FOOTPRINTS, FOOD WEBS, AND FACIES: UNRAVELING ECOSYSTEM DYNAMICS IN THE EARLY JURASSIC HARTFORD RIFT BASIN (USA)

Fluvial and marginal lacustrine sedimentary rocks of the Hartford basin (Connecticut and Massachusetts) are noted for abundant ichnofossils, such as tracks, trails, burrows, and root casts, produced by dinosaurs, crocodylomorphs, fishes, insects, crustaceans, mollusks, and plants. The high preservation potential and distribution of floodplain, playa, and lake shoreline paleoenvironments is also documented by exquisitely preserved sedimentary structures, including raindrop impressions, oscillatory and current ripples, desiccation cracks, evaporite crystal casts, rill marks, and more.

Tetrapod ichnofossil taxa are not evenly distributed across all facies; this may be attributable to facies-linked preservation bias or ecological partitioning. Among dinosaur tracks, the ichnogenus Eubrontes (large carnivorous theropod) is limited to lake shoreline sandstones and siltstones. In contrast, tracks made by large herbivorous prosauropods (Otozoum) are restricted to marginal fluvial facies and are notably lacking in lake margin deposits. Unlike these two facies-restricted ichnogenera, smaller theropod tracks (Grallator and Anchisauripus) and crocodylomorph tracks (Batrachopus) occur in a wide variety of facies, and may be found in association with both Eubrontes and Otozoum.

The combination of ichnofossils, body fossils, plant remains, and sedimentary features allows reconstruction of likely habitats and food webs, wherein certain patterns emerge. Of primary consideration is the apparent environmental partitioning of tracks of the dominant large predator, the Eubrontes trackmaker, from the footprints of terrestrial prey, in particular, the herbivorous forms represented by Otozoum. This raises a fundamental question: What did the abundant large theropods consume?

We suggest that the facies distribution of tracks, combined with the morphological features of similar large theropods, as well as rare occurrences of fish remains on track-bearing surfaces, indicates that the Eubrontes trackmakers primarily subsisted on fishes (semionotids and redfieldiids), abundant in the productive nearshore lake waters. During extended dry intervals, fishes trapped in rapidly shoaling shallows were easy prey for the opportunistic theropods.