile nests constructed by Onkotermes brevicorniger, which are described here in detail for the first time, enable us to interpret the fossil structures. They consist of connected chambers similar to the interconnected Laetolichnus. These were frequently exposed and broken by wind action resulting in loose fragments similar to the isolated Laetolichnus. The Celliforma ichnofacies represented at Laetoli, which contains L. kwekai, indicates arid or semiarid shrublands and woodlands. The distribution of O. brevicorniger also corresponds to arid and semiarid shrublands and dry woodlands of Argentina. Although the African termite producer of L. kwekai and the South American Onkotermes would be phylogenetically unrelated, the analogous structures probably reflect convergent nesting behaviors as an adaptation to similar arid to semiarid environmental conditions.